The invention relates to a retrofit kit assembly for converting a hydrocarbon gas combustion appliance, in particular a gas boiler, and more particularly for a condensing gas boiler, to a combustion appliance for combustion of fuel gas comprising more than 20 mol% hydrogen. Additionally, the invention relates to a combustion appliance comprising said retrofit kit assembly. Furthermore, the invention relates to the use of the retrofit kit assembly for converting a natural gas combustion appliance, in particular a natural gas boiler, into a combustion appliance, in particular boiler, for the combustion of pure hydrogen and to a method for retrofitting a combustion appliance.

The emission of carbon dioxide is one of the most relevant factors contributing to the pollution in environment. Since the contribution from the building sector is continuously increasing in the last decades, there is the need to reduce CO2 emissions from this sector. Heating of spaces and heating of water are the two major causes of energy consumption and CO2 emission from the building sector. Inefficient boilers and carbonintensive power can further worsen this problem.

Nowadays, the majority of boilers are gas boilers and are designed for natural gas, using hydrocarbon gases as fuel gas. Gas boilers combust gas fuel to heat water for domestic use and/or central heating systems in buildings. The market is looking into more sustainable alternatives with a lower 002-footprint to combusting natural gas. One of these alternatives is combusting pure hydrogen. It is noted that gas boilers combusting pure hydrogen (i.e. hydrogen boiler) are boilers to which fuel gas is supplied that comprises at least 98 mol% hydrogen. Currently, there are natural gas (or propane) boilers on the market which are only suitable to combust up to 20 mol% hydrogen into the gas blend (according to the specifications). In other words, current boilers on the market are not directly suitable for combustion of higher concentrations of hydrogen, in particular pure hydrogen, and important modifications are needed to possibly convert a standard natural gas boiler into a hydrogen gas boiler. These modifications commonly are expensive and time consuming. In addition, the converted natural gas boiler needs to run safely on hydrogen and needs to comply with safety critical requirements as safety defects can have serious consequences. Converting an existing natural gas appliance incorrectly can lead to defects of the resulting hydrogen appliance. One of the main sources of defects in a conversion is human error. There is a relationship between several types of human errors and defects in the appliance. Intentional errors, misunderstanding of instructions, forgetfulness, misidentification, inexperience, slowness, non-supervision, surprise. These human errors can lead to not following procedures, processing errors, errors in set up, missing parts, wrong parts, misoperation, and adjustment errors. Therefore, one of the main problems to be solved is the reduction or elimination of human errors in the conversion of natural gas boilers to hydrogen boilers.

Relying on training and work instructions to prevent errors alone is insufficient to reduce human error effectively. Available data indicates that no matter how much training a person receives or how well the process is documented, human error occurs. While application of standard work practices and training are valid methods for reducing the frequency of errors, they will not prevent errors from occurring. In addition, trying to rely only on training makes the conversion process slow, as there would need to be several safety checks.

Mistake-proofing or Poka Yoke ideally ensures that the product or process design itself prevents mistakes before they occur. Good Poka Yoke devices in addition are simple and inexpensive. By converting a natural gas boiler to a hydrogen boiler, there is a risk of explosion if there is a human error in the part setup regarding safety critical parts for combustion of hydrogen. Such human error can lead to defects in operation of the converted hydrogen boiler.

It is therefore desirable to obtain an easy and relatively low-cost conversion between a standard natural gas boiler and a hydrogen boiler which helps in reducing human error in the conversion. It is also desirable that the conversion is carried out providing behavior shaping constraints which reduce safety critical part related human errors.

EP 3 524 884 A1 is directed to providing a retrofit assembly for a fuel gas boiler that that reduces polluting emissions and/or increases the yield and/or reduces problems in the ignition phase and discloses a retrofit assembly for a fuel gas boiler, the boiler comprising a fuel gas burner, a feeding assembly for supplying fuel gas to the burner, and a control unit for controlling the feeding assembly. In particular, the retrofit assembly comprises a processing unit configured to acquire a first control signal of the feeding assembly configured to control the feeding assembly, a second signal correlated to the exhaust gas or fuel gas composition, the processing unit being configured to define a third control signal of the feeding assembly configured to control the feeding assembly and based on the second signal and on the first signal. The retrofit assembly is configured to be installed in the boiler and to control the flow rate of the fuel gas by means of the third signal.

Although directed to a conversion system for a gas boiler for reducing the polluting emissions of the boiler, this document only discloses the retrofit of a control system, i.e. the modification of the setting parameters for increasing the yield of the boiler. EP 3 524 884 A1 does not disclose how to reduce or eliminate human errors in the conversion of a natural gas boiler to a hydrogen boiler.

KR 2007 0097930A is directed to providing a gas burner and a flame detection method which ensures safe use when using and allowing for a visual check of the flame when ionization does not occur during combustion, by adding a fuel additive for generating an ion current during combustion and discloses a mixing pipe in which an air supply pipe and a fuel supply pipe are installed in communication with each other, an air supply fan connected to one end of the air supply pipe, and a gas burner that is connected to the other end of the air supply pipe to burn the mixture. A fuel additive supply pipe is installed in communication with the mixing pipe so that the fuel additive can be supplied and mixed. The fuel additive supply pipes are installed in the air supply pipe of the gas burner. The gas burner further comprises an air supply fan and a mixing pipe in which a fuel supply pipe for supplying fuel is installed, and a burner for generating a flame. The air supply fan is coupled to one end of the air supply pipe of the mixing pipe to supply air to the air supply pipe. The fuel supply pipe supplies hydrogen fuel, which is fuel. The fuel additive supply pipe is installed on one side of the outer circumferential surface of the fuel supply pipe to communicate with the fuel supply pipe. So, hydrogen fuel as fuel is supplied through the fuel supply pipe in the fuel supply direction, and hydrocarbon fuel (LNG, LPG, propane gas, butane gas, etc.) as a fuel additive is supplied in the fuel additive supply direction. By being supplied through the fuel additive supply pipe, it is mixed with the fuel, hydrogen fuel. In addition, the fuel supply pipe is installed on one side of the outer circumferential surface in communication with the air supply pipe, thereby supplying hydrogen fuel and a small amount of hydrocarbon fuel as a fuel additive to the inside of the mixing pipe. The mixing pipe further is connected to the fuel additive supply pipes spaced apart from each other on the outer circumferential surface of the air supply pipe so that the fuel additive is added inward in the fuel additive supply direction. A small amount of the supplied fuel additive is ionized during combustion in the burner combustion unit to generate an ion current in the flame, so that the flame detection unit can detect the ion current. The ignition device unit is composed of an ignition rod and an ignition transformer, and the flame detection unit is composed of a flame detection rod (frame rod) and an ion current measuring device. The ion current measuring device is connected to the controller. The ignition rod ignites the mixture discharged from the flame forming mat by high voltage discharge, and the flame detection rod senses ion current generated from the ignited flame. The ion current measuring device detects the magnitude of the ion current generated when hydrocarbon fuel, which is a fuel additive, is burned through the flame detection rod, and transmits it to the controller so that the controller controls the burner. KR 2007 0097930A discloses that by doing so, it is possible to ensure the safety of the burner.

The disclosed gas burner using hydrogen would not function for a conversion for natural gas boiler to a boiler allowing for the combustion of (pure) hydrogen as fuel as disclosed, because a fuel additive line is needed to ensure that a flame can be detected by ionization flame detection. This fuel additive line or source is not present in natural gas boilers. In other words, conversion of a natural gas combusting appliance to an appliance using the disclosed gas burner would not be contained to the conversion of the appliance itself as there would be additional fuel additive supply lines or sources needed as otherwise the flame detection would not be possible by ionization flame detection as disclosed in KR 2007 0097930A. Thus, without additional changes the converted appliance would be inherently unsafe within the meaning of KR 2007 0097930A.

DE 10 2020 117692 A1 is directed to providing a gas burner device which can also be operated with hydrogen and discloses a gas burner wherein the porosity of the burner surface portion is more than 35%. The disclosed gas burner device, which is particularly preferred a full premixing burner, consists in a known manner of a flame body 1 , which is provided with a burner surface portion 2 which has holes 2.1. A gas-air mixture is burned at the burner surface portion 2 to operate this gas burner device. Each hole 2.1 has a hole area and the burner surface section 2 has an area ratio porosity resulting from a sum total of the hole areas and the area of the burner surface portion 2 . The gas burner device has a modulation capability of 1 :5, i.e. it can work without problems in the range between 20 and 100% of its capacity. The porosity is particularly preferably provided that this less than 60%, preferably between 40% and 50%. The gas is also preferably provided that this at least 25%, preferably at least 50%, particularly preferably at least 75%, very particularly preferably more than 90% hydrogen is formed. The each remaining portion preferably consists of natural gas or the like. Remeha Selects System (2003-09-16) is a brochure for natural gas boilers and discloses the parts of the boilers including the part numbers for ordering the respective parts and their respective position in the boiler structure.

US 2022/003406 A1 is directed to a system and process for hydrogen combustion for industrial or steam generation applications and a burner design that is retrofitted into existing burner systems and discloses a system and process for hydrogen combustion, and more particularly to a combustion burner or retrofit kit combustion system and process having at least one burner nozzle using pure hydrogen as a primary fuel source. The system and process may also use the primary pure hydrogen fuel with one or more secondary fuels and/or a flame temperature reducing fluid for lowering a bulk flame temperature of the burner or retrofit kit combustion system. The combustion burner or retrofit kit combustion system and process can be incorporated into a boiler system of any type of design, such as firetube, watertube, utility, single burner, multiple-burner, side-fired, bottom-fired, roof-fired, tangentially-fired, either skid-mounted or field-erected, or a combination thereof. The hydrogen combustion burner or retrofit kit combustion system has a primary pure hydrogen fuel stream delivered to at least one burner nozzle where it burns with an oxidant. The disclosed hydrogen combustion burner or retrofit kit combustion system utilizes a single burner nozzle for combusting the primary pure hydrogen fuel and the oxidant (e.g., air, pure oxygen, oxygen-enriched air). A primary fuel flow valve and control selectively controls the primary fuel fluid flow to the burner for combustion with the oxidant.

Therefore, this and other prior art documents fail to address the problem of safely converting a hydrocarbon gas combustion appliance combusting a type of fuel gas, such as natural gas, into a gas boiler combusting another type of fuel gas, such as hydrogen, in particular pure hydrogen wherein the risk of human error is reduced.

One further specific risk in converting a natural gas boiler to a hydrogen boiler is that the control of the mixture velocity is more important when the boiler uses hydrogen as fuel gas rather than other fuel gasses such as methane. In fact, flashback can occur more easily in hydrogen boilers since the laminar flame speed of hydrogen air mixture is around eight seven times higher than the flame speed for methane air mixture (with reference to the stoichiometric condition). It is therefore desirable to mistake-proof the retrofitting of a natural gas boiler in such a way that also the hydrogen combustion risk is reduced in such a way, that the risk of human error in parameter setting or forgetting to change said parameter setting is reduced. The object of the invention is therefore to provide a retrofit kit assembly that is safe and easy to implement and that is effective in converting a natural gas combustion appliance, in particular a boiler (hydrocarbon gas boiler), into a hydrogen combustion appliance, particular a hydrogen gas boiler while reducing the risk of human error, in particular the simple, safe and effective reduction of the risk of flashbacks, in particular at the first operation of the retrofitted combustion appliance.

The object is solved by a retrofit kit assembly for converting a hydrocarbon gas combustion appliance, in particular a gas boiler, and more particularly for a condensing gas boiler, to a combustion appliance for combustion of fuel gas comprising more than 20 mol%, in particular more than 30 mol%, hydrogen, the kit assembly comprising: a frame structure fixable to a housing of the combustion appliance for, in particular fully, closing a burner chamber of the combustion appliance; and wherein the frame structure comprises a receiving portion for receiving at least one Poka Yoke configured hydrogen combustion component.

Thanks to the retrofit kit assembly, it is possible to convert a combustion appliance such as a hydrocarbon gas boiler into a hydrogen gas boiler in a very easy and safe way while reducing human error by way of the concept of prevention through design. The retrofit kit assembly according to the invention is in addition simple and inexpensive. In other words, the conversion can be realized in a very short time and only requires replacing a minimum number of components. In particular, it is not necessary to connect or disconnect a plurality of cables and/or to attach or detach a plurality of sensors which can lead to defects such as errors in part setup, missing parts, wrong parts, adjustment errors, mis-operation, or not following procedures due to human errors connected to these defects, in particular forgetfulness, inexperience and misidentification and above all inadvertent error which is strongly connected to most of these defects. Thus, the retrofit kit assembly according to the invention reduces a number of human error sources such as inadvertent error, misunderstanding, forgetfulness, misidentification, and inexperience by reducing the number of parts to be disconnected, replaced and connected to a minimum. In fact, the present retrofit kit assembly is ready to be mounted in a combustion appliance, in particular a (natural) gas boiler, by simply fixing the frame structure to a suitable housing of the combustion appliance. In addition, the modular nature of the assembly allows the possibility to provide a combination of different components suitable for a hydrogen gas combustion appliance depending on and optimized to the configuration and working principle of the respective (natural gas) combustion appliance to be converted. Thus, the retrofit-kit assembly according to the invention has overall lower skill-requirements for the conversion which leads to increased safety and reduced conversion time.

The retrofit kit according to the invention allows a constructively easy pre-determined orientation of the location of the manifold structure, in particular of the first and second connection, and thereby provides a technical behaviour shaping constraint in view of safety critical parts for the combustion of hydrogen, thereby reducing safety risks, such as the incorrect connection of hydrogen and air.

Such an embodiment has the advantage that it also ensures in an easy way that the requirements of the hydrogen-fire gas appliance guide PAS4444:2020 are fulfilled according to which a post blower mixing is needed. For this purpose, in the present retrofit kit assembly the manifold structure is provided with a connection for fuel gas (i.e. for a gas valve) that is located downstream the connection for receiving air (i.e. for a fan element). Downstream refers to the air flow in the manifold. The embodiment has the further advantage, that misidentification, forgetfulness, or inadvertent error are further reduced by way of product design, thus reducing defects caused by not following procedures, missing or wrong parts, improper setup, and errors in part setup itself.

In an alternative embodiment a pre blower mixing is possible. In that case the manifold structure is provided with a connection for fuel gas, in particular for a gas valve, that is located such that fuel gas is sucked by a fan. Thus, the fan has to consist of spark-free material.

The burner can be connected or is connected to the manifold structure at the outlet portion for receiving a gas mixture to be combusted. Thus, a compact shape retrofit kit assembly is achieved. In addition, this embodiment has the advantage that. Is decreases set-up time even further with associated reduction in set-up errors and thus even further improved quality and safety.

The frame structure, in particular the receiving portion, can be configured such that the hydrogen combustion component can be firmly fixed to the frame structure. Firmly fixed means that no relative movement is possible between the frame structure and the hydrogen combustion component. The receiving portion can be any kind of a portion of the frame structure in which the hydrogen combustion component can be at least partly arranged and/or to which at least a part of the hydrogen combustion component can be attached.

The pre-determined orientation and design of the frame structure of the conversion kit ensures that the correct frame structure is used and also ensures through a simple, constructive behaviour shaping constraint the correct hydrogen conversion kit is used to ensure the use of safety critical parts for proper combustion of hydrogen to mitigate an explosion risk due to an incorrect conversion kit.

Poka Yoke configured hydrogen combustion configured components according to the invention, are components which are safety critical for the correct combustion of hydrogen to prevent connecting natural gas components by mistake.

Poka Yoke within the meaning of the invention relates therefore to hydrogen combustion safety critical components which are physically, structurally, operatively and/or electrically configured to prevent inadvertent hydrogen combustion errors. Physical or structural configuration to prevent inadvertent errors comprises connections such as plugs which only fit in the correct port, so that an improper mounting is prevented physically. Operatively or electrically configured means that if the connection is not the proper connection, safety critical operation of the converted boiler, in particular combustion, is prevented and/or an error signal is given to also visualize the error. Suitable examples comprise that e.g. a UV sensor is mounted in a location of the frame which location is distinct from the position of an ignition probe of the natural gas boiler to be converted. The UV sensor preferably has a Poka Yoke configured plug which prevents insertion in the natural gas boiler control unit. Such a conversion kit makes it only possible to mount and connect the hydrogen combustion safety relevant components in the correct way. Thus, it is no longer necessary to include timeconsuming checks of every connection based on e.g. a manual and requires fewer connection to be disconnected and re-connected based on a manual, which also contributes to the reliability of the proper mounting, conversion and operation of the boiler. Preferably, in order to prevent the user from mistaking connection, mounting sequences and for visualizing the same in a constructively easy manner, the Poka Yoke configured components contribute to reliable and easy detection and resolving of hydrogen combustion safety related components during conversion. This enhances safety of the conversion of a natural gas boiler to a hydrogen boiler while saving time for error detection and individual checks. The Poka Yoke configured component can for example be a Poka Yoke configured connector. Such a Poka Yoke configured connecter according to the invention means a connector which is configured to prevent inserting of the Poka Yoke configured connector in a natural gas configured port.

It will be understood that for the purposes of the invention, it is not sufficient that just the polarity or orientation of a connector is correct as this is not sufficient to prevent human error in e.g. connecting the wrong sensor with the wrong control unit or an incorrect port on a control unit and thereby creating a safety risk. For example, if the control unit of the natural gas boiler is to be replaced with a hydrogen control unit, it would be insufficient if only polarity or orientation would be considered a Poka Yoke design for this invention. The polarity or orientation e.g. of a connector of a UV sensor could not prevent connection of the UV sensor to the natural gas control unit. In such a case, all parts could be connected to the natural gas control unit whilst the operation parameters remain the ones for natural gas, which could cause a safety risk at initial start-up or first ignition attempt, as the parameters could allow for a potentially explosive mixture of hydrogen and air.

Thus, Poka Yoke designs within the meaning of the application are designs which either prevent or detect the occurrence of an error in the setup regarding safety critical parts for combustion of hydrogen. The configuration of the Poka Yoke design, in other words, needs to be configured such that safety critical errors regarding the combustion of hydrogen are prevented during retrofitting or for easy detection before operation of the retrofitted hydrogen combustion appliance, such as a hydrogen boiler. Suitable examples of Poka Yoke designs comprise unique connectors which will allow for the fit of only the right e.g. sensor on the right board. This can be by hardware design, such as for example unique asymmetry or color coding which identifies the correct connection on both parts to be connected. Further examples of suitable Poka Yoke designs include software designs that, e.g., identify the sensor that has been connected to a control unit and will prevent operation and/or raise an error in case the wrong connection has been made. Further suitable examples can also comprise combinations of software and hardware designs.

Unique connectors within the meaning of this application comprise, for example, unique shapes. Unique shapes or configurations ensure that a unique part only matches with its predetermined counterpart. This ensures that the part can only be connected in the correct orientation and to the correct mating part. Further suitable designs for unique connectors include predetermined different sizes, keying features such as notches, grooves, or tabs, mechanical interlocks that only engage when the correct parts are connected, and/or color coding to help users identify the correct connections. Color coding is often used in conjunction with other unique features to provide a visual aid and add an extra layer of error-proofing, but can also be used on its own. Further suitable examples of Poka Yoke designs, comprise asymmetrical designs of the unique connectors or mechanical restrictors. It is understood that for a Poka Yoke design within the meaning of this application, the unique connector preferably comprises a unique shape and a further Poka Yoke design, such as a color-coding which identifies the connector and the respective part or parts to be connected.

Thus, it is ensured that the hydrogen consumption component can only be connected with a port of the control unit that is configured to receive said connector. Thus, a misconnecting of hydrogen consumption components to the control unit can be prevented in an easy manner.

The hydrogen combustion component can be any component of the kit assembly that is needed for hydrogen combustion and has to be electrically connected to the control unit.

The control unit can be a hydrogen combustion control unit configured to control a burner configured for hydrogen combustion. In said case the hydrogen combustion control unit is a control unit for controlling hydrogen combustion. Thus, the hydrogen combustion control unit is electrically connected with all hydrogen combustion components that are needed for the hydrogen combustion control. The hydrogen combustion component can be a UV-Sensor and/or thermal flame detection sensor.

The provision of the hydrogen combustion control unit for controlling the burner for hydrogen combustion has the advantage that the installer does not need to adapt and/or to exchange the natural gas combustion control unit. Thus, there is no risk that hydrogen combustion will be controlled on the basis of control signals sued for hydrocarbon gas combustion avoiding risky situations. The connection of the hydrogen combustion control unit and the frame structure can be such that the hydrogen combustion control unit is moved when the frame structure is moved.

The control unit can be a natural gas combustion control unit configured to control a burner configured for natural gas combustion. In the frame of retrofitting the hydrocarbon gas combustion appliance the natural gas combustion control unit can be removed or it can be electrically connected with the hydrogen combustion control unit that is configured to control the burner for hydrogen combustion.

The control unit, in particular the hydrogen combustion control unit and/or the natural gas combustion control unit, can be arranged in the receiving portion of the frame structure. Additionally or alternatively, the control unit can be mechanically connected to the frame structure.

Within the meaning of this application a retrofit kit assembly includes all the hardware and software that enables safely changing the content of the fuel supply line of an existing combustion appliance. In a preferred embodiment, the retrofit kit assembly is contained to the appliance to be retrofitted itself. In other words, there is no need to undertake major modifications outside the appliance, such as the addition of additional fuel or fuel additive supply lines or sources. In a further embodiment, the retrofit kit assembly is suitable for combustion appliances for providing domestic hot water and/or heating, in particular central heating.

In an embodiment the retrofit kit is suitable for retrofitting a combustion appliance for climatization of a building or at least one room within a building. Climatization of a building within the meaning of this application means that the appliance can provide central heating and/or domestic hot water. In a preferred embodiment the climatization of a building or at least one room within a building requires a heat output below 100 °C, in particular 40 to 85 °C for central heating and 40 to 75 °C for domestic hot water.

In a further embodiment, the combustion appliance is a modulating combustion appliance, in particular boiler, meaning that the heating output can be adapted to a heating demand. In other words, a modulating combustion appliance is able to modulate its power in real-time in order to provide the exact level of heat needed. A modulating combustion appliance, such as a boiler is more energy-efficient than a non-modulating boiler in terms of fuel gas usage. The modulating combustion appliance therefore doesn’t continuously work at 100% of its power. When the modulating combustion appliance operates, the modulating combustion appliance only uses a percentage of its power and only how much the modulating combustion appliance needs to maintain a set temperature. The amount the modulating combustion appliance uses depends on the ambient temperature, the external temperature, the number of people currently in the household, and other similar parameters. The modulating combustion appliance does not start, switch off or remain at full power unnecessarily. By avoiding unnecessary start- ups, which typically result in a fuel spike, energy efficiency can be improved compared to non-modulating combustion appliances. Modulating combustion appliances can operate with more or fewer modulation stages. The greater the number of modulation stages, the greater the energy efficiency. Suitable numbers of modulation stages comprise more than 1 modulation stage, in particular more than 1 modulation stage up to 10 modulation stages, in particular at least 3 modulation stages, in particular at least 4 modulation stages, in particular at least 5 modulation stages, in particular at least 10 modulation stages. Modulating combustion appliances include various parts which are configured to modulate in a predetermined number of stages, including for example modulating burners, modulating exchangers, modulating thermostats, modulating coils, modulating electronic cards, modulating controllers or control units, and modulating pumps.

In a further embodiment the combustion appliance is a heat and/or hot water appliance. In a further embodiment the combustion appliance is configured to be integrated in a building management system.

In an embodiment the combustion appliance is a heating appliance wherein the output temperature is > 85 °C, in particular 90 to 100 °C.

In a further embodiment the combustion appliance has a output power of 4 kW to 2 MW, in particular 4 kW to 100 kW, 4 kW to 80 kW, 4 kW to 60 kW, 4 kW to 50 kW, 4 kW to 40 kW; in particular 6 kW to 100 kW, 6 kW to 80 kW, 6 kW to 60 kW, 6 kW to 50 kW, 6 kW to 40 kW; in particular 12 kW to 100 kW, 12 kW to 80 kW, 12 kWto 60 kW, 12 kW to 50 kW, 12 kW to 40 kW; in particular 20 kW to 40 kW; in particular 30 kW to 2 MW, in particular 30 kW to 150 kW, in particular 30 kW to 320 kW, in particular 40 kW to 150 kW, in particular 40 kW to 320 kW, in particular 45 kW to 150 kW, in particular 45 kW to 320 kW.

The control unit, in particular the hydrogen combustion control unit and/or the natural gas combustion control unit, can comprise further ports. The further ports, in particular all further ports, are not configured to be configured with the Poka Yoke configured connector of the hydrogen combustion component. In particular, the further ports can be assigned to a specific hydrogen combustion component if the control unit is a hydrogen combustion control unit. Thus, it is ensured that each of the hydrogen combustion components is connected to the correct further port. If the control unit is the natural gas combustion component, the further ports can be connected to natural gas combustion components.

In an embodiment a hydrogen combustion control unit and a natural gas combustion control unit can be provided. The hydrogen combustion control unit can be electrically connected with the natural gas combustion control unit. The provision of the hydrogen combustion control unit has the advantage that the correct control signals are transmitted to the components of the kit assembly. In particular, the correct control signals for combusting fuel gas having more than 20 mol% hydrogen are provided. Thus, an explosion risk is reduced. A further advantage of the hydrogen combustion control unit is that the burner can always be controlled correctly.

As discussed above, the hydrogen combustion control unit comprises a port for receiving the Poka Yoke configured connector of the hydrogen combustion component, in particular a UV-sensor and/or flame detection sensor. The provision of such a port ensures that the hydrogen combustion component, in particular the UV sensor and/or flame detection sensor, can be easily connected with the hydrogen combustion control unit. The form of the port can be different of other ports of the hydrogen combustion control unit and/or of the ports of the natural gas combustion control unit. Thus, it is avoided that other electric components, in particular an ignition electrode, can be wrongly connected with the hydrogen combustion control unit.

For example, the frame structure can be fixable to the housing of a heat exchanger present in a combustion appliance, in particular a (natural) gas boiler. Generally speaking, a heat exchanger facilitates the transfer of heat derived from the combustion of fuel gas and air present in circulating conduits. Therefore, the housing of the heat exchanger usually contains the burner of the combustion appliance, in particular gas boiler, for combusting the fuel gas. It is noted that the main factors distinguishing a natural gas combustion appliance from a hydrogen gas combustion appliance are related to the combustion aspects of the fuel gas and that the functioning of the heat exchanger itself remains basically the same. Therefore, the present retrofit kit assembly is used to replace fundamental components for the combustion, such as the burner or a flame detection means, in order to convert a natural gas combustion appliance to a hydrogen gas combustion appliance. For instance, the present retrofit kit assembly further comprises a burner configured for hydrogen gas combustion. The burner configured for hydrogen combustion can preferably be fixed to the frame structure. The frame structure comprises a first portion and a second portion, wherein the burner is fixed to said first portion. The second portion extends longitudinally from the first portion. The frame structure being shaped as to cover at least partially, in particular fully, the housing, in particular a burner chamber, of the combustion appliance. In particular, the frame structure, and specifically the second portion of the frame structure is formed as a plate i.e. as a front cover for the internal housing of the combustion appliance.

In case the internal housing is the housing of a heat exchanger, the frame structure can work as a front cover of said heat exchanger. In particular, the first portion of the frame structure is interposed, in particular in flow direction of the air and fuel gas mixture, between the burner and the outlet portion of the manifold structure. This increases the compactness of the retrofit kit assembly. This has the additional advantage that it facilitates proper placement and detection of errors is simplified even further due to the fact that already the frame structure itself ensures proper placement of the retrofit kit assembly according to the invention and, thus, avoids misplacement by an installer. Given that the frame structure is a comparatively large structure, any misalignments or misplacements are at the same time made harder to do and at the same time makes detection very easy without requiring an in-depth analysis as would be required if all connections undone and done would need to be inspected to detect a defect in the conversion setup.

The retrofit kit assembly reduces the work amount for converting the combustion appliance to a hydrogen combustion appliance and reduces the danger that the retrofit kit assembly is assembled incorrectly. In particular, more components can be mounted during production, further reducing the time needed for conversion and further reducing the risk of human error in assembly.

A further advantage is that fewer individual parts need to be connected and disconnected and thereby further reducing potential errors, in particular in making a proper connection, or connecting the right parts. Additionally, the pre-assembled parts can already be tested in the factory and checked on leakage.

Combustion appliances such as gas boilers using natural gas are criticized due to their carbon dioxide emissions. The use of more than 30% hydrogen mixtures of natural gas up to 100 % hydrogen has the advantage to reduce the CO2 footprint in heating solutions. However, also the conversion needs to be sustainable. This is achieved by using the retrofit kit assembly and not exchange the entire combustion appliance, in particular boiler, which still has a lifespan of its own.

It is understood that within this application a combustion appliance can be a gas boiler. In this application references to a gas boiler are a mere reference to a preferred example of the term combustion appliance.

Fuel gas can comprise more than 20 mol% hydrogen. In particular, fuel gas can comprise more than 50 mol%, in particular more than 90 mol% hydrogen or be pure hydrogen. Pure hydrogen is defined as comprising at least 98 mol% hydrogen (hydrogen-fire gas appliance guide PAS4444:2020).

Natural gas is a naturally occurring hydrocarbon gas mixture, comprising methane and commonly further comprising varying amounts of among others higher alkanes, carbon dioxide, nitrogen, hydrogen sulfide or helium. The hydrocarbon gas can also comprise or consist of propane. In the application a hydrocarbon combustion appliance is an appliance in which natural gas is combusted.

According to an embodiment, the retrofit kit can optionally further comprise a burner configured for hydrogen combustion fixed to the frame structure. Thereby, more components can be mounted during production, further reducing the time needed for conversion and further reducing the risk of human error in assembly.

According to an embodiment the frame structure comprises a hydrogen combustion control unit configured to control a burner configured for hydrogen combustion wherein the control unit is connected to the frame structure. The hydrogen combustion control unit can be electrically connected with the natural gas combustion control unit. The provision of the hydrogen combustion control unit has the advantage that the correct control signals are transmitted to the components of the kit assembly. In particular, the correct control signals for combusting fuel gas having more than 20 mol% are provided. Thus, an explosion risk is reduced. A further advantage of the hydrogen combustion control unit is that the burner can always be controlled correctly.

The control unit can comprise a port for receiving the Poka Yoke configured connector of the UV-sensor. The provision of such a port ensures that the UV sensor can be easily connected with the hydrogen combustion control unit. The form of the port can be different of other ports of the hydrogen combustion control unit and/or of the ports of the natural gas combustion appliance control. Thus, it is avoided that other electric components, in particular an ignition electrode, can be wrongly connected with the hydrogen combustion control unit.

The hydrogen combustion component can be a UV-sensor and/or thermal flame detection sensor. The UV-sensor can comprise a Poka Yoke configured connector to prevent connecting of the UV sensor to a natural gas combustion appliance control unit. The thermal flame detection sensor can comprise a Poka Yoke configured connector to prevent connecting of the thermal flame detection sensor to a natural gas combustion control unit port. In this embodiment the Poka Yoke configured connector according to the invention means a connector which is configured to prevent inserting of the Poka Yoke configured connector in a natural gas boiler control unit port. Thus, it is ensured that the UV sensor and/or thermal flame detection sensor can only be connected with a control unit that is configured to control the burner for hydrogen combustion. This is explained below more in detail.

The natural gas combustion appliance control unit port is part of a natural gas combustion appliance control unit. Said control unit is configured to control the burner for natural gas combustion. In the frame of retrofitting the hydrocarbon gas combustion appliance said control unit can be removed or it can be electrically connected with the control unit that is configured to control the burner for hydrogen combustion.

In an embodiment, the retrofit kit further comprises a manifold structure comprising means for providing an air/gas mixture and the manifold structure further comprising an inlet portion and an outlet portion, wherein the inlet portion is configured to receive the air/gas mixture and wherein the inlet portion comprises a first connection for receiving at least fuel gas, and a second connection located downstream from the first connection and wherein the outlet portion is arranged such that the air/gas mixture exits the manifold structure through the outlet portion and wherein the outlet portion is connected to the frame structure.

The burner can be connectable or connected to the manifold structure at the outlet portion for receiving a gas mixture to be combusted. Thus, a compact shape retrofit kit assembly is achieved. In addition, this embodiment has the advantage that relevant parts are pre-mounted in production, which allows for thorough quality testing and thus leads to a lower number of parts needing to be assembled and tested during conversion. This decreases set-up time even further with associated reduction in set-up errors and thus even further improved quality and safety.

A gas burner configured for hydrogen needs be able to work at full power when there is a high heat demand. The gas burner should also be able to work at a lower power level, for example at 50% or 25% or 20% or 10% of the maximum power level, when there is only a low heat demand, another property of hydrogen is that the combustion temperature is about 300°C higher than the combustion temperature of methane. The burner deck temperature needs to stay below 585°C, the auto-ignition temperature of hydrogen at all times. In addition, a stable flame needs to be present taking account the high flame speed of hydrogen.

In an embodiment the gas burner can preferably be a pre-mixed gas burner, in particular a surface stabilized pre-mixed gas burner. Pre-mixed means, that a mixture of the fuel gas and the gas is supplied to the burner. In other words, the pre-mix gas burner is a type of burner where the fuel gas and the combustion air are mixed together before entering a perforated area and/or the combustion chamber. The fuel gas is preferably delivered at a constant pressure, and the combustion air is supplied through a separate air inlet. The fuel and air are mixed to form a homogeneous mixture before combustion. This burner has the advantage that it allows for efficient combustion, in particular by way of an easy control of the stoichiometric ratio. The burner also allows for low emissions of e.g. nitrogen oxide (NOx) due to the homogenous mixture of the fuel and air. In addition, it allows for a stable flame.

Pre-mix gas burners have the further advantage that a retrofit kit comprising a pre-mix gas burner enhances safety by constructively allowing for pre-selected components and the right adjustment for hydrogen as well as a pre-selected end-design of the converted combustion appliance which allows optimized performance for the gas combustion in terms of efficiency and emissions. It further does not require additional secondary fuels or temperature fluids to reduce unwanted emission profiles.

In a further embodiment the gas burner is a port injection burner. Port injection also referred to as late mixing means within this application that fuel is injected into the intake manifold or port and mixes with air just before entering the combustion chamber. This is still considered late mixing, as the fuel and air are not premixed, but rather combined just prior to combustion. In a further embodiment the gas burner is an injection or direct injection gas burner. Injection gas burner means that operates by injecting fuel gas into the combustion chamber or burner nozzle separately from the combustion air. The fuel gas and air mix and ignite within a combustion chamber. In a further embodiment the gas burner is preferably not an injection or direct injection gas burner. Injection burners may require additional measures to ensure flame stability, especially at low firing rates or when using certain fuel types. In addition, the combustion tends to be less efficient compared to a pre-mix gas burner.

The burner deck geometry can be adapted such that the temperature stays below the auto-ignition temperature of hydrogen at all times and that avoids a flame lift off. This can be achieved for example by a burner deck that comprises a sheet enclosing a chamber and having at least one protrusion with a through hole. The through hole is fluidically connected with the chamber wherein the protrusion comprises a concave section and/or a convex section, in particular concave section and convex section.

Due to this configuration of the protrusion, i.e. the presence of a concave section and a convex section, the flame front is maintained not so far from the burner deck - under the limit of the lift flame - and at the same time not so anchored on the deck surface - over the limit of the back flame. In this way, the burner deck according to the present invention focuses on a reduction of the risk of flashback and facilitates the lift instead of maintaining the flame attached to the burner. This is especially useful when employing a highly reactive gas, such as hydrogen, as fuel gas.

The concave and convex sections determine a particular aerodynamic of the protrusion and the corresponding through hole. In particular, a sort of Venturi effect is created when the gas mixture passes through the protrusion from the chamber of the burner to outside the gas burner. This aerodynamic helps the mixed flow to pass with a reduced local pressure loss and the flow is guided towards the outside without any recirculation. Additionally, the gas mixture that passes through the through hole of the protrusion maintains the temperature below the auto-ignition of the fuel gas, i.e. hydrogen. There are no local pressure drops that could cause hot spots, like it happens with the thin edge of a natural gas burner deck that has an anchoring effect for the flame. In this way, a flame lifting behaviour is prioritized instead of an anchor-feature. Accordingly, using such burner deck, a better fluid dynamic and thermal behaviour is obtained when and where the gas expands due to the combustion. Additionally or alternatively, the protrusion can protrude in a direction away from the chamber. In particular, the protrusion comprises a proximal portion close to the sheet, a distal portion away from the sheet and a middle portion located between the proximal and the distal portion. It is noted that the concave section of the protrusion includes the proximal portion and can include a part of the middle portion, whereas the convex section includes the distal portion and can include another part of the middle portion. Specifically, the transverse cross section of the distal portion, in particular at an end distal to the middle portion, is larger than the transverse cross section of the middle portion, and preferably the transverse cross section of the distal portion, in particular at an end distal to the middle portion, is larger than the transverse cross section of the middle portion and/or proximal portion, in particular at an end distal to the middle portion.

In the concave section the area of the transverse cross section is decreasing in a direction away from the chamber. In the middle section, the area of the transverse cross section is, in particular essentially, constant in the direction away from the chamber. In the convex section, the area of the transverse cross section is increasing in the direction away from the chamber. The transverse cross section corresponds with a plane that is orthogonal to a central axis of the protrusion.

Advantageously, the protrusion can have a Venturi shape and/or a double truncated cone shape. This is advantageous for further limiting the flashback. The concave section and the convex section can be arranged coaxially. Additionally, the burner deck is configured such that the gas-air mixture can merely flow out through the protrusion from the chamber to a combustion chamber of the gas burner.

In a further embodiment, the protrusion can extend over a length comprised between 15% to 25%, preferably 20%, of a thickness value of the sheet of the burner deck, in particular in radial direction with respect to a burner central axis. In this way, the risk of flashback is further reduced.

In an embodiment, less than 20%, in particular less than 19%, or less than 15%, for example less than 12.0% or for example less than 10.0% of the surface area of the burner deck is formed by a combined surface area of the holes. More than 5.0% of the surface area of the burner deck is formed by a combined surface area of the holes. Less than 7.0%, for example less than 5.0% or for example less than 4.0% of the surface area of the burner deck is formed by a combined surface area of the holes. More than 1.0% of the surface area of the burner deck is formed by a combined surface area of the holes. By having less than 20% of combined surface area of the holes, a stable combustion of hydrogen can be achieved even when modulating the gas burner, i.e. when changing the power level. A preferred range of the combined surface area of the holes is less than 20% and more than 15%, in particular less than 19% and more than 16%.

By providing a combined surface area of the holes in the burner deck of less than 20%, in particular less than 19%, or less than 15%, but more than 1 %, preferably more than 5%, low NOx is generated when hydrogen is combusted.

In a further embodiment, the perforated area preferably has a combined surface area of holes ratio to the total surface area of the burner deck of more than 0% to up to 20%, in particular more than 0% to up to 15%, in particular more than 0 % to up to 10%, in particular more than 0% up to 7 %, in particular more than 0% up to 5%, of the surface area of the burner deck. In particular the ratio is 0,5% up to 10%, 0,5% up to 7%, 0,5% up to 10%, in particular 1 % to 2%. Further suitable ranges are between 3% and 15%, preferably between 3% and 7% or between 5% and 15%.

In this matter it is to be mentioned that simply providing hydrogen to the known gas burner would not be successful. One of the reasons that this would not be successful is because of a difference in flame speed. Thus, the flow rate of the air-hydrogen mixture through the openings has to be chosen such that the combustion of the hydrogen can be stabilized on the burner deck of the gas burner. Another property of hydrogen that has to be considered is that the combustion temperature is about 300°C higher than the combustion temperature of methane. Thus, the burner deck becomes much too hot for materials typically used in gas burners. In particular, the burner deck can reach a temperature of about 585°C, so that hydrogen can auto-ignite.

Saying the aforementioned changing the amount of flow of the air-hydrogen mixture through a known gas burner, would cause one of 3 situations: i) there is too little flow, so the flash-back occurs, ii) there is too much flow, so no stable flame is created, because the flame is pushed too far away from the burner deck, or iii) a stable flame is created on the burner deck, but the temperature becomes too high as described above

According to an embodiment, the frame structure can cover the burner chamber in a sealing manner. Additionally or alternatively, the frame structure comprises a first portion and a second portion, wherein the burner is fixed to said first portion and the second portion extending longitudinally from the first portion, wherein the first portion of the frame structure is interposed between the burner and the outlet portion of the manifold structure.

According to the embodiment, the manifold structure can comprise a suppressor structure. The suppressor structure can be used to reduce the noise and/or the impact of flashback and/or can be an inlet silencer.

The first connection can be integrally connected to the manifold structure and/or can protrude from the manifold structure. This has the additional advantage that the retrofit kit assembly can be optimized either for mounting space or for further facilitation of the conversion by allowing for ease of access and recognition of the connection, e.g. in case of reduced visibility due to the original setup of the (natural) gas boiler to be converted.

The retrofit kit assembly can comprise a gas valve fixed at the first connection of the manifold structure and connectable to a gas conduit. This has the additional advantage that the safety is even further increased as even fewer connections need to be made as the gas valve will only have to be connected to the (natural) gas boiler to be converted. Therefore, even more connections can be quality controlled already during production of the retro fit kit assembly itself. This further reduces defects caused by human error, such as inadvertent error, inexperience, misidentification, or forgetfulness and thereby further reduces defects in the gas valve connection safety.

Additionally, the retrofit kit assembly can comprise a fan element fixed to the second connection of the manifold structure and connectable at least to an air conduit. This has the additional advantage that orientation and location of the fan element is predetermined such that the requirements of the hydrogen-fire gas appliance guide PAS4444:2020 are fulfilled by ensuring that ambient air is always sucked in in sufficient concentration I as needed. Additionally, it is prevented that an operator connects the fan element to wrong connection, namely the first connection resulting in pre blower mixing.

It is noted that the gas valve is hydrogen ready. Due to the small size of hydrogen molecules, conventional gas valves are prone to leak. Therefore, the gas valve used in the present retrofit kit assembly is more leak tight compared to the commonly used burners for natural gas. For example, to reach the same load with hydrogen compared to natural gas, the volume flow of gas is about three times bigger. Similarly, the fan element is hydrogen ready, meaning that no electro-static discharge is present. In case of hydrogen comprising fuel gas combustion electrostatic discharge can lead to unwanted ignition of the fuel gas.

To improve the safety even further, the manifold structure can comprise a, in particular Venturi shaped, mixer placed downstream the second connection, i.e. downstream the fan element. In this way, the volume of explosive hydrogen-air mixture is reduced. Since the air and gas flows in hydrogen combustion appliances might differ from the natural gas combustion, the, in particular Venturi shaped, mixer is configured to handle these flows without too much pressure drop. In this case the mixer is the means for providing the air/gas mixture.

In a particular example, to further improve safety, the gas valve is, in particular directly connected, to the, in particular Venturi shaped, mixer. That means, no further components are arranged in the gas flow path between the gas valve and the mixer. In this embodiment, even fewer parts need to be assembled during conversion making mounting even simpler and further reducing mounting errors.

For a natural gas combustion appliance, a certain working principle can be chosen, i.e. a pneumatic system or an electronic controlled system. For the conversion towards hydrogen gas the same working principle can be maintained or the working principle can be switched from pneumatic towards electronic or the other way around from electronic towards pneumatic. For this reason, the gas valve can be controlled electronically or pneumatically. Additionally, the fan and the gas valve can be controlled by the same electrical control unit or by separate control units.

Most of the current natural gas combustion appliances, in particular boilers, make use of an ionization probe to detect the flame. For hydrogen gas combustion appliances, it is not possible for high concentrations of hydrogen, in particular pure hydrogen, to use this ionization sensor to detect the flame due to the absence of carbon containing components in the gas mixture. Therefore, in one example, the assembly further comprises a flame detector sensor, in particular a UV sensor and/or a thermal sensor, wherein the flame detector is located at the outlet portion of the manifold. The ionization probe is the conventional flame detector for hydrocarbon combusting heating appliances, however, ionization probes do not detect hydrogen flames correctly or at all, in particular at high hydrogen concentrations. In particular when pure hydrogen is used, the flame can no longer be detected using a ionization probe. Therefore, a retrofit kit assembly comprising a UV sensor contains a further behavior-shaping constraint which facilitates that the correct safety critical sensor is included in the conversion without requiring additional checks and tests during conversion.

Alternatively or additionally, the assembly can comprise at least one of an optical sensor, a temperature sensor, a thermocouple or a catalytic sensor to function as flame detector. To improve the safety of the combustion appliance for which the present retrofit kit assembly is configured, the assembly can further comprise a thermocouple placed in the burner.

The retrofit kit assembly can comprise at least one, in particular more than one, sensor. The sensor can be a hydrogen detector. Alternatively, the sensor can be an oxygen sensor and/or flow sensor and/or a temperature sensor and/or a thermocouple and/or a catalytic sensor. The sensor or sensors can be used to detect the presence of hydrogen, in particular, the leakage of hydrogen which increases the safety in a simple and reliable way.

Alternatively or additionally, it is possible to control the combustion based on the sensor signals. For example, for an electronic controlled system, it is important to monitor the air to fuel ratio (lambda) and to control the combustion appliance based on that ratio. For this purpose, flow sensors, thermal conductivity sensors, O2 sensor, UV sensor or temperature sensor/thermocouple, or catalytic sensor can be used instead of or additionally to a ionization electrode commonly used in natural gas combustion appliances.

As a further safety measure, a controller or control unit that controls either the entire combustion appliance, including at least safety control function and comfort control function, or (in some embodiments) that controls at least the burner can be configured to detect the presence of appropriate sensors for use with a hydrogen gas combustion appliance. For example, if an ionization sensor is detected by a controller or control unit that has been configured to control a hydrogen combustion appliance, then the retrofit has not been properly completed and the unit may still be configured for use with natural gas. In such a situation, control software for a hydrogen gas combustion appliance may refuse to function and/or raise an error. If the control software detects appropriate sensors, such as a hydrogen leakage detector and UV sensor, then the control software can operate the hydrogen gas combustion appliance. In some embodiments, control software for a natural gas combustion appliance can refuse to operate and/or raise an error if sensors for use with a hydrogen gas combustion appliance are detected. This would indicate that a hydrogen retrofit kit has been installed, but the software and/or parameters have not been changed for use with a hydrogen combustion appliance or burner. In some embodiments, control software capable of controlling both a natural gas and a hydrogen gas combustion appliance may be used, with the detected sensors determining, at least in part, the mode in which appliance is operated.

To use the types of sensors that are connected to determine a mode of operation for the combustion appliance or burner (i.e. , as a natural gas or a hydrogen gas appliance or burner) and/or to refuse to operate and/or raise an error as described above, the sensor type should be determinable by the controller or control unit, e.g., using software. For example, sensors that are able to report their own type via their connection to the controller or control unit may be used. Alternatively, sensors may be connected to a sensor interface, which is able to determine the types of sensors that are attached and report this information to control software that operates the burner or appliance. In some embodiments, this sensor interface may be integrated with the controller or control unit. It should also be noted that for some types of sensors, the sensor type may be determined based on the signal provided by the sensor, without requiring further identification information. Additionally, the presence or absence of a sensor may be detected, which may permit use of, e.g., Poka Yoke sensor connectors to ensure that only appropriate sensors may be attached to particular sensor connectors on the controller or control unit. This permits the presence or absence of a sensor to be used as a detectable indication of the sensor type.

The outlet portion can comprise at least one receive portion for receiving a flame detector sensor and/or a sensor as discussed above. The manifold can also comprise a receive portion for receiving the sensor. In particular, the manifold can comprise a first receive portion for receiving a sensor. The sensor can be gas flow sensor for sensing a gas flow. The manifold can also comprise a second receive portion for receiving a sensor. The sensor can be an air flow sensor for sensing an air flow.

Also, the combustion appliance can comprise control components, in particular connecting cables, for the connection of the at least one of the above-mentioned additional components (i.e. flow sensors, thermal conductivity sensors, oxygen sensor, UV sensor or temperature sensor/thermocouple, or catalytic sensor) to the combustion appliance. The control components, in particular connecting cables, can be used to connect and/or connect sensors to a sensor interface which in turn is configured to be connected, in particular connected, to a control unit or controller or control unit. In one example, the frame structure is provided with a plurality of through holes arranged along the perimeter of the frame structure for receiving connecting means, in particular screws, to fix said frame structure to the internal housing of the combustion appliance, i.e. to the housing of the heat exchanger. In addition, to cope with possible noise issue, the assembly can further comprise an inlet silencer provided at the inlet portion of the manifold. Additionally or alternatively, the assembly can further comprise an inlet silencer provided at the inlet portion, in particular fluidically connecting the inlet portion with the mixer.

For hydrogen gas combustion appliance, a different burner is usually provided compared to the burners of natural gas combustion appliances. Since the flame speed of hydrogen is higher than for natural gas, the burner is more prone to flashbacks. Therefore, according to one example, the burner is suitable for hydrogen combustion. In this way, the outflow velocity can be configured to be greater than the flame speed. In another example, the burner can be suitable for the combustion of both natural gas and hydrogen.

In a further embodiment, the retrofit kit assembly can further comprise a data carrier comprising information which, when the data carrier is read out cause a computer or a control unit to carry out a method for controlling the operation of the combustion appliance. The method for controlling the operation of the combustion appliance preferably comprising parameters and/or parameter settings for the safe combustion of hydrogen such as lambda values, load values and/or settings for lambda and load for ignition and/or operation phase.

A suitable data carrier can for example be a QR code, an RFID carrier, or a label comprising a weblink. The data carrier can be arranged on a surface of the retrofit kit assembly, including for example a housing, a frame or a packaging of the retrofit kit. This allows for a particularly simple, cheap, and effective Poka Yoke solution.

Also, a computer readable data carrier is provided, the carrier having stored thereon the inventive computer program product. This product comprises instructions which, when the program is executed by a computer or control unit, cause the computer or the control unit to carry out the inventive method. In an example, the control unit is configured to perform the inventive method. The control unit can comprise at least one processor or be a processor. At the end a retrofit kit assembly is provided by means of which a natural gas combustion appliance can be retrofitted to a hydrogen gas combustion appliance. The retrofit kit assembly is configured in the aforementioned manner in order to reduce leakage and/or explosion risks.

According to one aspect of the invention, a combustion appliance and more particularly for a condensing gas boiler, is provided, the combustion appliance comprising an inventive retrofit kit assembly that is fixed to the housing. According to another aspect of the invention, a combustion appliance comprises a housing that has an interface configured to be connected with the retrofit kit assembly. The interface can be a mechanical interface so that the retrofit kit assembly can be mechanically connected to the housing of the combustion appliance. The connection can be a form-fitting or force fitting connection. In particular, the connection can be releasable. That means the connection can be released without destroying the retrofit kit assembly and/or the housing.

Examples of combustion appliances can include furnaces, water heaters, boilers, direct/in-direct make-up air heaters, power/jet burners and any other residential, commercial or industrial combustion appliance. In many cases, a combustion appliance can be modulated over a plurality of burner loads, with each burner load requiring a different flow rate of fuel resulting in a different heat output. At higher burner loads, more fuel and more air are typically provided to the burner, and at lower burner loads less fuel and less air are typically provided to the burner.

To improve the safety and to monitor important parameters during the functioning of the appliance, the at least one flame detector sensor and/or least one sensor be positioned such on the retrofit kit assembly that they sense physical values from the burner chamber. The burner chamber is at least partly delimited by the housing of the combustion appliance.

The retrofit kit assembly can comprise means for fixing the kit assembly to the housing, in particular the interface of the housing, of the combustion appliance. Accordingly, an operator would have all the required elements for converting a natural gas combustion appliance into a hydrogen combustion appliance. The retrofit kit assembly can further comprise a spacer for the housing of a combustion appliance to be retrofitted. This has the further advantage that additional volume inside the housing can be added in a constructively simple and safe way. The spacer can be fitted between a front cover of the combustion appliance and a housing part containing combustion relevant parts of the combustion appliance. Thereby, it is constructively easily possible to add additional volume to the mounting volume of the housing. This is for example needed when a hydrocarbon combustion appliance, such as a natural gas or propane combustion appliance, requires a change of the configuration of the components during retrofitting. This is for example needed in case a hydrocarbon combustion appliance, such as a natural gas or propane combustion appliance, has a pre-blower mixing configuration which is to be changed to a post-blower mixing configuration. The spacer thus allows for additional volume in the housing to be generated constructively easily if needed during retrofitting. In the context of this application, pre-blower mixing refers to a process wherein the fuel and the air are mixed before entering a fan (also referred to as blower). This has the disadvantage that a potentially ignitable mixture is accelerated and compressed by the fan, wherein a fan component has the potential risk of generating a spark. In particular hydrogen is fairly easily ignitable and therefore, the post-blower mixing is preferred for hydrogen appliances. Post-blower mixing within the context of this application therefore means that the fuel and the air are mixed after the air has passed through the fan.

The retrofit kit assembly can comprise a cable, in particular being part of a cable harness, that is electrically connected with at least one component of the kit assembly. Alternatively the kit assembly comprises a cable, in particular being part of a cable harness, that is electrically connected with at least one component of the kit assembly and is connectable with an electrical component of the combustion appliance. This has the further advantage that it prevents in a safe and easy manner that the wrong cable is connected to the wrong port on the PCB, resulting for example in a short cut. It also prevents that a sensor is connected to the wrong port resulting in faulty data and thus can lead to either a non-functioning boiler or poses a safety risk

In a further aspect of the invention, a combustion appliance, in particular a gas boiler, and more particularly a condensing gas boiler, ,is provided, wherein the kit assembly according to the invention and a housing is provided, wherein the combustion appliance comprises a combustion chamber wherein the kit assembly is fixed to the housing. Additionally or alternatively, a combustion appliance is provided with a housing comprising an interface configured to be connected with the retrofit kit assembly according to the invention.

In a further aspect of the invention, the use of the inventive retrofit kit assembly for converting a hydrocarbon gas combustion appliance into a combustion appliance for the combustion of pure hydrogen is provided. By using the present retrofit kit assembly, the combustion appliance conversion can be easy to realize and can be carried out in a very short time (for example less than one hour). Also, the conversion can be safe and effective for the operation of a hydrogen combustion appliance.

In another aspect of the invention, a method for retrofitting a combustion appliance, in particular a gas boiler, and more particularly for a condensing gas boiler, is provided. The combustion appliance has a burner for combusting a gas mixture including gaseous hydrocarbons, in particular natural gas or propane, and the method comprises: removing a front cover from an internal housing of the combustion appliance and removing the burner, installing an inventive retrofit kit assembly in the combustion appliance by fixing the frame structure to the internal housing of the combustion appliance.

The Poka Yoke behavior-shaping constraints by way of contact, meaning the use of shape, size, or other physical attributes for detection, ensures that the right conditions exist before a process step is executed, and thus preventing defects from occurring in the first place. The value of using the Poka Yoke is that they help people and processes work right the first time, which prevents in a simple and reliable way an improper part setup.

Optionally, the method can further comprise the step of updating the setting parameters of the combustion appliance for the combustion of pure hydrogen. By updating of setting parameters an improper operation can be prevented.

In one example, updating the setting parameters occurs automatically by detecting the presence of hydrogen being above a predetermined value, in particular 20 mol% or pure hydrogen, in the gas mixture. This can be carried out by measuring the amount of hydrogen in the gas mixture using for example a hydrogen detector conveniently placed in the combustion appliance. In another example, updating the setting parameters can occur automatically based on the presence of appropriate sensors, for example by connecting an additional sensor to the appliance. In a further example, updating the setting parameters can occur by detecting the absence of an ionization signal and by detecting a flame detection signal generated by a flame detector, in particular a UV sensor and/or a thermal sensor, and/or a ionization probe.

Other means may also be used to determine that the setting parameters should be automatically updated for the combustion of pure hydrogen. For example, a switch or electrical jumper on the controller or control unit may be used to indicate that the parameters should be updated. Additionally or alternatively, electrical contacts on the frame structure may serve as a switch to indicate that the retrofit kit has been installed, and the parameters should be updated for the combustion of hydrogen. For purposes of safety, in some embodiments, a combination of these means for determining that the parameters should be updated may be used. For example, updating the parameters may be carried out when the appropriate sensors are detected, and an electrical contact on the frame structure indicates the retrofit kit has been installed.

Additionally or alternatively, a code key, in particular a hydrogen specific code key, a or parameter key, in particular a hydrogen specific parameter key, can be supplied. The code key or parameter key can be detected by a control unit or a electronic safety unit. The code key or parameter key in particular comprises hydrogen related configuration information, in particular a configuration for combustion of hydrogen. The code key or parameter key can optionally further comprise additional parameter information, including user set parameters, counter values and/or information regarding malfunctions of the combustion appliance, in particular sensor and counter values, in particular at the time of the malfunction. In a further embodiment, the code key can comprise executable code.

According to another aspect a conversion and control unit is provided. The conversion and control unit is used for converting a hydrocarbon gas combustion appliance to a hydrogen combustion appliance combusting more than 20 mol% hydrogen, configured to take over control of the gas combustion appliance or to override a natural gas combustion control unit of the gas combustion appliance and/or to be inserted in between a natural gas combustion control unit of the hydrocarbon gas combustion appliance to be converted and at least one hydrogen combustion sensor. This unit can be comprised in the retrofit kit assembly.

Thanks to this solution, no software change such as change of parameter settings and/or algorithm change is needed for an already existing natural gas combustion control unit of the hydrocarbon gas combustion appliance, in particular a hydrocarbon gas boiler. For additional safety, in some embodiments, the natural gas combustion control unit may be configured to refuse to operate and/or to raise an error if the presence of a hydrogen retrofit kit is detected, e.g., through use of an electrical contact on the frame structure, but the conversion and control unit is not properly connected.

In an embodiment, the conversion and control unit can comprise at least one port for at least one hydrogen combustion sensor, in particular a UV sensor and/or a 02 sensor and/or a thermal sensor, in particular for flame check and/or a H2 leakage sensor, and a connector, in particular Poka Yoke, configured for a connection with a natural gas combustion control unit of the hydrocarbon gas combustion appliance to be converted.

In an embodiment, the conversion and control unit can comprise at least one, in particular Poka Yoke configured, connector, which allows for a natural gas combustion control unit to remain in the hydrocarbon combustion appliance to be converted and wherein the natural gas combustion control unit will be controlled by the conversion and control unit. This is possible as the conversion and control unit is connectable to the natural gas combustion unit by means of the connector and thus can send control signals to the natural gas combustion unit in order to control the natural gas combustion unit.

In an embodiment, the UV sensor and/or an 02 sensor is connected to the control unit.

In an embodiment, the conversion and control unit comprises at least one communication line configured for connection to the natural gas combustion control unit.

In an embodiment, the conversion and control unit is configured to comprise parameter settings for hydrogen combustion and/or parameter settings configured to detect a malfunction of the at least one hydrogen combustion sensor, in particular the UV sensor and/or the 02 sensor.

In an embodiment, the conversion and control unit is or comprises a printed circuit board (PCB).

The control unit preferably can comprise a safety control function - also referred to as a control safety unit. The safety control function controls the gas valve based on temperature signals and flame signal presence. Thus, the safety control function is related to safety critical operations of the combustion appliance, such as ignition and operation of the burner. Additionally the control unit can comprise a comfort control function - also referred to as primary control unit. The comfort control function comprises at least a fan control, in particular P/l/d controlling of the fan, a control of the pump and controls eating of the heating medium (such as water) to be used for instance in a central heating (CH) circuit and/or a domestic hot water (DHW) production circuit. In other words the comfort control unit controls a heating request, temperature settings for domestic hot water and the like. In a preferred embodiment, the control unit comprises at least the control safety unit and the primary control unit.

According to another aspect an electronic safety unit is provided. The electronic safety unit is used for converting a hydrocarbon combustion appliance to a hydrogen combustion appliance combusting more than 20 mol% hydrogen, configured to monitor combustion parameters and configured to prevent ignition and/or stop combustion of the hydrocarbon combustion appliance to be converted, wherein the electronic safety unit is configured to check that at least one hydrogen combustion parameter sent by a natural gas control unit for controlling a hydrogen burner is met. This unit can be comprised in the retrofit kit assembly.

The electronic safety unit can comprise or is a printed circuit board or processor. The electronic safety unit can be any unit that is configured to prevent an ignition and/or to stop a combustion dependent on the monitored combustion parameter. In particular the electronic safety unit can comprise mechanical and/or electric and/or electronical means for preventing ignition and/or stopping the combustion.

Suitable parameters for combustion control comprise proper burner deck temperatures, in particular whether the temperature is below the autoignition point of hydrogen in the combustion zone, air to fuel ratio and/or combustion performance profiles and/or H2 concentration in a predetermined room volume, in particular a cabinet in which the boiler is arranged, in other words H2 leakage.

Thanks to this solution, software changes such as change of parameter settings and/or algorithm change needed for an already existing control unit of the hydrocarbon gas boiler to safely combust hydrogen are controlled in a constructively easy, safe and inexpensive way and potential human errors are detected early and stop unsafe operation. According to another aspect a control unit for a gaseous hydrocarbon combustion appliance wherein the control unit is configured to detect a presence of at least one component of a retrofit kit assembly, in particular as described above, to refuse to operate and/or to raise an error if control software and/or control parameters for gaseous hydrocarbon combustion are operational in the control unit.

In a further embodiment, the control unit is configured to detect the presence of at least one component of the retrofit kit assembly by detecting at least the presence of a flame detector sensor for hydrogen combustion, in particular an optical sensor, in particular a UV sensor, a temperature sensor, a thermocouple or a catalytic sensor; additionally or alternatively of a hydrogen detection sensor, in particular a thermal conductivity sensor, in particular a temperature sensor and/or a thermocouple, and/or a catalytic sensor and/or an electrochemical sensor; additionally or alternatively of a hydrogen combustion control sensor, in particular a flow sensor, thermal conductivity sensor, 02 sensor, UV sensor or temperature sensor/thermocouple, or catalytic sensor; additionally or alternatively by detecting by determining electrical contact with the frame assembly closed, additionally or alternatively by detecting the position of a switch and/or jumper.

According to another aspect a control unit for a hydrogen combustion appliance is provided wherein the control unit is configured to refuse to operate and/or to raise an error if the control unit detects that a least one component of a retrofit kit assembly, in particular in a retrofit kit as described above, has not been installed or has not been correctly installed and/ or if control software and/or control parameters for gaseous hydrogen combustion is not installed or not correctly installed in the control unit.

In a further embodiment, the control unit is configured to detect the presence of at least one component of the retrofit kit assembly by detecting at least the presence of a flame detector sensor for hydrogen combustion, in particular an optical sensor, in particular a UV sensor, a temperature sensor, a thermocouple or a catalytic sensor; additionally or alternatively a hydrogen detection sensor, in particular a thermal conductivity sensor, in particular a temperature sensor and/or a thermocouple, and/or a catalytic sensor and/or an electrochemical sensor; additionally or alternatively a hydrogen combustion control sensor, in particular a flow sensor, thermal conductivity sensor, 02 sensor, UV sensor or temperature sensor/thermocouple, or catalytic sensor; additionally or alternatively by detecting by determining electrical contact with the frame assembly closed; additionally or alternatively by detecting the position of a switch and/or jumper. According to another aspect a control unit for a combustion appliance is provided wherein the control unit is configured to control a hydrogen combustion appliance only if a retrofit kit assembly, in particular a retrofit kit as described above, is completely and correctly installed and/or a control software and/or control parameters for hydrogen combustion are operational on the control unit.

According to another aspect a computer program product or computer readable medium is provided wherein the computer program product or computer readable medium includes instructions which when executed on a control unit for a gaseous hydrocarbon combustion appliance cause the control unit to detect a presence of at least one component of a retrofit kit assembly, in particular a retrofit kit assembly as described above, to refuse to operate and/or to raise an error if control software and/or control parameters for gaseous hydrocarbon combustion are still operational in the control unit.

In the context of the present specification, unless expressly provided otherwise, the expression “computer program product” or “computer-readable medium” and “memory” are intended to include media of any nature and kind whatsoever, non-limiting examples of which include magnetic storage, optical storage, solid-state memory. Suitable example of such storage include RAM, ROM, discs (CD-ROMs, DVDs, floppy disks, hard disk drives, etc.), USB keys, flash memory cards, solid-state-drives, tape drives, bar codes, and QR codes. The media can also be propagated signals in some embodiments.

In the figures, the subject-matter of the invention is schematically shown, wherein identical or similarly acting elements are usually provided with the same reference signs.

Figure 1 shows a schematic representation of a retrofit kit assembly according to an embodiment of the invention.

Figure 2 shows a perspective representation of the retrofit kit assembly according to another embodiment of the invention.

Figures 3A-B show a front view and a rear view of the retrofit kit assembly of Figure 2.

Figure 4 a perspective view of a UV-sensor and a control unit.

Figure 5 shows a flow chart of a method for retrofitting a combustion appliance according to an example. Figure 6 shows a retrofitted combustion appliance from the outside and with a closed housing, the housing comprising a spacer according to an example.

With reference to Figure 1 , a retrofit kit assembly 1 is shown. The assembly 1 comprises at least a frame structure 5, a manifold structure 10 and a burner 6 for the combustion of hydrogen. The manifold structure 10 serves to distribute the gas mixture and comprises an inlet portion 11 and an outlet portion 12. As can be shown in figure 2, the manifold structure 10 is integrally connected to the frame structure 5 at the outlet portion 12. The frame structure 5 has the shape of a plate and extends orthogonally from the manifold structure 10. In particular, the frame structure 5 comprises a first portion 7 and a second portion 14, wherein the frame structure 5 is connected to the manifold structure 10 at the first portion 7. It is noted that the second portion 14 extends longitudinally from the first portion 7. wherein the frame structure 5 comprises a receiving portion for receiving at least one Poka Yoke configured hydrogen combustion component 21. An example of the hydrogen combustion component 21 is shown in fig. 4.

The burner 6 is fixed to the frame structure 5 at the first portion 7. The burner 6 can be fixed to the frame structure 5 through suitable connecting means, such as screws or can be integrally connected to the frame structure 5 by welding. It is clear that at the connection region between the burner 6 and the frame structure 5, the first portion 7 of the frame structure 5 comprises at least an opening (not shown in the figure) for allowing the gas mixture coming from the manifold structure 10 to flow into the burner 6 for the combustion.

The inlet portion 11 of the manifold structure 10 is provided with a first connection 4 for receiving at least a first fluid, i.e. fuel gas (vertical arrow in the figure), and with a second connection 17 for receiving at least a second fluid, i.e. air (horizontal arrow in the figure). It is noted that the first connection 4 is located downstream the second connection 17 with respect to the air flow. Also, the first connection 4 is integrally connected to the manifold structure 10 and protrudes (extends longitudinally) from the manifold structure 10.

As mentioned above, the burner 6 is suitable for combustion of hydrogen. In this way, the retrofit kit assembly 1 can be used to convert a gas boiler such as a natural gas boiler into a hydrogen boiler. In fact, the retrofit kit assembly 1 can be coupled to a housing 3 of a combustion compliance 2. For example, the combustion compliance 2 can be a gas boiler, in particular a natural gas boiler, and the housing 3 can be the housing of a heat exchanger of the gas boiler. Specifically, the frame structure 5 of the retrofit kit assembly 1 can be fixed to a burner chamber 18 delimited by the housing 3. In particular, the burner chamber 18 is arranged within the housing and comprises an opening that is covered by the retrofit kit assembly 1 , in particular by the frame structure 5.

The retrofit kit assembly 1 consists of different components, which are connected to each other and in some cases are integrated in one single block element (i.e. the manifold structure 10, the frame structure 5 and the first connection 4). In this case, it is easy to replace the elements of the gas boiler to be converted with the present retrofit kit assembly 1. Specifically, the burner (i.e. from a burner suitable only for natural gas combustion to a burner suitable for pure hydrogen) as well as the arrangement of the connections for the inlet of gas and air (for hydrogen boilers, it is preferred a post blower mixing) are changed in order to carry out the conversion. The operator can simply remove the components to be replaced, i.e. the burner and the manifold, and fix the retrofit kit assembly 1 to the combustion appliance 2 (gas boiler), thereby modifying the general operation of the appliance.

The combustion appliance 2 comprises a natural gas combustion control unit 22. Said control unit 22 is configured to control a non-shown burner for combusting natural gas.

The retrofit kit assembly 1 comprises a hydrogen combustion control unit 23 that is attached to the manifold structure 23. Said control unit 23 is configured to control the burner 6 for combusting hydrogen. The natural gas combustion control unit 22 can remain in the combustion appliance 2 after the retrofitting. In particular, the natural gas combustion control unit 28 can be connected with the hydrogen combustion control unit 23.

The hydrogen combustion component 21 is electronically connected with one of the control units 22, 23.

Figure 2 illustrates a perspective view of the retrofit kit assembly 1 according to an example. The retrofit kit assembly 1 of figure 2 further comprises a gas valve 13 and a fan element 8. The gas valve 13 is fixed to the first connection 4 and is connected to a gas conduit 15 whereas the fan element 8 is fixed to the second connection 17 and is fluidically connected to ambient air. This particular arrangement of the first and second connections, i.e. of the gas valve 13 and the fan element 8, allows a post blower mixing of the fuel gas before entering into the burner 6 through the manifold structure 10. In order to reduce the noise, a suppressor structure 20 can optionally be provided at the inlet portion 11, for example at the fan element 8. More details of this advantageously arrangement can be gathered from figures 3A and 3B that illustrate a front view and a rear view of the retrofit kit assembly of figure 2. In this embodiment the control unit 23 is attached to the frame structure 5, in particular to the second portion 14 of the frame structure 5.

From the figures it is also clear the characteristics of the frame structure 5. The frame structure 5 is shaped like a plate or wall and can have a double function. In fact, the frame structure 10 can be used as a support element for the burner 6, the manifold structure 10 (and the components connected to the manifold structure 10) and can be used, at the same time, as a front cover for the housing 3 of the combustion appliance 2.

As shown in figure 2, the housing 3 is the housing of a heat exchanger of a gas boiler. The frame structure 5 is shaped to fit the edges of the housing 3 and to completely cover the burner chamber 18. When the retrofit kit assembly 10 is fixed to the housing 3, the burner 6 is inserted in the burner chamber 18, thereby replacing a burner previously present in the combustion appliance, i.e. in the housing of the heat exchanger. On the other hand, after fixing the retrofit kit assembly 10 to the housing 3, the manifold structure 10 and the components connected to the manifold structure 10 (i.e. the gas valve 13 and the fan element 8) are located outside the housing 3, thereby allowing possible connections for example with the gas conduit 15 and ambient air.

The fixing occurs through suitable connecting means, such as pins or screws. For this purpose, the frame structure 5 is provided with a plurality of through holes 19 arranged along the peripheral border of the frame structure 5, as clearly shown in figures 3A and 3B. Likewise, the housing 3 is provided with the plurality of through holes 19.

Figure 4 shows a perspective view of a hydrogen combustion component 21. The hydrogen combustion component can be a UV-sensor. The UV sensor comprises a sensing element 27 and a Poka Yoke configured connector 24. Fig. 4 also shows a control unit 22, 23. The control unit can be the hydrogen combustion control unit 23 or the natural gas combustion control unit 22. Both control units comprise a port 25 that is assigned to the Poka Yoke configured connector 24. That means, the connector 24 is assigned to said port 25 and only the UV-sensor can be electrically connected to said port 25. Thus, the poke yoke configured connector 24 and/or the port 25 is made such that it is different from a connector of a non-shown ignition electrode so that it is ensured that the ignition electrode cannot be connected with the control unit 22, 23 via the port 25.

The control unit 22, 23 comprises further ports 26. Said ports can be connected with other electrical components.

Figure 5 schematically illustrates the steps of a method 100 for retrofitting a combustion appliance 2. In particular, the method 100 can be used to convert a combustion appliance such as a natural gas boiler into a hydrogen boiler.

At step S101 , the method 100 comprises the step of removing a front cover from the housing 3 of the combustion appliance 2 and removing the burner. For example, at the step S101 the front cover of a heat exchanger is removed. At step S102, the method 100 comprises installing a retrofit kit assembly 1 as described in figures 1 to 3B in the combustion appliance 2. In particular, the step S102 occurs by fixing the frame structure 5 of the retrofit kit assembly 1 to the housing 3 of the combustion appliance 2, for example to the housing of the heat exchanger. At step S103, the method 100 comprises updating the setting parameters of the combustion appliance 2 for the combustion of pure hydrogen.

In one example, the update can be performed by automatically detecting pure hydrogen in the gas mixture, for example using a hydrogen detector suitably located in the combustion appliance 2. In another example, the parameters and/or software may be updated (or switched) automatically if the presence of sensors appropriate for a hydrogen boiler are detected, or if an electrical contact (not shown), e.g., on the frame structure 5 is closed. Alternatively, the updating of parameters (or software) can be carried out by the installer operator or by replacing the control board (PCB), or using a switch or jumper on the control unit to indicate that parameters and/or software for a hydrogen boiler should be used. In case different sensors are used or in case it is switched from a pneumatic to an electronic combustion (or vice versa), the control board can be interchanged.

In some embodiments, the electric control unit may include further safety features. For example, software in the control unit may refuse to operate and/or raise an error if the software or parameters are currently configured for a natural gas boiler, but the presence of sensors appropriate for use with a hydrogen boiler are detected and/or the presence of the retrofit assembly is detected (e.g., by an electrical contact on the frame structure 5). These conditions could indicate that the hydrogen retrofit assembly kit 1 has been installed, but that the parameters and/or software have not been properly configured for operating a hydrogen boiler.

Fig. 6 shows a retrofitted combustion appliance 2 from the outside and with a closed housing 29, the housing 29 additionally comprising a spacer 28 according to an example. The spacer 21 allows for additional volume in the housing 22 to be generated constructively easily if needed during retrofitting. The spacer 21 can be fitted between a front cover 30 of the combustion appliance 2 and a housing part 31 containing combustion relevant parts of the combustion appliance 2.

Reference Signs

1. Retrofit kit assembly

2. Combustion appliance

3. Internal housing

4. First connection

5. Frame structure

6. Burner

7. First portion

8. Fan element

9. Air conduit

10. Manifold structure

11. Inlet portion

12. Outlet portion

13. Gas valve

14. Second portion

15. Gas conduit

16. Retrofit kit

17. Second connection

18. Opening

19. Through holes

20. Suppressor structure

21. Hydrogen combustion component

22. Natural gas combustion control unit

23. hydrogen combustion control unit

24. Poka Yoke configured connector

25. port

26. further port

27. sensing element

28. spacer

29. housing

30. Front cover

31. Housing part containing combustion relevant parts of the combustion appliance