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Title:
A DOMESTIC WATER AND SPACE HEATING SYSTEM
Document Type and Number:
WIPO Patent Application WO/2016/042312
Kind Code:
A1
Abstract:
A system comprising a central heating circuit (2) and a hot water supply circuit (4). The central heating circuit comprises a boiler (1) and at least one space heater (2). The hot water supply system comprises a tank (3) with a cold water supply (31) and a hot water outlet (39) to supply domestic hot water. A secondary heat source (5) with a secondary heating coil (53) is in thermal contact with a bottom portion of the tank. The central heating circuit passes through a coil system (24, 27) in thermal contact with the tank. In a water heating mode, hot liquid from the boiler (1) is circulated through the coil system (24, 27) to heat the water in the tank. In a central heating heat recovery mode, liquid is circulated from a return line of the central heating circuit through the coil system (24, 27) to extract heat from the tank for the central heating circuit. The coil system comprises a primary heating coil (27) for the water heating mode and a heat recovery coil (24) below the primary heating coil (27) for the central heating heat recovery mode.

Inventors:
BUGLER MARK (GB)
GATAORA SANTOKH SINGH (GB)
KANE DAVID (GB)
Application Number:
PCT/GB2015/052666
Publication Date:
March 24, 2016
Filing Date:
September 15, 2015
Export Citation:
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Assignee:
IE CHP UK & EIRE LTD (GB)
International Classes:
F24D3/08; F24D11/00; F24D12/02; F28D20/00
Domestic Patent References:
WO2014051268A12014-04-03
Foreign References:
GB2457051A2009-08-05
NL8502349A1987-03-16
DE102010004984A12011-07-21
Attorney, Agent or Firm:
BOULT WADE TENNANT (70 Grays Inn Road, London Greater London WC1X 8BT, GB)
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Claims:
CLAIMS

1. A system comprising a central heating circuit and a hot water supply circuit;

the central heating circuit comprising a boiler for heating a liquid, at least one heat emitter which use the heated liquid to provide space heating, a pipe system connecting the boiler and radiators, a pump to circulate the liquid around the circuit, and a valve system to control the liquid flow;

the hot water supply system comprising a tank supplied with a cold water supply having an inlet in a lower region of the tank, a hot water outlet at an upper region of the tank, a water pipe system supplying hot water from the hot water outlet to domestic hot water taps;

characterised by a secondary heat source separate from the boiler for supplying heat, the secondary heat source heating the water in the tank via a secondary heating circuit, which includes a secondary heating coil in thermal contact with a bottom portion of the tank via which heat is transferred to the water in the tank; wherein the central heating circuit passes through a coil system in thermal contact with the tank; and wherein the valve system is arranged to control the flow of liquid in the central heating circuit such that, in a water heating mode, hot liquid from the boiler is circulated through the coil system to heat the water in the tank and in a central heating heat recovery mode, liquid is circulated from a return line of the central heating circuit through the coil system in thermal contact with the tank to extract heat from the tank for the central heating circuit, wherein the coil system comprises two coils, a primary heating coil for the water heating mode and a heat recovery coil for the central heating heat recovery mode, wherein the primary heating coil is in thermal contact with a first portion of the tank and the heat recovery coil and the secondary heating coil are in thermal contact with a second portion of the tank below the first portion of the tank.

2. A system according to claim 1, wherein the secondary heat source is a prime mover of a combined heat and power system.

3. A system according to claim 2, wherein the secondary heat source is a fuel cell. 4. A system according to any preceding claim, wherein the two coils are arranged in parallel and the valve system being arranged to selectively divert the flow of liquid between the primary heating coil and the heat recovery coil depending on the mode of operation.

5. A system according to claim any preceding claim, wherein the heat recovery coil at least partially overlaps with the secondary heating coil in the vertical sense. 6. A system according to any preceding claim, wherein a baffle is provided in the tank between the first portion of the tank and the second portion of the tank.

7. A system according to any one of the preceding claims, wherein the inlet for the cold water supply is beneath the secondary heating coil and is not directed directly towards it or the heat recovery coil.

8. A system according to any one of the preceding claims, wherein the cold water supply inlet is provided with a diffusing head to reduce its inlet velocity.

9. A system according to any one of the preceding claims wherein the boiler is connected in series with the coil system such that liquid fed through the coil system has first passed through the boiler, with the boiler being configured not to fire whilst in this mode.

10. A system according to any one of the preceding claims, wherein the secondary heating coil is positioned in the second portion of the tank.

11. A system according to any one of the preceding claims, wherein the coil system is in the tank.

12. A method of operating a domestic water and space heating system according to any of the preceding claims the method comprising the steps of:

determining the temperature of the tank; determining the hot water demand;

determining the temperature of the central heating return line; and

operating the valves to operate in central heating heat recovery mode if there is demand from the central heating, provided that there is sufficient thermal energy available in the tank, taking into account the water demand.

Description:
A DOMESTIC WATER AND SPACE HEATING SYSTEM

The present invention is directed to a domestic hot water and space heating system.

Such a system typically comprises a central heating circuit and a hot water supply circuit;

the central heating circuit comprising a boiler for heating a liquid, at least one heat emitter which use the heated liquid to provide space heating, a pipe system connecting the boiler and radiators, a pump to circulate the liquid around the circuit, and a valve system to control the liquid flow;

the hot water supply system comprising a tank supplied with a cold water supply having an inlet in a lower region of the tank, a hot water outlet at an upper region of the tank, a water pipe system supplying hot water from the hot water outlet to domestic hot water taps. Such a system will subsequently be referred to as "of the kind described".

The present invention is concerned with incorporating a secondary heat source into a system of the kind described in a manner which will lead to greater efficiency and

flexibility for the heating system. While the term secondary is used throughout the specification, this is for

nomenclature only. The secondary heat source may supply a greater portion of the heat to the system in some

embodiments . There is a growing trend for combined heat and power systems to be used in a domestic environment. The present invention may also be applicable to small scale industrial and commercial premises. In particular, prime mover technology such as fuel cell technology has allowed homeowners to efficiently generate their own electricity. This is highly efficient as it avoids transmission losses associated with a distributed power network. It is also beneficial to an end user as they can not only generate their own electricity at a relatively small cost, but they can also export

electricity back into the grid bringing them further

financial benefits.

Fuel cell based combined heat and power has previously been incorporated into heating systems. These have been

connected in series with or parallel to the boiler.

However, the use of a fuel cell in this way is limited as the temperature of the liquid becomes too high for operation of the fuel cell which then has to be turned down or

switched off or to have a heating circuit that is supplied by a boiler at temperatures to match the fuel cell in which case the heating system operates at lower temperatures, such as underfloor heating systems. This can be overcome by using a separate water circuit for the fuel cell. To date, both the boiler and fuel cell have been used to provide the heat directly to the hot water tank and to the central heating system sometimes via an intermediary buffer vessel with limitations on the maximum allowable return temperature from the central heating of 40-45°C (typically) . This then restricts the use of the fuel cell system to low temp central heating systems, or to time periods where a weather compensation or optimised start control loop is in

operation. It is also known to use the waste heat from a fuel cell to provide central heating alone. However, this is relatively low grade heat (typically at a temperature of less than 60°Cand is therefore only useful for low temperature heating systems such as an underfloor heating system or air-based heating system. The present invention aims to provide more effective use of the low grade heat and to incorporate the use of this heat into a high grade heat system in a simple manner .

According to a first aspect of the present invention, a system of the kind described is characterised by a secondary heat source separate from the boiler for supplying heat, the secondary heat source heating the water in the tank via a secondary heating circuit which includes a secondary heating coil in thermal contact with the bottom portion of the tank via which heat is transferred to the water in the tank;

wherein the central heating circuit passes through a coil system in thermal contact with the tank; and wherein the valve system is arranged to control the flow of liquid in the central heating circuit such that, in a water heating mode, hot liquid from the boiler is circulated through the coil system to heat the water in the tank and in a central heating heat recovery mode, liquid is circulated from a return line of the central heating circuit through the coil system in thermal contact with the tank to extract heat from the tank for the central heating circuit , wherein the coil system comprises two coils, a primary heating coil for the water heating mode and a heat recovery coil for the central heating heat recovery mode, wherein the primary heating coil is in thermal contact with a first portion of the tank and the heat recovery coil and the secondary heating coil are in thermal contact with a second portion of the tank below the first portion of the tank.

Effectively, the present invention makes far greater use of the energy in the domestic hot water tank. It can therefore be easily fitted into an existing system replacing a tank which is already in situ (with a tank of a similar size) or it can be supplied as a whole system. The tank has an additional heat input in the form of the secondary heat source which can efficiently and economically accept low or high grade heat, thereby reducing the heat input to the tank from the boiler. It also has opportunity to provide a use for this heat beyond the traditional hot water supply in that heat can be extracted for use in the central heating system.

A combined heat and power (CHP) system with a prime mover such as a fuel cell becomes thermally constrained, in that thermal output cannot be utilised or dissipated, hence the electrical output needs to be reduced or stopped altogether. CHP Prime Movers, such as fuel cells, are generally at its most efficient when used to produce a constant output over a long period of time. This reduces the losses and

degradation (lifetime) issues associated with start-up and shutdown, reduces losses due to transient performance in response to modulation events, provides an opportunity to avoid inefficient operation at very low part-load ranges, and generates electricity for longer periods (potentially maximising on-site utilisation of electricity) . By using the waste heat from the fuel cell to provide first stage heating in the hot water tank, the heat is used in a way which is highly compatible with the efficient running of the fuel cell, and allows additional electrical generation through reduction of thermal constraints, therefore the CHP system can provide financial benefits to the homeowner, and to the electricity grid at times of peak demand by

eliminating demand on the grid & exporting to the grid. For example, the electricity grid can suffer stain at peak times, such as early evening. By managing the heat

effectively, the fuel cell can run for longer periods and support most of the homes electricity needs, effectively taking the strain off the grid and in most cases, can also export any surplus generation to support the network.

The ability to extract heat from the tank for the central heating system provides further efficiency benefits. When heat is available at a suitable temperature in the tank when heat input is required to the central heating system, this heat can be extracted from the tank and supplied to the central heating system without operating the boiler. Once this available heat has been used (which may be determined by a system controller in a manner which leaves sufficient spare capacity in the tank in order to satisfy an

anticipated hot water requirement), the boiler can be fired to satisfy this requirement. However, in the meantime, the tank, which is essentially acting in a similar way to a thermal store but is a domestic hot water store, has been able to supply this heat is either waste heat or is heat which has been previously generated at high efficiency.

Although the invention has been motivated by the need to make efficient use of the waste heat from a combined heat and power unit based on, for instance, a fuel cell, the inventors have recognised that it may also be practical with other efficient low grade heat sources such as a heat pump. In the case of a heat pump, this can provide an efficient source of low grade heat during summer and winter months which can provide a significant contribution towards heating the hot water. Heat pumps are the most efficient when the ground or air temperature outside is above freezing. The invention will optimise the use of the heat pump operation and use the boiler when efficiency drops. In cooler months it can still provide heat for the domestic hot water, along with a significant contribution to the central heating requirement. When the temperature drops towards freezing, it would be appropriate to switch off the heat pump as, at that point, the differential temperature that it is able to generate cannot be used economically and efficiently in the central heating system. The invention would allow a heat pump to operate within a high temperature central heating system whilst retaining a low flow and return temperature. These temperatures, along with the differential between these temperatures and the source (i.e. outside air/ground) temperatures are a key driver to heat pump Co-efficient of Performance and thermal output level.

The coil system may be a single coil with the valve system optionally being arranged to reverse the direction of flow through the single coil depending on the mode of operation either to provide heat into the tank or withdraw heat from the tank.

However, preferably, the coil system comprises two coils, a primary heating coil for the water heating mode and a heat recovery coil for the central heating heat recovery mode, the coils being arranged in parallel and the valve system being arranged to selectively divert the flow of liquid between the primary heating coil and the heat recovery coil depending on the mode of operation. The benefit of using two coils is that the position of the coil within the tank and the direction of flow through the coil can be optimised for each of the two coils.

In certain embodiments, the secondary heating coil and the heat recovery coil are the same coil.

Preferably, the primary heating coil is above the heat recovery coil. The primary heating coil will generally heat the water in the upper zone of the tank to a higher

temperature than the temperature in the lower part of the tank such that the heat recovery coil is positioned to make better use of the lower grade heat in the lower part of the tank. In order to enhance the effect further, the heat recovery coil preferably at least partially overlaps with, but extends slightly above the secondary heating coil in the vertical sense.

In order to promote the maintenance of stratified layers at different temperatures within the tank, a baffle is

preferably provided in the tank between the primary heating coil and the heat recovery and secondary heating coils. The baffle reduces the convection currents within the tank thereby reducing the mixing of different temperature water. To further enhance the stratification, it is preferable to generate as little turbulence as possible. Therefore, preferably, the inlet for the cold water supply is beneath the secondary heating coil and is not directed directly towards it. By generally introducing the water at a low point in the tank, the turbulence can be minimised. Jetting cold water across the secondary heating coil introduces forced convective cooling, which introduces control

instability issues as the fuel cell thinks that the return temperature has dropped dramatically, and ramps up to 100% load, but once the domestic hot water draw off ceases, the return temperature very quickly stabilises at a much higher temperature, causing a quick modulation event. The cold water inlet may be provided with a diffusing head to reduce its inlet velocity.

The boiler may be connected in parallel with the coil system such that, in central heating heat recovery mode, the liquid bypasses the boiler. However, preferably, the boiler is connected in series with the coil system such that liquid in the coil system has first passed through the boiler, with the boiler being configured not to fire whilst in this mode. This vastly simplifies the valve and configuration According to a second aspect of the present invention there is provided a method of operating a domestic hot water and space heating system according to a first aspect of the present invention, the method comprising the steps of:

determining the temperature of the tank; determining the hot water demand; determining the temperature of the central heating return line; and operating the valves to operate in central heating heat recovery mode if there is demand from the central heating, provided that there is sufficient thermal energy available in the tank, taking into account the water demand. Examples of a system and method in accordance with the present invention will now be described with reference to the accompanying drawings : Fig. 1 is a schematic view of the system in the mode in which the boiler heats the tank;

Fig. 2 is a similar view in which the boiler heats the central heating system;

Fig. 3 is a similar view in which the tank heats the central heating system; and

Fig. 4 is a simplified schematic view of the system showing a two-coil set-up.

The system will be described below with reference to Fig. 1 and the different modes of operation will then be described with reference to the three drawings.

In broad terms, the system comprises a boiler 1 and its associated pipe work and valves which provides the primary heat source. The central heating system 2 with associated pipe work and valves provides space heating. A water tank 3 provides the domestic hot water service (potable water within the tank) and the duties of a thermal store and ties the remaining parts of the system together. A domestic hot water system 4 provides tap water to the domestic taps. A secondary heat source in the form of a prime mover 5 provides heat to the tank.

A system controller receives inputs from various sensors around the system and controls the system accordingly as described below. The various components of the system will be described below .

The boiler 1 may be supplied as part of the system or may be an existing domestic boiler. One of the benefits of the invention is that it can be installed in a domestic (or small industrial or commercial) environment using any existing boiler, central heating system, and hot water supply system that uses pumped circulation with minimal modification to the tank as described below and the addition of a secondary heat source. The existing cylinder would be replaced, and new controls added in addition to those existing on the central heating/DHW system. The invention can also be used in a system with a combi boiler, but, in this case, a tank would need to be added to the system.

The boiler 1 may be provided with a communications interface (e.g. OpenTherm) which can link the boiler to the system controller. Water from the boiler is circulated via a primary system pump 10. Again this can be an existing component and may be integrated within the boiler case. If access to the wiring between the pump 10 and boiler 1 control panel is not available, an additional circulation pump can be included. An optional boiler bypass loop 11 is provided to maintain the minimum flow rate of water through the boiler's heat exchanger and to allow boiler pump overrun protection to function. This is a typical component in such a heating system. The control of the hot water from the boiler into the central heating system is achieved by flow valve arrangement 12. This selectively directs the flow either to the central heating system 2 or to the tank 3 as described below. The valve arrangement shown in Fig. 1 is a three-port diverter valve, but alternative valve arrangements include a 2 x 2 port valve, or a 3-port mid-position valve. The valves will typically include electrical switches (that make contact as the valve reaches end or mid-positions) .

The central heating system 2 is simply a standard

arrangement of radiators and flow return pipework arranged in one of several standard formats. The system can work with a range of central heating systems from low temperature (e.g. underfloor heating) to higher temperature (e.g.

standard UK design parameters of 82°C flow - 71°C return) . The central heating system may be sealed or vented. A central heating return sensor 20 detects the return

temperature in the central heating circuit and is used to decide whether the tank can be used to heat the central heating system. The return valve arrangement 21 controls the return flow from the central heating system back to the boiler. This return valve arrangement 21 is a three-way valve, or 2 x 2- port valves, which diverts the flow to a boiler primary return line 22 or a heat recovery loop 23 leading to a heat recovery coil 24. If it is determined that heat is required from and available in the tank 3, the flow is diverted by the return valve arrangement 21 via the heat recovery flow loop 23 up through the heat recovery coil 24 before rejoining the boiler primary return line 22. Otherwise, it is simply directed along the boiler primary return line 22. As well as being arranged to heat the central heating system 2, the boiler 1 can also heat the water in the tank 3. This is done via a cylinder primary input loop 26 which

circulates hot water via a primary heating coil 27 in the upper portion of the tank 3. The flow through the cylinder primary input loop is controlled by the flow valve

arrangement 12 when the system controller determines that high grade heat is required in the tank 3 as described below. Under these circumstances, the flow valve

arrangement 12 diverts flow to the cylinder primary input loop and the hot water flows downwardly through the primary heating coil 27 heating the water in the tank before being returned to the boiler primary return line 22. The system also has a secondary heat source 5 in the form of a prime mover of a combined heat and power unit, such as a fuel cell, Stirling engine, internal combustion engine, etc. In practice this may form a separate unit which may be installed outside. It may also be a heat-only source, such as a heat pump. This is provided with the necessary gas supply 51 and electrical connections 52 for distribution and control as is known in the art. This also incorporates sensors and controls to maintain system operation to control flow temperature, electrical and thermal output versus return temperature, reporting temperatures and output levels to the system controller. This control could be built into the heat source. If it is built into the heat source, the system controller may also provide some or all features of this control, or leave it to the heat source to manage.

Where a communications interface is provided, aspects of the heat source control may be turned off. A communications interface may link the prime mover control to the system controller .

The heat output from the secondary heat source is circulated via a secondary heat loop 53 under the control of a

secondary heat source bypass valve 54 via a secondary heating coil 55 in the lower part of the tank 3. As will be appreciated from Fig. 1, the secondary heating coil 55 and the heat recovery coil 24 are arranged such that their coils overlap with one another allowing efficient removal of heat from the secondary heat source zone in the tank, where the portion of the heat recovery coil 24 positioned above the secondary heating coil 55 is used to heat the water in the coil to a slightly higher temperature (to be more useful to central heating system) due to stratification in the tank. Effectively, the secondary heat loop 53 provides three main functions. It removes heat from the secondary heat source 5 to allow the prime mover to continue to operate (for example to generate electricity) . It pre-heats the cold water in the lower part of the tank 3 in order to reduce the load on the boiler for domestic hot water provision and it can heat the coil 24 indirectly via the domestic hot water in the tank to pre-heat the central heating system 2. An optional secondary heating source bypass loop 56 is provided to prevent overheating of the lower part of the cylinder. The water in the secondary heat loop is circulated by a pump (not shown) that is part of the prime mover package.

In certain embodiments, the secondary heat source 5 may be provided with bypass lines directly connected to either the boiler 1 or central heating system 2. In this manner the secondary heat source 5 may provide heat to any part of the circuit .

Turning to the tank 3 itself, the three coils 24, 27, 55 present in the tank have been described above. If using a conventional tank, the system will need to be modified to receive the new tank and valve arrangement. Other than the return valve arrangement 21, and controls, no further hardware may be needed to adapt an existing system, although a secondary heat source, and secondary heat loop may need to be added if not already present. In certain embodiments a thermostatic mixing valve may be added.

In addition, the tank has a cold water inlet 31 in a lower region. This may have a defusing head which is designed to minimise the turbulence in the lower part of the cylinder. Preferably, it should be positioned so as not to direct water directly onto the coils 24, 55. A cylinder pre-heat sensor 32 senses the temperature in the lower part of the cylinder. Positioned between a primary heating coil 27 and the secondary heating and heat coils 55 and 24 is an annular baffle plate 33 which has a central orifice 34 and is supported away from the edge of the tank by a number of spokes in order to leave an annular gap 35 between the edge of the baffle plate 33 and the wall of the tank 3. This is designed to reduce convection currents and subsequent mixing of the water between the hot upper part of the tank and the relatively cooler lower part. Other designs of baffle plate may be used to minimise these currents. A thermostat 36 controls the temperature in the top of the tank that is heated by the boiler in order to maintain a minimum volume of water to maintain water service. An overheat thermostat 37 provides a safety feature for an unvented domestic hot water configuration and can be applied in other

configurations if desired. This will cut power to the boiler if the cylinder starts to overheat and de energise valve 54 so not further heat from the prime mover can enter the tank and create a temperature and pressure increase. As a final backup, a pressure relief valve 38 is provided at the top of the tank. The tank 5 may also be provided with an immersion heater as is well known in the art to provide an additional back up heating supply.

An outlet 39 for the hot water is provided at the top of the tank 3. This feeds hot water to an optional thermostatic mixing valve 41 where it is mixed with cold water from a cold water supply 42 which reduces its temperature to a level suitable for supply to the domestic hot water taps along line 43.

In addition to the sensors described above, a number of additional sensors may be present in the system. These include the following. A room air temperature sensor. This may be the existing room air thermostat or a newly installed one. However, an additional thermostat may be provided to provide additional control features or combined into one unit to provide both functions. A household electrical power sensor may be used to monitor the electrical load to allow a user to take advantage of variable tariffs or smart grid requests or to monitor and report on household energy consumption .

The central heating system 2 may be provided with

temperature and flow rate sensors. Additional temperature sensors may be provided in the tank 3 in order to monitor the temperature of the tank at numerous levels, not just at a single point in each of the two zones as described above. Sensors may be provided in the cold water inlet 31, the hot water outlet 39 and the line 43 supplying the domestic hot water taps. Additional flow and temperature sensors may be provided in the secondary heat loop to monitor the

temperature and flow rate of the secondary heat source. This data could come from sensors present in the secondary heat source, supplied by a communications link to the system controller

All of these sensors provide additional information to the controller allowing more sophisticated control of the system, optimising performance, and/or as a means for improved diagnostics.

The primary operational modes are described below. In all of the subsequent drawings, the valve ports which are shown in outline represent an open valve and the valve ports which are shown as a solid block indicate a closed valve. The arrows on the flow lines implicate the direction of flow .

With reference to Fig. 1, this shows a water heating mode. In this arrangement, the boiler 1 fires and the flow valve arrangement 12 is set to direct hot water, pumped by the primary system pump 10, around the cylinder primary input loop 26 such that the hot water flows through the primary heating coil 27 to provide high grade heat to the water in the upper part of the tank 3. Depending on the domestic hot water demand, the central heating system is not simultaneously supplied with hot water from the boiler under the control of the flow valve arrangement 12. Circulation to the central heating system may be suspended for the duration of the tank heating, to avoid raising the temperature of the central heating return, unless occupant comfort will be unduly impacted.

Meanwhile, should the system controller determine that it is efficient to run the prime mover (either because it is desirable to generate electricity in the case of a CHP system or because environmental and system conditions are favourable for the operation of the heat pump) , lower grade heat is supplied to the lower part of the tank 3 via the secondary heating coil 55. At this time, the return valve arrangement 21 blocks any flow through the heat recovery loop 23. The secondary heating coil 55 therefore just heats the tank 3. In this mode of operation, heat is not removed from the tank.

Fig. 2 shows a configuration in which the boiler 1 heats the central heating system 2. In this case, the flow valve arrangement 12 is switched to prevent flow through the cylinder primary input loop 26 but opens flow though the central heating system 2. This flow simply circulates back through the boiler primary return pipework 22. At this time, the heat recovery flow loop 23 remains closed under the control of the return valve arrangement 21 such that the central heating system operates independently of the tank 3. At the same time, the secondary heat source 5 may continue to supply heat to the lower part of the tank via the

secondary heating coil 55 should the conditions be favourable to operate the prime mover 5 and should the temperature in the tank 3 not yet have reached its target level . Fig. 3 shows an arrangement in which the tank 3 heats the central heating system 2. In this arrangement, the boiler 1 is not fired. However, the primary system pump 10 continues to operate. The flow valve arrangement 12 diverts the flow around the central heating system 2 and not through the cylinder primary input loop 26. The return valve

arrangement 21 now shuts the direct access to the boiler primary return pipework 22 and diverts the flow around the pre-heat loop 23 in which it absorbs heat via the pre-heat coil 24. This then passes back into the boiler primary return line 22, circulates through the (unfired) boiler and is pumped back around the central heating system 2 to provide space heating. This mode of operation can only be realised when there is sufficient thermal capacity (in its simplest form we may only measure the temperature) within the tank 3 taking into account current or predicted future hot water demand. This is determined by the differential between the cylinder pre-heat temperature sensed by the sensor 32 and the central heating return temperature sensed by the central heating return sensor 20. If necessary, once the available heat in the tank 3 has been depleted, the boiler may be fired and the system can revert to the

configuration shown in Fig. 2 in order to continue to satisfy the central heating demand from the boiler. Fig. 4 shows an arrangement which uses two coils 44, 47. The return line of the top coil 47 is indicated in dashes for clarity. The secondary heat source 5 and the boiler 1 are connected in parallel to one another. Alternatively, they can be connected in series. Valve units 42, 50, 49, 21 allow for each or both of the boiler 1 and secondary heat source 5 to provide the water tank 3 and the central heating system 2 with heat .

In this embodiment, the secondary heat source 5 can provide heat to the tank 3 via the coils 44 or 47 depending on the relative temperatures of the secondary heat source 5 and tank 3 and on the demand. Heat can also be taken out of the tank by the central heating system via the coils 44 or 47 in a heat recovery mode.

In this manner the two sets of coils 24, 55 of the first embodiment can be replaced by the single set of coils 44 for the embodiment shown in this Figure.

Other modes of operation are envisaged. For example, if heat is available for the secondary heat source, but there is no demand from the central heating system, heat may be provided to the central heating system in any event during a "setback period". This is where the building's heated airspace is heated to a temperature lower than the comfort temperature (i.e. the temperature set by the user on the main thermostat to provide the target temperature for the building airspace during the heating period) . This allows the secondary heat source to continue to operate and

provides a useful use for the heat as it pre-heats the house ahead of a comfort heating period. If, during a setback time, further heat is available and the desire remains to run the secondary heat source, again, this heat can be supplied to the central heating system even though there is no immediate demand for it provided that the temperature of the building airspace does not exceed the comfort

temperature. This allows the secondary heat source to continue to run providing the above described efficiency benefit and electrical generation while using the heat in a manner which does not inconvenience the user as it does not exceed the comfort temperature and is potentially beneficial as it further pre-heats the house ahead of a comfort heating period .