TECHNICAL FIELD

The invention relates to a hydronic system, to a controller for use with a hydronic system and a method of controlling a hydronic system.

BACKGROUND ART

Many property managers spend fortunes dealing with complaints about the indoor climate. This may be the case even in new buildings using the most recent control technology. These problems are widespread: some rooms never reach the desired temperatures; room temperatures oscillate, particularly at low and medium loads, even though the terminals have sophisticated controllers; although the rated power of the heat production unit(s) may be sufficient, design power can't be transmitted, particularly during start-up after weekend or night setback.

According to a manual of Tour & Andersson AB. Balancing of Radiator Systems. 3rd edition. Edited by PETITJEAN, Robert. Ljung, Sweden: Sandberg, 2003 these problems frequently occur because incorrect flows keep controllers from doing their job. Controllers, such as thermostatic radiator valves, can control efficiently only if design flows prevail in the hydronic system when operating at design conditions. A solution to eliminate above identified problems is to balance the flows in the system to make sure all terminals can receive at least design flow, regardless of the total average load on the plant.

Hydronic home heating systems have the same problems mentioned above. In some rooms the temperature oscillates and in other rooms the desired temperature could only be reached after a very long period.

Hydronic systems are designed to perform well under nominal conditions in a building. In nominal conditions the heat loss of the building depends on the outdoor temperature. Therefore in current weather-dependent systems, the control unit controls the temperature of the fluid in the supply path, wherein the temperature of the fluid in the supply path depends on the outdoor temperature. A heating curve defines the relation between the outdoor temperature and the desired fluid temperature in the supply path. In Fig. 2 an example is given of a prior art heating curve. By controlling the temperature of the fluid in the supply path continuously a predefined amount of heat is transported to the radiator units. The amount of heat that is not used by the radiator units flows back to the central unit through the bypass path. The prior art system is energy efficient when the predefined amount of heat flowing to the radiator units corresponds to the nominal heat loss of the building. However, a building, i.e. rooms of a building, is not always used under nominal conditions. To reduce energy, at night the room temperature could be lower than the room temperature at nominal conditions. It has been found that to arrive within a reasonable period of time from night temperature at the nominal temperature in the room, the temperature of the fluid in the supply path is too low.

SUMMARY OF INVENTION

It is an object of the invention to provide an improved hydronic system, to obviate at least one of the disadvantages, described above.

According to a first aspect of the invention, there is provided a hydronic system having the features of claim 1.

The hydronic system comprises a central unit and a number of radiator units each provided with a thermostatic valve. The central unit is configured for heating and/or cooling a fluid. The fluid is a heat-transfer medium. A supply path transports the fluid from the central unit to the number of radiators. A return path transports the fluid from the number of radiator units back to the central unit. The hydronic system further comprises a bypass path having an input coupled with the supply path and an output coupled with the return path. The bypass path comprises a bypass valve. An outdoor temperature sensor configured for sensing the outside temperature generates an outdoor temperature signal. A control unit is configured to receive the outdoor temperature signal and to control the central unit in response to the outdoor temperature signal. The system is characterised in that the only path for fluid from an output of a radiator or the output of bypass path to an input of a radiator or the input of the bypass path is along subsequently the return path, central unit and supply path and the system further comprises a sensor attached to the return path (8) downstream the number of radiators (4) and the bypass path (10), the sensor is configured to generate a heat signal indicative for the minimal amount of heat actually circulated in the hydronic system. The control unit is further configured to receive the heat signal and to generate the control signal in response to the heat signal and the outdoor temperature signal to maintain the minimal amount of heat at a level which depends on the outdoor temperature.

The invention is based on the insight that the system has a minimum volume of fluid that is always circulating in the system when all thermostatic radiator valves are closed. This minimum volume represents a minimum amount of heat in the system that could be supplied directly to a radiator as soon as the thermostatic valve of the radiator opens. The minimum volume is the volume of fluid in the loop formed by the central unit, the part of the supply path between central unit and bypass part, the bypass part and the part of the return path between bypass part and central unit. By controlling a minimum amount of heat circulating in the system given a specific outdoor temperature, there is always sufficient heat available in the loop described above to be transported to the radiators to maintain the desired temperature and to increase a room temperature on request. In this way, the loop function acts as a heat buffer. As soon as the estimated amount of heat stored in the buffer is below a predefined value which depends on the outdoor temperature, the central unit will be activated. When it is detected that the estimated amount of heat is above the predefined value the central unit will be deactivated.

In an embodiment of the invention, the sensor is a fluid temperature sensor attached to the return path downstream the number of radiators and the bypass path. In use, the fluid in the system downstream the number of radiators and the bypass path has always the lowest temperature in the loop. The temperature in this part of the system is determined by mixing the fluid through the bypass path and the fluid from the radiators. As the temperature of the fluid through the bypass path, which corresponds to the temperature in the supply path, is always higher than the temperature of the fluid coming from a radiator, the temperature of the fluid in the return path downstream the bypass path and radiators could not be higher than the temperature in the supply path and not be lower than the temperature of the fluid coming from the radiators. Consequently, the temperature sensed by the temperature sensor at the return path is a good measure to determine the minimum amount of heat circulating in the loop.

The terms "upstream" and "downstream" relate to an arrangement of items or features relative to the propagation of fluid through the system. Another advantage of this embodiment is that there is always a relative long delay time between the moment the central unit is activated to heat fluid and the moment at which the heated fluid passes the sensor and the sensed temperature rises above the desired temperature. As a result of this, more heat is added to the loop than is actually needed by the system. Subsequently, the additional heat in the loop is gradually supplied to the radiators and it will take some time before the central unit is activated again. In this way, a short cycling problem of the boiler is prevented.

Yet another advantage of this embodiment is that when the central unit is activated, i.e. is heating the fluid, the temperature of the fluid in the supply path is higher than the temperature that is actually needed to maintain the nominal temperature in a room. As a consequence, the temperature in the room will rise and the thermostatic valve is closed slightly. In case a room is not at the desired room temperature, the flow of fluid through the radiator increases and the heat output of the radiator increases. In the system according to the invention, there is less need to balance the radiators as the temperature of the fluid in the supply path is in principle not limited by the controller as in current systems, and thus the heat transported to the radiators is not limited and the thermostatic valves in the system ensure that sufficient heat is transported to rooms that need to be heated.

In an embodiment, the control unit is further configured to determine a threshold value (thr) which is a function of at least the outdoor temperature (tout), to compare the heat signal (t _re t) with the threshold value (thr) to obtain a control signal (ctrl) to control the central unit. This allows improving the performance of the hydronic system at each outdoor temperature.

In a further embodiment, the function of at least the outdoor temperature to determine the threshold value (thr) has a non-linear characteristic. It has been found that a function with a non-linear characteristic provides a heating system with a better performance than a linear function.

In an embodiment, the function of the outdoor temperature (tout) to determine the threshold value (thr) has a slope at each outdoor temperature value (tout). The slope of the function decreases with a decrease of the outdoor temperature (tout), and at a point wherein the outdoor temperature (tout) corresponds to a reference room temperature (t _re t) the slope is at least four times steeper than the slope at a point wherein the outdoor temperature (tout) is 20 degrees below the reference room temperature (t _re t). These features enable to supply fluid to the radiators with an average temperature, which makes optimal use of the heat output characteristic of a radiator.

In an embodiment, the function provides at an outdoor temperature (tout) which equals a reference room temperature (t _re t) a threshold value (thr) that corresponds to said reference room temperature (t _re t). This feature prevents that the fluid in the system is heated unnecessarily. When the outdoor temperature equals the reference room temperature, a building will not lose energy and consequently the temperature of the fluid in the system will be at least the actual room temperature. As a result, the central unit and pumping unit can be switched off. This will reduce energy consumption.

In an embodiment, the control unit comprises a first user adaptable setting to adapt the reference room temperature (t _re t). The relationship between the reference room temperature (t _re t) and the threshold (thr) is defined by the following equation:

thr = tret + CI X f (tref - tout)

wherein f(...) is a function with a non-linear characteristic and t _ou t is the outdoor temperature (t _ou t) and a is a constant. These features allow to adapt the reference room temperature and to use one function f(...) to determine the optimal threshold value for different reference room temperatures.

In a further embodiment, the control unit comprises a second user adaptable setting to adapt the constant a. These features allow a user to adapt the determination of the threshold value to the ratio of the design heat capacity of the radiators and the heat loss of the building at a reference minimum outdoor temperature. Again, only one function f(...) is needed to determine for different ratios the optimal threshold value to control the system.

In an embodiment, when all the thermostatic valves are closed, a volume of fluid is circulating through at least a part of the supply path, the bypass path and at least a part of the return path. The volume of fluid circulating corresponds to the volume in at least 20 meters of piping with a diameter of the supply path. It has been found that such volume of fluid is sufficient to prevent a short cycling problem of the boiler. In a further embodiment, the bypass path further comprises a fluid buffer. This feature allows to position the bypass valve near the heating unit and to provide a sufficient amount of circulating fluid to prevent short cycling of a boiler. In an embodiment, the control unit is configured to use in a first mode a first function of at least the outdoor temperature to determine the threshold value (thr) and to use in a second mode a second function of at least the outdoor temperature to determine the threshold value (thr). In a range of the outdoor temperature the threshold value determined in the second mode is higher than the threshold value determined in the first mode. The control unit is further configured to switch from the first mode to the second mode when the heat signal drops below the threshold value minus a pre-set value. When, a person has increased the room temperature setting of one of the thermostatic radiator valves, the temperature of the fluid in the return path will decrease due to the increased flow through the radiator. By monitoring the minimum amount of heat circulating in the system by means of the fluid temperature in the return path, it is possible to determine such a demand for increase of the room temperature. The features of this embodiment allows reducing the period of time needed to arrive at the desired room temperature, by increasing the amount of heat circulating in the system.

In a further embodiment, below a predefined outdoor temperature (tout) given a specific reference room temperature (t _re t) the difference between the threshold value determined in the first mode and the threshold value determined in the second mode decreases as the outside temperature declines. Experiments have shown that this feature improves the efficiency of the system with respect to energy consumption in combination with the desired period of time to arrive the desired room temperature if someone increases the room temperature setting of a thermostatic radiator valve.

In a further embodiment, both the function of the first mode and the second mode provides at an outdoor temperature (tout) which equals a reference room temperature (t _re t) a threshold value which corresponds to said reference room temperature (t _re t). This feature reduces the energy consumption.

In an embodiment, the control signal (ctrl) has an on-value which activates the central unit and an off-value which deactivates the central unit. The control unit is further configured to determine the period of time the control signal has the off-value after the controller switches from the first mode to the second mode, and configured to switch from the second mode to the first mode when the period of time exceeds a predefined period of time. This feature provides a simple algorithm to estimate when the rooms in the building have arrived at the desired room temperature. In a further embodiment, the control unit is further configured to derive from the heat signal a first signal indicating whether the central unit is active or inactive. The control unit further derives from the first signal a second signal indicating whether or not the central unit has not been active for more than a predefined continuous period of time. And from the heat signal the control unit derives a third signal indicating whether the heat signal is above or below a reference room temperature minus a predefined value and disables the second mode for a period which starts when the third signal indicates that the heat signal is above the reference room temperature minus the predefined value and the second signal indicates that the central unit has not been active for said predefined continuous period of time. In an embodiment, the period in which the second mode is disabled ends when the control signal changes from on-value to off-value. These features further reduce the energy consumption. When the central unit is switched off at night, the temperature of the fluid in the system will decrease to the room temperature. If in the morning the central unit is switched on and the fluid temperature is not more than the predefined value below the reference room temperature, this is an indication that not so many heat is necessary to start-up the system and to heat the rooms to the desired room temperature. In this situation there is no need to use the second mode as this would result into too much heat in the system resulting in a temperature overshoot of the room temperature.

According to a second aspect of the invention there is provided a controller for use in a hydronic system. A controller having the features described above has the advantage that it can replace weather-dependent controllers in existing hydronic systems easily and that it improves the heating comfort in buildings while at the same time reduces energy consumption.

According to a third aspect of the invention, there is provided a method of controlling a central unit of a hydronic system. An embodiment of the method comprises:

- sensing the outdoor temperature with an outdoor temperature sensor to obtain an outdoor temperature signal;

- generating a heat signal indicative for the minimal amount of heat actually circulating in the hydronic system by means of a sensor;

- generating by a control unit a control signal to control the central unit in response to the heat signal and the outdoor temperature signal. In a further embodiment, the sensor is a fluid temperature sensor attached to the return path downstream the radiators and the bypass path. In an embodiment, the fluid temperature sensor is placed as closely as possible to the central unit. The fluid temperature sensor could also be placed at the return path in the central unit.

It should be noted that US6062485A discloses a return water temperature sensor. A control unit monitors the temperature in the return line through the return water temperature sensor and operates the modulating valve to close the valve if the water temperature in the return line falls below a lower limit. The control unit monitors the temperature of the water returning to the boiler to prevent condensation and thermal shock within the boiler.

Other features and advantages will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, various features of embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects, properties and advantages will be explained hereinafter based on the following description with reference to the drawings, wherein like reference numerals denote like or comparable parts, and in which:

Fig. 1 is a block diagram showing schematically a hydronic system;

Fig. 2 is a graph illustrating examples of a heating curve according to the invention and a prior art heating curve;

Fig. 3 is a graph comparing the maintenance heating curve with a normalized curve showing the heat output of a radiator as a function of the flow through the radiator;

Fig. 4 is a graph, illustrating an example of heating curves used in different modes; and,

Fig. 5 is a block diagram, illustrating an example of a control unit.

DESCRIPTION OF EMBODIMENTS

Fig. 1 shows schematically a block diagram of a hydronic system 1 according to the invention. The system comprises a central unit 2, a number of radiator units 4, a supply path 6, a return path 8, a bypass path 10, an outdoor sensor 14, a control unit 16 and a sensor 18. The central unit 2 is configured for heating and/or cooling a fluid. The fluid is used as a heat-transfer medium. In a hydronic system in the form of a central heating system, the central unit could be in the form of a boiler and the fluid is water. Each of radiator units 4 in the system is provided with a thermostatic radiator valve TRV. A thermostatic radiator valve (TRV) is a self- regulating valve fitted to hot water heating system radiators, to control the temperature of a room by changing the flow of hot water to the radiator. A TRV comprises a valve part and a control part. The control part could be removable from the valve part and controls the valve in response to the temperature in the room. The supply path 6 is configured for transporting the fluid from the central unit to the number of radiators and the return path 8 is configured for transporting the fluid from the number of radiator units to the central unit 2. The bypass path 10 has an input coupled with the supply path 6 and an output coupled with the return path 8. The bypass path comprises a bypass valve 1 1 . The bypass valve 1 1 opens to direct fluid via the bypass path when the pressure in the supply path exceeds a predefined pressure limit. The pressure will exceed the predefined limit when all thermostatic radiator valves are closed. This situation defines a minimum amount of fluid that is always circulating in the system when a pumping device 22 is pumping fluid through a loop of the system 1 , which loop includes the central unit, supply path, bypass path and return path. It should further be noted that if the outdoor temperature is below the desired room temperature the pumping device is continuously pumping fluid through the system.

The outdoor temperature sensor 14 is configured for sensing the outdoor temperature and to generate an outdoor temperature signal . The control unit 16 is configured to receive the outdoor temperature signal t _ou t and to control the central unit 2 in response to the outdoor temperature signal t _ou t.

The sensor 18 is attached to the return path downstream the number of radiators 4 and the bypass path 10. Thus, at a position in the return path 8 along which all the fluid in the system has to flow before it reaches the central unit 2. The terms "upstream" and "downstream" relate to an arrangement of items or features relative to the propagation of fluid through the system. Advantageously, the sensor 18 is placed as closely as possible to the central unit 2. The sensor 18 could even be placed at the return path in the central unit 2. The sensor 18 is a temperature sensor measuring the temperature in the return path 8. The fluid at the position is a mixture of the fluid coming from the radiators and the fluid flowing through the bypass path. The temperature of the fluid in the supply path could not be lower than the temperature of the fluid in the return path. The temperature flowing through the bypass path has a temperature which is about the temperature of the fluid in the supply path. The fluid flowing out of a radiator could not be higher than the fluid supplied to the radiator. Therefore, the temperature in the return path could never be higher than the fluid in the supply path and never lower than the average temperature of the fluid coming from the radiators. As there is always a minimum amount of fluid circulating in the system, the temperature measured by the sensor 18 could be used to generate a heat signal t _re t indicative for the minimal amount of heat actually circulated in the hydronic system 1. The control unit 16 is further configured to receive the heat signal t _re t and to control the central unit 2 in response to the heat signal t _re t and the outdoor temperature signal tout.

The control unit 16 is a weather-dependent controller that controls the central unit 2 in dependency of both the outdoor temperature sensed by the outdoor sensor 14 and the fluid temperature measured in the return path by the sensor 18, which is an indication of the minimum amount of heat that could be supplied to the radiators. This minimum amount of heat is stored in a volume of fluid which is circulating through at least a part of the supply path 6, the bypass path 10 and at least a part of the return path 8 and central unit 2 when all thermostatic valves are closed. This volume of fluid is a heat buffer that is used to supply continuously heat to the radiators even when the central unit 2 is idle. The heat capacity of the heat buffer should be large enough to prevent a short cycling problem of the central unit.

It has been found that a volume of fluid corresponding to the volume in at least 20 meters of piping having the largest diameter of the supply path 6 is sufficient to reduce the short cycling problem significantly. If the volume of fluid in the loop formed by central unit, supply path, bypass path and return path is less than the volume of 20 meter of piping having the diameter of the supply path, the volume of fluid in the loop could be enlarged by positioning a fluid buffer 12 in the bypass path.

A hydronic system is most energy efficient when the radiators in the building are balanced and temperature of the fluid in the supply path has the lowest possible temperature necessary to keep the room temperature constant at the design room temperature. The term "balanced" means that the radiators in a system have correct flows in each radiator at design condition. In the present description it is assumed that the design room temperature is the desired room temperature. Weather-dependent controllers available on the market use this principle to control the central unit. The controllers use a heating curve with a function as shown in Fig. 2 and indicated with "Prior art" to control the control unit. With these prior-art controllers, when the temperature of the fluid in the supply path falls below the temperature corresponding to the outdoor temperature, the central unit is switched on. To reduce short cycling of the central unit, the temperature in the supply path has a differential gap of at least 12°C before the central unit is switched off. Even with a differential gap of 16°C it is not always possible to avoid short cycling of the boiler. In the latter case the controller switches the central unit on when the temperature of the fluid in the supply path falls below 80°C and the controller switches the central unit off when it exceeds 96°C. As a result of the differential gap and the requirement to heat up a room by 3 to 4°C per hour, the temperature of the fluid in the supply path to switch on the central unit is at low outdoor temperatures much higher than the lowest possible temperature in the supply path to keep the room temperature constant at the design temperature, which is for example 20°C.

By using the temperature of the fluid in the return path, it is possible to use a minimal differential gap of circa 3°C, however not much lower than that, because otherwise short cycling can occur. The differential gap depends on the amount of fluid circulating in the aforementioned loop and the time needed to transport fluid from the central unit 2 to the sensor 18. When heated fluid passes the sensor, the temperature of the fluid in the supply path is at least 15°C higher than the fluid passing the sensor 18. This amount of fluid forms a heat buffer, which is used to supply heat to the rooms when the central unit is switched off.

Short cycling is the problem that the time between two subsequent moments the central unit switches on is shorter than acceptable by the internal controller of the central unit. The loop of central unit, supply path, bypass path and return path is a suitable heat buffer for all outdoor temperatures. At outdoor temperatures just below the reference room temperature, a predefined amount of additional heat is added to the loop by the central unit in a short period as the capacity of the central unit is relatively high compared to the heat demand of the building. This predefined amount of additional heat is defined by the temperature increase needed to switch off the central unit after the central unit is switched on. The amount of heat enables supplying heat to the rooms for the relative long period when the central unit is off. At low outdoor temperatures, the amount of heat in the buffer is relatively low. However, most of the capacity of the central unit is used to supply heat to the rooms and less heat is available to add the predefined amount of heat to the loop. Consequently, more time is needed to add the predefined amount of heat to the loop, whereas the period of time the central unit is off is then much shorter. But the time period between two subsequent moments the central unit switches on is large enough to reduce short cycling significantly.

Optionally the hydronic system 1 comprises a room temperature sensor 20. The room temperature sensor senses the room temperature in a reference room of the building. The sensor could be in the form of a room thermostat. The room sensor could be used to switch the climate in the building between day and night modes. In night mode, the sensor 20 will inhibit the control unit 18 to supply an on- signal to the central unit 2 as long as the room temperature is above a pre-set value, for example 15°C.

Fig. 2 shows three embodiments of heating curves according to the invention wherein the desired minimum temperature of the fluid in the return path is a function of the outdoor temperature. The value of the function at a specific outdoor temperature used in the controller is a threshold value. It can be seen that the heating curve used in a prior art weather-dependent controller is almost linear whereas the heating curve used in a heating system according to the invention is non-linear. If the temperature of the fluid falls below the threshold value the central unit is switched on. The curves have been obtained by experiments. The curve identified with "nominal" corresponds to the function that should be used to control the central unit in response to the temperature of the fluid in the return path when the nominal capacity of the radiators corresponds to the heat loss of the building at -10°C outdoor temperature. The curve identified with "Undersized" should be used when the nominal capacity of the radiators is a predefined factor lower than the heat loss of the building at -10°C. The curve identified with Oversized" should be used when nominal capacity of the radiators is a predefined factor higher than the heat loss of the building at -10°C. The nominal capacity or power of the radiators used in the experiment are determined by a supply temperature of 80°C, a temperature drop over the radiator of 20 degrees Kelvin and a room temperature of 20°C. Furthermore, the nominal capacity is needed at a temperature difference of 30°C between room temperature and outdoor temperature. The curve "Nominal" of Fig. 2 corresponds to a reference room temperature of 20°C and a hydronic system with radiators having a nominal heat emission which corresponds to the heat loss of the building at -10°C. The nominal heat emission of the radiators is determined with a supply temperature of 80°C and a return temperature of 60°C in a room with a temperature of 20°C. The curves "Undersized" and Oversized" could be obtained by the following equations: fund8rsiz8d(tout) ⁼ tref ⁺ Qi X (fnominal(tout)-tref)

fov8rsiz8d(tout) = tref + θ2 X (fnominal(tout)-tref) wherein is the reference room temperature, fnominai( . . .) is a curve "Nominal" and t _ou t is the outdoor temperature ai and a2 are a constant, ai> 1 and θ2< 1 .

It was surprising that at outdoor temperatures up to about 12°C below the reference room temperature, the desired temperature in the return path is higher than the desired temperature in the supply path when using a current weather- dependent controller. Further, it was surprising that with each degree the outdoor temperature decreases the increase in minimum temperature decreases. In other words, the slope of the function decreases with a decrease of the outdoor temperature. It has further been found that the slope at a point wherein the outdoor temperature (t _ou t) corresponds to a reference room temperature, which is 20°C in Fig.2, is at least four times steeper than the slope at a point wherein the outdoor temperature is 20 degrees below the reference room temperature. In Fig. 2, 20 degrees below the reference room temperature is an outdoor temperature of 0°C.

Furthermore, it was surprising, that the heating curve has a curvature that corresponds to the curvature of the heat output of the radiators used in the building as a function of the flow through the radiator, i.e. the radiator heating curve. Fig. 3 shows a heating curve identified with maintenance and the heat output curve of a radiator as function of the flow q through a radiator. The vertical axis of the heating curve corresponds to the temperature that has to be added to the reference room temperature to obtain the threshold value of the heating curve in Fig. 2. The horizontal axis associated with the heating curve corresponds to the difference between reference room temperature and outdoor temperature. The relation between the heating curve in Fig. 3 and the heating curve in Fig. 2 is defined by the following equation:

th r = tref + f(tref - tout)

wherein thr is the threshold value used to control the central unit, f(...) is the function Maintenance, t _re t is the reference room temperature and t _ou t is the outdoor temperature (t _ou t) .

With respect to the heat output curve of a radiator, on the vertical axis the heat output (in P%) is given and on the horizontal axis the flow (in q%). The heat output curve of a radiator could be obtained by supplying a fluid with a constant predefined temperature to the radiator, for example 80°C, maintaining the temperature in the room at a predefined room temperature, for example 20°C. Subsequently for each flow through the radiator the stable temperature at the output of the radiator is measured. The temperature drop over the radiator and the flow enables to determine the heat output of the radiator at the corresponding flow. The temperature drop over the radiator is the difference between supply and return temperature of the fluid at the input and output of the radiator. A heat output of 100% and flow of 100% corresponds to the emitted power of the radiator and flow through the radiator when a flow of fluid with a temperature of 80°C is supplied to the radiator and a flow of fluid with a temperature of 60°C is coming out of the radiator.

Another characteristic of the function used to control the central unit in dependence of the outdoor temperature and the fluid temperature is that at an outdoor temperature (t _ou t) which equals the reference room temperature (which is 20°C in Fig. 2) a threshold value is used that corresponds to said reference room temperature.

The functioning of the control unit will be described with reference to Fig. 5 which illustrates a block diagram of the functions performed in the control unit to generate the control signal Ctrl which controls the central unit 2, i.e. boiler, of a hydronic system.

The control unit 16 comprises a first input for receiving a signal t _ou t representative for the outdoor temperature supplied by the outdoor temperature sensor 14 and a second input for receiving a second signal t _re t representative for the temperature of the fluid in the return path supplied by the temperature sensor 18. The control unit further comprises three user adaptable settings. A first user adaptable setting 57 is the reference room temperature . A second user adaptable setting 58 is a correction factor a to correct the heating curve for oversized or undersized radiators in a building. How to determine the oversizing or undersizing factor of a radiator is common knowledge of a person skilled in the art. An optional third user adaptable setting 59 is an offset value offsetl that is used to switch the controller from a first mode to a second mode. In the first mode a different function is used to determine the threshold value thr than in the second mode. Fig. 4 shows an embodiment of the functions used in the first mode (Maintenance) and the second mode (Heat up) to determine the threshold value thr.

The control unit 16 is configured to perform the following signal processing. In unit 50, which could be in the form of a subtractor, the outdoor temperature tout is subtracted from the reference room temperature t _re t to obtain a difference signal, which is supplied to unit 51. Unit 51 performs a table-look-up function and could be in the form of a memory in which the functions as shown in Fig. 4 are stored. Furthermore, unit 51 receives the correction factor a. The output signal of unit 51 is supplied to unit 52. In unit 52, which could be in the form of an adder, the output signal of unit 51 is added to the value of the reference room temperature t _re t to obtain the value of threshold signal thr. The threshold signal corresponds to a signal that has been obtained by the following equation:

thr = tret + CI X f (tret - tout)

wherein is the reference room temperature, f(...) is a function with a non-linear characteristic and tout is the outdoor temperature and a is a constant.

In unit 53, which could be in the form of a comparator, the threshold value thr is compared with the temperature t _re t of the fluid in the return path and a control signal Ctrl is generated to control the central unit 2 of the hydronic system. If t _re t < thr a control signal is generated with an on-value that sets the central unit 2 in the active state. If t _re t > thr+hys a control signal with an off-value is generated which sets the central unit 2 in the idle state. The value hys is a value in the range 3 - 6, wherein a value of 3 corresponds to 3°C. The value hys enables to add an additional amount of heat to the loop which reduces the short cycling of the central unit 2 as described above.

As described above, by monitoring the temperature of the fluid in the return path, it is possible to detect an additional request for heat in the building, for example, when a person increases the temperature setting of one or more thermostatic radiator valves. In that case, the temperature of the fluid in the return path will drop significantly, which is an indication that one or more rooms have to be heated up and more heat has to be generated by the central unit 2, to heat up said room in a reasonable time and to maintain the other rooms at the desired temperature. To perform this function and the switching between a first mode and a second mode, the control unit comprises the units 54, 55 and 56.

Unit 54, which could be in the form of a subtractor, subtracts the value of offsetl from the threshold value signal thr to obtain a second threshold value signal thr2. The value of offsetl corresponds to the number of degrees the temperature of the fluid in the return path should fall below the minimum temperature defined by threshold value thr to generate a control signal to set the central unit in the active state. It has been found that an offset value in the range of 5 - 15 degrees could be used. Subsequently, unit 55, which could be in the form of a comparator, the second threshold value thr2 is compared with the temperature t _re t of the fluid in the return path. When t _re t < thr2 unit 55 generates a signal which triggers unit 56 to switch the control unit from a first mode to a second mode. In response to said trigger unit 56 generates a mode signal indicating that unit 51 should use a second function to obtain its output signal. Furthermore, unit 56 starts a procedure to count the period of time the control signal Ctrl has the off-value after the last change of the controller into the second mode. The off-value indicates that the central unit 2 should be in the idle state. As soon as the counted period of time exceeds a predefined period value, unit 56 generates a mode signal indicating that unit 51 should use a first function to obtain its output signal that is supplied to unit 52. A period value of 30 minutes has been found very suitable.

A central unit 2 could have two or more control inputs. For example, a central unit could have a first control input for coupling a room thermostat which uses a protocol to communicate with the central unit and to control the heat output of the central unit, and a second control input for receiving an ON/OFF signal. Open Therm is an example of such a communication protocol. The control unit 16 shown in Fig. 5 is attached to the second control input. If at least one of the signals supplied to the first control input or the second control input indicates that the central unit has to be switched off, the central unit will go to the idle state. The central unit will go to the active state as soon as both signals at the first and second control input indicate that the central unit has to become active. In the event the room thermostat is set to a room temperature which is below the room reference temperature and setting of the TRV's in the room, the room thermostat controls the central unit and thus the temperature in the room. If the room thermostat is set to a room temperature above the room reference temperature and setting of the TRV's, the control unit 16 controls the central unit 12 and the TRV's control the temperature in the room.

It has been found that if the room thermostat is used at night to lower the night temperature in the room without changing the setting of the TRV's, the temperature in the room has an overshoot after the room thermostat is set to day temperature and the temperature in the room has fallen about three degrees below the room reference temperature . Surprisingly, this was due to the use of the second mode (heat up) causing the central unit to supply too much heat to the radiators. By adding the units 61 ... 65 to the circuitry of the control unit 16, the overshoot could be reduced significantly. They generate a control signal M2-off which is supplied to unit 56. The control signal instructs the unit 56 when it might not to generate a mode signal indicating that unit 51 should use a second function to obtain its output signal.

Unit 61 determines whether the return water temperature t _re t is increasing or not. Increasing means that the central unit is active. Unit 61 derives from the heat signal t _re t a first signal S 1 indicating whether the central unit 2 is active or inactive. Unit 61 could be implemented with a differential detector or slope detector. The first signal S 1 is supplied to unit 62. Unit 62 derives from the first signal S 1 a second signal S2 indicating whether or not the central unit 2 has been inactive for more than a predefined continuous period of time. This could be implemented by a counter which counts up when the first signal S 1 indicates that the central unit is inactive and which is reset to zero when the first signal S 1 indicates that the central unit is active. When the count value reaches a predefined count value corresponding to a predefined continuous period of time, a value for signal S2 will be generated that indicates that event. The predefined count value could be a user adaptable setting with values in a range corresponding to 2 - 12 hours. Units 63 and 64 use the heat signal t _re t, the reference room temperature and a user adaptable offset value offset2 to derive a third signal S3 indicating whether the heat signal t _re t is above or below the reference room temperature minus the offset value offset2. The value of offset2 could be in the range of 0 - 8°C. An offset value of 3°C has been found very suitable. Unit 63 could be a subtractor and unit 64 a comparator. Unit 65 generates a control signal M2-off with a value which disables the second mode for a period which starts when the third signal S3 indicates that the heat signal is above the reference room temperature minus the predefined value and the second signal S2 indicates that the central unit has not been active for the predefined continuous period of time. The period in which the second mode is disabled ends when the control signal Ctrl changes from on-value to off-value.

The units 61 ... 65 functions as follows. When the control value of the room thermostat is decreased more than the value offset 2 below the reference room temperature , the room thermostat will set the central unit in idle mode irrespective of the control signal Ctrl generated by the control unit 16. The pump might also be stopped. The return water temperature will decrease and the control unit 16 will generate a control signal Ctrl indicating that the central unit should become active. However, this is not possible due to the control signal from the room thermostat. During the night, the return water temperature will decrease to a temperature similar to the actual room temperature. When the outdoor temperature t _ou t is 5°C below the room reference temperature , a return water temperature with a value similar to the room reference temperature will already be 15°C below the desired return water temperature (see Fig. 4). In normal conditions at day, this would result in using the second mode. However, if the room temperature will not fall below the reference room temperature minus offset2 during the night period, this is an indication that the outdoor temperature is such that a normal amount of heat is needed to heat-up the building rooms in a reasonable period of time. When in the morning the room thermostat is set to a temperature above the reference room temperature, unit 65 will then disable or inhibit the use of the second mode. As soon as the value of control signal Ctrl changes from active to idle, this is an indication that the return water temperature is at a temperature corresponding to the desired return water temperature. This event, changes the control signal M2-off to a value which allows unit 56 to generate a mode signal with a value to use the second mode as soon as the return water temperature falls below the desired return water temperature minus the value of offset 1. This could happen when the desired room temperature of the TRV's in a room is increased. However, if the room temperature falls more than the value offset2 below the reference room temperature this is an indication that more heat is needed to heat- up the rooms in a reasonable period of time. In that case, when in the morning the room thermostat is set to a temperature above the reference room temperature, unit 65 will not disable or inhibit the use of the second mode. Similar, when during the night, the room thermostat has to activate the central unit, this is an indication that it is cold outside. As a consequence, more than a normal amount of heat is necessary to heat-up the rooms and the second mode could be used directly when the room thermostat is set to a value above the reference room temperature .

It should be noted that the control unit 16 as shown in Fig. 5 could be a stand-alone unit which can be added to an existing heating system to improve the climate in the rooms and to reduce the energy consumption. However, if the control unit 16 has an input to obtain directly the status of the control signal of the room thermostat, unit 61 could be removed and the status of the control signal could then be combined with the control signal Ctrl generated by the control unit to obtain the input signal S1 for Unit 62.

In an alternative embodiment, the control unit comprises an input to receive a temperature signal that is representative for the temperature of the rooms in the building. This could be a signal from a temperature sensor in one of the rooms. The value of the representative room temperature is then supplied to unit 64 instead of the temperature of the return water tret.

Fig. 4 shows an exemplary embodiment of the first function and second function for use in the first mode and second mode respectively. The first function is indicated with "Maintenance" and is used in the first mode. The second function is indicated with "Heat up" and is used in the second mode. The first function in Fig. 4 corresponds to the function that is used to obtain the heating curve "Nominal" in Fig. 2 and wherein the reference room temperature is 20°C. From Fig. 4 it is clear that both the first and second function have a non-linear characteristic. Furthermore, Fig. 4 shows the difference Delta between the value of the second function "Heat up" and the first function "Maintenance". The first function provides the hydronic system with the optimal energy efficiency when the temperature in the building is substantially stable, i.e. the temperature in the rooms corresponds to the temperature of the thermostatic radiators valves. The second function, which is used when one or more rooms in the building have to heat up to the desired temperatures indicated on the thermostatic radiator valves, provides the hydronic system the requested characteristic with respect to "heat up" time of one or more rooms in the building through which the desired comfort in the rooms is achieved in a reasonable period of time. With the second function as shown in Fig. 4 it is possible to heat up a room by at least about 4°C/hr. This is not dependent on the outdoor temperature. From Fig. 4 can further be derived that below a predefined outdoor temperature tout given a specific reference room temperature t _re t the difference between the threshold value determined in the first mode and the threshold value determined in the second mode decreases as the outside temperature declines. This is the case when the difference between reference room temperature and outdoor temperature is more than about 10°C. It can further be deduced that at an outdoor temperature tout which corresponds to the reference room temperature t _re t both the function of the first mode and the second mode provides a threshold value that corresponds to the reference room temperature t _re t.

In summary a hydronic system according to the invention performs a method comprising the following actions:

- sensing the outdoor temperature with an outdoor temperature sensor 14 to obtain an outdoor temperature signal ;

- generating a heat signal t _re t indicative for the minimal amount of heat actually circulating in the hydronic system by means of a sensor 18;

- generating by a control unit 16 a control signal to control a central unit 2 in response to the heat signal t _re t and the outdoor temperature signal t _ou t to maintain the minimal amount of heat actually circulating in the system at a level which depends on the outdoor temperature signal and the reference room temperature.

The sensor 18 is a fluid temperature sensor attached to the return path

8 downstream the number of radiators 4 and the bypass path 10 of the hydronic system.

It should be noted that for each of the "Maintenance" mode and "Heat up" mode only one non-linear curve is used in the control unit 16 to obtain the optimal threshold values for different reference room temperatures and dimensioning of the radiators in relation to the heat loss of the building. The non-linear curve of the "maintenance" mode has a curvature which corresponds to the curve of the heat emission P% of a radiator as a function of the flow q% as shown in Fig. 4. This means that in a heating system with 80/60 radiators a heating curve can be used that corresponds to the curve of the heat emission as a function of the flow in P% (radiator heating curve) for 80/60 type radiators, whereas if 75/65 type radiators are used in a heating system a radiator heating curve that corresponds to 75/65 type radiator can be used. It should further be noted, that the principle of the invention could be used for hydronic systems in different climates. For example, in colder regions, where a hydronic system is designed to be sufficient for outdoor temperatures of -40°C and a room reference temperature of 20°C, the curves shown in Fig. 4 could be used. The only difference is that the value at temp x°C on the x-axis in Fig. 4 is then used at temp 2x°C. Thus, the threshold values at outdoor temperatures of 20°C, 10°C ,0°C, - 5°C and -10°C in Fig. 2 are than used at outdoor temperatures of 20°C, 0°C, -10°C, - 20°C and -40°C, respectively.

It should further be noted that parts of the function of the control unit could be implemented on a processor instead of dedicated hardware components.

The examples of the hydronic system described above relate to a central heating system. The idea of the invention could also be used in a hydronic cooling system. In that case, the cooling capacity of the radiators as a function of the flow through the radiators has to be determined; and this in relation to the heat- transfer medium.

While the invention has been described in terms of several embodiments, it is contemplated that alternatives, modifications, permutations and equivalents thereof will become apparent to those skilled in the art upon reading the specification and upon study of the drawings. The invention is not limited to the illustrated embodiments. Changes can be made without departing from the idea of the invention.