In a first operation mode or winter operation mode of the gas burner appliance 10 the heated service water 14 is provided to the heating system 15 and/or to the secondary heat exchanger 18 depending on the position of the valve 20. If no heat demand for domestic hot water is present during the first operation mode or winter operation mode of the gas burner appliance 10, the heated service water 14 is only provided to the heating system 15 by switching the valve 20 to a first position. If a heat demand for domestic hot water is present during the first operation mode or winter operation mode of the gas burner appliance 10, the valve 20 is switched to a second position and the heated service water 14 is partially or completely provided to secondary heat exchanger 18. In a second operation mode or summer operation mode of the gas burner appliance 10 the heated service water 14 is only provided to the secondary heat exchanger 18 to provide domestic hot water 19. In the second operation mode or summer operation mode there occurs no heat demand from the heating system. In the second or summer operation mode of the gas burner appliance 10 the gas burner 1 1 is usually only started if a heat demand for domestic hot water is present.

As mentioned above, a gas/air mixture is combusted by the gas burner 1 1 . The gas/air mixture has a defined mixing ratio of gas and air. The gas/air mixture having the defined mixing ratio of gas and air is provided by mixing an air flow 21 with a gas flow 22. The quantity of the gas/air mixture provided to the gas burner 1 1 is adjusted by a fan 23. So, the modulation of the gas burner load is effected by controlling the speed of the fan 23. The defined mixing ratio of gas and air within the gas/air mixture is controlled by a controller 24 on basis of a signal provided by an electrical or electronic sensor 25. In Figure 1 , the electrical or electronic sensor 25 is provided by a flame ionization sensor. The controller 24 generates a control variable for an actuator 26 of a gas valve 27 to control the defined mixing ratio of gas and air of the gas/air mixture based on the signal provided by the electrical or electronic sensor 25.

A gas burner appliance making use of a signal provided by an electrical or electronic sensor to control the mixing ratio of gas and air within the gas/air mixture is also called electronic gas/air-control gas burner appliance.

It should be noted that other electrical or electronic sensors can be used to establish an electronic gas/air-control. Example given, a signal of a pressure sensor or flow meter or a signal of a temperature sensor can be provided to the controller 24 in order to generate the control variable for controlling the defined mixing ratio of gas and air within the gas/air mixture. EP 1 084 369 B1 and EP 1 179 159 B1 each disclose a method to control the mixing ratio of gas and air within the gas/air mixture on basis of a signal provided by an electrical or electronic pressure sensor or flow meter. An actual value corresponding to a pressure ratio between a gas pressure in a gas duct and an air pressure in an air duct or corresponding to a pressure ratio between the gas pressure in the gas duct and the air pressure at the reference point is provided by the electrical or electronic sensor, wherein this actual value is compared with a nominal value. A control variable for a gas valve assigned to the gas duct is generated on basis of the control deviation between the actual value and nominal value, wherein the gas valve is adjusted on basis of this control variable in order to control the defined mixing ratio of gas and air in the gas/air mixture.

Electronic gas/air-control gas burner appliances need to be calibrated example given to compensate different gas qualities to ensure an optimum and complete combustion of the gas. DE 10 2004 051 270 A1 discloses a method to calibrate an electronic gas/air-control gas burner appliance on basis a signal provided by a flame ionization sensor. DE 10 2008 031 979 A1 also discloses a calibration of the defined gas air mixing ratio of a gas/air mixture to compensate different gas qualities on basis of a signal provided by a flame ionization sensor.

In EP 1 084 369 B1 and EP 1 179 159 B1 the calibration can be effected on basis of the signal provided by the electrical or electronic pressure sensor or flow meter.

In the first operation mode or winter operation mode of the gas burner appliance the calibration of a gas burner appliance, namely the calibration of the mixing ratio of gas and air within the gas/air mixture to e.g. compensate different gas qualities, can be effected without any loss of comfort for the end user when the gas burner is running in order to provide hot service water for the heating system. However, in the second operation mode or summer operation mode of the gas burner appliance the calibration of a gas burner appliance would result into a loss of comfort for the end user, because the calibration requires a burner start which would usually only occurs when a heat demand for domestic hot water is present. During the calibration of the gas burner appliance a modulation of the burner load is not allowed which would then cause the loss of comfort for the end user.

Against this background, a novel method for operating a gas burner appliance is provided.

The method for operating a gas burner according to the present application is defined in the claim 1 .

In the second operation mode or summer operation mode of the gas burner appliance the calibration of the defined mixing ratio of gas and air of the gas/air mixture is initiated in such a way that a value being representative of a number of starts and/or of an operating time of the gas burner and a temperature being representative of the temperature of the gas burner are monitored.

If the value being representative of the number of gas burner starts and/or of the operating time reaches or crosses a first threshold, and if further the temperature being representative of the gas burner temperature reaches or falls below or is below a second threshold, then the gas burner is started without presence of a heat demand to calibrate the defined mixing ratio of gas and air of the gas/air mixture. The method according to the present application allows the calibration of a gas burner appliance without loss of comfort for the end user.

If the value being representative of the number of gas burner starts and/or of the operating time reaches or crosses a security threshold, and if no heat demand for domestic hot water is present, and if the temperature being representative of the gas burner temperature reaches or falls below or is below the second threshold, then the gas burner appliance is switched to the first or winter operation mode and thereafter the gas burner is started without presence of a heat demand for domestic hot water to calibrate the defined mixing ratio of gas and air of the gas/air mixture. This allows the calibration of a gas burner appliance without loss of comfort for the end user.

If the value being representative of the number of gas burner starts and/or of the operating time reaches or crosses the security threshold, and if a heat demand for domestic hot water is present, and if the temperature being representative of the gas burner temperature reaches or falls below or is below the second threshold, then the gas burner is started to calibrate the defined mixing ratio of gas and air of the gas/air mixture. However, if in this scenario the temperature being representative of the gas burner temperature is above the second threshold, then the gas burner start is delayed and the gas burner becomes started when the temperature being representative of the gas burner temperature reaches or falls below the second threshold. If the calibration has been started when a heat demand for domestic hot water is present, a modulation of the gas burner loads is delayed and allowed after the calibration is finished. This allows the calibration of a gas burner appliance with almost no loss of comfort for the end user.

The temperature being representative of the temperature of the gas burner is also monitored after the gas burner and the calibration have been started, wherein the gas burner is stopped either if the calibration is finished or if during calibration the temperature being representative of the gas burner temperature reaches or ex- ceeds a third threshold. This avoids an overheating of the gas burner during calibration of the gas burner appliance.

Preferred developments of the invention are provided by the dependent claims and the description which follows. Exemplary embodiments are explained in more detail on the basis of the drawing, in which: Figure 1 shows a schematic view of a gas burner appliance;

Figure 2 shows a schematic signal flow diagram illustrating the method according to present application. As already explained above, Figure 1 shows a schematic drawing of an exemplary embodiment of a gas burner appliance 10.

The gas burner appliance 10 comprises a gas burner 1 1 for combusting a gas/air mixture. Figure 1 shows flames 12 resulting from the combustion of the gas/air mixture. The gas burner appliance 10 further comprises a primary heat exchanger 13 for heating service water 14 by the combustion of said gas/air mixture. The heated service water 14 can be provided to a heating system 15 having at least one radiator 1 6. A pump 17 circulates the service water 14. The gas burner appliance 10 further comprises a secondary heat exchanger 18 for heating domestic water 19 by the heated service water 14. The gas burner appliance 10 further comprises a valve 20. The valve 20 is preferably a three-way valve. The gas/air mixture provided to the gas burner 1 1 has a defined mixing ratio of gas and air. The gas/air mixture is provided by mixing an air flow 21 with a gas flow 22. The quantity of the gas/air mixture provided to the gas burner 1 1 is adjusted by a fan 23. So, the modulation of the gas burner load is effected by controlling the speed of the fan 23.

The defined mixing ratio of gas and air within the gas/air mixture provided to the gas burner 1 1 is controlled by a controller 24 on basis of a signal provided by an electrical or electronic sensor 25. In Figure 1 , the electrical or electronic sensor 25 is provided by a flame ionization sensor. The controller 24 generates a control variable for an actuator 26 of a gas valve 27 to control the defined mixing ratio of gas and air of the gas/air mixture based on the signal provided by the electrical or electronic sensor 25.

The gas burner appliance 10 can be operated in a first operation mode, also often called winter operation mode, or in a second operation mode, also often called summer operation mode. Usually, an end user chooses the operation manually by pressing a button on the controller 24.

In the first operation mode or winter operation mode of the gas burner appliance 10 the heated service water 14 is provided to the heating system 15 and/or to the secondary heat exchanger 18 depending on the position of the valve 20. In the first operation mode or winter operation mode of the gas burner appliance 10 there is usually a heat demand from the heating system 15 present causing that the gas burner 1 1 is started and running.

If no heat demand for domestic hot water is present during the first operation mode or winter operation mode of the gas burner appliance 10, the heated service water 14 is only provided to the heating system 15 by automatically switching the valve 20 to a first position.

If a heat demand for domestic hot water is present during the first operation mode or winter operation mode of the gas burner appliance 10, the valve 20 is automatically switched to a second position and the heated service water 14 is partially or completely provided to secondary heat exchanger 18.

In the second operation mode or summer operation mode of the gas burner appliance 10 the heated service water 14 is only provided to the secondary heat exchanger 18 to provide domestic hot water 19. In the second operation mode or summer operation mode of the gas burner appliance 10 the valve 20 is switched to a position in which the heated service water 14 is completely provided to secondary heat exchanger 18. In the second operation mode or summer operation mode there occurs no heat demand from the heating system 15. In the second operation mode or summer operation mode of the gas burner appliance 10 the gas burner 1 1 is usually only started if a demand for domestic hot water is present.

The present application relates to a method to calibrate the defined mixing ratio of gas and air of the gas/air mixture in the second operation mode or summer operation mode of the gas burner appliance 10, namely when the end user has chosen the second operation mode or summer operation mode and when no heat demand from the heating system 15 occurs.

For the calibration of the defined mixing ratio of gas and air of the gas/air mixture in the second operation mode or summer operation mode of the gas burner appliance 10 a value being representative of the number of starts of the gas burner 1 1 and/or being representative of the operating time of the gas burner 1 1 is monitored, and further a temperature being representative of the temperature of the gas burner 1 1 is monitored.

If the value being representative of the number of gas burner starts and/or being representative of the operating time of the gas burner 1 1 reaches or crosses a first threshold, and if further the temperature being representative of the gas burner temperature reaches or falls below or is below a second threshold, then the gas burner 1 1 is started for the calibration without presence of a heat demand for domestic hot water to calibrate the defined mixing ratio of gas and air of the gas/air mixture to compensate example given different gas qualities. This allows the calibration of a gas burner appliance 10 when the second operation mode or summer operation mode of the same has been chosen without loss of comfort for the end user.

Further details of the invention will be described under reference to the flow diagram 100 of Figure 2. Preferably, a value being representative of a number of starts of the gas burner 1 1 is monitored. This can be accomplished by monitoring a value of a down counter which reduces the counter value by 1 with each burner start. It is also possible to monitor a value being representative of the operating time of the gas burner 1 1 . In the following description making reference to Figure 2 it is assumed that a value being representative of a number of starts of the gas burner 1 1 is monitored. Block 101 of Figure 2 illustrates that the value of the down counter is compared with a first threshold, In block 102 of Figure 3 the value of the down counter is compared with a security threshold being smaller than the first threshold.

If the value being representative of the number of gas burner starts reaches or crosses, namely falls below the first threshold (block 101 Y - YES), and if the value being representative of the number of gas burner starts does not reach the security threshold (block 102 N - NO), then in block 103 the temperature being representative of the gas burner temperature is compared with the second threshold.

If the temperature being representative of the gas burner temperature reaches or falls below or is below a second threshold (block 103 Y - YES), then for calibration the pump 17 is automatically started in block 104 and the gas burner 1 1 is automatically started in block 105 without presence of a heat demand for domes- tic hot water. A calibration routine becomes automatically executed. Calibration routines as such are known to the person skilled in the art.

After the gas burner 1 1 and calibration routine have been started and during the calibration routine is executed, the temperature being representative of the gas burner temperature is still monitored. In block 106 it is checked if the temperature being representative of the gas burner temperature reaches or exceeds a third threshold or if the temperature being representative of the gas burner temperature does not reach or exceed the third threshold.

If the temperature being representative of the gas burner temperature does not reach or exceed the third threshold (block 106 Y - YES) and if the calibration is in block 107 determined to be finished (block 107 Y - YES), then the gas burner 1 1 , namely the combustion of the gas/air mixture by the gas burner 1 1 , is stopped in block 108. If the temperature being representative of the gas burner temperature reaches or exceeds the third threshold (block 106 N - NO), then then the gas burner 1 1 , namely the combustion of the gas/air mixture by the gas burner 1 1 , is also stopped in block 108. If in block 107 the calibration is determined to be not finished (block 107 N - NO), the calibration is continued by turning back to block 106.

The gas burner appliance 10 may be operated in a comfort mode during the second operation mode or summer operation mode. Such a comfort mode keeps the secondary heat exchanger 18 at a defined temperature so that domestic hot water can be provided in a short time frame after a heat demand for domestic hot water occurs. If the gas burner appliance 10 can be operated in such a comfort mode, the comfort mode is automatically switched off in optional block 109 before block 103 is executed.

If the value being representative of the number of gas burner starts reaches or crosses, namely falls below the first threshold (block 101 Y - YES), and if the value being representative of the number of gas burner starts reaches or crosses the security threshold (block 102 Y - YES), then in block 1 10 the presence of a heat demand for domestic hot water is checked. If the gas burner appliance 10 can be operated in a comfort mode, the comfort mode is automatically switched off in optional block 109 before block 1 10 is executed.

In no heat demand for domestic hot water is present (block 1 10 N - NO), then in block 103 the temperature being representative of the gas burner temperature is compared with the second threshold.

If the temperature being representative of the gas burner temperature reaches or falls below or is below a second threshold (block 103 Y - YES), then the gas burner appliance 10 is switched in block 1 1 1 automatically to the first operation mode or winter operation mode by switching the valve 20 to the first position, and afterwards blocks 104, 105, 106, 107 and 108 are executed as described above. The switching of the gas burner appliance 10 to the first operation mode or winter operation mode while no heat demand for domestic hot water is present is allowed only for a limited number of times, preferably only once. So, the blocks framed by dashed lines in Figure 1 will therefore be executed only for the limited number of times, preferably only once.

If the value being representative of the number of gas burner starts reaches or crosses, namely falls below the first threshold (block 101 Y - YES), and if the value being representative of the number of gas burner starts reaches or crosses the security threshold (block 102 Y - YES), and if no heat demand for domestic hot water is present is (block 1 10 N - NO), and if the limited number of times is reached, then is it assumed a heat demand would be present and block 1 10 is continued with the path Y-YES.

Is a heat demand for domestic hot water is present or assumed to be present (block 1 10 Y - YES), then in block 103 the temperature being representative of the gas burner temperature is compared with the second threshold. If the temperature being representative of the gas burner temperature reaches or falls below or is below a second threshold (block 103 Y - YES), then the pump 17 is automatically started in block 104 and the gas burner 1 1 and calibration is automatically started in block 105.

If the temperature being representative of the gas burner temperature is above the second threshold (block 103 N - NO) while a heat demand for domestic hot water is present or assumed to be present (block 1 10 Y - YES), then the pump 17 is automatically started in block 104 and the start of gas burner 1 1 in block 105 is delayed until it is detected in block 103a that the temperature being representative of the gas burner temperature reaches or falls below the second threshold. After the gas burner 1 1 and calibration 1 1 have been started in block 105 than blocks 1 12, 1 13 become executed.

In block 1 12 (corresponding to block 106) it is checked if the temperature being representative of the gas burner temperature reaches or exceeds a third thresh- old or if the temperature being representative of the gas burner temperature does not reach or exceed the third threshold.

In block 1 1 3 (corresponding to block 107) it is checked in the calibration is finished.

If it is detected that the temperature being representative of the gas burner temperature does not reach or exceed the third threshold (block 1 12 Y - YES) and if it is further detected that the calibration is finished (block 1 13 Y - YES), then in block 1 14 the modulation of the gas burner load is allowed to provide domestic hot water according to the corresponding heat demand. If it is detected that the temperature being representative of the gas burner temperature reaches or exceeds the third threshold (block 1 12 N - NO), then the gas burner appliance 10 is switched in block 1 1 1 automatically to the first operation mode or winter operation mode by switching the valve 20 to the first position, and afterwards blocks 1 12a and 1 13a are executed as described above. Block 1 12a corresponds to block 1 12 and block 1 13a corresponds to block 1 13.

After the calibration is finished (block 1 13a Y - YES), then in block 1 15 the gas burner appliance 10 is switched back in block 1 15 automatically to the second operation mode or summer operation mode by switching the valve 20, and afterwards in block 1 14 the modulation of the gas burner load is allowed to provide domestic hot water according to the corresponding heat demand.

If it is detected that the temperature being representative of the gas burner tem- perature reaches or exceeds the third threshold (block 1 12a N - NO), then the gas burner 1 1 is stopped in block 108.

List of reference signs

10 gas burner appliance

1 1 gas burner

12 flame

13 primary heat exchanger

14 service water

15 heating system

16 radiator

17 pump

18 secondary heat exchanger

19 domestic water

20 valve

21 air flow

22 gas flow

23 fan

24 controller

25 ionization sensor

26 actuator

27 gas valve

100 flow diagram

101 block

102 block

103 block

103a block

104 block

105 block

106 block

107 block

108 block 109 block

110 block

112 block

112a block

113 block

113a Block

114 block

115 block