WO2006034529A1

WO2006034529A1 - Improvements to water heater control systems, burner control systems and diffusers for gas burners

Info

Publication number: WO2006034529A1
Application number: PCT/AU2005/001459
Authority: WO
Inventors: Quentin Arthur Carl Adam
Original assignee: Rheem Australia Pty Limited
Priority date: 2004-09-30
Filing date: 2005-09-22
Publication date: 2006-04-06

Abstract

The present invention provides a control system for a water heater (1 ), said water heater including a burner (5) to combust an air and combustible gas mixture; a delivery means to deliver said mixture to said burner; a gas metering means to control the amount of combustible gas entering an air stream to form said mixture; a variable speed fan (6) to form said air stream and ultimately deliver said mixture to said burner; a temperature sensor (T2) to provide a signal representative of the temperature of the water exiting said wate heater; a flow sensor (64) to provide a signal representative of the rate of flow of water exiting the water heater, and a control unit (60) to receive said signals from said temperature sensor and said flow sensor; the control unit producing a signal to vary the fan speed in response to said signals to modulate the heat output of said burner, said burner being a type having a turn down ration in the rage of 8:1 to 25:1.

Description

Improvements to water heater control systems, burner control systems and diffusers for gas burners

Field of the invention

[001] The present invention relates to water heaters which can be of the instantaneous type and can also be of the type used in combination boilers which provide potable hot water and hot water for central heating purposes. These latter type are sometimes known as combi-boilers. More particularly, the invention relates to water heater control systems, air gas mixture diffusers and burner control systems for these water heaters.

Background of the invention

[002] Forced draft instantaneous gas water heaters are well known but utilise generally expensive and complicated burner control systems to regulate the heat output of the burner and the emissions therefrom.

[003] These complicated systems add to an increased cost of the instantaneous water heater.

[004] Any reference herein to known prior art does not, unless the contrary indication appears, constitute an admission that such prior art is commonly known by those skilled in. the art to which the invention relates, at the priority date of this application.

Summary of the invention

[005] The present invention provides a control system for a water heater, said water hearer including a burner to combust an air and combustible gas mixture; a delivery means to deliver said mixture to said burner; a gas metering means to control the amount of combustible gas entering an air stream to form said mixture; a variable speed fan to form said air stream and ultimately deliver said mixture to said burner; a temperature sensor to provide a signal representative of the temperature of water exiting said water heater; a flow sensor to provide a signal representative of the rate of flow of water exiting said water heater, and a control unit to receive said signals from said temperature sensor and said flow sensor; the control unit producing a signal to vary the fan speed in response to said signals to modulate the heat output of said burner, said burner being a type having a turn down ratio in the range of 8 : 1 to 25:1.

[006] The gas metering means produces an air to combustible gas ratio which does not vary substantially as the fan speed changes. [007] The control system can modulate the heat output of said burner in the range of 6: 1 to 15:1. More preferably the control system can modulate the heat output of said burner in the range of 8:1 to 12:1. ^• Even more preferably the control system can modulate the heat output of said burner by approximately 10: 1.

[008] The burner can be of a woven mesh type or foam ceramic type.

[009] The delivery means includes a diffuser forming a passage between said fan and said burner.

[010] The diffuser can include an inlet opening which is it 90° to the outlet opening.

[011] The mixture from said fan is made to enter said diffuser at an angle approximately

90° to the direction which is normal to the surface of the burner.

[012] The diffuser can include a redirection formation to pass said flow through 90°.

[013] The diffuser can include a flow splitter t split a portion of the flow and to direct said portion across said diffuser in a direction approximately 90° to the direction which is normal to the surface of the burner.

[014] Preferably the diffuser is of a single piece construction.

[015] Preferably the diffuser is formed by die casting.

[016] The diffuser can include a depression to receive said fan.

[017] Preferably the combined height f said fan when attached to said diffuser is less than the height of said diffuser added to the height of said fan, the heights being measured when said fan and diffuser are in the orientation they would have when assembled and in use.

[018] The diffuser can be divergent in the direction from said inlet to said outlet. The diffuser can also diverge in two orthogonal directions.

[019] Water exiting the water heater passes through a control, valve or governor, which can upon receiving a signal from the controller to increase or decrease the rate of flow of water exiting said water heater.

[020] The present invention also provides a water heater having a control system, as described above wherein there is included a heat exchanger of the type made from a series of plates wherein water passages are provided between pairs of plates and combustion passages are formed between adjacent pairs of said plates. [021] The water heater can also include a governor to maintain water pressure in said heat exchanger so as to raise the temperature at which water will boil in said heat exchanger.

[022] The present invention further provides a difruser for delivering a flow of air and combustible gas mixture to a burner, said diffuser being adapted to be located between a fan outlet and a burner, said difruser having an inlet for connection to said fan outlet and an outlet which is adapted to be positioned near to an upstream side of a burner, said inlet being oriented at 90° to the outlet opening.

[023] The mixture enters said diffuser at an angle approximately 90° to the direction which is normal to the surface of the burner.

[024] The diffuser can include a redirection formation to direct said flow through an arc of at least 90°.

[025] The diffuser can include a flow divider to divide said flow so that a portion of said flow travels across said diffuser in a direction substantially parallel to surface of the burner.

[026] The diffuser applies a substantially even pressure over the surface area of the burner.

[027] The diffuser can be of a single piece construction.

[028] The diffuser can be formed by die casting.

[029] The diffuser can include a depression to receive a fan.

[030] The diffuser can be such that the combined height of said fan when attached to said diffuser is less than the height of said diffuser added to the height of said fan, the heights being measured when said fan and diffuser are in the orientation they would have when assembled and in use.

[031] The diffuser can be divergent in the direction from said inlet to said outlet.

[032] The diffuser can be divergent in two orthogonal directions.

[033] The present invention also provides a method of controlling a water heater burner system in a water heater, said water heater burner system including: a burner to combust an air and combustible gas mixture, said burner having a turn down ratio in the range of 8:1 to 25:1; a delivery means to deliver said mixture to said burner; a gas metering means to control the amount of combustible gas entering an air stream to form said mixture; a variable speed fan to form said air stream and ultimately deliver said mixture to said burner; said method including steps of providing a signal representative of the temperature of water exiting said water heater; providing a signal representative of the rate of flow of water exiting said water heater, utilising a control unit to receive and process said signals representative of said temperature and said flow sensor; the control unit producing only a signal to vary the fan speed in response to said signals to modulate the heat output of said burner.

[034] The gas metering means can produce an air to combustible gas ratio which does not vary substantially as the fan speed changes.

[035] The control system can modulate the heat output of said burner in the range of 6:1 to 15:1. Preferably the control system can modulate the heat output of said burner in the range of 8:1 to 12:1. More preferably, the control system can modulate he heat output of said burner by approximately 10:1.

[036] The present invention further provides a method of controlling a water heater, said water heater including a heat exchanger, of the type made from a series of plates wherein water passages are provided between pairs of plates and the heat exchange is provided with combustion passages which are formed between adjacent pairs of said plates, said water heater having a cold water inlet to said heat exchanger, and a hot water outlet from said heat exchanger, and a control system to control at least the burner associated with said water heater, and a flow governor adapted to receive a signal from said control system, said flow governor being located down stream of or at said hot water outlet, said methos including the step of opening or closing said governor as required to maintain a water pressure in said heat exchanger so as to raise the temperature at which water will boil in said heat exchanger.

[037] The invention also provides a water heater including a heat exchanger of the type made from a series of plates wherein water passages are provided between pairs of plates and the heat exchange is provided with combustion passages which are formed between adjacent pairs of said plates, said water heater having a cold water inlet to said heat exchanger, and a hot water outlet from said heat exchanger, and a control system to control at least the burner associated with said water heater, and a flow governor being located down stream of or at said hot water outlet.

[038] The governor can open or close as required by said control system to maintain a water pressure in said eat exchanger so as to raise the temperature at which water will boil in said heat exchanger. Brief description of the drawings

[039] An embodiment or embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

[040] Figure 1 illustrates a perspective view of the components of a water heater;

[041] Figure 2 illustrates a perspective view of the diffuser as used in the water heater of figure 1;

[042] Figure 3 illustrates a more forward perspective view of the apparatus of figure 2;

[043] Figure 4 illustrates an underneath perspective view of the diffuser of figure 2;

[044] Figure 5 illustrates a schematic diagram of a burner control system for use with the water heater of figure 1.

Detailed description of the embodiment or embodiments

[045] Illustrated in figure 1 is a water heater 1; having a cold water inlet 8; heat exchanger 3; gas inlet pipe 4; burner 5; fan 6; hot water outlet pipe 2; all mounted within an enclosure having a rear panel 9 of which is visible and the base 10 with the side walls and top panel having been removed.

[046] The heat exchanger element is of the type described in co-pending applications

PCT/AU99/00473, PCT/AU01/00659 preferably which are generally of a plate type construction. The heat exchanger 3 forms a combustion chamber 11 which is surrounded by water passages so that an optimal amount of heat can be extracted from the combustion products.

[047] Above the combustion chamber 11 is a rectangular burner 5. A preferred burner is of the foam ceramic type but other burners can be utilised. Best performance is achieved by a burner which is capable of a turn down ratio in the range of 8:1 to 20:1 but most preferably of the range 10:1 to 15:1. Other types of burners which could be utilised include woven mesh burners which have similar turn down ratios.

[048] The burner 5 has its rectangular periphery sealed to the open end of the combustion chamber 11 at the top of the heat exchanger 3. Above the burner 5 is positioned the rectangular flange 20 of the diffuser 7 which, with a suitable sealing compound therebetween, will prevent the ingress of air via the flange 20 into the upstream side of the burner.

[049] As illustrated in figures 1 to 5, the diffuser 7 has an offset shape with an inlet 22 in a vertical plane whereby any air and gas mixture entering the diffuser 7 must travel in a direction which is generally parallel to the surface of the burner 5, which in this case is a substantially horizontal direction, to enter the diffuser. The inlet 22 is contained within a generally vertical face 24 which intersects with a curved surface 26 to define a recessed portion within which to receive the lower portions of the fan 6 when the fan 6 is bolted to the vertical face 24 via a series of four bolt holes 28.

[050] Upon entry into the diffuser 7, the first portion encountered by the airstream is a quarter spheroid portion 30 which is defined internally by a spheroidal shaped wall 32. The wall 32 helps to direct the air/gas mixture ravelling in the direction of arrow 34 and assist in splitting the air/gas flow into three directions. The first direction, generally indicated by the arrow 36 I in a downward direction and is generally perpendicular to the upstream surface of the burner 5. A second direction being generally indicated by arrow 38 travels around the rear of the diffuser and a third direction 40 being around to the front. The air travelling in the direction of arrows 38 and 40 are directed to the left side 42 of the diffuser by travelling along channels 44 and 46 formed by the underneath portion of the surface 26 and the undersurface of raised side walls 46 and 48. The air and gas mixture flow in the direction of arrow 34 is also assisted in its redirection in the direction of arrows 38 and 40 by a curved protruding portion 50. The diffuser 7 being shaped as it is and being able to direct flow in the directions of arrows 36, 40 and 38, has been found to be particularly effective in providing an air/gas mixture which is applied at a generally even pressure to the upstream surface of the burner 5. Another advantage of the diffuser 7 is that the curved surface 26 and the relative positioning of the vertical face 24 allow the fan 6 to be mounted to the diffuser whereby the joined height of the diffuser and fan is less than the height of the fan plus the height of the diffuser. This allows for a more compact arrangement coupled with the additional benefit of a generally even air gas mixture pressure.

[051] In figure 5, there is illustrated a schematic of the control system which includes a controller printed 60 comprising a circuit board which receives inputs from a flame sensor 62; the output temperature sensor T2; a flow rate signal from a flow meter 64; a user defined set temperature T-set indicated by the numeral 66, and a fan speed signal Fv.

[052] A cold water input temperature Tl is also indicated as an input, and, whilst being preferable, is not essential in that the system can function without knowing the actual temperature Tl. For example, the temperature Tl can be provided by a look up table or an assumed cold water input temperature could be provided. If the cold water input temperature Tl is assumed, then the likelihood is that the controller will require an iterative algorithm in order to achieve the required set temperature 66. Whereas if the cold water input temperature Tl is in fact measured, an iterative approach would not be required as will be described below. .[053] The cold water input temperature is optional. This is because the flow rate is measured and output temperature T2 is also measured, thus the controller 60 can estimate the inlet water temperature and adjust the fan speed and thus the heat generated by the burner 5. Because the fan speed is known, the heat generated by the burner 5 is known either via a look up table or equation, and as output temperature T2 is measure, if T2 not equal to Tset (the required "set point") the controller will successively recalculate and change the fan speed, until T2 equals Tset, and in this way Tl can be calculated (note Tl = ground water temperature which is normally pretty stable). Therefore having both Tl and T2 provides some redundancy because the Power Input = ((mass flow rate)*(spec heat constant)*(T2-Tl))/efficiency.

[054] The micro-controller printed circuit board 60 has four outputs. The first output being to an igniter 68, the second output to a gas solenoid 70, the third output to a fan speed control printed circuit board 72, and an optional fourth output to a flow governor 74 which is located on the hot water outlet.

[055] The igniter 68 is only activated to start the burner, and once lit, the flame sensing signal will switch off the igniter 68.

[056] The output signal to the gas solenoid 70 is a safety interlock. That is the gas valve 70 is enabled once the correct start up sequence is executed. The correct start up sequence can be that water flow is detected by flow meter 64, the diffuser 7 is pre-purge with air, air/gas valve 76 is enabled via the solenoid 70, the igniter 68 is fired and a signal representing the presence of a. flame is sent by flame sensor 62 back to the controller 60. While solenoid 70 keeps the air/gas valve 76 disabled, any extraneous air flows, which may be present due to wind or other atmospheric conditions, will prevent both the combustion chamber 11, and volume of the diffuser 7 from containing an air gas , mixture. This will prevent any possibility of an explosive start up. As an additional safety measure the diffuser 7 and consequently the combustion chamber 11 is subjected to a pre-purge air stream from the fan 6 as discussed above.

[057] The governor 74 is used to control the flow of water out of the heat exchanger 3.

This helps to provide and or maintain the outlet water temperature under most operational circumstances, such as high flow rate with a low temperature rise, or alternatively a low flow rate with a high temperature rise. Another advantage of the governor 74, being located downstream of the heat exchanger 3, is that the governor 74 can be used to control the water pressure in the heat exchanger 3. By the governor 74 maintaining pressure in the heat exchanger, water in the heat exchanger 3 will be less likely to be brought to boiling point This will help maintain the efficiency and integrity of the heat exchanger 3. [058] The air gas mixture control valve 76 is of a type that can mix a combustible gas with an air flow in a substantially constant ration for a specified range of speeds of the airflow passing through the control valve 76. The speed of the airflow will be controlled by the fan speed.

[059] The air gas mixture control metering valve 76 can be one such as that manufactures by the SIT SpA group and designated as a 391 AGM. Another appropriate valve can be that manufactured by Dungs GmbH and their BCS 300 valve.

[060] The required or "set point" outlet temperatures Tset can be the temperature required by the manufacturer, regulatory authorities or the user. The controller 60 compares T2 with Tset, then samples the flow rate and either "guesses" Tl or measures it. Next it calculates the required change in burner output which is then resolved into a change in fan speed signal. The gas control valve automatically meters the combustible gas added to the air stream as the air flow rate is detected via pneumatic communication namely a pressure signal from the fan inlet actuates a diaphragm in the gas control valve, thus maintaining substantially the same air/gas ratio.

[061] Thus from T2, Tset and flow rate data, the controller 60 calculates the fan. speed and then simply drives the fan at this speed. The controller 60 can send a signal to an on board fan speed controller printed circuit board. The fan speed signal can be a Pulse Width Modulation (PWM) signal sent from the controller 60. Alternatively, if desired this could be a voltage frequency signal depending on the type of on board fan speed controller and the control signal required. The fan speed control components within the controllerόO, whereby the fan motor windings would be connected directly to the controller 60 with speed feedback to the controller 60.

[062] It must be emphasised that this control system utilises only the fan speed as the output from the controller 60, to control the burner's heat output. His is done by the controller 60 using flow and outlet temperature as controller's inputs.

[063] In figure 5, the fan speed feedback Fv is illustrated as an input to the controller

60, but it must be emphasised that it is only a preferment and is not essential to the operation of the control system.

[064] Thus, once the burner 5 is fired and the fan speed initially controlled and then the controller 60 uses feedback to do any fine adjustment so that T2 = Tset. [065] A hybrid feed forward plus the use of feedback is the preferred control algorithm utilised. Using only PID feedback type controllers is much slower than controllers having hybrid feed forward plus the use of feedbacks, because they use T2 minus Tset as the feedback "error: that the controller acts on. Using feed forward in the controller 60 means that the controller can start or get t very quickly, an appropriate fan speed, then uses a feedback approach to control inaccuracy.

[066] With the control system as illustrated in figure 5 the only signal required to control the burner output, once the burner is lit and functioning is a fans peed signal whereby to increase or decrease the heat output of the burner 5, only the fan speed need be increased or . decreased respectively.

[067] It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text or drawings. Al of these different combinations constitute various alternative aspects of the invention.

[068] The foregoing describes embodiments of the present invention and modifications, obvious to those skilled in the art can be made thereto, without departing from the scope of the present invention.

[069] Where ever it is used, the word "comprising" is to be understood in its "open" sense, that is, in the sense of "including", and thus not limited to its "closed" sense, that is the sense of "consisting only of. A corresponding meaning is to be attributed to the corresponding words "comprise", "comprised" and "comprises" where they appear.

[070] It will be understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text. All of these different combinations constitute various alternative aspects of the invention.

[071] While particular embodiments of this invention have been described, it will be evident to those skilled in the art that the present invention may be embodied in other specific forms without departing from the essential characteristics thereof. The present embodiments and examples are therefore to be considered in all respects as illustrative and not restrictive, and all modifications which would be obvious to those skilled in the art are therefore intended to be embraced therein.

Claims

A control system for a water heater, said water heater including a burner to combust an air and combustible gas mixture; a delivery means to deliver said mixture to said burner; a gas metering means to control the amount of combustible gas entering an air stream to form said mixture; a variable speed fan to form said air stream and ultimately deliver said mixture to said burner; a temperature sensor to provide a signal representative of the temperature of water exiting said water heater; a flow sensor to provide a signal representative of the rate of flow of water exiting said water heater, and a control unit to receive said signals from said temperature sensor and said flow sensor; the control unit producing only a signal to vary the fan speed in response to said signals to modulate the heat output of said burner, said burner being of a type having a turn down ratio in the range of 8: 1 to 25: 1.

2. A control system as claimed in claim 1, wherein said gas metering means produces an air to combustible gas ratio which does not vary substantially as the fan speed changes.

3. A control system as claimed in claim 1 or 2, wherein said control system can modulate the heat output of said burner in the range of 6: 1 to 15: 1.

4. A control system as claimed in clam 3, wherein said control system can modulate the heat output of said burner in the range of 8 : 1 to 12 : 1.

5. A control system as claimed in claim 3 or 4, wherein said control system can modulate the heat output of said burner by approximately 10:1.

6. A control system as claimed in claim 5, wherein said burner is of a woven mesh type or foam ceramic type.

7. A control system as claimed in claims 1 to 6, wherein said delivery means includes a diffuser forming a passage between said fan and said burner.

8. A control system as claimed in claim 7, wherein said diffuser includes an inlet opening which is at 90° to the outlet opening.

9. A control system as claimed in claim 7 or 8, wherein said mixture from said fan enters said diffuser at an angle approximately 90° to the direction which is normal to the surface of ' the burner.

10. A control system as claimed in any one of claims 7 to 9, wherein said diffuser includes a redirection formation to pass said flow through 90°.

1. A control system as claimed in any one of claims 7 to 10, wherein said diffuser includes a flow splitter to split a portion of the flow and to direct said portion across said diffuser in a direction approximately 90° to. the direction which is normal to the surface of the burner.

.2. A control system as claimed in any one of claims 7 to 11, wherein said diffuser will apply a substantially even pressure over the surface area of the burner.

13. A control system as claimed in any one of claims 7 to 12, wherein said diffuser is of a single piece construction:

14. A control system as claimed in any one of claims 7 to 13, wherein said diffuser is formed by die casting.

15. A control system as claimed in any one of claims 7 to 14, wherein said diffuser includes a depression to receive said fan.

16. A control system as claimed in claim 2\15, wherein the combined height of said fan when attached to said diffuser is less than the height of said diffuser added to the height of said fan, the heights being measured when said fan and diffuser are in the orientation they would have when assembled and in use.

17. A control system as claimed in any one of claims 7 to 16, wherein said diffuser is divergent in the direction from said inlet to said outlet.

18. A control system as claimed in claim 17, wherein said diffuser is divergent in two orthogonal directions.

19. A control system as claimed in any one of claims 1 to 18, wherein water exiting the water heater passes through a control valve or governor, which can upon receiving a signal from the controller to increase or decrease the rate of flow of water exiting said water heater.

20. A water heater having a control system as claimed in any one of claims 1 to 19, wherein there is included a heat exchanger of the type made from a series of plates wherein water passages are provided between pairs of plates and combustion passages are formed between adjacent pairs of said plates.

21. A water heater having a control system as claimed in clam 19, wherein said governor also maintains a water pressure in said hear exchanger so as to raise the temperature at which water will boil in said heat exchanger.

22. A diffuser for delivering a flow of air and combustible gas mixture to a burner, said diffuser being adapted to be located between a fan outlet and a burner, said diffuser having an inlet for connection to said fan outlet and an outlet which is adapted to be positioned near to an upstream side of a burner, said inlet being oriented at 90° to the outlet opening.

23. A diffuser as claimed in claim 22, wherein said mixture enters said diffuser at an angle approximately 90° to the direction which is normal to the surface of the burner.

24. A diffuser as claimed in claim 23, wherein there is included a redirection formation to direct said flow through an arc of 90°.

25. A diffuser as claimed in any one of claim 22 to 24, wherein said diffuser includes a flow splitter to divide said flow so that a portion of said flow travels across said diffuser in a direction substantially parallel to surface of the burner.

26. A diffuser as claimed in any one of claims 22 to 25, wherein said diffuser will apply a substantially even pressure over the surface area of the burner.

27. A diffuser as claimed in any one of claims 22 to 26, wherein said diffuser is of a single piece construction.

28. A diffuser as claimed in any one of claims 22 to 27, wherein said diffuser is formed by die casting.

29. ^' A diffuser as claimed in any one of claims 22 to 28, wherein said diffuser includes a depression to receive a fan.

30. A diffuser as claimed in any one of claims 22 to 29, wherein the combined height of said fan when attached to said diffuser f less than the height of said diffuser added to the height of said fan, the heights being measured when said fan and diffuser are in the orientation they would have when assembled and in use.

31. A diffuser as claimed in any one of claims 22 to 30, wherein said diffuser is divergent in the direction from said inlet to said outlet.

32. A diffuser as claimed in any one of claims 22 to 31, wherein said diffuser is divergent in two orthogonal directions.

33. A method of controlling a water heater burner system in a water heater, said water heater burner system including; a burner to combust an air and combustible gas mixture, said burner having a tirn down ratio in the range of 8: 1 to 25: 1 ; a delivery means to deliver said mixture to said burner; a gas metering means to control the amount of combustible gas entering an air stream to form said mixture; a variable speed fan to form said air stream and ultimately deliver said mixture to said burner; said method including the steps of providing a signal representative of the rate of flow of water exiting said water heater, utilising a control unit to receive and process said signals representative of said temperature and said flow sensor; the control unit producing only a signal to vary the fan speed in response to said signals to modulate the heat output of said burner.

4. A method of controlling a water heater burner system in a water heater as claimed in claim 33, wherein said gas metering means produces an air to combustible gas ratio which does not vary substantially as the fan speed changes.

S5. A method of controlling a water heater burner system in a water heater as claimed in claim 33 or 34, wherein said control system can modulate the heat output of said burner in the range of 6:1 to 15:1.

36. A method of controlling a water heater burner system in a water heater as claimed in claim 35, wherein said control system can modulate the heat output of said burner in the range of 8:l to l2:l.

37. A method of controlling a water heater burner system in a water heater as claimed in claim 35 or 36, wherein said control system can modulate the heat output of said burner by approximately 10:1.

38. A method of controlling a water heater, said water heater including a heat exchanger of the type made from a series of plates wherein water passages are provided between pairs of plates and the heat exchange is provided with combustion passages which are formed between adjacent pairs of said plates, said water heater having a cold water inlet to said heat _■gXfihangsr, and a hot water outlet from said heat exchanger, Αnή^jβcmlτcύ^tfemJiiJxmixxύ-aϊ, least the burner associated with said water heater, and a flow governor adapted to receive a signal from said control system, said flow governor being located down stream of or at said hot water outlet, said method including the step of opening or closing said governor as required to maintain a water pressure in said heat exchanger so as to raise the temperature at which water will boil in said heat exchanger.

39. A water heater including a heat exchanger of the type made from a series of plates wherein water passages are provided between pairs of plates and the heat exchange is provided with combustion passages which are formed between adjacent pairs of said plates, said water heater having a cold water inlet to said heat exchanger, and a hot water outlet from said heat exchanger, and a control system to control at least the burner associated with said heat exchanger, and a flow governor adapted to receive a signal from said control system, said governor being located down stream of or at said hot water outlet. 0. A water heater as claimed in claim 39 wherein said governor will open or close as required by said control system to maintain a water pressure in said heat exchanger so as to raise the temperature at which water will boil in said heat exchanger.