GB2156963A - Gase-fired water heaters - Google Patents

Abstract

A water heater having a heat exchanger (1) is connected to supply hot water to a central heating system and, for domestic purposes, via a calorifier (7). The flow of heating water to the calorifier is controlled by a diverter valve (9) actuated by a demand for hot water. The diverter valve (9) operates to perm it flow of heating water to the calorifier (7) and to interrupt the flow of hot water to the central heating system on demand for hot water. Gas flow to the gas burner (2) of the heater is regulated by a modulating gas flow valve (6), under the control of a thermally responsive element (16) in the inlet to the heat exchanger, to maintain the water temperature constant. The element (16) is connectible into one or other of two temperature control circuits in dependence upon the state of the diverter valve (9). <IMAGE>

Description

SPECIFICATION Improvements in or relating to gas-fired water heaters This invention relates to gas-fired water heaters and has particular but not exclusive reference to gas-fired water heaters that supply hot water both for domestic etc. purposes and for central heating purposes.

In some forms of such water heaters, hot water for domestic andlor sanitary etc. purposes, for example, bathing, washing-up, washing machines, is supplied from a hot water tank fed from the water heater. In that case, it is necessary to maintain a relatively large volume of water at a relatively high temperature to be available as required and this is generally wasteful of energy even when the hot water tank is thermally insulated.

Other forms of water heater employ the so-called instantaneous water heater in which water is heated only when hot water is required. This can be more economical as regards energy usage but control of water temperature is difficult especially when the same water heater is also required to supply hot water for a central heating system.

According to the present invention, a water heater comprises a water path with an inlet and an outlet, a heat exchanger for transferring heat from a gas burner to water flowing along the path, a gas flow control valve for regulating the rate of flow of gas to the gas burner, a thermally responsive device exposed to the temperature of water at the inlet, and in which the valve is controlled by the device in a manner such that changes in said water temperature result in changes in the rate of flow of gas to the gas burner that maintain the said water temperature at substantially a pre-set value.

The water heater may provide hot water for domestic purposes and/or a central heating system.

In the case in which hot water is provided for both domestic purposes and central heating, a water flow responsive device is incorporated which responds to a demand for domestic hot water and isolates the central heating system from the water heater.

In the case referred to in the preceding paragraph, the thermally responsive device may regulate the gas valve via a control system having a first control for setting the said temperature for central heating purposes and a second control for setting the said temperature for hot water supply purposes. In that case, the water flow responsive device may operate means for rendering the first control inoperative and the second control operative when the device responds to a demand for hot water.

By way of example only, an embodiment of the invention will now be described in more detail with reference to the accompanying drawings of which: Figure 1 shows, in block schematic form only, a water heating system forming part of the embodiment, Figure 2 shows, in schematic form and on a larger scale, a component of the system, Figure 3 is an explanatory graph, and, Figure 4 is a circuit diagram of a control unit of the embodiment.

The water heater comprises a heat exchanger having a length of finned pipe 1 heatable by a gas burner 2 equipped with a pilot safety device 3 of any suitable form that is, in use, heated by a pilot burner 4. Supply of gas from an inlet pipe 5 to the burners 2 and 4 is controlled by a valve 6 to be described in more detail below.

It will be understood that other forms of gas fired water heater could be used, for example those including fully sequential controls giving automatic spark ignition etc. on actuation of a start control by a user.

The heat exchanger is connected to supply hot water both to a central heating system and to a calorifier 7. The outlet pipe 8 of the heat exchanger is connected via a diverter valve 9 to the inlet 10 of the calorifier 7 and to a connecting pipe 11 leading to the central heating system consisting, for example, of a number of radiators (not shown) of conventional form.

The outlet 12 of the calorifier 7 is joined to the inlet pipe 13 of the heat exchanger 1 as is the return pipe 14 of the central heating system. Connected in pipe 13 is a water pump 15 for circulating water through the calorifier 7 and the central heating system. Also fitted to the inlet pipe 13 is a thermally responsive element 16.

Inside the calorifier 7 is a coiled pipe 17 of considerable length through which flows a domestic water supply from a cold water inlet pipe 18 via the diverter valve 9 to a hot water outlet pipe 19.

The pipe 19 is joined to hot water taps in kitchens, bathrooms and cloakrooms as necessary and also to any domestic equipment, for example washing machines, requiring a supply of hot water.

It will be understood that the water heater is connected to ducting for carrying away combustion products, the ducting being shown diagtrammatically at 20 and connected to a flue hood 21 including a flue break 22. Alternatively, the flue break could be located at a suitable point in the ducting. Alternatively, the water heater may be of the room sealed type having separate air inlet and combustion products outlet ducts terminating in a suitable flue terminal. The room sealed heater may include a circulating fan so enabling the duct sizes to be reduced, appropriate fan controls and safety devices being included.

The water heating system as now described will also include other components commonly found in such systems including an expansion tank 23, a bleed valve or air vent 24, a pressure relif valve 25 and a user controlled by-pass valve 26 for diverting a proportion of the hot water flow from the central heating system to maintain a required temperature differential across the central heating system.

The diverter valve 9 shown in more detail in Fig.

2, comprises a water chamber 27 divided into two parts 28, 29 by a flexible diaphragm 30 connected in suitable manner to a two-position control switch 31 and water flow control valves 32 and 33. The valves 32, 33 are loosely mounted upon a shaft 34 operatively connected to diaphragm 30 and resiliently urged against stops on the shaft 34 by a spring 35 that encircles the shaft between the valves. Valve 32 coacts with a valve seat 36 located at one exit point from the diverter valve 9 of hot water to pipe 11. Valve 33 coacts with a seating 37 located at the other valve exit to control the flow of hot water from pipe 8 to pipe 10.

Gas flow control valve 6 is of the well known modulating type and is eiectro-magnetically operated under the control of an electro-magnet forming part of the circuit shown in Fig. 4 and which will be described in more detail below.

Typically, the valve 6 is of Type V8600N manufactured by Honeywell fitted with an electromagnet to vary the position of the servo-pressure regulator and hence the position of the main valve and so regulate the rate of flow of gas to the burner 2.

The operation of the valve is illustrated in Fig. 3 which shows the relationship between water temperature and gas flow rate to the burner 2. At maximum water and gas flow rates, the system is designed to produce a rise of 20"C or thereabouts between the temperature of water entering and leaving the heat exchanger.

Thus, if for example element 16 is set to respond to a water temperature of 60"C, this will produce an outlet water temperature of 80"C.

The results of increases in water temperature sensed by element 16 are illustrated by line A. Gas flow to the burner 2 is so controlled that, as the temperature sensed by element 16 rises, the rate of gas flow to the burner reduces as shown by line A. At a sensed temperature of 67"C, it will be seen that the rate of gas flow has dropped to 40% of maximum. For the purposes of this specification, the change of +7"C is regarded as of little significance and the inlet water temperature is thus maintained at substantially the preset value of 60"C.

Line B illustrates the drop in outlet water tem perature as the rate of gas flow reduces. At the sensed temperature of 67"C, the outlet temperature is 75"C.

Fig. 3 also shows how the mean radiator temper ature - Line C - varies over the range of inlet tem perature variations referred to above. The radiator system is arranged so that at full gas flow the mean radiator temperature at a particular ambient temperature is 70"C. At the higher temperature of 67"C sensed by element 16, the mean temperature has risen by 1" only. The mean radiator temperature is thus substantially independent of variation in gas flow rate over the range of temperatures shown in Fig. 3.

The valve 6 does not reduce the rate of gas flow below the 40% value even though sensed inlet temperature rises above 67"C. In the event of a malfunction and sensed inlet water temperature continues to rise, the gas flow to the burner 2 is shut off automatically when a predetermined inlet temperature is reached. This will be described in more detail later.

Figs. 1 and 2 show the diverter valve 9 in the po sition for supplying hot water to the central heating system only. Valve 33 is in its closed position and there is no flow of hot water from outlet pipe 8 to the inlet 10 of the calorifier 7.

If now it is desired to draw off hot water for domestic use, water commences to flow from inlet pipe 18 through the calorifier coiled pipe 17 to the outlet pipe 19. A pressure difference is thus set up across an orifice plate or venturi in the diverter 9 which deflects diaphragm 30 causing shaft 34 to move to the right as seen in Figs. 1 and 2 thereby moving valve 32 on to its seating 36 and valve 33 off its seating 37. Thus, the flow of hot water to the central heating system is stopped and hot water starts to flow through the calorifier via inlet 10 and returns to the heat exchanger via pipes 12 and 13.

Diaphragm 30 is resiliently loaded, by means not shown, to an extent sufficient to allow it to flex to a maximum as soon as the flow of water through pipe 18 exceeds a predetermined low value.

it is found that, in a typical domestic installation, hot water is very seldom drawn off for a period long enough for the drop in ambient temperature resulting from the cessation of hot water flow to the radiators to become apparent. Of course, if the central heating system is not in use, there is no problem.

The control system for the heater is designed to give the user control over the temperature of water circulated through the central heating system and, separately, control over the temperature of water drawn off for domestic purposes. Changeover from one control to the other is effected automaticaily by the switch 31 in a manner described below in connection with Fig. 4 which is a circuit diagram of a control circuit.

The power supply for the circuit is obtained via a step-down transformer T1 whose secondary is connected across the input terminals of a full wave rectifier bridge FWR. The positive d.c. output of bridge FWR is smoothed by electrolytic capacitor C1 and powers the energising coil EC of the modulating valve 6. Current flow through the coil EC is controlled by a MOSFET transistor M1 or a bi-polar or other suitable transistor in series connection between coil EC and the negative bridge output via resistor R14.

The junction C1, EC is joined via a voltage dropping resistor R1 to a zener diode D1 which provides a stabilised voltage supply at the junction R1, D1. Smoothing and decoupling of the stabilised supply is provided by electrolytic capacitor C10 and capacitor C3.

The stablished smoothed power supply also powers four operational amplifiers VC1...4 forming a quad operational amplifier. That supply is also joined to two-position switch 31 which, depending upon the position of its movable contact connects the supply either to contact CTl or CT2. The mova ble contact of the switch is normally in the position shown in Fig. 4 in which power is applied to semiconductor analogue switches SWB and SWC which are rendered conductive thereby applying the potentials at the sliders of potentiometers VR2 and TR2 to be applied respectively to the non-inverting terminal of amplifier VC1 and the inverting termi nal of amplifer VC3.

Amplifier VC1 operating as a voltage follower with a very high input impedance isolates the semi-conductor swich SWB from operational amplifier VC2 in order to prevent current flow through resistors R3, VR1 and VR2. The output of amplifier VC1 is joined via resistor R7 to inverting input of amplifier VC2 to whose non-inverting input is connected the negative output of the bridge FWR.

The output of amplifer VC2 is connected via resistor R10, potentiometer TR2 and semi-conductor switch SWC to the non-inverting input of amplifer VC3.

The thermally responsive element 16 of Fig. 1 is shown in Fig. 4 as NTC thermistor TH1 joined to resistor R6 across negative earth and an auxiliary power supply derived from the low voltage a.c. input to bridge FWR via electrolytic smoothing capacitor C6, rectifying diode D6 and dropping resistor R2. Capacitor C2 acts as a charge storage device to provide a negative potential line during positive half cycles of the mains supply. The potential between dropping resistors R2 and R6 is clamped by diode D7 and smoothed by electrolytic capacitor C5.

Changes in the potential at junction R6, TH1 are conveyed to the inverting input of amplifer VC2 via resistor R8.

The output of amplifer VC2 is joined via resistor R15 to the non-inverting input of amplifer VC4 acting as a voltage comparator whose inverting input is connected to the junction VR2, R5 which together with resistor R3 are connected across the stabilised d.c. power supply and negative earth.

Positive feedback between the output of comparator VC4 and the non-inverting input thereof is obtained via resistor R16.

The output of comparator VC4 controls the conductivity of a second MOSFET, bi-polar or other suitable semi-conductor switch M2 which in turn controls the energisation via resistor R17 of another energising coil EC2 acting on a separate valve in the modulating valve 6 in such manner that when energised the armature of the coil closes the separate valve fully and supply of gas to the burner 2 and the pilot burner 4 is shut-off. This is the safety control referred to above and itss operation will be described in more detail below.

When hot water is to be drawn off, switch 31 changes over and contact CT2 is connected to the stabilised smoothed power supply line at the junction R1, D1. Power is then supplied to semi-conductor analogue switches SWA and SWD.

Switch SWA connects the slider of potentiometer VR1 to the non-inverting input of amplifer VC1.

Switch SWD connects the slider of potentiometer TR1 with the non-inverting input of amplifer VC3.

Thus, it will be appreciated that control over MOSFET M1 can be exercised by two different routes depending upon the position of switch 31, that position being determined by whether or not hot water is being drawn off from the water heating system.

In the normal condition of switch 31 shown in Figs. 1, 2 and 4, contact CTl is powered and the selection of a desired water temperature is set by a user by operating the slider of potentiometer VR2 via a suitable control knob. The position of the slider determines the value of the voltage that is superimposed upon that at the junction TH7, R6.

As the water temperature sensed by thermistor TH1 increases, the voltage at the junction TH1, R6 changes and these changes are transmitted through operational amplifiers VC2 and VC3 to control the drain current of MOSFET M1 and thus the degree of energisation of coil EC and thereby the extent to which modulating valve 6 reduces the rate of flow of gas to the burner 2.

Under conditions of hot water draw off, contact CT2 is powered and semi-conductor analogue switches SWA and SWD are rendered conducting.

A desired hot water temperature can now be selected using potentiometer VR1. Subsequently, control of the drain current of MOSFET M1 is as just described. Potentiometer VR1 operates in a manner similar to that of potentiometer VR2 to preset the value of the voltage to be superimposed on that at the junction TH1, R6.

In the event that the temperature sensed by sensor TH1 exceeds a preset maximum, the output of amplifier VC2 will differ by a predetermined amount from a preset potential at the junction VR2, R5 and will cause amplifier VC4 to respond and via MOSFET M2 will de-energise coil EC2 thereby terminating the flow of gas to burner 2.

Potentiometer TR1 and TR2 are merely trimmers that are preset in the factory or by an installer to give maximum gas flow to the burner 2 at maximum permitted water flow rates.

It will be appreciated that the invention may be incorporated in a water heating system providing central heating only or domestic hot water only.

Claims (21)

1. A water heater comprising a water path with an inlet and an outlet, a heat exchanger for transferring heat from a gas burner to water flowing along the path, a gas flow control valve for regulating the rate of flow of gas to the gas burner, a thermally responsive device exposed to the temperature of water at the inlet, and in which the valve is controlled by the device in a manner such that changes in said water temperature result in changes in the rate of flow of gas to the gas burner that maintain the said water temperature at substantially a preset value.

2. A water heater as claimed in claim 1 in which the water flow path includes a water flow diverter valve operable to divert water flowing along the path from a first outlet to a second outlet.

3. A water heater as claimed in claim 2 and further comprising a calorifier having a hot water inlet connected to the second outlet and a water outlet connected to the inlet of the water path, the calorifier also including a further water flow path in heat exchanging relationship with water flowing from the hot water inlet of the calorifier to the water outlet of the latter.

4. A water heater as claimed in claim 3 in which the diverter valve includes means responsive to water flow along the further water path and operable on sensing water flow along the latter path to cause the diverter valve to divert water along the first mentioned path to the second outlet.

5. A water heater as claimed in claim 4 in which the means responsive to water flow includes a flexible diaphragm exposed to the pressure differential across a restrictor in the further water flow path.

6. A water heater as claimed in claim 5 in which the restrictor is an orifice plate or a venturi.

7. A water heater as claimed in either claim 5 or claim 6 in which the diaphragm is operatively connected to first and second water flow control valves operable to control water flow to the first outlet and to the second outlet respectively.

8. A water heater as claimed in claim 7 in which the first and second water flow control valves are mounted upon a common shaft that is operatively connected to the diaphragm.

9. A water heater as claimed in any one of the preceding claims in which means are provided for terminating gas flow to the burner in the event that said water temperature exceeds another pre-set value.

10. A water heater as claimed in any one of claims 2-9 and further comprising a control circuit including the thermally responsive device, a first control means for setting the temperature of water for supply to the first outlet, and a second control means for setting the temperature of water for supply to the second outlet, and an arrangement responsive to operation of the diverter valve for rendering one or other of the control means inoperative in dependence upon the condition of the diverter valve.

11. A combined central heating/hot water supply system including a gas fired water heater as claimed in any one of claims 1-10.

12. A system as claimed in claim 11 when appended to any one of claims 2-10 and in which the central heating includes one or more radiators connected between the first outlet and the inlet of the first mentioned water flow path.

13. A system as claimed in claim 12 in which an adjustable by-pass is connected between the first outlet and the inlet of the first mentioned water flow path.

14. A system as claimed in any one of claims 11-13 in which the thermally responsive element comprises a negative temperature coefficient thermistor.

15. A system as claimed in claim 14 in which the thermistor is an element in a voltage dropping chain and connected to provide a voltage indicative of the sensed temperature for controlling the conductivity of a semi-conductor device for regulating the current flow through an electromagnet which operatively controls the gas valve and thereby flow to the gas burner.

16. A system as claimed in claim 15 in which means are provided for superimposing on said voltage a further voltage determined in accordance with the pre-set value.

17. A system as claimed in claim 15 in which means are provided for superimposing on said voltage one of two further voltages each determined in accordance with one of two pre-set values.

18. A system as claimed in claim 17 and further including switch means for determining which of the two further voltages shall be superimposed on said voltage.

19. A system as claimed in claim 18 in which said switch means are operatively controlled by said diverter valve.

20. A gas-fired water heater substantially as herein described with reference to and as illustrated by Figs. 1 and 2 of the accompanying drawings.

21. A combined central heating/hot water system substantially as herein described with reference to and as illustrated by the accompanying drawings.