GB2230623A

GB2230623A - Fuel rate control for heating appliance

Info

Publication number: GB2230623A
Application number: GB9006135A
Authority: GB
Inventors: Humphrey Bolton Hollins
Original assignee: Baxi Partnership Ltd
Current assignee: Baxi Partnership Ltd
Priority date: 1989-04-19
Filing date: 1990-03-19
Publication date: 1990-10-24
Also published as: GB8908815D0; GB9006135D0

Abstract

The temperature of fluid supplied to a circulating fluid heating system by a fuel burning heating appliance is monitored, and the rate of change of the monitored temperature is measured. The rate of fuel supply is adjusted in dependence upon a comparison of the measured rate of change with a predetermined rate of change such that the rate of fuel supply decreases with increasing measured rates of change thus reducing cycling. The rate of change may be calculated over different sampling periods. The predetermined rate of change may be a function of the measured temperature, and perhaps of the outside temperature. The rate of fuel supply may also be reduced progressively in stages as the temperature of the fluid supplied to the system or the temperature of a room heated by the system passes preset thresholds. <IMAGE>

Description

FUEL SUPPLY CONTROL The present invention relates to a method and apparatus for controlling the rate of supply of fuel to a fuel burning heating appliance supplying heated fluid to a circulating fluid heating system.

Circulating water domestic and industrial heating systems have been widely installed. In most domestic systems, a boiler fueled by gas or oil heats water in a boiler compartment and that water is then distrobuted to a domestic hot water cylinder and to radiators located in rooms to be heated. Water is circulated to the hot water cylinder as a result of gravity feed or is pumped.

Water is circulated to the radiators by a pump, the pump being turned on and off by an electrical signal provided by a room thermostat. Fuel is supplied to the boiler at a constant rate whenever a time clock indicates that heating is required unless the temperature of the water in the boiler exceeds a predetermined limit. Thus, unless the load on the boiler is greater than the predetermined boiler output which results from the predetermined rate of fuel supply the boiler turns itself on and off in a cyclical manner as the water temperature oscillates about the predetermined, temperature.

The thermal efficiency of boilers is greatest when operating continuously and falls off considerably when the boiler cycles between its on and off conditions. Furthermore, the boiler emits a relatively large amount of noise each time it turns on and, once turned on, the higher the rate of fuel supply the greater is the amount of noise produced.

These efficiency and noise problems are particularly acute in light load conditions that exist for the greatest part of the year.

It is an object of the present invention to provide an improved apparatus and method for controlling the rate of supply of fuel to a fuel burning heating appliance such that the problems outlined above are obviated or mitigated.

According to the present invention there is provided an apparatus for controlling the rate of supply of fuel to a fuel burning heating appliance supplying heated fluid to a circulating fluid heating system, comprising means for monitoring the temperature of fluid supplied to the heating system by the appliance, means for measuring the rate of change of the monitored temperature, and means for adjusting the rate of fuel supply in dependence upon the measured rate of change such that the rate of fuel supply decreases with increasing measured rates of change.

The invention also provides a method for controlling the rate of supply of fuel to a fluid heating appliance in a circulating fluid heating system, wherein the temperature of fluid supplied to the heating system by the appliance is monitored, the rate of change of the monitored temperature is measured, and the rate of fuel supply is adjusted in dependence upon the measured rate of change such that the rate of fuel supply decreases with increasing measured rates of change or vice versa.

Preferably the rate of change of the monitored temperature is compared with a predetermined rate of rise and the rate of fuel supply is adjusted in dependence upon the difference between the compared rates of rise. The predetermined rate of rise may be a function of the instantaneous boiler output temperature.

Preferably the rate of rise is measured over a plurality of periods of time, the resulting rate of rise being compared with a different predetermined rate of rise in respect of each period. Thus a rate of rise which is measured over a short period will only result in an adjustment to the rate of fuel supply if it differs very much from the predetermined rate of rise whereas a rate of rise measured over a prolonged period will result in a change in the rate of fuel supply if the measured rate of rise is only slightly different from the predetermined rate of rise. This enables the system to respond rapidly to rapid changes in the rate of rise without risk of "hunting" in the event of short term fluctuations in the monitored temperature during periods when the rate of rise changes only slowly.

According to a second aspect of the present invention, there is provided an apparatus for controlling the rate of supply of fuel to a fuel burning heating appliance supplying heated fluid to a circulating fluid heating system, comprising means for monitoring the temperature of fluid supplied to the heating system by the appliance, means for detecting when the monitored temperature exceeds a predetermined limit, and means for adjusting the rate of fuel supply such that the rate of fuel supply is reduced but maintained' above zero when the monitored temperature exceeds the predetermined limit.

The present invention also provides a method for controlling the rate of supply of fuel to a fuel burning heating appliance supplying heated fluid to a circulating fluid heating system, wherein the temperature of fluid supplied to the heating system by the appliance is monitored, and the rate of fuel supply is reduced but maintained above zero in the event of the monitored temperature exceeding a predetermined limit.

Preferably at least three predetermined temperatures are defined including minimum and maximum predetermined temperatures, and the rate of supply is reduced from a first level to a lower second level when the minimum temperature is exceeded, from the second level to a third level when the next higher predetermined temperature is exceeded, and to zero when the maximum temperature is exceeded. Thus as the boiler approaches its maximum safe temperature the rate of fuel supply is automatically cut back, thereby enabling the boiler to operate continuously at close to its maximum output temperature if necessary in a variety of load conditions. This reduces cycling.

Preferably the or each predetermined temperature is adjustable in response to an output provided by a thermostat, whereby different predetermined temperatures can be applied for example when a circulating pump is turned off and the boiler is only providing water by gravity feed to a hot water cylinder as compared with when a circulating pump is turned on.

According to a third aspect of the present invention, there is provided an apparatus for controlling the rate 'of supply of fuel to a fuel burning heating appliance supplying heated fluid to a circulating fluid heating system, comprising a thermostat sensing the temperature in a room heated by the system, and means for adjusting the rate of fuel supply to the appliance in the event of the sensed temperature exceeding a preset temperature such that the rate of fuel supply is reduced but maintained above zero. Thus the thermostat does not operate as a simple on/off switch controlling for example a water circulating pump but rather as a temperature sensor the output of which controls the rate at which energy is supplied to the system.The thermostat could be arranged to set two control points, that is a first temperature at which the rate of energy supply is cut back from a maximum level to a minimum level, and a second temperature at which the rate of energy supply is cut back to zero. In many operating conditions the system will thus simply switch between its maximum and minimum energy supply rates with the boiler never being turned off. Even if the minimum energy supply rate is greater than the then load on the system the frequency at which the boiler is turned on and off will be greatly reduced.

Furthermore noise associated with the expansion and contraction will be greatly reduced.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic illustration of the controls of a conventional domestic circulating water central heating system; Figure 2 illustrates the relationship between the post-ignition running noise and the thermal output of a boiler operating in a conventional system such as that illustrated in Figure 1; Figure 3 and 4 respectively illustrate the variation of noise output with time in a conventional system operating with pumped water circulation and gravity water circulation; Figure 5 illustrates the variation in the rate of temperature rise with output temperature in a conventional system operating with a constant rate of fuel supply; Figure 6 illustrates the variation in instantaneous output temperature with time in a conventional system;; Figure 7 schematically illustrates the controls of the heating system incorporating features of the present invention; Figure 8 schematically illustrates timing cycles which may be used in embodiments of the present invention to monitor output temperature rate of rise; Figure 9 illustrates a relationship between rate of temperature rise and temperature which is used to derive control parameters in accordance with an embodiment of the present invention; Figures 10 and 11 illustrate the relationship of noise output to time in systems according to the present invention operating with pumped circulation and gravity circulation respectively; Figure 12 is a simplified flow diagram illustrating the operation of an embodiment of the present invention; and Figure 13 is a schematic illustration of the electrical components of a system incorporating features of the present invention.

Referring now to Figure 1, in a conventional domestic hot water system a boiler 1 is controlled by a local thermostat 2 'such that when the boiler is turned on fuel is supplied to the boiler at a constant rate until the output temperature of the boiler reaches the temperature set by the thermostat 2. The fuel supply to the boiler is then turned off until the output temperature drops to a predetermined level at which point the boiler is turned on again.

Primary control of the boiler is effected by a time switch 3 which is often remotely located and a room thermostat 4 which is in nearly all circumstances remotely located. First the boiler is turned on by the time switch 3 and when so turned on fuel is supplied to the boiler until the temperature set by the thermostat 2 is exceded. The room thermostat 4 controls a pump (not shown) which pumps water heated by the boiler through a system of radiators (not shown). As soon as the temperature in the room in which the thermostat 4 is located reaches the temperature setting of the thermostat 4 the pump is turned off and in some systems the boiler is also turned off.

The system also incorporates a hot water cylinder 5 the contents of which are heated by water circulating to the cylinder from the boiler. When the pump is turned on there is generally some assistance to the flow of water from the boiler to the cylinder.

When the pump is turned off gravity circulation of water continues from the boiler to the cylinder. A thermostat 6 is provided on the cylinder which indicates to the boiler when the cylinder has been heated to a predetermined temperature. In some conventional systems if the room thermostat 4 and the cylinder thermostat 6 both indicate that the set temperature has been reached the boiler is turned off regardless of the setting of the time switch 3.

Thus it will be seen that in the conventional system the user has four physically separated control elements 2, 3, 4 and 6. These are interconnected by cabling which generally carries mains voltages and therefore the cable must be of substantial dimensions and carefully installed to protect it against damage.

Referring now to Figure 2, this indicates that the greater the thermal output which is of course related to the rate of fuel supply the greater is the noise output. Thus operation of the boiler at a relatively low thermal output would reduce the instantaneous noise output.

Referring now to Figure 3, this illustrates the noise output variation with time in a system such as shown in Figure 1 operating with the pump running.

Each spike represents the short-term increase in noise as a fuel supply valve is opened and the fuel is ignited. The noise level then drops back but slowly increases as the water temperature of the boiler increases to the boiler thermostat limit. The boiler is then turned off and the noise level rapidly falls away until the next ignition cycle.

With respect to Figure4, this illustrates noise output when the circulation pump is off, that is to say when either the sensed room temperature has reached the room thermostat setting or the system is operating to provide domestic hot water only, that is to maintain the temperature of the cylinder 5. Again as the boiler turns on there is a short-term larger noise output. The noise then falls back before rising gradually again until the boiler thermostat setting is reached and the boiler switches off. The noise level then decays until the next ignition cycle Clearly it would be highly advantageous to operate the boiler such that ignition cycling was reduced. It would also be advantageous to minimize the rate at which fuel is supplied to the boiler so that during operation of the boiler the ouput noise is minimized.

Referring now to Figure 5, this illustrates the rate of rise of boiler output temperature with varying output temperatures given a constant rate of fuel supply. The rate of rise is relatively rapid when the boiler is started up from cold but decreases as the output temperature of the boiler increases.

This is because when the water within the boiler and its associated pipework is at ambient temperature the heat output from the system into its environment is effectively zero and all the energy delivered to the system as a result of feeding fuel to the boiler is absorbed by the system, that is primarily by the water within the system. As the water temperature rises the rate at which energy is lost from the system to the environment increases. Thus the relationship illustrated in Figure 5 results assuming a constant rate of energy supply to the system.

Ideally the boiler should operate at a fuel supply rate set so that the boiler temperature does not rise sufficiently high to cause the boiler to cycle on and off. In accordance with one aspect of the present invention this is achieved by monitoring the rate of temperature rise and adjusting the fuel supply by reference to that rate of temperature rise.

In accordance with a second aspect of the present invention, the boiler temperature is monitored as it approaches the maximum set temperature and the fuel supply is cut back in a series of steps before that temperature is reached. In accordance with a third aspect of the invention, a room thermostat cuts back the maximum fuel supply rate as sensed room temperature approaches a predetermined level.

Figure 6 illustrates variations in the temperature of the water close to the boiler flow outlet with time, the variations resulting from low flow velocities with the boiler and stray thermal currents. The thick line of Figure 6 shows the mean temperature of the water leaving the boiler whereas the thinner line indicates the instantaneous variation in temperature of a temperature sensor located in the boiler output. It will be seen that because of unpredictable flow conditions within the boiler the output temperature fluctuates very markedly over the short term.

The boiler load, that is the rate at which energy is supplied to the boiler and at which energy is lost by or absorbed in the system, is equal to the sum of the system load (energy delivered to the environment by the system) plus the rate of system temperature rise load, that is the product of the rate of temperature rise and a constant which can be considered as the equivalent water weight of the parts of the system the temperature of which changes with the output temperature of the boiler. This does not include for example any expansion tank.

Furthermore room radiators cannot be considered as fully dynamic as there is a significant time lag before these components reach that mean temperature, especially in systems where water is distributed to radiators in series rather than in parallel.

The radiator load may be calculated at for example a reference temperature. So too may the equivalent water weight. If the ratio between these two calculations is approximately constant then the dynamic performance of the system can be predicted reliably. In the majority of systems this will be true, however in exceptional circumstances the first cycle can be used to recalculate these factors to accurately predict the relationship between energy supply to the system and the rate of temperature rise.

The rate of temperature rise is difficult to measure because of the temperature/time relationship illustrated by Figure 6. Short-term temperature fluctuations are smaller if the water temperature is measured on its return to the boiler but the return temperature responds very slowly to changes in the operation of the system, for example the turning on or off of the pump. Thus it is desirable to be able to measure the output temperature of the boiler (the flow temperature) in a way which enables the system to respond rapidly to changes in the system conditions without following every short-term fluctuation in the boiler output temperature.

Figure 7 schematically illustrates an embodiment of the present invention in which a boiler 1, a room thermostat 4 and a temperature sensor mounted on the cylinder 5 are connected to a control unit 7 at which the system user is able to set the desired temperature of the room in which the room thermostat is situated, and the desired temperature of the cylinder 5. Thus the user can control the entire system from a single point. It is not necessary for the user to set the maximum boiler temperature as this is not a functional part of the control system.

Figure 8 illustrates schematically the approach adopted to measuring the rate of temperature rise at the output flow from the boiler. Essentially the temperature change of the boiler output flow is measured over periods of 14, 28, 70 and 140 seconds.

The shorter the sampling periods (14, 28, 70 or 140 seconds) the greater is the possible proportional error due to short-term temperature fluctuations. By monitoring the combination of these four different measurements however a true representation of the rate of change in the monitored temperature can be achieved in a way which enables fast system response where such a response is appropriate without introducing undue instability. During stable operation, the rate of temperature rise is measured in successive cycles each 140 seconds long. In the event of a fuel rate change in response to a change in the system configuration, for example, a fresh cycle is initiated so that temperature change information is not accumulated over a period which spans different operating conditions.For example in the case illustrated in Figure 8, the system operates stably for the first full cycle of 140 seconds and for the next 28 seconds but then the system configuration changes and thus a third cycle is initiated which in its turn is terminated at 70 seconds.

Bearing in mind the relationship between noise output, thermal output and time illustrated by Figures 2 to 4, and that system thermal efficiency reduces in the event of the boiler being turned on and off repeatedly, it is desirable to be able to adjust the rate at which fuel is supplied so that the boiler output is matched to the rate at which energy is being lost from the system to the environment. The rate of energy loss is a function of the way the system is being used, for example demands for hot water or increased room temperature, a function of external temperature which adjusts the rate at which energy must be supplied to maintain stable room temperatures, and a function of the time at which the system has been operated in a particular configuration.For example the rate at which energy must be supplied is greater when the system is first turned on as compared with when the system has been operating for many hours. The present invention proposes monitoring the rate of temperature rise to give an indication of the true energy requirements of the system at any particular time. The precise relationship between the rate of temperature rise and the rate at which energy is supplied to the system is not itself of fundamental importance and it is affected for example by the required rate of response of the system to configuration changes and the like.

The slower the response time required, the lower can be the maximum rate of fuel supply in any particular set of condidions. Taking the curve of Figure 5 as an example, it may be decided by the system designer that an ideal rate of temperature rise would be that indicated by the curve of Figure 5. An alternative would be a simple straight line between the two points where the curve of Figure 5 cuts the axes. It will be appreciated that other linear and non-linear alternatives could be considered as being suitable however to the required system performance or to fit available equipment. For example it is generally easier to write sofware to operate on the basis of linear functions and therefore with some equipment it would be much easier to use a straight line relationship or a relationship based on a plurality of straight line relationships. Such an example is illustrated in Figure 9 by the line 8, this line indicating a selected rate of temperature rise in the full range of operating temperatures of the boiler ouput.

Referring to Figure 9, the system is set up such that the rate at which fuel is supplied is dependant upon the relationship between measured rate of temperature rise and the rate of temperature rise represented by the line 8. For example, assuming that the rate of fuel supply is represented by G and that the rate of fuel supply can be set at either zero or Gl,.Gl + C, G1 + 2C,...G1 + 9C, the rate of fuel supply can be switched up or down this ten component range by reference to the four measured rate of temperature rises, that is the measurements made every 14 seconds, every 28 seconds, every seventy seconds, and every 140 seconds.For example if the measured rate of temperature rise in a 14 second period exceeds four times the rate of rise indicated by the line 8, that is the rate of temperature rise is above that indicated by line 9, the rate of fuel supply is reduced by one step. If the measured rate of temperature rise in a 14 second period is more negative than - 50C per minute, that is below the -50C per minute line in Fig.9, then the rate of fuel supply is increased by one step. If the rate of temperature rise in a 28 second period is more negative than - 10C per minute, that is below the -1OC/min line in Fig. 9, again the fuel supply rate is increased by one step. If the rate of temperature rise in a 70 second period is greater than 1.3 times the rate indicated by the line 8, that is it exceeds the level indicated by line 10, the rate of gas supply is reduced by one step.If the measured rate of temperature rise is less than half that indicated by line 8, that is it is below line 11 in Figure 9, the rate of gas supply is increased by one step.

Finally if the rate of temperature rise in a 140 second period is greater than the level indicated by the line 8, the rate of gas supply is reduced, whereas if it is less than two thirds of the level indicated by line 8, that is it is below the level indicated by line 12, the rate of gas supply is increased by one step. Thus the system responds to measurements taken over a relatively short period only if those measurements indicate very large fluctuations. The system response time is thus reasonably short without risk of instability.

The system can also be arranged to control the fuel supply rate in dependence upon sensed room temperature. For example, if a user selects a room temperature of 200C, the maximum fuel supply rate can be reduced in predetermined steps as the sensed room temperature approaches 200C. In a simple system, the fuel supply rate would be cut back to a minimum rate at a sensed temperature of 190C, and the pump would be switched off at a sensed temperature of 200C.

As a further degree of control, steps can be taken to progressively reduce the rate of gas supply in the event of preset temperature limits being approached by the boiler output flow. The following limits could be applied for example; OC Gas supply rates less than 820C full range (0-G1 + 9C) 82-82.50C 0-G1 + 7C 82.5-830C 0-G1 + 5C 83-840C 0-G1 + 3C 84-860C 0-G1 86-0 0 The flow temperature taken as a control input for this arrangement could be the instantaneous temperature taken every 1.4 seconds for example or an average of a series of successive temperature measurements. Averaging would avoid any tendency to instability due to short-term fluctuations in output temperatures.

In certain heat exchanges where different responses are required in dependance upon whether or not the system pump is operating, the controlling flow temperature can be biased from the figures given above, for example the temperature range can be increased by 10C when the pump is operating and decreased by 60C when the pump is not operating. This approach compensates for differences in the rate at which the output temperature changes as between pump and gravity circulation.

Referring now to Figures 10 and 11, this shows the noise output from the system operating as described with reference to Figure 9. As can be seen from Figure 10 when the pump is operating continuously even in relatively low load conditions the pump stays on all the time with only minor variations in noise as the fuel level is adjusted in small Steps. As shown in Figure 11, when the system pump is not working even the minimum rate of fuel supply is generally too great for all the energy supplied to be absorbed and accordingly cycling still occurs. In such circumstances, ie the cylinder requires heat but the room does not, the "rate of rise" control is switched off and the boiler is held at its minimum fuel supply rate. The frequency of cycling is however considerably reduced and the average rate of supply of fuel during any one supply period is also reduced.The noise output is therefore reduced and in addition the system efficiency is increased as the rate of cycling of the boiler on and off is reduced.

Referring now to Figure 12, this is a simplified flow diagram illustrating the operation of the system as described above with reference to Figure 9. In the flow diagram, a counter is used to count periods of 1.4 seconds; thus each count of 10 corresponds to 14 seconds. A flow diagram of this sort can be implemented in a simple microprocessor system.

Referring now to Figure 13, this is a schematic electrical diagram of a system capable of operating according to the above. A central control unit 13 incorporates an ignition circuit 14, a flame sense amplifier 15, a mains supply transformer 16, a check switch 17, a gas burning appliance 18, and a gas supply network comprising valves 19 and 20 which can be switched to control the rate at which the gas is supplied. All of the components 13 to 20 are located at the boiler and connected to a controller 21 by appropriate low voltage wiring. A room thermostat 22, operating as a temperature sensor only rather than an on/off switch, a cylinder temperature sensor 23, and a pump 24 are linked to the controller 21 which also receives its own mains supply. The controller incorporates a display 25 and an array of buttons 26 linked to a microprocessor (not shown).The manipulation of the buttons 26 enables the settings of the desired room and cylinder temperatures to be determined, and the programming of a timer controlling the periods of operation of the system.

The check switch 17 enables tests to be made on the boiler by an installer without having to repeatedly return to the controller 21. The microprocessor incorporated in the controller 21 is set up to operate in accordance with the flow diagram illustrated in Figure 12. Thus an efficient and relatively quiet operation results.

The techniques described above which can be used to adjust the rate of fuel supply to the boiler make it possible to introduce extra control features. For example, rather than relying upon a fixed rate of rise limit as indicated by line 8 in Figure 9, the rate of rise limit may be itself a function of other parameters. For example, in very cold conditions it may be desirable to increase the rate of temperature rise and this could be achieved by sensing the outside temperature and increasing the rate of rise limit in inverse proportion to the sensed outside temperature.

Claims

1. An apparatus for controlling the rate of supply of fuel to a fuel burning heating appliance supplying heated fluid to a circulating fluid heating system, comprising means for monitoring the temperature of fluid supplied to the heating system by the appliance, means for measuring the rate of change of the monitored temperature, and means for adjusting the rate of fuel supply in dependence upon the measured rate of change such that the rate of fuel supply decreases with increasing measured rates of change.

2. A method for controlling the rate of supply of fuel to a fluid heating appliance in a circulating fluid heating system, wherein the temperature of fluid supplied to the heating system by the appliance is monitored, the rate of change of the monitored temperature is measured, and the rate of fuel supply is adjusted in dependence upon the measured rate of change such that the rate of fuel supply decreases with increasing measured rates of change or vice versa.

3. A method according to claim 2, wherein the rate of change of the monitored temperature is compared with a predetermined rate of change and the rate of fuel supply is adjusted in dependence upon the difference between the compared rates of change.

4. A method according to claim 3, wherein the predetermined rate of change is a function of the instantaneous boiler output temperature.

5. A method according to claim 3 or 4, wherein the rate of change is measured over a plurality of periods of time, the resulting rate of change being compared with a different predetermined rate of change in respect of each period.

6. A method according to any one of claims 2 to 5, wherein the rate of fuel supply is reduced but maintained above zero in the event of the monitored temperature exceeding a predetermined limit.

7. A method according to claim 6, wherein at least three predetermined temperatures are defined including minimum and maximum predetermined temperatures, and the rate of supply is reduced from a first level to a lower second level when the minimum temperature is exceeded, from the second level to a third level when the next higher predetermined temperature is exceeded, and to zero when the maximum temperature is exceeded.

8. A method according to claim 6 or 7, wherein the or each predetermined temperature is adjustable in response to an output provided by a thermostat.

9. A method according to any one of claims 2 to 8, wherein the temperature in a room heated by the system is sensed, and the rate of fuel supply to the appliance is adjusted in the event that the sensed temperature exceeds a preset temperature such that the rate of fuel supply is reduced but maintained above zero.

10. A method according to claim 9, wherein if the sensed room temperature rises above a first temperature the rate of fuel supply is reduced from a maximum level to a minimum level, and if the sensed room temperature exceeds a second temperature the rate of fuel supply is reduced to zero.

11. An apparatus for controlling the rate of supply of fuel to a fuel burning heating appliance supplying heated fluid to a circulating fluid heating system, comprising means for monitoring the temperature of fluid supplied to the heating system by the appliance, means for detecting when the monitored temperature exceeds a predetermined limit, and means for adjusting the rate of fuel supply such that the rate of fuel supply is reduced but maintained above zero when the monitored temperature exceeds the predetermined limit.

12. A method for controlling the rate of supply of fuel to a fuel burning heating appliance supplying heated fluid to a circulating fluid heating system, wherein the temperature of fluid supplied to the heating system by the appliance is monitored, and the rate of fuel supply is reduced but maintained above zero in the event of the monitored temperature exceeding a predetermined limit.

13. An apparatus for controlling the rate of supply of fuel to a fuel burning heating appliance supplying heated fluid to a circulating fluid heating system, comprising a thermostat sensing the temperature in a room heated by the system, and means for adjusting the rate of fuel supply to the appliance in the event of the sensed temperature exceeding a preset temperature such that the rate of fuel supply is reduced but maintained above zero.

14. An apparatus for controlling the rate of supply of fuel to a fuel burning heating appliance substantially as hereinbefore described with reference to Figs. 7 to 13 of the accompanying drawings.

15. A method for controlling the rate of supply of fuel to a fuel burning heating appliance substantially as hereinbefore described with reference to the Figs. 7 to 13 of the accompanying drawings.