CONTROL OF A CENTRAL HEATING SYSTEM

Abstract: The present invention relates to the control of a central heating system. The system includes a condensing boiler 1, a pump 3 operable to pump hot flow water leaving the boiler 1 around a circuit having two branches 6 and 7 and an electrically operated diverter valve 4 operable to direct the flow water to flow into one or other of the branches. The branch 6 includes radiators 8 to provide space heating by release of heat from the flow water and the branch 7 includes a calorifier 10 for heating water stored in a cylinder 9. The control unit includes a cylinder thermostat 16, a room termostat 17 and two pipe thermostats 18 and 19. The thermostat 18 is mounted on the pipework in the space heating branch 6 downstream of the radiators 8 and the thermostat 19 is mounted on the pipework in the hot water branch 7 upstream of the cylinder 9. When both hot water and space heating are required the boiler is operated and hot water is directed by the valve 4 to flow into the space heating branch 6 until the water reaches a preset level at the point where the thermostat 18 is mounted. At this stage the thermostat 18 switches from position e to f and the valve 4 directs the water to flow into the hot water branch 7. When the water temperature reaches a higher preset level at the point where the thermostat 19 is mounted it switches from position g to h. If thermostat 18 has switched back to position e because the temperature of the water at the thermostat 18 has fallen to below its preset level, the valve 4 directs the water to flow into the branch 6. The process is then repeated.

Description: Control of a Central Heating System This invention relates to the control of a central heating system of the type in which a condensing-type fuel burning appliance is operable to heat a fluid to serve as a heat source to supply space heating and hot water in a circuit through which the fluid is circulated at least during operation of the appliance and which has a first branch incorporating space heating means for releasing heat from the fluid and a second branch for providing heating of the hot water stored in a reservoir. In a conventional central heating system of the above type the condensing-type fuel burning appliance usually takes the form of a fuel fired boiler, for instance, a gas boiler. The fluid heat source is usually water and in boilers of the condensing type at least a proportion of the combustion products leaving the boiler are caused to condense by heat exchange with the water entering or leaving the boiler. This increases the overall heat content of the boiler water and therefore the overall efficiency of the system. However, condensing boilers work most efficiently when the temperature of the water with which the combustion products exchange their heat is as low as possible (preferably no more than 55"C) commensurate with satisfactory operation. In practice with present systems the temperature of the return water to the boiler may be higher than that necessary to provide efficient boiler operation. One method by which the efficiency of the boiler may be raised during periods when both space heating and domestic hot water are required is to provide a priority supply of boiler heated water to the branch in which reservoir water is being heated. In this case, the flow of the heating water to each branch is separated but heated water is supplied preferentially to the domestic hot water (DHW) branch until the temperature of the stored water in the reservoir has reached a preset level. At this point, the flow of heated water is diverted to the space heating (SH) branch until the ambient temperture has reached a preset level or the temperature of the stored water has fallen below its preset level. The heated water is then diverted to the DHW branch once more and the process continues. This procedure does at least ensure that the temperature of the return water to the boiler is relatively low (less than 7O0C) during at least part of the heating-cycie. The main disadvantage with this type system however is that for long periods there nay be no space heating at all and this is generally undesirable especially on start up of the system from cold when the requirement for space heating outweighs the requirement for hot water. It is therefore an object of the present invention to provide control of a central heating system of the type defined in which during periods when both space heating and domestic hot water required the temperature of the return water to the boiler from the space heating branch is as low as possible without the disadvantages of the conventional systems. According therefore to one aspect of the present invention, there is provided a method for controlling a central heating system of the type defined, the method comprising operating the burner and, while there is a demand for the heated fluid for space heating and for heating the stored water, directing the heated fluid to flow by way of the space heating branch until the temperature of the fluid at a point in the space heating branch downstream of the space heating means is at or above a preset level, then directing the fluid to flow by way of the domestic hot water branch until the fluid temperature at a point outside the space heating branch is at or above a preset level and providing that the temperature of the fluid at the point in the space heating branch is below the present level directing the fluid to flow by way of the space heating branch and repeating the procedure. According to another aspect of the present invention there is provided apparatus for controlling a central heating system of the type defined, the apparatus comprising means for operating the burner appliance, means for circulating the fluid at least during operation of the appliance, a first sensor for sensing the temperature of the fluid at a point in the space heating branch downstream of the space heating means, a second sensor for sensing the temperature of the fluid at a point outside the space heating branch, and fluid flow director means responsive to the sensors and operable, while there is a demand for the heated fluid to provide space heating and heat the stored water, to direct the fluid to flow by means of the space heating branch until the temperature sensed by the first sensor is at or above a preset level then by way of the domestic hot water branch until the temperature sensed by the second sensor is at or above a preset level providing that the temperature sensed by the first sensor is below the preset level then by way of the space heating branch once again and thereafter repeating the procedure. An embodiment of the present invention will now be particularly described with reference to the drawings, in which: Figure 1 is a schematic diagram of a central heating system in accordance with the present invention and Figure 2 is a schematic diagram of an electric circuit for controlling the system. Referring to Figure 1, the system comprises a condensing gas-fired boiler 1 which is operable to heat water to serve as a heat source system for supplying both hot water and space heating. The system includes a circuit 2 through which water heated by the boiler 1 is circulated by a pump 3 at least while the boiler 1 is operating. The circuit includes an electrically operated diverter valve 4 by means of which the heated flow water flowing in the flow pipe 5 may be diverted either into a first branch 6 or a second branch 7. The first branch 6 provides circulation of the heated water through space heating means in the form of radiators 8 connected in parallel in the branch 6 and by means of which heat from the heated water may be released to provide space heating. The second branch 7 provides circulation of the heated water through a reservoir in the form of a storage cylinder 9 to enable water stored in the cylinder 9 to be heated by a conventional calorifier 10 by heat exchange with the circulating water. The cylinder 9 is supplied with gravity fed cold water by a supply pipe 11 extending downwardly from a header tank 12 mounted above the cylinder 9. Stored hot water may be drawn off as required for domestic purposes by a pipe 13 via taps 14. Return water from either branch returns to the boiler 1 via the return pipe 15 as is the usual practice. While not shown the boiler 1 as is conventional incorporates gas burners for heating the circulating water in a heat exchanger in the boiler, a gas inlet pipe for supplying gas to the burners, an ignition device such as a pilot light for igniting the gas and an electrically operated valve for controlling flow of gas through the inlet pipe to the burners. The boiler 1 as is the usual practice also incorporates a thermostat which controls the gas inlet valve in response to the temperature of the water leaving the boiler 1. This determines the safe maximum water temperature in the boiler 1. As conventional the system also includes a cylinder thermostat 16 for controlling the temperature of the stored hot water and a room thermostat 17 for controlling the ambient temperature inside the building or dwelling from a central point. The system also includes a first pipe thermostat 18 for mounting on the pipework within the space heating branch 6 downstream of the radiators 8 and a second pipe thermostat 19 for mounting on the pipework in the hot water branch 7 upstream of the cylinder 9. Theae thermostats serve as water temperature sensors for the purposes to be described. Finally the system described also includes a conventional clock programmer 20 and control unit 21 and these too will be described in more detail subsequently. Referring to Figure 2 where similar components bear identical reference numerals to Figure 1, the programmer 20 is mains operated and as conventional can be set to switch the system on and off at certain preselected times and can be manually overridden to provde electrical power as desired. The programmer 20 enables electrical power to be supplied either to the space heating (SH) line 22 or to the domestic hot water (DHW) line 23 or to both simultaneously. These lines are provided with individual switches (not shown) to switch them on and off individually as required. The circuit also comprises the cylinder, room and pipe thermostats 16, 17 18 and 19 respectively, relays R1 and R2, two position relay contacts R11,R111 and R11ll controlled by R1 and relay contact R21 controlled by R2. When R1 is deactivated the contacts Rill, R111 and Rllll are in the position a shown in Figure 2. When R1 is activated the contacts Rill, R111 and Ri111 are in the position b. Relay R1 is activated when the cylinder thermostat 16 is closed and power is on line 23. When R2 is deactivated contact R21 is in the position c shown in Figure 2 whereas when R2 is activated R21 is in the position d. The pipe thermostat 18 (for the SB branch) can switch between two positions e and f depending upon the temperature of the water at the point downstream of the radiators where it is mounted. In Figure 2 the pipe thermostat 18 is in position e when the temperature of the water at the mounting point is below the preset level. The thermostat 18 switches to position f when the temperature at the mounting point is at or above the preset level. The pipe thermostat 19 (for the D branch) can switch between two positions g and h depending upon the temperature of the water at the point upstream of the cylinder 18 where it is mounted. In Figure 2 the pipe thermostat 19 is in position g when the temperature of the water at the mounting point is below the preset level. The thermostat 19 switches to the position h when the temperature at the mounting point is at or above the preset level. The preset temperature levels for the thermostats 18 and 19 are set during manufacture or installation. However the applicants have found that for efficient operation of the boiler the pipe thermostat 18 should be set to switch between positions e to f at a temperature no higher than 60"C preferably no higher than 50"C. The pipe thermostat 19 should be set to switch between positions g and h at a higher temperature than this but preferably a few degrees lower than the temperature at which the boiler thermostat switches the boiler off. This will prevent the boiler from cycling, that is switching on and off during a heating cycle when both hot water and space heating are demanded and thus improve the overall efficiency of operation. The boiler thermostat may be set to switch the boiler off at a water temperature of 820 C. In this case the pipe thermostat 19 may be set to switch between positions g and h at say 790C. Relay R2 is initially activated when power is on line 22, the room thermostat 17 is closed and the pipe thermostat 18 is in position f. Once activated it can be maintained activated when pipe thermostat 19 is in position g irrespective of the condition of the pipe thermostat 18. Provided the boiler thermostat is closed, the boiler gas inlet valve will be opened to fire the boiler if power is on line 22, the room thermostat 17 is closed and R11 is in position a. Alternatively, or in addition, the valve will be opened for firing if power is on line 23 and Rll is in position b. The pump 3 will operate if power is on line 22 and R111 is in position a. Alternatively the pump 3 will operate if power is on line 23 and the R11 is in position b. When the diverter valve 4 is unpowered it directs the flow water to flow into the DItW branch 7 of the heating circuit (see Figure 1) and prevents flow water from entering the SH branch 6. When however the diverter valve 4 is powered up, it directs the flow water to flow into the SB branch 6 and prevents flow water from entering the D branch 7. The diverter valve 4 is powered up only when there is power on line 22 and either Rllll is in position a or the room thermostat 17 is closed, R21 is in position c and Rl is in position b. Suppose the clock programmer 20 is set so that both domestic hot water and space heating are required. Both lines 22 and 23 will be switched on but if the components are in the state shown in Figure 2 with the cylinder and room thermostats 16 and 17 open, the boiler gas inlet valve will be closed, the diverter valve 4 will be unpowered and therefore open to the hot water branch but the pump 3 will be operating as it will be receiving power from line 22 via position a of Rill. Consequently water is being circulated but is not being heated by the boiler 1. Suppose now that both the cylinder thermostat 16 and the room thermostat 17 are closed ie. there is a demand both for hot water and space heating. Since the cylinder thermostat 16 is closed, relay R1 is activated and each of the relay contacts Rill, R111 and Rllll will be in position b. The boiler gas inlet valve will be receiving power from line 23 via position b of Rll. If the boiler thermostat is also closed, the boiler will be firing and heating up the flow water. The pump will be powered up from line 23 via R111 and water will be circulating. If the temperature of the water at the point in the space heating branch where the SH pipe thermostat 18 is mounted is below the preset switching level (say 500 C) the pipe thermostat 18 will be in position e. The temperature of the water at the point in the DIIW branch where the DHW pipe thermostat 19 is mounted will also be below the preset switching level (say 790C) and the thermostat 19 will be in position g. Consequently relay R2 will not be activated and R21 will be in position c. As a consequence the diverter valve 4 will be powered up as it will be receiving power from line 22 by way of the closed room thermostat 17, R21 in position c and Rllll via position b. Therefore the diverter valve 4 will direct the heated flow water to flow into the SH branch to which the valve 4 will be open and no flow water will enter the domestic hot water branch. This state of affairs will continue until the temperature of the flow water in the SB branch at the downstream point where the pipe thermostat 18 is mounted reaches its preset level. At this stage the pipe thermostat 18 will switch over from position e to position f. This will connect the relay R2 to line 22 (by way of the pipe thermostat 18 and the closed room thermostat 17) and relay R2 will be activated. Consequently R21 will switch from position c to position d. This will disconnect the diverter valve 4 from line 22 which thus will be unpowered and will open to the domestic hot water branch to direct water to flow around this branch. This state of affairs will exist until the temperature of the water at the point in the SH branch where the pipe thermostat 18 is mounted falls below its preset level and the temperature of the water at the point in the D branch where the pipe thermostat 19 is mounted rises to its preset level. In this event the pipe thermostat 18 will switch over from position f to position e and the pipe thermostat 19 will switch over from position g to position h. This will cause relay R2 to be deactivated since it is now no longer receiving any power from line 22. Consequently R21 will switch over from position d to position c and the diverter valve 4 will be powered up once again from line 22 via the closed room thermostat 17, R21 (in position c) and R1111 in position b. The diverter valve 4 will open once again to the SH branch to direct the flow water to flow by way of this branch. While there is a demand for hot water to heat the stored water and for space heating this process will repeat. Should the room thermostat open at any time because the ambient temperature has risen to its preset level while the cylinder thermostat is still closed, power to the diverter valve 4 from line 22, via R21 and R1111 will be disconnected. Consequently the diverter valve 4 will open to the DHW branch if not already open. Should the cylinder thermostat 16 open at any time because the temperature of the stored water has reached the preset level and the room thermostat 17 is still closed, diverter valve 4 will receive power from line 22 via Rllll in position a irrespective of the position of R21. Consequently the diverter valve 4 will open to the SB branch if not already open. In the event that the temperature of the water in the boiler 1 reaches its preset level the boiler thermostat will open, the boiler gas inlet valve will close and the boiler 1 will cease firing. However since the pump 3 is powered continuously either from line 22 (via contact R1ÚÚ in position a) or from line 23 (via contact R111 in position b) the water will continue to be circulated by way either of the D branch or the SH branch. In the event that only domestic hot water is required, the space heating line 22 is switched off while the domestic hot water line 23 remains on. If the cylinder thermostat 16 is closed R1 is activated and contacts Rill, RI11 and Rllll are in position b. Consequently the boiler 1 receives power from line 23 via Rll in position b and if the boiler thermostat is closed, the boiler will fire. The pump will be operating as it will be receiving power from line 23 via Rlll in position b. The diverter valve 4 will be unpowered since the SH line 22 is switched off and thus water heated by the boiler 1 will circulate only in the DHW branch. When the cylinder thermostat 16 opens, R1 will be deactivated and R11, R11l and R1111 will move to position a. Thus the boiler 1 and the pump 4 will cease operation as they are unpowered but the diverter valve 4 will still be open to the DHW branch as this too is unpowered. In the rare event that only space heating is selected, line 23 is switched off while line 22 is switched on. Therefore R1 is deactivated and the contacts Rll, Rlll and R11ll are in position a. If the room thermostat 17 is closed the boiler 1 is powered up from line 22 via the room thermostat 17 and Rll in position a. The boiler 1 will fire if its thermostat is closed. The pump 4 will be operating since it will be powered up by line 22 via R1ÚÚ in position a. The diverter valve 4 will also be powered up from line 22 via Rllll in position a. Thus the diverter valve 4 will be open to the SH branch. Water heated by the boiler 1 will be circulated in the SH branch. If the room thermostat 17 opens, power to the boiler 1 will be disconnected and it will cease operation. The pump 3 however will continue to operate since it is receiving power from line 22 via R11l in position a. It will therefore be appreciated that the system operates independently of the pipe thermostats 18 and 19 where only domestic hot water or space heating is required. It is during those periods when both domestic hot water and space heating is required that the operating efficiency of the boiler is increased. In this case, during the cycle of operation when water is directed through the SH branch the return water temperature is no higher than the level preset by the SH pipe thermostat 18 (preferably 500C or less). Thus the. efficiency of the boiler is maintained at a high level as the return water temperature is relatively low. Furthermore the system reduces the frequency of boiler cycling, i.e. the number of times the boiler switches on and off during a heating period. This also improves the efficiency of boiler operation.

Claims: CLAIMS 1. A method for controlling a central heating system of the type defined, the method comprising operating the burner and, while there is a demand for the heated fluid for space heating and for heating the stored water, directing the heated fluid to flow by way of the space heating branch until the temperature of the fluid at a point in the space heating branch downstream of the space heating means is at or above a preset level, then directing the fluid to flow by way of the domestic hot water branch until the temperature of the fluid at a point outside the space heating branch is at or above a preset level and providing that the temperature of the fluid at the point in the space heating branch is below the preset level directing the fluid to flow by way of the space heating branch and repeating the procedure. 2. A method claimed in claim 1 in which the point outside the space heating branch is in the domestic hot water branch. 3. A method as claimed in claim 2 in which the point is upstream of the reservoir. 4. A method as claimed in any of the preceding claims in which if the ambient temperature is at or above a preset level then irrespective of the fluid temperature at the respective points, the fluid is directed to flow by way of the domestic hot water branch. 5. A method as claimed in any of the preceding claims which if the temperature of the water in the reservoir is at or above a preset level then irrespective of the fluid temperature at the respective points, the fluid is directed to flow by way of the space heating branch. 6. A method as claimed in any of the preceding claims in which if the ambient temperature and the temperature of the water in the reservoir are both at or above their preset levels, the fuel-burning appliance is caused to cease operation. 7. Apparatus for controlling a central heating system of the type defined, the apparatus comprising means for operating the burner appliance, means for circulating the fluid at least during operation of the appliance, a first sensor for sensing the temperature of the fluid at a point in the space heating branch downstream of the space heating means, a second sensor for sensing the temperature of the fluid at a point outside the space heating branch, and fluid flow director means responsive to the sensors and operable, while there is a demand for the heated fluid to provide space heating and to heat the stored water, to direct the fluid to flow by means of the space heating branch until the temperature sensed by the first sensor is at or above a preset level then by way of the domestic hot water branch until the temperature sensed by the second sensor is at or above a preset level providing that the temperature sensed by the first sensor is below the preset level then by way of the space heating branch once again and thereafter repeating the procedure. 8. Apparatus as claimed in claim 7 in which the second sensor is located at a point in the domestic hot water branch. 9. Apparatus as claimed in claim 8 in which the second sensor is upstream of the reservoir. 10. Apparatus as claimed in any of claims 7 to 9 in which a sensor is provided for sensing the ambient temperature and the fluid flow director means is responsive to the ambient temperature sensor such that if the ambient temperature sensed is at or above a preset level the fluid flow director means is operable to direct the fluid to flow by way of the domestic hot water branch irrespective of the fluid temperatures sensed by the first and second sensors. 11. Apparatus as claimed in any of claims 7 to 10 in which a sensor is provided for sensing the temperature of the water in the reservoir and the fluid flow director means is responsive to the reservoir water temperature sensor such that if the water temperature sensed is at or above a preset level the fluid flow director means is operable to direct the fluid flow by way of the space heating branch irrespective of the fluid temperatures sensed by the first and second sensors. 12. Apparatus as claimed in any of claims 7 to 11 in which if the temperatures sensed by the ambient and water temperature sensors have reached their preset levels the fuel-burning appliance is caused to cease operation. 13. A method substantially as hereinbefore described with reference to the drawings. 14. Apparatus substantially as hereinbefore described with reference to the drawings.

Inventors: ROYSTON, GEORGE DANN
Applicants: BRITISH GAS CORPORATION
Application year: 1984
Main classification: F24D19
IPC codes: F24D19