GB2090023A

GB2090023A - Central Heating Control System

Info

Publication number: GB2090023A
Application number: GB8041015A
Authority: GB
Original assignee: British Gas Corp
Current assignee: British Gas Corp
Priority date: 1980-12-22
Filing date: 1980-12-22
Publication date: 1982-06-30
Also published as: GB2090023B

Abstract

A control system is provided for controlling the operation of a wet central heating system of the type in which water can be pumped around a space heating loop and optionally a domestic hot water loop. The control system includes switches 30 and 31 which are turned on so as to permit hot water if demanded to be supplied to the space heating and domestic hot water loops. If the cylinder thermostat 32 or the room thermostat 33 or both are closed, the boiler 1 is fired for a heating cycle as conventional and also as conventional the heated water is circulated by the pump 11 around either or both loops. However, after a heating cycle even when both thermostats 32 and 33 are open, the control arrangement is such that hot water is still circulated around the space heating loop. This is because the pump 11 is still caused to operate as it receives current on lines 46 and 42 because a relay contact RL2''' is closed. Similarly the space heating zone valve 13 is still open because it receives current on lines 52 and 45 since relay contact RL2'''' is closed. Thereby the boiler is enabled to operate more efficiently as the residual heat stored in the boiler's thermal capacity is pumped to the radiators. The pump is caused to operate at a first speed during heating and at a slower speed after the heating cycle so as to circulate water in to the system to dissipate boiler heat content and prevent overheating. <IMAGE>

Description

SPECIFICATION Improvements In Control Systems The present invention relates to a control system for operating a wet central heating system of the type in which water can be forcibly circulated within at least a space heating loop during a boiler heating cycle.

Conventional wet central heating systems of the type defined have a cyclically fired boiler, that is, one fired usually with a fluid fuel such as oil or gas, but not with a solid fuel and the boiler supplies heated water for heat exchange with water circulating in a first loop from which domesoic hot water may be drawn off when required and to a second loop or further loops including radiators which supply space heating to the dwelling when supplied with hot water.

In these systems hot water for domestic hot water purposes is supplied from a gravity fed cylinder, the water in the cylinder being heated indirectly by a heat exchanger or calorifier through which heated water circulates to and from the boiler in the domestic hot water loop. In older systems the water in the domestic hot water loop circulates solely by gravity while the water in the space heating loop is circulated forcibly to and from the boiler by an electric pump. In more modern systems the pump is arranged forcibly to circulate the water around both the space heating and the domestic hot water loops and the loops are then provided with electrically operated valves or a valve serving both loops. Domestic hot water gravity fed systems can also be provided with such a valve or valves.The valves or valve can be shut off selectively to prevent water circulating within either or indeed both loops.

The control system to which the present invention relates is of the type providing a mode of operation of the central heating system generally for cold weather use and permitting water to be circulated during a boiler heating cycle within at least the space heating loop. In this mode of operation some control systems will permit water to be supplied solely to the space heating loop independently of the domestic hot water loop. Other control systems will not permit hot water to be independently supplied to the space heating loop without the water being available if required to the domestic hot water loop as well. In both these types of systems there is another mode of operation generally for warm weather use in which water can be supplied solely to the domestic hot water loop independently of the space heating loop.In the first system described the two modes can be combined so that hot water is available to both loops if required.

In the existing control system of the type defined the control system controls the electric pump for permitting the forcible circulation of water around the space heating loop during boiler heating cycles and, in more modern systems, around the domestic hot water loop as well. In older systems the water circulates by gravity in the domestic hot water loop. The control systems also control where fitted the electrically operated valves or valve serving both loops so that the valves or valve open when the pump is operating.

In these control systems an electrical circuit is provided to enable the boiler to be fired for a heating cycle. In this case the circuit opens a fuel inlet valve to the boiler and the fuel emerging from the valve is ignited by means of a pilot light, for instance, to heat water in the boiler.

In the conventional wet central heating systems of the type defined in which the boiler has a high thermal capacity, the boiler may contain between and after boiler heating cycles when no water is being circulated in the loops, a large volume of residual hot water which remains in the boiler until the next heating cycle is initiated. The stored thermal energy in the residual hot water is lost between heating cycles mainly as waste heat up the boiler flue and additionally as waste heat conducted through the boiler case.

It is therefore an object of the present invention to provide a control system of the type defined which improves the efficiency of boiler usage by providing a use for this residual heat.

According to one aspect of the present invention a control system is provided for controlling the operation of a wet central heating system of the type in which water can be forcibly circulated within at least a space heating loop, the control system being of the type providing a mode of operation of the central heating system permitting water to be forcibly circulated during a boiler heating cycle within the sapce heating loop, wherein the control system when providing that mode of operation is adapted to cause the forcible circulation of water within the space heating loop after a boiler heating cycle.

In this way the residual hot water remaining in the boiler after a heating cycle is circulated within the space heating loop to provide additional space heating. This additional space heating, while perhaps not demanded, will only raise the room temperature marginally and will at the same time reduce the overall total period of boiler heating cycles, that is, the total period when the boiler is fired during the period when the central heating system is operative. Thus the total consumption of energy during that period will be reduced.

In one embodiment of the invention the control system includes means, such as relays in an electrical circuit, the means being operable to cause the forced circulation of water within the space heating loop after a boiler heating cycle when the control system is providing the respective mode of operation.

Preferably the control system includes a room thermostat operable to cause the boiler to fire and force the water to circulate within the space heating loop during the boiler heating cycles.

Preferably, also the control system includes a hot water cylinder thermostat operable to cause the boiler to fire to heat water for circulation within the domestic hot water loop during the boiler heating cycles.

In the case where the central heating system is of the type in which water can be forcibly circulated within the domestic hot water loop during a boiler heating cycle, the cylinder thermostat is operable to cause the water heated during a heating cycle to be forcibly circulated within the domestic hot water loop.

Preferably the control system includes a programmable clock to operate the control system for a selected period.

In a modification of the invention, the control system is adapted to cause water to be forcibly circulated at a first speed during a boiler heating cycle and at a second speed around the space heating loop after a boiler heating cycle when the control system is providing the respective mode of operation, wherein the second speed is slower than the first speed.

According to another aspect of the present invention, a wet central heating system of the type in which water can be forcibly circulated within at least a space heating loop includes the control system defined above.

An embodiment of the invention will now be described by way of non-limiting example only with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a conventional wet central heating system and Figure 2 is a schematic representation of a control system for controlling the operation of the central heating system shown in Figure 1, and Figure 3 is a schematic representation of a modification to the control system shown in Figure 2.

Referring to Figure 1 , the wet central heating system includes a cyclically fired boiler 1 , which in this case is gas fired, for supplying hot water to a domestic hot water loop 2 and a space heating loop 3.

While not shown, the boiler 1 incorporates as is conventional gas burners for heating water circulating internally in a heat exchanger in the boiler 1, a gas inlet pipe for supplying gas to the burners, 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 domestic hot water loop 2 incorporates a calorifier 4 for enabling water contained in a conventional domestic hot water cylinder 5 to be heated by the hot water circulating in the loop 2.

The cylinder 5 is supplied with cold water under gravity from a cold water storage tank 6, via a pipe 7 and subsequent hot water in the cylinder 5 may be drawn off as required via pipework 8 and taps 9.

The space heating loop 3 includes a number (three are shown) of metal radiators 10 eg steel, aiuminium or cast iron radiators which when supplied with circulating hot water radiate heat to the environment as is conventional.

The central heating system shown in Figure 1 also incorporates electrically operated means for circulating water within the loops 2 and 3. the circulating means comprises a conventional electric pump 11 and electrically operated motorised zone valves 1 2 and 13 situated respectively in the domestic hot water loop 2 and space heating loop 3.

In the system shown the pump 11 is able to pump water around the domestic hot water loop 2 and the space heating loop 3 when the valves are open.

The above central heating system is of conventional design and its general construction and operation will be readily apparent to one skilled in the art.

Referring to Figure 2 any components shown which are included in Figure 1 bear reference numerals identical to those of Figure 1. The control system shown in Figure 2 includes an electrical circuit having a live line 20 which branches to a line 21 and a line 22. Line 22 branches to a line 23 and to a line 24 which controls the supply of hot water to the space heating loop. Line 23 branches to a line 25 which controls the supply of hot water to the domestic hot water loop and to a line 26 which is connected via the motor 27 of a clock circuit 28 to neutral line 29. Both lines 24 and 25 are controlled by an on/off switch 30 and when switch 30 is closed, line 25 conducts current but line 24 will not conduct current unless a further switch 31 in line 24 is closed.

This is a conventional arrangement and enables the circuit to be operated in two modes in which in one mode when switch 30 is on but switch 31 is off, the domestic hot water loop can be supplied with hot water and in the other mode when both switches 30 and 31 are on, both loops can be supplied with hot water.

On/off switch 30 and switch 31 are incorporated in a conventional programmable clock circuit 28 such as the Switchmaster 350 programmer to enable heating to be provided for selected periods according to the clock setting.

The circuit also includes relays RL1 and RL2, a thermostat 32 located on the domestic hot water cylinder and a room thermostat 33. Relay RL1 has a coil RL1' which when energised can actuate contacts RL1" and RL1"' respectively to close.

Relay RL2 has a coil RL2' which when energised can respectively actuate contact RL2" to close, RL2"' and RL2"" to open and RL2""' to close.

Line 21 supplies a live supply to contacts RL1"' and RL2nn' independent of clock unit 28 via branched lines 34 and 35 respectively, line 35 being connected to the live terminal inside the boiler 1. Line 25 itself branches to lines 36 and 37, line 36 including thermostat 32 and line 37 including contact RL2". Line 36 branches to a line 38 which is connected to neutral line 29 across the motor of the domestic hot water zone valve 12 which in this case is of the motorised type.

Line 36 also branches to line 39 which is connected to neutral line 29 and includes a switch 40 and relay coil RL2'. Switch 40 which is an integral part of valve 12 is closed by the motor 41 of the zone valve 12 as this valve 12 opens.

Line 24 branches to lines 42, 43, 44 and 45 respectively. Line 42 includes relay contact RL2"' and joins line 37 in a new line 46 which is connected across the electric pump 11 to neutral line 29. Line 43 includes room thermostat 33 and branches at the free terminal 47 of the room thermostat switch 48 to two further lines 49 and 50 which are both connected to the neutral line 29. Line 49 includes the relay coil RL1 ' while line 50 includes a resistor 51 which forms part of the room thermostat 33 and is connected to the terminal 47 of the thermostat 48. Line 44 includes relay contact RL1" and joins line 45 which includes relay contact RL2"", to form a new line 52.

Line 52 is connected to neutral line 29 across the space heating zone valve 13 which is of the motorised type. A switch 53 is closed by a motor 54 of the zone valve 13 as this valve 1 3 opens.

The switch 53 is located in line 34 as is contact RL1"' and is connected via line 55 to the live terminal in the boiler 1.

As is conventional the live and neutral terminals within the boiler 1 are connected via a line 56 including a boiler thermostat 57 and an electrically operated gas inlet valve 58 for supplying gas to the boiler burners (not shown).

The valve 58 opens when the boiler thermostat 57 closes provided current is being supplied to either line 35 or line 55 and gas supplied to the burners is ignited by a pilot light (not shown). The valve 58 closes when the thermostat 57 opens.

To operate the control system the switch 30 must be turned on either manually or via the clock 28. If only domestic hot water is required switch 31 is turned to the "off" position. If both domestic hot water and space heating are required switch 31 is turned on.

Suppose that only domestic hot water is required, then switch 30 is turned on while switch 31 is in the off position so that no current flows in line 24. This therefore constitutes one mode of operation of the control system and in this case the behaviour of the circuit depends upon whether the cylinder thermostat 32 is open or closed, the thermostat being closed when heating is required.

Only Domestic Hot Water Required a) If the thermostat 32 is open the circuit and the components are in the following state: (i) Zone valve 12 is closed, (ii) Switch 40 is open, (iii) Coil RL2' is de-energised, (iv) Contact RL2" is open, (v) Contact RL2"' is closed, (vi) Pump 11 is at rest, (vii) Coil RL1'is de-energised, (viii) contact RL1 " is open, (ix) Contact RL2"" is closed, (x) Zone valve 1 3 is closed, (xi) Switch 53 is open, (xii) Contact RL1 "' is open, (xiii) Contact RL2""' is open, (xiv) Boiler gas inlet valve 58 is closed and the boiler is not fired.

Therefore no water is heated by the boiler and no water circulates in either the domestic hot water loop or the space heating loop.

b) If the thermostat 32 is closed the circuit and the components are in the same state as above except that: (i) Zone valve 12 is now open, (ii) Switch 40 is closed, (iii) Coil RL2' is energised, (ivr Contact RL2" is closed, (v) Contact RL2"' is open, (vi) Pump 1 1 is operating, (vii) Contact RL2"" is open, (viii) Contact RL2""' is closed, and (ix) Boiler gas inlet valve 57 is open and the boiler is fired provided that switch 57 is closed.

Therefore water is heated by the boiler and this heated water is circulated only in the domestic hot water loop.

Suppose that both domestic hot water and space heating are required then both switch 30 and 31 are turned on. This therefore constitutes another mode of operation of the control system and in this case the behaviour of the circuit depends upon whether the cylinder thermostat 32 is open or closed and upon whether the room thermostat 33 is open or closed.

Domestic Hot water and Space Heating Required c) If both thermostat 32 and 33 are closed the circuit and the components are in the following state: (i) Zone valve 12 is open, (ii) Switch 40 is closed, (iii) Coil RL2' is energised, (iv) Contact RL2" is closed, (v) Contact RL2"'is open, (vi) Pump 11 is operating, (vii) Coil RLl' is energised, (viii) Contact RL1" is closed, (ix) Contact RL2"" is open, (x) Zone valve 13 is open, (xi) Switch 53 is closed, (xii) Contact RL1"' is closed, (xiii) Contact RL2""' is closed, (xiv) Boiler gas inlet valve 58 is open and the boiler is fired provided switch 57 is closed.

Therefore water is heated by the boiler and is circulated within both the domestic hot water loop and the space heating loop.

d) If cylinder thermostat 32 is closed the room thermostat 33 is open the circuit and the components are in the same state as in (c) above except that: (i) Coil RL1'is de-energised, (ii) Contact RL1" is open, (iii) Zone valve 13 is closed, (iv) Switch 53 is open and, (v) Contact RL1 "' is open.

e) If cylinder thermostat 32 is open but room thermostat 33 is closed the circuit and the components are in the same state as in (c) above except that:- (i) Zone valve 12 is closed, (ii) Switch 40 is open, (iii) Coil RL2' is de-energised, (iv) Contact RL2" is open, (v) Contact RL2"' is closed, (vi) Contact RL2"" is closed, and (vii) Contact RL2"' is open.

Therefore water is heated by the boiler and this heated water is circulated only in the space heating loop, It will be appreciated that thus far described the control system operates the central heating system in a conventional manner.

fl If however, both the cylinder thermostat 32 and the room thermostat 33 are open as they would be after boiler heating cycles, the circuit and the components are in the following state: (i) Zone valve 12 is closed, (ii) Switch 40 is open, (iii) Coil RL2' is de-energised, (iv) Contact RL2" is open, (v) Contact RL2"' is closed, (vi) Pump 11 is still operating however because Contact RL2"' is closed and receives current on line 24, (vii) Coil RL1' is de-energised, (viii) Contact RL1" is open, (ix) Contact RL2"" is closed, (x) Zone valve 13 is open however because contact RL2'," is closed and receives current on line 24, (xi) Switch 53 is closed, (xii) Contact RL1' is open, (xiii) Contact RL2""' is open and (xiv) Boiler gas inlet valve 58 is closed since there is no current through either contact RL1 Dl or RL2""' to the live terminal within the boiler and thus the boiler is not fired.

Therefore in this case even though the boiler is not fired, residual heat remaining in the boiler heat exchanger between and after heating cycles is circulated within the space heating loop since zone valve 13 is open and pump 11 is still operating while zone valve 12 in the domestic hot water loop is closed preventing heat being circulated out of the cylinder 5 (Figure 1) and into the domestic hot water loop. It is therefore at this point that the present control system differs from conventional control systems since these latter do not permit any circulation of water between and after heating cycles. Thus in these systems any residual hot water remaining in the boiler between and after heating cycles loses its thermal energy mainly as waste heat up the boiler flUe.

The applicants however, have discovered that if this residual hot water is circulated between and after heating cycles within the space heating loop even though the room thermostat is open, there is only a marginal increase in the room temperature and that this is effective over a period to reduce the time during which it is necessary to fire the boiler. There is thus a considerable saving in overall energy and the saving increases as the thermal capacity of the boiler increases and therefore as its capacity to lose heat as waste increases.

It will be appreciated that as conventionally the gas inlet valve 58 cannot open when the boiler thermostat 57 is open no matter what demands are made by the remainder of the control system.

Therefore the boiler will only fire in conditions (b) to (e) above if the thermostat 57 is closed.

Referring to Figure 3 parts identical to those in Figures 1 and 2 bear identical reference numerals.

Basically the modified circuit shown in Figure 3 includes all the components shown in Figure 2 with the addition of two further relay contacts RL2""" and RL1"". Line 46 leading from contact RL2" and RL2"' is connected to a first winding 59 of the pump 11. This winding 59 branches to lines 60 and 61. Line 60 includes relay contacts RL1"', and RL2""" and is connected to a second winding 62 of the pump 11 while line 61 is connected to one side of a capacitor 63. The second winding 62 of the pump 11 branches to a line 64 connected to the other side of the capacitor 63 and to a line 65 which is connected to the neutral line 29.

In addition line 46 has a branch line 66 which forms a top contact 67 for relay contact RL1"" which is normally closed on lower contact 68.

Line 42 also now branches to line 69 and 70 where line 69 is connected to line 46 beyond contact RL2" and line 70 forms a top contact 71 for the relay contact RL2"" which is normally closed on a lower contact 72.

The modified circuit enables the pump 11 to be operated at a first or full speed during a boiler heating cycle when the control system is set to supply to the domestic hot water loop alone or to both the domestic hot water loop and the space heating loop and at a second speed after a boiler heating cycle only if the control system is set to heat water for supply to both loops, the second speed being slower than the first. Thus the circuit enables electricity to be saved when the boiler is not firing. This pump speed variation is achieved by means of the relay contacts RL1"" and RL2""" which in operation switch the pump windings 59 and 62 from series connection when the pump runs at a slow speed to a parallel connection when the pump 11 runs at full speed.

To illustrate the operation of the modified circuit in Figure 3 it will be convenient to refer back to conditions (a) to (f) in the description of Figure 2.

Only Domestic Hot Water Required (a) Switch 30 on, switch 31 off but cylinder thermostat 32 open. Since in this condition both switch 31 and relay contact RL2" are open no current can reach the pump 11 and it is at rest.

(b) Switch 30 on, switch 31 off and cylinder thermostat 32 closed. In this condition current flows in parallel through the pump windings 59 and 62 and the pump is therefore operating at full speed. One current path is through line 22, switch 30, lines 23, 25 and 37, contact RL2" (which is now closed), line 46, winding 59 and line 61 to one side of the capacitor 63 the other side of which is connected to neutral line 29 via lines 65 and 64. The other current path is also through line 22, switch 30, lines 23, 25 37 and contact RL2" and divides through lines 69 and 70, contact RL2""" which is engaging the top contact 71 as coil RL2' has been energised, pump winding 62 and line 65 to neutral. Both pump windings 59 and 62 receive the full voltage available and the pump 11 therefore runs at full speed.

Both Domestic Hot Water and Space Heating Required (c) Sqitch 30 on, switch 31 on, both thermostats 32 and 33 closed. In this condition current again flows in parallel through the pump windings 59 and 62. The current paths are the same as described in (b) above but now relay contact RL1"" is engaging its top contact 67 because relay coil RL1' has been energised.

(d) Switch 30 on, switch 31 on, cylinder thermostat 32 closed and room thermostat 33 open.

In this condition current again flows in parallel through the pump windings 59 and 62 as described in (b) above and the current paths are also the same. In this case relay contact RL2"N" is engaging its top contact 71 because relay coil RL2' has been energised.

(e) Switch 30 on, switch 31 on, cylinder thermostat 32 open, and room thermostat 33 closed.

In this condition current again flows in parallel through the pump windings 59 and 62. The current paths are through line 22, switches 30 and 31, lines 24 and 42, relay contact RL2"' and lines 69 and 46 where the paths divide. One path is through winding 59 and line 61 to one side of the capacitor 63, the other side of which is connected to the neutral line 29 via lines 65 and 64. The other current path is through line 66, and relay contact RL1"" via top contact 67 (since relay coil RL1' has been energised), relay contact RL2""" which is in its normally closed position, pump winding 62 and line 65 to neutral.

(f) Switch 30 on, switch 31 on but both thermostats 32 and 33 open. In this condition current flows in series through the pump windings 59 and 62 from the live line 20 to the neutral line 29 and the pump 11 is therefore operating at slower than full speed. The current path is through line 22, switches 30 and 31, lines 24 and 42 via contact RL2"t lines 69 and 46, pump winding 59, line 60 via relay contacts RL1"" and RL2""" both in the normally closed position, pump winding 62 and line 65. The residual heat left in the hot water remaining in the boiler after the burner firing cycle is therefore circulated at a slow speed through the space heating loop although the boiler is not firing.

It will be appreciated that the central heating system described is only an example of a conventional system for which the control system is suitable. The control system described however can be modified for use with central heating systems in which the hot water in the domestic hot water loop is circulated under gravity and with those central heating systems having no zone valves or having one zone valve of the type which serves both the domestic hot water loop and the space heating loop.

It will be further appreciated that while the control system has been described as controlling the supply of heated water to the space heating loop by the use of a room thermostat incorporated into the electrical circuit of the control system, the control system can be easily adapted for use with conventional thermostatic radiator valves as well as or instead of a room thermostat.

In addition it will be appreciated that while electrical relays have been specified as an example of the means by which the control system causes the pump to operate and the space heating zone valve to open after a firing cycle so that hot water can be supplied to the space heating loop, the control system can be easily modified to achieve the same result utilising alternative devices. For instance, instead of relays the control system could incorporate an electrical timing device to operate the pump and open the space heating zone valve for a preset duration after the termination of a boiler firing cycle.

Alternatively, the control system could incorporate a further thermostat for location in the central heating system to operate the pump and open the space heating zone valve after the boiler firing cycle until a preset lower water temperature is reached in the system.

It will also be appreciated that the relays, where used, can be located in a compact wiring unit with appropriate terminals for connection to the various components of the central heating system such as the clock, thermostats, pump, zone valves and the gas inlet valve. While the use of relays has been described it will be appreciated that other equivalent solid state electronic components can be used.

Claims

1. A control system for controlling the operation of a wet central heating system of the type in which water can be forcibly circulated within at least a space heating loop, the control system being of the type providing a mode of operation of the central heating system permitting water to be forcibly circulated during a boiler heating cycle within the space heating loop, wherein the control system when providing that mode of operation is adapted to cause the forcible circulation of water within the space heating loop after a boiler heating cycle.

2. A control system as claimed in Claim 1 in which the system includes means incorporated in an electrical circuit and operable to cause the forced circulation of water within the space heating loop after a boiler heating cycle when the control system is providing the respective mode of operation.

3. A control system as claimed in Claim 2 in which the means comprise relays.

4. A control system as claimed in any of Claims 1 to 3 including a room thermostat operable to cause the boiler to fire and force the water to circulate within the space heating loop during the boiler heating cycles.

5. A control system as claimed in any of Claims 1 to 4 including a hot water cylinder thermostat operable to cause the boiler to fire to heat water for circulation within the domestic hot water loop during the boiler heating cycles.

6. A control system as claimed in Claim 5 in which where the central heating system is of the type where the water can be forcibly circulated within the domestic hot water loop during a boiler heating cycle, the cylinder thermostat is operable to cause the water heated during a heating cycle to be forcibly circulated within the domestic hot water loop.

7. A wet central heating system including a circulating pump for the forcible circulation of water and incorporating the control system as claimed in any of the preceding claims.

7. A control system as claimed in any of the preceding claims including a programmable clock to operate the control system for a selected period.

8. A control system as claimed in any of the preceding claims in which the control system is adapted to cause water to be forcibly circulated at a first speed during a boiler heating cycle and at a second speed around the space heating loop after a boiler heating cycle when the control system is providing the respective mode of operation, wherein the second speed is slower than the first speed.

9. A control system substantially as hereinbefore described with reference to Figure 2 or Figure 3.

10. A wet central heating system of the type in which water can be forcibly circulated within at least a space heating loop and incorporating the control system as claimed in any of the preceding claims.

New Claims or Amendments to Claims filed on 28 July 1981.

Superseded Claims 1 to 10.

New or Amended Claims:

1. A control system for controlling the operation of a wet central heating system including a circulating pump for the forcible circulation of water, the system being adapted to activate the pump to circulate water at a first speed during a boiler heating cycle and at a second slower speed after a boiler heating cycle.

2. A control system as claimed in Claim 1 in which the system is adapted to close a control valve to the domestic hot water loop when the pump is activated after a boiler heating cycle.

3. A control system as claimed in Claim 1 or Claim 2 in which the system includes means for switching the pump between high power operation for circulation of water at the first speed and low power operation for circulation of water at the second speed.

4. A control system as claimed in Claim 3 in which the switching means causes the pump windings to switch between parallel connection for high power operation and series connection for low power operation.

5. A control system as claimed in any of the preceding claims including a programmable clock including a "Hot Water Only" setting which prevents the operation of the pump after a boiler heating cycle.

.6. A control system substantially as hereinbefore described with reference to Figure 3.