GB2148552A

GB2148552A - Central heating control system

Info

Publication number: GB2148552A
Application number: GB08328191A
Authority: GB
Inventors: Royston George Dann
Original assignee: British Gas Corp
Current assignee: British Gas Corp
Priority date: 1983-10-21
Filing date: 1983-10-21
Publication date: 1985-05-30
Also published as: GB8328191D0; GB2148552B

Abstract

Cylinder, room and pipe thermostats 15, 16 and 17 control the boiler gas inlet valve 23, the circulating pump 14 and the diverter valve 5 which directs hot water to flow either into a domestic hot water (DHW) branch or into a space heating (SH) branch. Hot water circulates preferentially around the DHW branch if the cylinder thermostat 15 is closed. Otherwise if the cylinder thermostat 15 is open when the room thermostat 16 is closed and the pipe thermostat 17 is in position c the diverter valve 5 directs the hot water to flow by way of the SH branch. When the pipe thermostat 1 is in position c the temperature of the water circulating in the SH branch 3 is below a preset level. If the cylinder thermostat 15 is open and either the room thermostat 16 is open or the pipe thermostat 17 is in position d the boiler inlet valve 23 will close and the boiler will cease firing. When the pipe thermostat 7 is in position d the water circulating in the SH branch is at or above the preset level. <IMAGE>

Description

SPECIFICATION Condensing boiler control system This invention relates to controlling the operation of a central heating system of the type including an appliance which is operable to heat a fluid and a circuit which contains two parallel branches forming part of the circuit and around which the fluid is caused to flow at least during operation of the appliance.

In a conventional central heating system of the above type, one branch provides domestic hot water and the other space heating, the heated water being supplied by a boiler appliance. Until required the domestic hot water is stored in a storage cylinder incorporated within the domestic hot water branch.

In order to maintain the temperature of the water in the cylinder at a selected satisfactory minimum level, the temperature of the water leaving the boiler must be higher than this level. Typical values for these temperatures are 60"C for the stored cylinder water and 80'for the water leaving the boiler. Consequently the boiler thermostat is set at the higher level, for instance 80"C, while the temperature of the return water to the boiler will usually be between the boiler thermostat and the cylinder water temperatures.

The requirement specified above leads to problems in the operation of central heating systems of the type in which the temperature of the- water in the domestic hot water branch of the circuit is necessarily higher than the temperature of the water supplied to the space heating branch. A typical system of this type is the underfloor type heating system in which the space heating branch comprises a number of coiled hot water pipes laid under the floor, the heat radiating from the pipes serving to heat the rooms with the hot water being supplied from the boiler. The maximum allowable comfortable floor temperature is a few degrees above room temperature for example 28"C .To achieve such a floor temperature the temperature of the water in the underfloor pipes must be no more than 5OdeC. However, in order to provide stored cylinder water at the satisfactory minimum level, usually 60"C, the boiler must supply the hot water at the much higher temperature of say 80"C and this will tend to increase the water temperature in the pipes to a level greatly exceeding 50"C. On the other hand if the temperature of the water leaving the boiler is reduced to close to 50"C to provide water of the appropriate temperature level in the hot water pipes, the temperature of the stored cylinder water will be reduced to an unsatisfactorily low level, well below 60'C.

One solution to the above problem is to allow a proportion of the water returning from the underfloor system to the boiler to be diverted to mix with a proportion of the water flowing from the boiler before the flow water enters the system. In this solution the remaining proportion of the boiler flow water is allowed to mix with the remaining proportion of the water returning from the underfloor system.In this way the temperature of the water in the underfloor pipes can be reduced to a satisfactory low level (50"C). Consequently the floor temperature can be maintained at its required level (28"C). Domestic hot water can then be supplied from the boiler at 80'C ensuring that the temperature of the water stored in the cylinder is at a satisfactory level (60"C). However this solution involves the use of costly additional components such as pumps, valves and pipework to form bypasses and undesirably increases the overall cost of the system.

In addition if this solution is applied to an underfloor system operating with a condensing boiler then most of the benefits derived by using such a boiler are lost. In a condensing boiler, at least a proportion of the combustion products 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 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. Unfortunately, in systems employing the type of water mixing arrangement described the temperature of the water entering or leaving the boiler is relatively high being as high as 80 C when leaving the boiler and as high as 70'C when entering the boiler.Consequently the efficiency of the boiler is considerably reduced.

Another solution to the problem described is to provide two boilers to provide respectively hot water for the domestic hot water branch and for the space heating branch. In this case one boiler can supply water at the high temperature (80"C) required for the domestic hot water and the other can supply water at the low temperature (50"C or less) for space heating. Clearly, this is an expensive solution and is impractical for that reason.

It is therefore an object of the present invention to control the operation of a central heating system of the type in which the temperature of the water supplied to the domestic water branch is higher than the temperature of the water supplied to the space heating loop without the use of costly water mixing systems or an additional boiler.

According to one aspect of the present invention, there is provided a method for controlling the operation of a central heating system of the type including an appliance which is operable to heat a fluid and a circuit, which contains two parallel branches forming part of the circuit and around which the fluid is caused to flow at least during operation of the appliance, the method comprising sensing the temperature of the fluid at a point in a first branch of the circuit and at a point in the circuit outside the first branch, causing the appliance to operate whenever the temperature sensed at one or other of the points has fallen to a level preset for the points, directing the fluid to flow around the circuit preferentially by way of the first branch whenever the temperature sensed at the point in this branch has fallen to the level preset for this point and directing the fluid to flow around the circuit by way of the second branch whenever the temperature sensed at the point in the circuit outside the first branch has fallen to the level preset for this point while simultaneously the temperature sensed at the point in the first branch has risen to the level preset for this point.

According to another aspect of the present invention, there is provided apparatus for controlling the operation of a central heating system of the type including an appliance which is operable to heat a fluid, a circuit which contains two parallel branches forming part of the circuit and circulating means for causing the fluid to flow around the circuit at least during operation of the appliance, the apparatus comprising sensors for sensing the temperature of the fluid at a point in a first branch of the circuit and at a point in the circuit outside the first branch, switch means for causing the appliance and the circulating means to operate whenever the temperature sensed at one or other of the points falls to a level preset for that point and for causing the appliance to cease operation whenever the temperatures sensed at both points have simultaneously risen to the levels preset for the points and fluid flow director means for directing the fluid to flow around the circuit preferentially by way of the first branch whenever the temperature sensed at the point in this branch has fallen to the level preset for this point and to flow around the circuit by way of the second branch whenever the temperature sensed at the point in the circuit outside the first branch has fallen to the level preset for this point while simultaneously the temperature sensed at the point in the first branch has risen to the level preset for this point.

An embodiment of the 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 a circuit for controlling the system shown in Figure 1.

Referring to the drawings, the system shown in Figure 1 comprises a cyclically fired condensing boiler appliance 1, which in this case is gas fired, for supplying hot water to a domestic hot water (DHW) branch 2 and a space heating (SH) branch 3.

While not shown, the boiler 1 incorporates as is conventional gas burners for heating water circulating internally in a heat ex changer 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 boiler 1 operates on the condensing principle such as is fully described in our co-pending UK Patent Application No. 2103341A.

Flow water leaving the boiler 1 flows by a pipe 4 to an electrically operated diverter valve 5 which diverts the flow water either into the DHW branch 2 or the SH branch 3.

The DHW branch 2 incorporates a calorifier 6 to enable hot flow water to heat the water stored in the storage cylinder 7. The cylinder 7 is supplied with cold water under gravity from a cold water tank 8 by way of a supply pipe 9 and hot water from the cylinder 7 may be drawn off as required by way of the pipework 10 and the taps 11.

The SH branch 3 incorporates several (in this case three) parallel connected coiled pipes 12 for supplying space heating to the building or dwelling.

The branches 2 and 3 join to form a return water pipe 1 3 within which is located an electrically operated pump 14 for pumping the water around the system.

Thus far described the system is of conventional design and operation and it will be appreciated that the boiler may be of conventional non-condensing design.

Also as conventional, the system includes a cylinder thermostat 1 5 for controlling the temperature of the stored water and a room thermostat 16 for controlling the ambient temperature inside the building or dwelling from a central point.

The system also includes a pipe thermostat 1 7 for mounting on the pipework within the SH branch 3 for the purposes to be described.

While not shown 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 temperture in the boiler.

Referring to Figure 2, where similar components bear identical reference numerals to Figure 1, the circuit includes a conventional mains operated clock programmer 20 which can be set to switch the system on and off at certain pre-selected times and can be manually overridden to provide electrical power when desired.

The programmer 20 enables electrical power to be supplied either to the space heating line 21 or to the domestic hot water line 22 or to both simultaneously.

These lines are provided with individual switches (not show) to switch them on and off individually as required.

The circuit also comprises the cylinder, room and pipe thermostats 1 5,1 6 and 1 7 respectively, relays R1 and R2, two-position relay contacts R11, R1 " and R1111 controlled by R1, relay contact R2' controlled by R2 and a switch S1.

When R1 is deactivated the contacts R1', Rill and R1"' are in the position a show in Figure 2. When R1 is activated the contacts R1', R1" and R1"' are in position b.

When R2 is deactivated R21 is open as shown in Figure 2. R21 is closed when R2 is activated. However, if R2 is deactivated after previous activation, the contact R21 remains closed for a preset further period.

The pipe thermostat 1 7 can switch between two positions c and d depending upon the temperature of the water in the SH branch.

The pipe thermostat 1 7 operates so as to occupy the position c show in Figure 2 when the temperature of the water in the SH branch is below the preset temperature and to occupy the position d when the temperature of the water in the SH branch is at or above the preset temperature.

The Switch S1 when closed connects the pump 14 to line 21 so that if power is on line 21 the pump 14 operates continuously irrespective of the state of the other components.

Provided the boiler thermostat is closed the boiler gas inlet valve 23 is opened and the relay R2 is activated when the relay contact R1' is in position a, the room thermostat 1 6 is closed, the pipe thermostat 1 7 is in position c and power is on line 21. Alternatively or in addition, provided the boiler thermostat is closed the valve 23 is opened and relay R2 is activated when relay contact R1' is in position b and power is on line 22.

In the powered up state the diverter valve 5 is closed to the DHW branch but open to the SH branch. This occurs when relay R111 is in position a and power is on line 21. In the off state the diverter valve 5 is closed to the SH branch but open to the DHW branch. This is when contact R1 " is in position b or when there is no power on line 21.

The pump 14 operates when power is on line 21 and the pipe thermostat 17 is in position d or the switch SI is closed or R21 is closed and R1" is in position a. The pump 14 also operates when power is on line 22 and R111 is in position b.

Relay R1 is activated only when both power is on line 22 and the cylinder thermostat 1 5 is closed.

Suppose the clock programmer 20 is set so that space heating and domestic hot water are required. Lines 21 and 22 will be powered up but if the components are in the state show in Figure 2 with the cylinder and room thermostats 1 5 and 1 6 open, the boiler gas inlet valve 23 will be closed, the diverter valve 5 will be open to the space heating branch and the pump 14 will not be operating.

Consequently no hot water is being produced or circulated.

Suppose now that both the cylinder ther mostat 1 5 and the room thermostat 1 6 are closed ie. there is a demand both for domestic hot water and space heating. Since the cylinder thermostat 15 is closed, R1 will be acti vated. Consequently contacts R1', Ri and R111 will be in position b.

The boiler gas inlet valve 23 will be receiving power from line 22 via contact b of R1 1 Relay R2 will also be activated for the same reason and therefore contact R21 will be closed. If the boiler thermostat (not shown) is also closed the gas inlet valve 23 will be open and the boiler will be firing. The pump 14 will be receiving power from line 22 via contact b of R1111 and will thus be operating.

The diverter valve 5 will be open to the DHW branch.

Consequently the hot water will be directed to flow around the domestic hot water branch.

This state of affairs will continue until the cylinder thermostat 1 5 opens at the preset temperature. R1 will now be deactivated and R1', R111 and R1"' will be in position a.

If the pipe thermostat 1 7 is in position c ie.

the water in the space heating branch is below the preset temperature, the boiler gas inlet valve 23 will be receiving power from line 21 by way of contact a of R1', the closed room thermostat 1 6 and the pipe thermostat 17. If the boiler thermostat is still closed, the inlet valve 23 will be open and the boiler will be firing.

The pump 14 will now be receiving power from line 21 via the closed contact R2' and R111 by way of position a.

The diverter valve 5 will be receiving power from line 21 by way of contact R111 (position a) and will thus now open to the SH branch.

Consequently hot water will now be directed to flow around the SH branch.

This state of affairs will continue until either: (a) The cylinder thermostat 1 5 closes once again when hot water will again be preferentially directed around the domestic hot water branch as previously described or (b) the pipe thermostat 1 7 moves from position c to position d or (c) the room thermostat 1 6 opens.

In the event of either (b) or (c) but not (a) occurring power from line 21 is disconnected from the boiler gas inlet valve 23 which will close and the boiler will cease firing. R2 will also be deactivated and R2' contact will open after the preset delay. During the delay period, the pump 14 will still be receiving power from line 21 so that it will continue to operate for the preset period and hot water will continue to circulate in the SH branch although the boiler will not be firing.

In the event of (b) occurring the pump 14 will continue to operate because it is receiving power from line 21. This will continue until the pipe thermostat 1 7 returns to position c.

If the pipe thermostat 1 7 returns to position c before the end of the delay period of relay R2, the pump 14 stays on by that means instead.

Movement of the pipe thermostat from position c to position d indicates that the water in the SH branch has reached the preset temperature.

If the switch S1 is closed the pump 14 will operate continuously irrespective of whether the boiler is firing or not since the pump 14 will be continuously powered from line 21.

In the event that space heating is not required, for example in the summer, power on line 21 is switched off and the boiler will only fire when receiving power from line 22 via contact R 11 in position b in response to the relay R1 being activated by closure of the cylinder thermostat 1 5.

In this event the pump 14 will now only operate when receiving power from line 22 via contact R1"' in position b. The pump 14 will cease operation when the cylinder thermostat 1 5 opens and deactivates R1 with the consequent movement of contact R1" from position b to position a and the disconnection of line 22 from the pump 14.

The diverter valve 5 will permanently remain open to the DHW branch since it is not receiving power from line 21 which is switched off.

In the rare event that only space heating is required on line 22 is switched off while power is retained on line 21.

Thus Ri is deactivated and the contacts R1', R1" and R1"1 are in positions a shown in Figure 2. Consequently the boiler will only fire when the room thermostat 1 6 is closed and the pipe thermostat 1 7 is in position c to provide power from line 21. If switch S1 is closed the pump 14 operates continuously. If switch S1 is open the pump 14 is operated by power via Ri 111 (position a) and closed contact R21. Contact R21 is closed by activation of R2 which is activated by the same path as the boiler gas inlet valve 23.

Since Ri 11 is in position a the diverter valve 5 is always open to the space heating branch as it is powered from line 21.

Power from line 21 to the boiler gas inlet valve 23 is disconnected either when the room thermostat 1 6 opens or the pipe thermostat 1 7 moves from position c to position d as previously described or the boiler thermostat is open. This causes the valve 23 to close and the boiler to cease firing. R2 is also deactivated and R21 opens after the preset delay. This causes the pump 14 to be disconnected from line 21 and to cease operation.

Instead of the diverter valve show, each branch can be provided with its own electrically operable zone valve with appropriate modification to the circuit such that when one valve is open the other is closed and vice versa to provide alternating flow of water through the branches with preference given to the DHW branch. Any other suitable types of valves could also be used.

The pipe thermostat 1 7 is of conventional design and can be of a type similar to a cylinder thermostat so as to be mountable on the SH pipework. While in the embodiment described, the pipe thermostat 1 7 has been show as mounted on the pipework of the SH branch it can be mounted on any other suitable component. While preferably the pipe thermostat 1 7 is mounted in the SH branch, it could be mounted at any other convenient position in the central heating system except in the DHW branch. For instance it could be mounted on pipework beyond the junctions between two branches either upstream or dowstream in the system.

The system describes permits the boiler to supply high temperature water to the DHW branch preferentially and, as required, low temperature water to the SH branch when the DHW requirements have been satisfied. It therefore promotes safe and efficient operation of a central heating system of the underfloor space heating type especially when the system incorporates a condensing boiler.

Claims

1. A method for controlling the operation of a central heating system of the type including an appliance which is operable to heat a fluid and a circuit, which contains two parallel branches forming part of the circuit and around which the fluid is caused to flow at least during operation of the appliance, the method comprising sensing the temperature of the fluid at a point in a first branch of the circuit and at a point in the circuit outside the first branch, causing the appliance to operate whenever the temperature sensed at one or other of the points has fallen to a level preset for that point and to cease operation whenever the temperatures sensed at both points have risen to the levels preset for the points, directing the fluid to flow around the circuit preferentially by way of the first branch whenever the temperature sensed at the point in this branch has fallen to the level preset for this point and directing the fluid to flow around the circuit by way of the second branch whenever the temperature sensed at the point in the circuit outside the first branch has fallen to the level preset for this point while simultaneously the temperature sensed at the point in the first branch has risen to the level preset for this point.

2. A method as claimed in Claim 1 in which fluid is directed to flow around the circuit by way of the second branch for a period after the appliance has ceased operation.

3. A method as claimed in Claim 1 or Claim 2 comprising sensing the ambient temperature, and, if the temperature sensed at the point in the first branch has risen to its preset level, causing the appliance to cease operation whenever the ambient temperature sensed has risen to a preset level.

4. A method as claimed in any of Claims 1 to 3 in which the point outside the first branch of the circuit at which the temperature is sensed is dowstream of the upstream junction between the branches.

5. A method as claimed in Claim 4 in which the sensing point is in the second branch.

6. A method as claimed in any preceding claims comprising sensing the temperature of the fluid at a point within the appliance and causing the appliance to cease operation whenever the temperature of the fluid at this point has risen to a preset level.

7. Apparatus for controlling the operation of a central heating system of the type including an appliance which is operable to heat a fluid, a circuit which contains two parallel branches forming part of the circuit and circulating means for causing the fluid to flow around the circuit at least during operation of the appliance, the apparatus comprising sensors for sensing the temperature of the fluid at a point in a first branch of the circuit and at a point in the circuit outside the first branch branch, switch means for causing the appliance and the circulating means to operate whenever the temperature sensed at one or other of the points falls to a level preset for that point and for causing the appliance to cease operation whenever the temperatures sensed at both point have risen to the levels preset for the points and fluid flow director means for directing the fluid to flow around the circuit preferentially by way of the first branch whenever the temperature sensed at the point in this branch has fallen to the level preset for this point and to flow around the circuit by way of the second branch whenever the temperature sensed at the point in the circuit outside the first branch has fallen to the level preset for this point while the temperature sensed at the point in the first branch has risen to the level preset for this point.

8. Apparatus as claimed in Claim 7 in which the switch means is adapted to cause the circulating mans to operate for a period after the appliance has ceased operation and while the flow director means is directing fluid to flow by way of the second branch.

9. Apparatus as claimed in Claim 7 and Claim 8 in which a sensor is provided to sense the ambient temperature and the switch means is adapted to cause the apppliance to cease operation if the ambient temperature has risen to a preset level provided that temperature sensed at the point in the first branch has risen to its preset level.

1 0. Apparatus as claimed in any of Claims 7 to 9 in which the sensor for sensing the temperature at the point outside the first branch is located dowstream of the upstream junction between the branches.

11. Apparatus as claimed in Clairn 10 in which the sensor is located in the second branch.

1 2. Apparatus as claimed in any of Claims 7 to 10 in which a sensor is provided for sensing the temperature of the fluid at a point within the appliance and the switch means is responsive to the sensor to cause the appliance to cease operation whenever the temperature sensed at the point in the appliance has risen to a preset level.

1 3. A method for controlling the operation of a central heating system substantially as hereinbefore described with reference to the accompanying drawings.

14. Apparatus for controlling the operation of a central heating system substantially as hereinbefore described with reference to the accompanying drawings.