GB2113427A

GB2113427A - Central heating systems

Info

Publication number: GB2113427A
Application number: GB08234107A
Authority: GB
Inventors: Gerald Dandridge Ffiske
Original assignee: STOCK ELECTRONICS Ltd
Current assignee: STOCK ELECTRONICS Ltd
Priority date: 1981-12-10
Filing date: 1982-11-30
Publication date: 1983-08-03
Also published as: GB2113427B

Abstract

A hot water central heating system has a boiler 1, a circulating pump 4, and a controller 5 which varies the speed of the pump between pre-set maximum and minimum speeds in accordance with the difference in temperature of the water leaving and returning to the boiler as measured by temperature sensors TH1 and TH2 in the supply and return pipes 2 and 3 respectively. The system has conventional high and low thermostat contacts 11 and 12 for controlling the boiler 1, and also has contacts 13 in parallel with the contacts 12 and arranged to open to shut off the boiler when the pump 4 operates at minimum speed. The boiler switches on again if the speed of the pump increases, thereby closing the contacts 13, or if the contacts 12 are closed. <IMAGE>

Description

SPECIFICATION Central heating systems This invention relates to central heating systems in which the hot water is circulated by means of a pump. Temperature control in such systems is normally obtained by thermostatic control of the boiler itself, in conjunction with an on/off thermostat to control the pump. Such control is extremely sluggish in that there is inevitably a considerable delay in the response to changed temperature conditions. Thus if the temperature becomes too high and the themostat operates to reduce the heat supply from the boiler, there is an appreciable time lag before the water in the boiler starts to cool down and the effect of this cooling is felt at the radiators making up the system. This arrangement is wasteful on energy because of unnecessary boiler over-run.

According to the present invention, the heat supplied to the radiators and/or hot water cylinders is controlled by regulating the speed of the circulating pump by means of a speed controller responsive to the temperature difference between the supply and return pipes to the boiler as measured by sensors on the respective pipes. As long as there is a steady demand of heat from the system then the temperature of the return pipe will be less than that of the supply pipe and the pump will run at a speed determined by the temperature differential. If, however, the heat demand decreases, the temperature difference between the supply and return pipes decreases accordingly and this causes the speed of the pump to decrease in a corresponding manner so that a reduced amount of heat is supplied to match the reduced demand.

It will be appreciated that the response of such a system is virtually instantaneous in that as soon as the reduced demand for heat is reflected by a reduced temperature difference between the supply and return pipes, the speed of the pump and hence the heat supplied is reduced without delay. Thus, although the temperature of water in the boiler may not have changed to any appreciable extent at all, the heat supplied is nevertheless reduced in accordance with the reduced speed of the pump.

The boiler itself will still require the usual thermostatic control to avoid over heating when the heat demand is low and the temperature of this water may therefore vary between limits determined by the boiler water temperature thermostat, but this variation has no direct effect on the heat supplied which is controlled by the pump speed. If, for example, a large heat demand is experienced at a time when the water temperature is somewhat low, this is compensated for by increased pump speed and in consequence the boiler will switch on unless prohibited by another switching device.

A particular advantage of a system in accordance with the invention arises when used in conjunction with individual thermostatically controlled radiators.

As each room reaches a satisfactory temperature level, so the thermostat turns off that radiator. In a conventional system this could lead to an increasing acceleration of water circulation, particularly after all radiators have been satisfied. By controlling pump speed in accordance with the invention, the excessive system noise which could otherwise be caused, is avoided. Other forms of conventional system include a modulating valve to vary water temperature whilst keeping the pump running at a constant speed. Control of the rate of water flow in accordance with the invention eliminates the need for such a device, and replaces it by much simpler and cheaper controls. A further conventional alternative is to leave the pump running at maximum speed regardless of water circulation requirements, thus using excessive power for that purpose.As an alternative to leaving the pump running continuously, a thermostat is sometimes introduced to control the pump. The siting of this thermostat is often arbitrary and difficult to locate to sense an average building condition. By reducing pump speed in accordance with the invention, a power is reduced and there is therefore a direct saving of electricity.

In addition to the normal high temperature ther mostaticcontrolsforthe boiler, in a system according to the invention, the boiler temperature may be additionally controlled in accordance with the flow through the pump so that, after a period of low heat demand, the boiler temperature may be maintained at a value below that set by the high temperature thermostat. This may be achieved either by direct measurement of flow or indirectly by means of a switching device responsive to pump speed.

An example of a system operating in this way is illustrated in the accompanying drawing, which is a schematic diagram.

The boiler shown as 1 has a supply pipe 2 and a return pipe 3. The supply pipe includes a tempera ture sensor TH 1 and the return pipe includes a temperature sensorTH2. The supply is controlled by a rotary pump 4 and the speed of this pump is controlled in accordance with the difference of temperature sensed by the sensors TH1 and TH2.

The signals from these two sensors are supplied a control signal to the pump 4. As already explained, the speed of the pump increases with the heat demand and when the demand approaches zero, the two sensors sense substantially the same temperature and the controller 5 causes the pump motor to run at minimum speed orto be switched off altogether if required.

The value of the temperature differential at which this occurs can be adjusted by a potentiometer 6 in the controller 5, being set at a low level, e.g. 4"C, in warm periods and at a higher level, e.g. 11"C, in cold periods. The minimum speed of the pump motor can be set by a second potentiometer 7. The value of the change in the temperature differential which has to take place to bring the pump gradually up to maximum speed, known as "speed" temperature can be varied by a third potentiometer 8.

The boiler 1 has a normal electrically controlled heat source 10 controlled by a thermostat having the normal two pairs of contacts, i.e. high temperature contacts 11 and low temperature contacts 12. In contrast to the normal arrangement, the low temperature contacts 12 are arranged in parallel with contacts 13 which, as illustrated, are controlled by a flow switch 14 in the return pipe 3. Alternatively, the same effect can be obtained by controlling the contacts 13 by a device responsive to pump speed.

The high temperature contacts 11 are normally closed and open to interrupt the supply to the heater 10 only when the boiler temperature reaches a pre-set maximum. As long as the pump 4 is operating above its minimum speed, the contacts 13 of the flow switch 14 remain closed and thus by-pass the normally open low temperature contacts 12 of the thermostat, to maintain the supply to the heater 10.

As explained above, when the heat demand is zero, the pump 4 runs at a speed set by the potentiometer 7 and at this speed the flow switch 14 opens its contacts 13 to interrupt the supply to the heater 10. The boiler 1 thus starts to cool and if there is no further heat demand in the meanwhile, the supply is restored to the heater 10 by closing of the contacts 12 when the temperature of the boiler reaches its pre-set minimum value. If, before this point is reached, there is a further heat demand as sensed by the sensors TH 1 and TH2, the pump 4 is controlled to run at a greater speed, the flow switch 14 closes its contacts 13 and supply to the heater 10 is re-established.

Instead of using a lowertemperaturethermostat, the contacts 12 may be the contacts of a timed relay and may thus be closed to restore supply to the heater 10 when a predetermined delay has elapsed without any heat demand from the system.

In the absence of the flow switch 14, supply to the heater 10 would be maintained until interrupted by the high temperature contacts 11 and the boiler 1 would be maintained at an uneconomically high temperature in the absence of any heat demand. The inclusion of the contacts 13 avoid the supply of unnecessary heat to the boiler in the absence of any heat demand, as detected either by the flow switch or by the low pump speed.

Claims

1. A central heating control system comprising sensors for measuring the difference of temperature between the supply and return pipes to a boiler and a speed controller responsive to the measured temperature difference for adjusting the speed of a circulating pump connected to the boiler in accordance with the temperature difference.

2. A central heating control system according to claim 1 for use with a boiler including contacts for re-establishing supply after a period of low heat demand, the system including a switch having contacts for connection in parallel with the boiler contacts and responsive either to liquid flow or to pump speed.

3. A central heating control system according to claim 2 in which the boiler contacts are controlled by a low temperature thermostat.

4. A central heating control system according to any one of the preceding claims in which the speed controller includes a potentiometer for adjusting the value of the temperature differential for which the speed of the pump is a minimum.

5. A central heating control system according to any one of the preceding claims, in which the speed controller also includes a potentiometerforadjust- ing the change in the temperature differential which has to take place to bring the pump up to maximum speed.

6. A central heating system including a boiler and an electrically driven circulating pump and fitted with a control system according to any one of the preceding claims.