GB2091445A - An electrical control circuit for a heating system - Google Patents

Abstract

The electrical control circuit is connected in an electrically actuated heating system between one terminal of a conventional time switch and a room thermostat. The control circuit is energised by operation of the time switch and delays the actuation of the heating system in dependence upon a variable such as ambient temperature, so as to avoid the room reaching its set temperature prior to a desired time. The ambient temperature is sensed by a thermistor. The control circuit thus delays actuation of the heating system as the ambient temperature rises, which gives considerable fuel savings. As described the delay is determined by means of a counter fed by an oscillator whose frequency increases as the sensed temperature drops, thus shortening the delay. Other arrangements are, however, envisaged. Other variables such as wind speed, humidity, and incident solar radiation may alternatively or additionally be used.

Description

SPECIFICATION An electrical control circuit for a heating system The present invention relates to an electrical control circuit for controlling the actuation of a heating system by a programmable time switch.

Many heating systems, particularly domestic central heating systems are actuated by a programmable time switch which switches the heating system on and off at preset times. Such a time switch enables the heating system to be operated with an economical use of fuel.

However, the time switch is generally preset to switch the heating system on at a time which will ensure that the building to be heated by the heating system is at a predetermined temperature by a predetermined time. However, in crder to ensure that the required temperature is achieved in time under all conditions it is necessary to preset the time switch in accordance with the coldest ambient temperature conditions likely to be experienced. Accordingly, if the ambient temperatures are higher the time switch will switch on the heating system earlier than is necessary to achieve the required temperature in time and there will be a wastage of fuel.

It has been appreciated that considerable fuel savings can be made if the time at which the heating system is switched on is variable and takes account of the ambient temperature.

Sophisticated controllers, generally incorporating microprocessors, are now available and may be used in place of the conventional time switches.

However, these devices which calculate the time at which the heating system is to be switched on are expensive and are thus only suitable for use with large heating plants, for example, in commercial premises and muiti-storey buildings.

They are too expensive for domestic use not only because of their initial cost but also because of their installation cost.

A simpler electro-mechanical unit which varies the time at which a heating system is switched on is also known but as this includes mechanical moving parts it is only capable of coarse adjustments to the switch on time and as it is subject to wear it requires periodic maintenance.

Furthermore, these known devices are only responsive to ambient temperature.

It is an object of the present invention to provide an electrical control circuit for controllling the actuation of a heating system which is simple and reliable such that it is suitable for use with a domestic heating system.

According to the present invention there is provided an electrical control circuit for controlling the actuation of a heating system by a programmable time switch, said control circuit comprising sensing means arranged to produce a variable electrical signal dependent upon at least one parameter of the environment, an electrical delay circuit responsive to said electrical signal and arranged to determine a delay period dependent upon the electrical signal, and means for coupling said control circuit to a heating system having a programmable time switch for delaying actuation of said heating system until the end of said delay period.

Preferably, the sensing means are sensitive to one or more parameters which affect the heat loss from a building, for example, internal temperature, external temperature, wind speed, humidity, and radiant heat, for example from direct sunlight.

Accordingly, a control circuit of the invention can be provided to take account of those parameters which in any particular application are the most significant.

Any suitable coupling means may be provided to enable communication between a control circuit of the invention and the heating system.

In a preferred embodiment, the coupling means comprises a first terminal for connection to the time switch, a second terminal for connection to the heating system, and switching means for connecting said first terminal to said second terminal, wherein the control circuit is arranged such that upon the arrival of an actuating signal or supply at said first terminal the switching means keep the first terminal disconnected from the second terminal and the delay circuit commences to determine the delay period, and wherein at the end of the delay period said delay circuit actuates the switching means to connect said first terminal to said second terminal.

In use, the first terminal of the electrical control circuit will be connected to the time switch whilst the second terminal will be connected to the heating system. When the time switch switches on at its preset time it will apply the electrical supply to said first terminal and thereby actuate the delay circuit The switching means will remain, in their normal condition in which the first terminal is disconnected from the second terminal such that actuation of the heating system will be prevented. At the end of the delay period determined in dependence upon the or each parameter sensed by the sensing means, the switching means are actuated to connect the first terminal to the second terminal and thus to connect the time switch directly to the heating system.The heating system will thus be actuated by the supply from the time switch and will be controlled in the normal manner by the time switch and by any thermostats or other control devices provided.

It will be appreciated that an electrical control circuit of the invention is simple and it can be easily coupled to existing heating systems controlled by a conventional programmable time switch. As it is designed simply to control the connection between the time switch and the heating system it is fully compatible with any type of heating system controlled by any time switch.

In an embodiment, the sensing means comprises a temperature sensitive element, for example, a thermistor, which may be mounted internally or externally of the building to be heated by the heating system. Preferably, the delay circuit includes adjustable means for varying its characteristics in dependence upon the nature and location of the sensing means, for example, upon whether an internal or an external temperature sensitive element is to be used. The delay circuit may also be provided with further adjustable means so that its characteristics may also be varied in dependence upon the characteristics of the building.

In an embodiment, the delay circuit comprises an oscillator circuit arranged to oscillate at a frequency dependent upon the electrical signal produced by the sensing means. Preferably, where a temperature sensitive element is provided, the frequency of oscillation will be arranged to increase as the temperature sensed by the temperature sensitive element decreases. Thus, as only a minimal delay period will be required where low temperatures are sensed but a significant delay period is required where higher temperatures are sensed, the number of pulses produced by the oscillator circuit can be counted directly and the delay period taken to be the time taken by the oscillator circuit to produce a predetermined number of pulses. This time will be inversely related to the temperature sensed as is required.In an embodiment, the pulses produced by the oscillator circuit are fed to a counter connected to actuate the switching means once a predetermined total has been counted. The counter is arranged to be automatically reset to zero upon the arrival of an actuating signal from said programmable time switch.

In an alternative embodiment, rather than counting the pulses produced by the oscillator circuit their mark-space ratio could be directly monitored or the mean level could be measured and the variations therein used to determine the delay period.

Alternatively, the electrical signal produced by the sensing means could be integrated and compared with a reference level to determine the delay period.

The present invention also extends to a heating installation comprising an electrically actuated heating system, a programmable time switch for controllling the actuation of the heating system, and an electrical control circuit coupled to the time switch and the heating system, the control circuit comprising sensing means arranged to produce a variable electrical signal dependent upon at least one parameter of the environment, an electrical delay circuit responsive to said electrical signal and arranged to determine a delay period dependent upon said electrical signal, and coupling means for enabling connection of said time switch directly to said heating system wherein the control circuit is arranged such that when the time switch operates to actuate the heating system the coupling means prevent actuation of the heating system and the delay circuit commences to determine the delay period, and wherein at the end of the delay period the delay circuit actuates the coupling means whereby the time switch is connected directly to the heating system which is actuated thereby.

An embodiment of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows schematically an electrically actuated heating system controlled by a programmable time switch and a control circuit of the present invention, Figure 2 shows an electrical circuit diagram of an electrical control circuit of the invention, and Figure 3 is a graph showing the switch on times of a heating system both with and without the use of a control circuit of the invention and indicating the potential energy savings.

Figure 1 shows an electrically actuated heating system controlled by a conventional time switch which, as it is well known, is not illustrated in detail. The terminals 1 to 11 of the time switch, which is connected to an electrical supply, are shown. In the embodiment illustrated, the terminal 2 of the time switch is connected to the neutral lead of the supply whilst the terminal 10 is connectable to the live lead of the supply. The heating system comprises an electrically actuated pump 12 connected to the terminal 2 and to the terminal 10 by way of a room thermostat 13. The room thermostat 13 is also connected by way of a cylinder thermostat 14 to an electrically actuated boiler 15 which is connected to the terminal 2. Of course, any fuel can be used to heat the water in the boiler 15.The heating system so far described is conventional and it will be appreciated that at a first preset time the time switch will effect the connection of the terminal 10 to the live lead of the supply such that the heating system is actuated and will similarly disconnect the heating system from the supply at a second preset time. In addtion, the cylinder thermostat 14 is connected to terminal 6, which is also connectable to the live lead of the supply, so that water can be heated by the boiler 1 5 even when the heating system is not actuated.

An electrical control circuit 1 6 of the present invention has first terminal 1 7 connected to the terminal 10 and second terminal 1 8 connectable to the room thermostat 13; In addion, the control circuit 16 has a terminal 21 connected directly to the terminal 2. This control circuit 1 6 controls the actuation of the heating system in dependence upon the ambient temperature as determined by a thermistor 1 9 which is connected to the control circuit 16. The thermistor 19 may be mounted externally of the building to be heated by the heating system in which case it is connected to the control circuit 1 6 by flying leads 20 as is illustrated in Figure 1. However, if required the thermistor 19 may be mounted internally of the building and in this case is preferably mounted in a housing (not shown) together with the control circuit 1 6.

A circuit diagram of the control circuit 1 6 is shown in Figure 2. The control circuit 1 6 includes a delay circuit for delaying actuation of the heating system by the time switch by a period dependent upon the temperature sensed by the thermistor 19. In the embodiment illustrated, this delay circuit is an oscillator circuit including a Programmable Unijunction Transistor (P.U.T) Q4 arranged to oscillate at a frequency dependent upon the resistance of the thermistor 1 9 and hence upon the temperature sensed thereby. The transistor Q4 will only conduct when its anode voltage is greater than the voltage on the gate.

Thus, when the voltage on the gate is increased the anode voltage required to "fire" the transistor Q4 is increased. The gate of the transistor 4 is connected by way of a diode D10 and a resistor R6 to the thermistor 1 9. Thus, the voltage on the gate is increased when the resistance of the thermistor 1 9 decreases, that is, when the sensed temperature increases. The anode of the transistor Q4 is connected to a capacitor C4 which is connected in series with a potentiometer P, and a resistor R4.Accordingly, if the anode voltage of the transistor Q,, is increased the voltage on the capacitor C4 has to be greater and because of the time constant of the series combination of C4,R4 and P1 the time taken to charge the capacitor C4 to the greater voltage will. be increased and the frequency of oscillation of the oscillator circuit will be lower. Thus, an increase in the temperature sensed by the thermistor 1 9 lowers the oscillator frequency. Similarly, a decrease in the temperature sensed by the thermistor 1 9 increases the oscillator frequency.

It will be appreciated that the frequency of the oscillator circuit can be varied by adjusting the potentiometer P1. This potentiometer P1 will generally be adjusted once the control circuit has been installed and is in operation so that allowance can be made for the 'u' factor of the building to be heated, that is, the type of building and its fabric.

The thermistor 1 9 is connected in series with a resistor R8 and this series combination is connected with a series combination of a potentiometer P2 and a resistor R7 to form a potential divider connected to the gate of the transistor Q4. It will be appreciated that the frequency of oscillation of the oscillator circuit can also be adjusted by adjustment of the potentiometer P2. Generally this potentiometer P2 will be preset before installation of the control circuit in accordance with whether the thermistor 1 9 is to be mounted internally or externally of the building to be heated.

If for any reason the voltage on the gate of the transistor Q4 went below that of its cathode, for example, if the thermistor 1 9 goes open circuit, the oscillator circuit would cease oscillating. To prevent this, diodes D5, D6 and D7 and a transistor Q1 are connected in the gate circuit and act as a voltage clamp.

A diode D9 and a current limiting resistor R3 provide a stable voltage for the negative terminal of the capacitor C4 and the cathode of the transistor Q4. The diode D9 also offsets the voltage drop across emitter resistor P9 of transistor Q1.

The diode D9 has the effect of producing a greater percentage change in the gate voltage of the transistor Q4 due to a change in the resistance of the thermistor 1 9 whereby the sensitivity of the oscillator circuit is increased.

When the transistor Q4 is "fired" its gate is momentarily shorted down to the cathode voltage while the transistor Q4 is conducting such that a short negative going pulse is produced at the gate.

These negative going pulses are connected to the base of an output transistor Q2 by way of a capacitor C5.

The output transistor 2 is normally biassed ON by resistors R10 and R11. When a negative going pulse is applied to its base the transistor Q2 is turned OFF whereby the voltage at its collector is pulied high by resistor R12. When the negative pulse dies, the transistor Q2 is turned ON again and pulls the collector voltage low again.

Accordingly, a train of positive going pulses are produced at the collector of the transistor Q2 having the same frequency as that of the oscillator circuit. These output pulses are fed to a counter 22.

The counter 22 is arranged to reset to zero and become operative when the supply is connected to the terminals 17 and 21 by the time switch. In this respect it receives the necessary bias voltage by way of a transistor Q3 which is normally biassed ON. The transistor Q3 also supplies the necessary voltage to cause the oscillator circuit to operate. The counter 22 then receives and counts the pulses generated by the oscillator circuit and generates an output signal when a predetermined number of pulses have been received. This output signal is fed by way of a resistor P15 to the base of the transistor Q3 and turns the transistor Q3 OFF.

Whilst it is conducting the transistor Q3 shortcircuits a relay RLA which is connected in series with a resistor R13. However, when the transistor Q3 is turned OFF current flows through the resistor R13 and the relay RL1 and the relay RL1 is thereby energized. Turning the transistor Q3 OFF also causes the oscillator circuit to cease to oscillate.

The relay R" controls two contacts RL1A and RL1B which are actuated when the relay RLA is energized. The contact RL1B is a normally closed contact connected in the collector circuit Q3 which is opened when the relay is energized to thereby prevent the transistor Q3 from being turned ON so that the relay R,1 is retained in its energized condition.

Normally the second relay contact RL1A connects the terminal 17 to an open terminal 23.

It will be appreciated that in this position the terminal 18, to which the room thermostat 13 is connected, is open circuit. Accordingly, even though the time switch has switched the supply on the heating system has not been actuated thereby. However, upon energization of the relay R" the contact RL1A comes into contact with the terminal 18 such that the supply is fed to the heating system to enable actuation thereof. As the supply is stili applied to the terminals 17 and 21 the relay RL1A remains energized until the time switch switches off the supply at which time the control circuit 1 6 is reset to its initial condition.

It will thus be appreciated from the above that each time the time switch switches on the supply the heating system is not immediately actuated.

The heating system is only actuated after a delay period determined by the time taken for a predetermined number of pulses to be received by the counter 22. As the frequency of the pulses is inversely related to the temperature sensed by the thermistor 19 this delay period increases as the temperature increases.

The control circuit illustrated includes a manual override facility in the form of a normally closed push-button switch P5 connected in the collector circuit of the transistor 03. Operation of this switch P5 opens the collector circuit and the relay R,1 is thereby energized to enable actuation of the heating system.

The control circuit illustrated also includes indicators for indicating the state of the circuit. A light emitting diode LED2 indicates the state of the time switch and is illuminated when the supply is connected to the terminals 17 and 21 by the time switch.

At the end of the delay period a further light emitting diode LEDl, connected in series with the relay R", is illuminated to indicate that actuation of the heating system has been enabled.

Figure 3 is a graph of temperature against time indicating the energy savings which can be made using a control circuit of the present invention.

When a time switch is used alone the switch will be set to actuate the heating system at time to ensure that the building is heated to the required temperature TR by time th. Of course, the required temperature TR will be achieved earlier the warmer the ambient temperature. This is illustrated on the graph by three heating curves A, B, C marked in full lines. The curve A relates to a lower than average ambient temperature, the curve B relates to an average ambient temperature and the curve C relates to an above average ambient temperature. The graph also shows in dashed lines the corresponding heating curves A', B', C', obtained where the actuation of the heating system is delayed by a variable period dependent upon the ambient temperature. It will be seen that for below average temperatures the heating curves C and C' are substantially the same.

However, at higher ambient temperatures there can be a substantial delay before the heating system is actuated and the required temperature TR can still be achieved in time. The shaded area enclosed by each heating curve A and B is an indication of the energy which can be saved using a control circuit of the invention.

In the embodiment illustrated and described above the delay period is established in dependence upon the internal or external ambient temperature. However, the temperature sensitive element, that is, the thermistor 19, may be replaced by or used in conjunction with one or more other sensors sensitive to other parameters which affect the heat loss from the building.

Furthermore, although the embodiment illustrated has a switching circuit including a relay for providing connection between the control circuit and heating system this may be replaced by any suitable coupling means.

Claims (12)

1. An electrical control circuit for controlling the actuation of a heating system by a programmable time switch, said control circuit comprising sensing means arranged to produce a variable electrical signal dependent upon at least oneparameter of the environment, an electrical delay circuit responsive to said electrical signal and arranged to determine a delay period dependent upon the electrical signal, and means for coupling said control circuit to a heating system having a programmable time switch for delaying actuation of said heating system until the end of said delay period.

2. A control circuit as claimed in Claim 1, wherein said sensing means are sensitive to one or more of the following parameters, namely, temperature, wind speed, humidity, and radiant heat.

3. A control circuit as claimed in any preceding claim, wherein the sensing means comprises a temperature sensitive element.

4. A control circuit as claimed in Claim 3, wherein the temperature sensitive element is a thermistor.

5. A control circuit as claimed in any preceding claim, wherein said delay circuit includes adjustable means for varying its characteristics in dependence upon the nature and location of the sensing means.

6. A control circuit as claimed in any preceding claim, wherein said coupling means comprises a first terminal for connection to the time switch, a second terminal for connection to the heating system, and switching means for connecting said first terminal to said second terminal, and wherein said control circuit is arranged such that upon the arrival of an actuating signal or supply at said first terminal the switching means keep the first terminal disconnected from the second terminal and the delay circuit commences to determine the delay period, and wherein at the end of the delay period said delay circuit actuates the switching means to connect said first terminal to said second terminal.

7. A control circuit as claimed in Claim 6, wherein the delay circuit comprises an oscillator circuit arranged to oscillate at a frequency dependent upon the electrical signal produced by the sensing means.

8. A control circuit as claimed in Claim 7, wherein the sensing means comprises a temperature sensitive element, and the frequency of oscillation of the oscillator circuit is arranged to increase as the temperature sensed by the temperature sensitive element decreases.

9. A control circuit as claimed in Claim 7 or 8, wherein the pulses produced by the oscillator circuit are fed to a counter connected to actuate the switching means once a predetermined total has been counted.

10. A heating installation comprising an electrically actuated heating system, a programmable time switch for controlling the actuation of the heating system, and an electrical control circuit coupled to the time switch and the heating system, the control circuit comprising sensing means arranged to produce a variable electrical signal dependent upon at least one parameter of the environment, an electrical delay circuit responsive to said electrical signal and arranged to determine a delay period dependent upon said electrical signal, and coupling means for enabling connection of said time switch directly to said heating system, wherein the control circuit is arranged such that when the time switch operates to actuate the heating system the coupling means prevent actuation of the heating system and the delay circuit commences to determine the delay period, and wherein at the end of the delay period the delay circuit actuates the coupling means whereby the time switch is connected directly to the heating system which is actuated thereby.

11. An electrical control circuit for controlling the actuation of a heating system by a programmable time switch substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

12. A heating installation substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.