GB2040080A - Control apparatus for central heating systems - Google Patents

Abstract

Control apparatus for a central heating system includes one or more thermistors 25-28 sensing the temperature in a space heated by the system; a digital processor 1 stores a set value of temperature required in the space during an operating period of the system and derives a digital temperature signal representing the temperature sensed by the thermistor. The processor controls the supply of heat during the operating period to maintain the temperature in the space at the set value. The processor includes a data memory 2 for storing set values of temperature for several heating zones and for hot water, and a programming memory 4. Set values of time and temperature are put in using a keyboard 7 and display 8. A clock 5 provides signals for the processor and for a current time and day register 6. The processor outputs control a boiler 34 and pump 35 and one or more zone valves 36. <IMAGE>

Description

SPECIFICATION Control apparatus for central heating systems This invention relates to improvements in control apparatus for central heating systems. The invention finds particular application in, but is not exclusively limited to, domestic central heating systems.

Known central heating systems utilise thermostatic switch devices to sense room and hot water temperatures in the system for control purposes.

These thermostatic devices, which include adjustable movable electrical contacts, are however generally inaccurate and unreliable, and the actual temperature at which the contacts of the device open may be siginificantly different, i.e. may deviate by several degrees, from the nominal temperature at which the device is set.

In known systems, the time, or so-acalled programmer, used in the system to set the operating periods of the system is usually in the form of an electric clock and mechanical contacts actuated by the clock. Such timers again tend to be relatively inaccurate and prone to failure because of the moving parts involved. Moreover, the known timers provide for a minimal selection of the periods for which the system is operated, usually provision being made for no more than two periods of operation in a 24 hour period. Furthermore, unless the timer is adjusted from day to day, the programmed periods of operation are the same for each day of the week, although the required pattern of operation of the system at a weekend may be significantly different from that required on a weekday.

Known central heating systems are usually singlezone systems, that is to say heat is supplied to all or some of the radiators of the system. Normally the temperature of a single room is sensed by a single sensor and used to control operation of the entire heating part of the system. As conditions and requirements may vary considerably from room to room, such control may result in underheating of some rooms or wasteful overheating of other areas.

Similarly, the required periods of operation for different zones served by the system and possibly comprising a plurality of rooms, e.g. bedrooms and living rooms in a domestic household, may be completely different. It is true that, during operation of the system as demanded by the single sensor, the heat supply to individual radiators of the system may be controlled by manual or thermostatic valves but this does not enable an underheated space to be supplied during periods of non-operation signalled by the single sensor.

All these defects of known central heating control systems can give rise to unsatisfactory and ineffi cient operation of the system.

It is an object of the invention to provide control apparatus for a central heating system which enables improved accuracy, reliability and flexibility of control.

Accordingly, the present invention provides control apparatus for a central heating system, including a a thermistor sensing the temperature in a space heated by the system, and a digital processor storing a set value of temperature required in the said heated space during an operating period of the system, the processor deriving a digital temperature signal representing the temperature sensed by the thermistor and controlling the supply of heat by the system during the operating period to maintain the temperature in the said heated space at the set value.

An analogue temperature signal provided by the thermistor may be converted to the digital temperature signal in an analogue-to-digital converter within the processor or an external converter controlled by the processor.

The processor may store a plurality of set values of temperature corresponding to respective operating periods during a day. Preferably, the processor stores set values of temperature corresponding to respective operating periods during a plurality of successive days.

The control apparatus may comprise a plurality of thermistors each sensing the temperature of a respective space heated by the system and the processor may derive a digital temperature signal representing an average of the temperatures sensed by the thermistors.

The processor may include a programmable memory for storing each set value of temperature to be maintained during each operating period and data input means, such as a keyboard, may be provided for entering the set values of temperature in the memory.

Display means, such as an LED or LCD display, may be provided for displaying the data entered by means of the data input means, and preferably the prevailing temperature sensed by any thermistor and the time according to a digital clock contained in the processor.

The apparatus may also include a thermistor sensing the temperature of a hot water supply provided by the system and providing a digital hot water temperature signal. The digital processor may then store a set value of the hot water temperature required during an operating period of the system, the processor deriving a digital hot water temperature signal representing the sensed hot water temperature and controlling the heating of the hot water supply to maintain the hot water temperture at the set value.Alternatively, the apparatus may include a poteniometer for setting a desired hot water temperature signal and a comparator for comparing the sensed hot water temperature signal from the thermistor with the desired tempertature signal from the potentiometer and delivering to the processor a control signal indicating whether the sensed temperature is higher or lower than the desired temperature.

The digital processor may control the operation of a boiler and a pump of the central heating system by controlling switches in the power circuits of these components. The power switches may be electronic switches, such as TRIACS, and desirably means are then provided for ensuring operation of the switches at a zero crossing of the power circuit current or voltage. The power switches may however be relays, means being provided for electrically isolating the relays from the digital processor.

In control apparatus for a multi-zone system heating a plurality of zones the temperatures of which are to be individually controlled, a thermistor may be provided in a space in each zone and the processor may store a set value of temperature for each zone during each operating period, the processor receiving the digital temperature signal from each zone and controlling the supply of heat to the zone by the system during the operating period to maintain the space in the zone at the set value.

Each zone of a multi-zone system may include a plurality of thermistors each sensing the temperature of a respective space in the zone and the processor may derive for each zone a digital temperature signal representing an average of the temperatures served by the thermistors in the zone.

In order that the invention may be readily understood, embodiments thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a basic block diagram illustrating the principle of a control apparatus embodying the invention for a central heating and hot water supply system, showing how the apparatus can be extended for use in a system in which the temperatures of a plurality of central heating zones are to be controlled independently; Figure 2 is a more detailed diagram of the Figure 1 apparatus, showing the coupling of thermistors to a digital microprocessor of the apparatus and showing the coupling of a keyboard and digital display to the microprocessor; Figure 3 shows the coupling of the microprocessor to control a boiler, pump and zone valves of the system; Figure 4 shows a power supply arrangement of the apparatus;; Figure 5 is a basic block diagram of a second embodiment of a control apparatus according to the invention; and Figure 6is a more detailed diagram of the Figure 5 embodiment.

Referring to Figure 1, a control apparatus for a central heating and hot water supply system includes a microprocessor 1 having a data memory 2 for storing set values of temperature required for the hot water supply and central heating zones during operating periods of the system. The microprocessor 1 further includes a processing unit 3 controlled by a programme stored in a programme memory 4.

A clock Sin the processor 1 provides timing signals for the operation of the processor and also provides a a time base for a time register 6 holding the current time of day and day of the week.

A keyboard device 7 is connected to the microprocessor 1 for entering the set temperature values in the memory 2, the data entered being displayed on a digital display 8. The keyboard 7 includes: ten keys 9 to 18 for entering the decimal digits 0 to 9; a programme key 19 for selecting a first or weekday (days 2 to 6) or a second or weekend (days 1 and 7) scheme of operating periods; a day key 20 and a time period key 21 for selecting the day and time period whose temperature is being set; a zone key 22 for selecting the zone whose temperature is being set, the hot water supply being regarded as zone 0; a temperature key 23 for indicating that a temperature is being set; and a laod key 24 for loading the data entered in a display memory 2A of the processor into the memory 2. The keyboard 7 can also be used to demand display of current operating period and temperature conditions.Up to six time periods may be selected, for example: t1 = 8.00 hrs., dressing and breakfast; t2 = 9.30 hrs., morning at work; t3 = 13.00 hrs., lunch; t4 = 14.30 hrs., afternoon; t5 = 16.00 hrs., evening; t6 = 23.00 hrs., bedtime.

In Figure 1, the display 8 shows a set temperature of 220C being programmed for zone 1 during time period t1 of day 3.

The current temperatures of the hot water supply and a first central heating zone are detected by four thermistors 25 to 28. Thermistors 25 to 27 each sense the temperature in a respective space in the first central heating zone and the thermistor 28 senses the temperature of the hot water supply. The thermitors 25 to 28 are connected to the microprocessor 1 via a gate 29 which connects a selected one of the thermistors to the microprocessor in response to selection signals delivered by the microprocessor on line 291. Each further central heating zone is provided with up to four thermistors 30 each sensing the temperature in a respective space in the zone and connected to the microprocessor by a gate 31 in similar manner to thermistors 25 to 28.

Two outputs 32 and 33 from the microprocessor are connected to control the operation of a boiler 34 and a pump 35 of the system. Further outputs 37 may also be provided to control the operation of one or more central heating zone valves 36 controlling the supply of heat to the individual zones.

In operation of the described apparatus, the microprocessor converts analogue electrical signals from the thermistors into digital signals representing the temperatures of the hot water supply and the central heating zones. These temperature signals are compared by the microprocessor with the set temperatures valves and the boiler, pump and zone valves to maintain the set temperatures. The boiler 34 is controlled so that it is on when either the hot water supply temperature is below its set value or the temperature of any central heating zone is below its set value. The pump 35 is on when the temperature of any central heating zone is below its set value. The zone control valve 36 of any central heating zone is open when the temperature of that zone is below its set value. To avoid instability, it is preferable to arrange that the boiler, pump or control valve is only switched off if the relevant sensed temperatures exceed the respective set temperature values by 0.5 C and is only turned back on when the temperature drops below the set temperature value minus O.50C.

The boiler and pump may be switched on 4xT minutes before the programmed time, where T is the set value or average or maximum set value of the central heating zones. Hot water and central heating override switches may be provided to provide hot water supply and central heating independently of the programme. Provision may be made for ensuring that the temperature of the water cannot fall below a predetermined temperature (such as 200C or 5 C) at any time to provide frost protection.

During normal operation of the apparatus, display 8 displays the current time of day. In the event of an interruption by operation of the keyboard to enter data or deman display of further data, the current time display is temporarily stopped while the entry of data or display of further data takes place.

The boiler and pump may be switched off altogether for a number of days, for example during a holiday period, and set to come on again automatically upon registering of the set time lapse.

Figure 2 shows an arrangement for serially coupling the thermistors 25 to 28 for the hot water supply and the first central heating zone to a line 50 via an MOS gating device 29 under the control of a 2 to 4 bit decoder 40 decoding thermistor selection output signals 41,411 from the microprocessor 1.

The individual analogue temperature signals from the thermistors pass via a passive bridge 42 to the inverting signal input of an operational amplifier43 in which the analogue signal is converted into a digital temperature signal by 8-bit digitizing signals supplied from the microprocessor on an 8-bit data output bus 51 and applied to the non-inverting amplifier input via an R12R network 44. The individual signals from the central heating thermistors 25 to 27 are averaged in the microprocessor, while the single digital hot water supply signal is used as the current hot water supply temperature.

Figure 2 shows in dashed lines how the basic system of one central heating zone may be expanded to cater for further central heating zones.

The thermistors 30 of each further central heating zone are connected to a respective MOS gating device 31 for serial connection to line 50 under the control of decoder 40. The gating devices 29,31 are individually enabled by respective outputs of a second 2 to 4 bit decoder 47 which decodes zone selection signals 48,481 from microprocessor 1.

system selection switch 46 permits device 29 to be permanently enabled by connection to Ov if only one central heating zone is required or to the respective output of decoder 47 if more than one zone is employed. The illustrated arrangement provides for a group of four central heating zones but, if required, a further decoder 47 may be provided to control up to four further gating devices 31. A zone group selector switch 49 may then be provided to permit decoder 47 to be permanently enabled by connection to Ov if only one group of four zones are required or to be selectively enabled by a zone group selection signal 52 from the microprocessor.

In a basic system having a hot water supply and a single central heating zone it may be more conve nient to perform the analogue to digital conversion by a suitable converter provided in the microprocessor.

Figure 2 also shows the coupling of the keyboard device 7, in the form of a push button key-pad, to the microprocessor via a four bit data input bus 53 and to the display 8, in the form of a seven-segment LED or LCD display of 7 characters 54. A group 55 of four characters are for displaying data, such as temperature values and time of day and a group 56 of three characters are allotted to the display of the operating status, such as day, operating period and heating zone. The individual characters 54 are controlled via common anode character drivers 60 by inputs from a scan generator 57 which takes the form of an 8-bit shift register receiving from the microprocessor a clock signal 58 at its clock input and a programme signal 59 at its clear input.The segments of the characters are driven to display the required data via a segment driver 61 supplied with data signals from the microprocessor on the data output bus 51. A decimal point input 62 is connected to Ov.

The keyboard can be extended to include 20 pushbuttons and an additional external programmable memory 63 can be provided as shown in dashed lines in Figure 2. The external memory 63 is used to store set values for central heating zones after the first and for possible use with extra operating periods or auxiliary domestic uses such as solar panel control and timed burglar alarm systems.

Figure 5 is a basic block diagram of a second, simplified embodiment of the control apparatus according to this invention. The Figure 5 apparatus is very similar to the apparatus of Figure 1 and like references numerals have been used to denote like components. In the Figure 5 apparatus, however, the internal data memory 2 of the microprocessor 1 stores only set values of temperature required for a single central heating zone during operating periods of the system, the regulation of the hot water temperature being carried out separately as hereinafter explained. As in the Figure 1 embodiment, the microprocessor includes a processing unit 3 controlled by a programme stored in a programme memory 4 and a clock 5 provides timing signals for operation of the processor and for time register 6.

Keyboard device 7 is connected to microprocessor 1 for entering the set temperature values in memory 2 and the entered data is displayed on display 8.

Keyboard 7 has the same keys as in Figure 1, except for the zone key 22 which is not required. Up to 4 time periods may be selected, for example : t1 = 8.00 hrs, dressing and breakfast; t2 = 9.30 hrs, morning at work; t3 = 16.00 hrs, evening; t4 = 23.00 hrs, bedtime.

The current temperature of the single central heating zone is detected by two thermistors 25 and 26, the thermistors 25 and 26 each sensing the temperature in a respective space in the single central heating zone.

Outputs 32 and 33 from the microprocessor are connected to control the operation of boiler 34 and pump 35 via a status display device 100 which also receives signals on line 101 indicating whether the weekday or weekend scheme of operating periods is being operated.

In the Figure 5 embodiment, the temperature of the hot water supply is sensed by a separate thermistor 102 whose analogue output signal is compared in a comparator 103 with a desired hot water temperature signal set by a potentiometer 104 calibrated in degrees centigrade to provide a hot water control signal to the microprocessor on line 105 indicating whether the sensed temperature is above the desired temperature.

The operation of the second embodiment is similar to that of the Figure 1 embodiment except that the temperature signals from thermistors 25,26 are only compared with set central heating temperature values and a separate control signal is received from hot water thermistor 101. The boiler 34 is controlled so that it is on when either the set temperature in a period is higher than in the immediately preceding period or the central heating temperature is below its set value. The pump 35 is on when the temperature of the central heating is below its set value. If the hot water temperature is above its desired value and the set point central heating temperature has been achieved, then both pump and boiler are turned off.

The display of data on display 8 may take place as described for Figure 1.

Figure 6 shows in more detail the construction of apparatus according to Figure 5, using an Intel 8748 microprocessor. Figure 5 is similar in many respects to Figure 2 and only the difference will be described, like parts having been given the same reference numerals as in Figure 2.

The coupling and signal digitisation of the central heating temperature thermistors 25,26 is similar to Figure 2, except that an active wheatstone bridge 106 is used. As regards the hot water temperature, the control signal from comparator 103 is applied to input 107 of microprocessor 1.

The coupling and operation of the keyboard device 7 are similar to that in the apparatus of Figure 2, except that a digital selector 108 fed by a control bus 109 of the microprocessor is used for energising the keyboard device and the characters 54 of the display 8. Figure 6 shows by way of further example, a display 8 which is expanded by comparison with that of Figure 5 to include a group 110 of four characters for permanently displaying the time of day, a single character 111 for displaying the day, a single character 112 for displaying the time period, and a group 113 of two characters for displaying current or set temperature values. A detector 114 detects when a row of the keyboard device 7 changes logic state to provide an output at 115 which indicates that a key has been pressed and is applied to input 116 of microprocessor 1 to initiate an interrupt routine.

The status display device 100 comprises four LEDS 120-123 which are illuminated via drivers 119 by respective output signals on a status bus 124 to indicate respective output signals on a status bus 124 to indicate respectively operation on the week day scheme, operation on the weekend scheme, operation of the boiler and pump are coupled via an optocoupler device 125 to control operation of relays (not shown) in the power circuits of the boiler and pump.

If desired, a 500 ns drive pulse may be provided at output 126 of the microprocessor upon entry into operating period t1 and may be used to operate a device, such as an alarm or tea-making appliance, in the early morning.

If the apparatus of Figure 6 is to be expanded to provide additional central heating zones or the like, an external programmable memory may be used and the logic state of input 127 arranged to indicate the presence of such external memory.

Claims (20)

1. Control apparatus for a central heating system, including a thermistor sensing the temperature in a space heated by the system, and a digital processor storing a set value of temperature required in the said heated space during an operating period of the system, the processor deriving a digital temperature signal representing the temperature sensed by the thermistor and controlling the supply of heat by the system during the operating period to maintain the temperature in the said heated space at the set value.

2. Apparatus according to claim 1,wherein the processor stores a plurality of set values of temperature corresponding to respective operating periods during a day.

3. Apparatus according to claim 2, wherein the processor stores set values of temperature corresponding to respective operating periods during a plurality of successive days.

4. Apparatus according to any one of claims 1 to 3, including a plurality of thermistors each sensing the temperature of a respective space heated by the system, the processor deriving a digital temperature signal representing an average of the temperatures sensed by the said plurality of thermistors.

5. Apparatus according to any preceding claim, including a programmable memory for storing each set value of temperature to be maintained during each operating period and data input means for entering desired set values of temperature in the memory.

6. Apparatus according to claim 5, wherein the data input means is a keyboard.

7. Apparatus according to claim 5 or 6, wherein display means are provided for displaying the data entered by means of the data input means.

8. Apparatus according to claim 7, wherein the display means also serves to display the prevailing temperature sensed by the thermistor or thermistors and the time according to a digital clock contained in the processor.

9. Apparatus according to any preceding claim, including a thermistor sensor the temperature of a hot water supply provided by the system and providing a sensed hot water temperature signal.

10. Apparatus according to claim 9, wherein the processor stores a set value of the hot water supply temperature required during an operating period of the system, the processor deriving a digital hot water temperature signal representing the sensed hot water temperature and controlling the heating of the hot water supply to maintain the hot water temperature at the set value.

11. Apparatus according to claim 9, including a potentiometer for setting a desired hot water temperature signal and a comparator for comparing the sensed hot water temperature signal from the thermistor with the desired temperature signal from the potentiometer and delivering to the processor a control signal indicating whether the sensed temperature is higher or lower than the set temperature.

12. Apparatus according to any preceding claim, wherein the digital processor controls the operation of a boiler and a pump of the central heating system by controlling switches in the power circuits of these components.

13. Apparatus according to claim 12, wherein the power switches are electronic switches and means are provided for ensuring operation of the switches at a zero crossing of the power circuit current or voltage.

14. Apparatus according to claim 12, wherein the power switches are relays and means are provided for electrically isolating the relays from the digital processor.

15. Apparatus according to any preceding claim for a multi-zone system heating a plurality of zones the temperatures of which are to be individually controlled, wherein a thermistor is provided in a space in each zone and the processor stores a set value of temperature for each zone during each operating period, the processor receiving the digital temperature signal from each zone and controlling the supply of heat to the zone by the system during the operating period to maintain the space in the zone at the set value.

16. Apparatus according to claim 15, wherein each zone of the multi-zone system includes a plurality of thermistors each sensing the temperature of a respective space in the zone and the processor may derive for each zone a digital temperature signal repesenting an average of the temperatures served by the thermistors in the zone.

17. Apparatus according to any preceding claim, wherein an analogue temperature signal provided by the or each thermistor is converted to a digital temperature signal in an external analogue-to-digital converter controlled by the processor.

18. Control apparatus for a central heating system substantially as hereinbefore described with reference to Figures 1 to 16 of the accompanying drawings.

19. Control apparatus for a central heating system substantially as hereinbefore described with reference to Figure 17 of the accompanying drawings.

20. Any novel feature or combination of features herein disclosed.