GB2303718A

GB2303718A - Electronic control for a storage heater

Info

Publication number: GB2303718A
Application number: GB9515128A
Authority: GB
Inventors: Paul Anthony Harder
Original assignee: DIMPLEX
Current assignee: DIMPLEX
Priority date: 1995-07-24
Filing date: 1995-07-24
Publication date: 1997-02-26
Anticipated expiration: 2015-07-24
Also published as: GB9515128D0; GB2303718B

Abstract

An electronic control for a storage heater 1 has room and core temperature sensors 5,6 from which respective digital values are derived. Potentiometer 8 allows the user to set a required room temperature and provides output U to microprocessor 7. Switches 16,17 are set to provide different modes of operation of the storage heater (1), whereby different look-up tables 18, having predetermined values, are selected. A core heating element 3 is switched off when the room and core temperatures match appropriate values in the look-up tables 18, whereby the core 2 is charged with the amount of heat predictably required for room heating. A convector 11 and fan 4 are switched in the different operating modes.

Description

ELECTRONIC CONTROL FOR A STORAGE HEATER The invention relates to storage heaters and more particularly to an electronic control for a storage heater including a core for storing heat; a core heating element connected, via a time controlled switch, to an electrical supply from which off-peak electricity is available on a given tariff; and means for controlling an air flow which is heated by the core.

A problem facing the invention is to provide accurate control of charge acceptance of a storage heater, depending on the amount of heat in its core and the temperature of the room in which it is installed. For example, in cold weather, the core will dissipate more heat and the room temperature will fall to a lower temperature when the core has discharged most of its heat. In these circumstances, the core requires (or accepts) a "full" charge of energy from the off-peak supply. (A "full" charge would be a predetermined maximum depending on the design of the storage heater.) In comparatively warmer weather, the core discharges less heat and the room temperature will remain higher. Consequently, perhaps only a half charge will then be required.In each case, the following days weather is assumed to be approximately the same as before but, if it is not, the charge acceptance will be adjusted by a charge controller, according to the temperatures reached by the core and the room. Therefore, the charge accepted by the heater will "track" the weather.

In some designs of storage heater, a fan may be used to induce an air flow through the core. The amount of heat extracted from the core being greater when the fan operates at higher speeds. The operating state of the fan will clearly affect both the core temperature and the room temperature. This in turn will clearly affect the charge acceptance of the storage heater.

In some cases, storage heaters are fitted with an auxiliary heater. For example, an additional convector may be incorporated in the design of heater, the convector being switched on manually, or automatically to supplement the heat dissipated by the storage heater when the core temperature is low. This will affect room temperature hence also affecting charge acceptance.

Charge controllers also need to contend with variations in tariffs in different areas of the country. In some areas, off-peak electricity is available for comparatively longer periods than in other areas, i.e. in a 24 hour cycle.

In some areas, off-peak electricity is available for a predetermined period during the day and this enables the core to be "topped up" with a charge in colder weather.

However, as storage heaters are made in accordance with a particular design, the design may be more suited to one tariff than another. This creates problems of adjusting charge controllers so that an adequate charge is stored during the off-peak period.

A further problem encountered with the design of charge controllers is due to what may be termed "relative comfort".

For example, elderly people may prefer warmer rooms than younger people and consequently a heavier demand is made on the stored heat in the former case and also on auxiliary heating.

In short, it can be seen that the temperature of the core and the room can fluctuate widely depending on (a) the weather, (b) the available off-peak tariff, and (c) the particular demands made on the heater in order to achieve relative comfort. There is also the problem that one design of storage heater may suit one set of operating conditions but not another.

Attempts to solve this problem by employing simple controls, such as those using thermostats which are operated hydraulically and/or with bimetallic strips have been reasonably successful. However, these forms of control are relatively crude and hence susceptible to inaccuracies (e.g.

due to tolerance of components).

The invention solves the above-mentioned problems by providing an electronic control comprising: a room temperature sensor for providing an analog value representing the temperature of a room in which the storage heater is installed; a core temperature sensor for providing an analog value representing the temperature of the core; user temperature setting means for providing an analog output representing a required room temperature; switching means for selecting different operating modes of the storage heater; and microprocessor means, which includes data storage means having stored sets of predetermined values for comparison with digital representations of said analog values, and which is responsive to said switching means and to select the appropriate set of values to cause the supply of offpeak electricity to be interrupted when the digital representations match the selected set of values, and so that the core is charged only with the amount of heat that is predictably required for subsequent room heating, depending on a predetermined relationship between core temperature and room temperature.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Fig. 1 schematically illustrates the main features of a storage heater according to an embodiment of the invention, Fig. 2 is a table showing operating states used in the storage heater of Fig. 1, and Fig. 3 is a graph illustrating the relationship between core temperature, room temperature and the temperature at which the core element is switched off.

Generally speaking, the storage heater of the preferred embodiment of the invention incorporates an electronic control having two main functions; the control of core and room temperature. It preferably incorporates a fan for inducing a flow of air through the core. It also preferably incorporates auxiliary heating means, in the form of a convector. Both the fan and the convector are switched on and off in accordance with space heating requirements.

Core temperature and room temperature are both sensed. A user control enables adjustment of room temperature, but no user operated charge control is available and switching means are used instead to alter the relationship between room temperature, core temperature and charge acceptance of the core, in accordance with an available tariff and/or the relative comfort requirements of the user. The electronic control incorporates a microprocessor which responds to various inputs in order to control the heating elements and the fan and this may be programmed for either automatic or manual operation. Further details of the storage heater are given below.

Fig. 1 is only a schematic diagram in simplified form for the sake of explanation.

Storage heater 1 has a core 2 heated by an element 3.

A fan 4 induces an air flow through the core 2. Core temperature and room temperature are sensed by respective thermistors 5,6, which provide output signals C and R, that are supplied to microprocessor means 7. The microprocessor means 7 effectively compares room temperature (R) with the user setting (U) to control the fan (and convector) to achieve the required room temperature. A potentiometer 8 enables a room temperature to be selected by a user and it provides an output U to microprocessor means 7. The range of temperatures that can be selected may be such that in a minimum position (1) the room temperature is controlled to 50C. There is then a linear scale to a maximum (7) which corresponds to 300C. These are approximate temperatures.

Relay devices 9,10 respectively control currents supplied to core element 3 and to a convector element 11.

The convector element provides auxiliary heating, i.e.

independently of the heat output from core 2. Devices 9 and 10 are represented diagrammatically. Microprocessor 7 can sense the ON/OFF state of the core and convector element respectively. Signals P1, P2, supplied from microprocessor means 7, operate the relays 9, 10 in order to turn the core element and/or convector element on or off. Switch 12 operates the convector manually (in manual mode).

Fan 4 has a motor connected to a triac-controlled proportional controller 13 having a multi-voltage output for controlling the speed of fan 4 and hence the air flow.

Microprocessor 7 senses the voltage supplied to the fan 4 and hence its operating state (speed of the fan). Signal P3 from microprocessor 7 controls the triac-controlled proportional controller 13 and hence the voltage and on/off states of the fan. The fan motor will vary in size depending on the model of the storage heater. The maximum rating can be 25w with a minimum rating of 12w.Full proportional control of the fan can be obtained as follows: SET POINT FAN VOLTAGE set point 1.0 higher than room temp 240V set point 0.5 to 1.00 higher than room temp 180V set point 0 to 0.5 higher than room temp 120V set point = room temp 0V Part proportional control is as follows: FAN VOLTAGE set point 0.50 higher than room temp 180V set point 0 to 0.5 higher than room temp 120V set point = room temp 0V The proportional control of the fan is not available when the convector element is operating.

The fan will switch off if the sensed core temperature is < 20 C.

The operation of the fan during a charge period is determined by a selector switch (see below).

Device 14 detects when off peak electricity is available and it supplies a signal, indicating this condition, to microprocessor means 7. The power supply 15 is usually through a time-controlled switch (not shown).

A three-position selector switch 16 (not shown connected) provides inputs S1, S2, S3 to microprocessor means 7. The switch is used to control the convector element 11 and the fan 4 as follows: position 1 convector only available position 2 convector (restricted use when automatic mode is selected) and fan available position 3 fan only available Switch 16 provides different modes of operation when the off-peak supply is available under different tariffs.

The operation of the convector element 11 is as follows, assuming that the rating of the convector element is 2 kW max.

1. During an off-peak period with the selector switch 16 in position 1 the convector 11 can be switched on only if it is selected and only if the sensed room temperature is below a predetermined set point.

2. During an off-peak period with the selector switch 16 id position 2, (automatic mode) the convector 11 switches on and off automatically, and only if the sensed core temperature is below a predetermined value and the sensed room temperature is below the set point. In manual mode, the convector can be switched on and off, at will, by the user.

3. During an off-peak period, the convector element is always switched off with the selector switch in position 3.

4. If at any time the sensed core temperature is 800 or above, the convector element will be cycled (by microprocessor 7) to give approximately half power (on for 30 seconds and off for 30 seconds).

5. When an off-peak supply is not available (due to the time of day), the convector (if selected) will operate as follows: (a) if the sensed core temperature is below a predetermined value, the convector will be on if the set point is higher than the sensed room temperature, (b) if a switch is set for manual operation, see dil switch below, the convector will operate if selected and the set point is higher than the sensed room temperature, (c) when the sensed core temperature is below 200C and the convector is switched on, the convector will operate.

The convector should then remain on and the fan off until the next off-peak period. The temperatures should then revert to normal operation.

When the convector is operating, the proportional control of the fan is disabled. An indicator, such as a neon, can be illuminated to show that the convector is operating.

Dil switch 17 (not shown connected) has three switches, each of which can be set to an ON or OFF position to control the amount of charge and to control the manual/automatic operation of the convector as shown in the table illustrated by Fig. 2. With reference to the table, part proportional control means that the fan starts to operate at a working voltage of 180V (as noted above).

The microprocessor means 7 incorporates A/D converters (not shown) and other appropriate signal processors (not shown) to enable digital values to be generated representing the analog inputs C,R,U. The microprocessor is programmed to compare the digital representations of R with U and to generate outputs P2 and P3 to control convector 11 and fan 4 accordingly. It is also programmed to compare the digital representations of core temperature and room temperature (C,R) with appropriate stored values and to generate the outputs P1 to control charge acceptance. The stored values may be provided in look-up tables 18, the relevant table being selected depending on the setting of switches 16 and 17. The stored values are predetermined by experiment, or calculation or both.This enables the room temperature to be controlled in accordance with the set point adjusted by the user (on potentiometer 8) depending on the sensed temperatures and operating states in the storage heater. The operating states of the core element, convector element and fan can be sensed to "tell" the microprocessor 7 what operating states are presently available and this is taken into account in the automatic mode of operation.

The microprocessor employs an algorithm, based on the graph shown in Fig. 3, tc determine charge acceptance such that if the room temperature is 20"C or below, a "full" charge would be taken by the core. At this setting the core would not accept a charge if the room temperature reached 280C (see Fig. 2).

If the room temperature thermistor 6 is short circuited, or open circuited, the core will only be charged to its maximum level, i.e. 100"C as sensed by the core thermistor 5 and, in this condition, fan 4 and convector element 3 are switched off.

Referring to the graph shown in Fig. 3, the central curve (shown in full line) represents normal operation of the storage heater. By way of example, the core element will be switched off with a combination of a core temperature of 640C and a room temperature of 240C. The look-up table 18 stores values representing the required operating curve and the algorithm effectively uses the curve to generate output P1 to control charge acceptance.

The operating curve may be shifted either to the left (broken line) or to the right (chain and dot line) depending on relative comfort requirements. This will shift the operating temperatures at which the core element is switched off. This shift can be accommodated by using different look-up tables 18 in the microprocessor means 7. However, in a modification, sets of look-up tables 18 are stored and a further switch (not shown) is incorporated which can be set into different states, to achieve the following: Position A "normal" - 100% at room temperature of 200C or below Position B "maximum" - 100% at room temperature of 230C or below.

Position C "minimum" - 100% at room temperature of 170C or below.

The figure 100% indicates a "full" charge (i.e. a predetermined maximum), having regard to the respective curves shown in Fig. 3 (i.e. the central curve being "normal") The amount of charge taken by the core 2 is continually monitored during the charge period, by the thermistors 5,6.

If the convector or fan are operating during a charge period, the "calculated" sensed core temperature should be increased by 10 C. If the core thermistor is open circuited, the electronic control will be inoperative.

Essentially, the microprocessor will select the appropriate look-up table so that the room temperature is controlled by switching the fan and convector, and so that the appropriate charge is stored. The manner of operation of the microprocessor to operate in this fashion will be understood by those skilled in the art and no further detailed explanation will be given. In the case of automatic operation, the microprocessor 7 issues appropriate commands with the aim of automatically controlling the room temperature to the required value. With manual operation, the user is free to turn on and off the fan and convector in accordance with the control parameters mentioned above.

The core thermistor may be located at the top of the storage heater and the room temperature thermistor may be located on the side of the storage heater. However, these are typical points of location, which can be varied in order to suit particular requirements.

Instead of using a fan 4, a flap-type of control can be used to regulate the flow of air through the core.

Other forms of auxiliary heating can be employed instead of a convector.

One advantage of using the digital form of control described above is that temperature control is more ac curate since there is a more linear relationship (in practice) between sensed temperatures so that switching is more reliable. A further advantage is that the switching temperatures can be easily adjusted by simply varying the values stored in the look-up tables. In particular, charge acceptance can be made automatic and more accurate.

Storage heaters can therefore be made to operate ore economically, thereby saving the cost of electricity.

Claims

CLAIMS:

1. An electronic control for a storage heater including: a core for storing heat; a core heating element connected, via a time controlled switch, to an electrical supply from which off-peak electricity is available on a given tariff; a fan for inducing an air flow which is heated by the core; and the electronic control comprising: a room temperature sensor for providing an analog value representing the temperature of a room in which the storage heater is installed; a core temperature sensor for providing an analog value representing the temperature of the core; user temperature setting means for providing an analog value representing a required room temperature; switching means for selecting different operating modes of the storage heater; and microprocessor means, which includes data storage means having stored sets of predetermined values for comparison with digital representations of said analog values, and which is responsive to said switching means to select the appropriate set of values to cause the supply of off-peak electricity to be interrupted when the digital representations match the selected set of values, and so that the core is charged only with the amount of heat that is predictably required for subsequent room heating, depending on a predetermined relationship between core temperature and room temperature.

2. An electronic control according to Claim 1 including auxiliary heating means for providing heat additional to the core and further including first switching means for selecting manual, or automatic operation, wherein: (a) the auxiliary heating means operates either manually, or automatically; (b) the core can be charged to a maximum, or minimum, or intermediate level; (c) the means for controlling an air flow operates at either maximum or minimum; said microprocessor means being also responsive to switching states in said first switching means so as to select the appropriate sets of values accordingly.

3. An electronic control according to Claim 1 or 2 including second switching means operable: (a) to isolate the means for controlling the air flow during an off-peak period; (b) to enable the auxiliary heating means and the means for controlling the air flow to operate during an offpeak period; (c) to allow the means for controlling the air flow alone to operate during an off-peak period; the switching states (a), (b) and (c) being selected, on installation of the storage heater, depending on the available tariff; said microprocessor means also being responsive to switching states in said second switching means.

4. An electronic control according to Claim 3, when dependent on Claim 2 wherein predetermined sets of values are stored in look-up tables in said data storage means and wherein the switching states of the first and second switching means determine which look-up table is selected, by the microprocessor means, so as to interrupt the supply of off-peak electricity to said element during said charging period.

5. An electronic control according to any of the preceding claims, wherein third switching means are provided for selecting different sensed room temperatures at which, depending on the sensed core temperature, the core heating element is switched off.

6. An electronic control according to any of the preceding claims in which the means for controlling the air flow comprises a fan and a proportional controller for operating the fan at different speeds.

7: An electric control according to Claim 6 wherein the auxiliary heating means comprises a convector.

8. An electronic control according to any of the preceding claims and further including means for detecting that offpeak electricity is available, the latter means providing an output to the microprocessing means to signal off-peak availability.

9. A storage heater including the electronic control of any of Claims 1-8.