WO1990006477A2

WO1990006477A2 - An electric boiler and controls therefor

Info

Publication number: WO1990006477A2
Application number: PCT/GB1989/001458
Authority: WO
Inventors: Peter Joseph Newton
Original assignee: DIMPLEX HEATING Ltd
Current assignee: DIMPLEX HEATING Ltd
Priority date: 1988-12-06
Filing date: 1989-12-06
Publication date: 1990-06-14
Anticipated expiration: 1991-06-06
Also published as: WO1990006473A2; WO1990006477A3; EP0533658A1; DE68921988D1; ES2070310T3; EP0447438A1; EP0447438B1; DE68921988T2; WO1990006473A3

Abstract

The invention relates to an electric boiler comprising a core (2), at least one electric element (10) for heating the core (2), a heat exchanger for transferring the stored core heat to a secondary system (8), an auxiliary heater (12) adapted to heat the secondary system directly, and control means (14) for controlling heat supply to the core and supply of heat from the core (2) and the auxiliary heater (12) to the secondary system. In a preferred embodiment, the control means operates the boiler in either a first mode in wich the heat supply to the core (2) is terminated when the core temperature reaches a first temperature (for winter running) or a second mode in which heat supply to the core (2) is terminated when the core temperature reaches a second temperature (for spring/autumn running), lower than the first temperature.

Description

AN ELECTRIC BOILER AND CONTROLS THEREFOR

The invention relates to boilers in which water is heated by electricity, through the medium of a core of material capable of storing heat, the core being heated by electrically powered heating elements and heat from the core being transferred to a secondary circuit in which a fluid, commonly water, is circulated, as in the case of a domestic central heating or hot water system.

The present invention also relates to the controls for such an electric boiler.

In our co-filed PCT application, based on UK applications Nos. 8828422.9, 8901281.9 and 8922306.9, is described an

embodiment of an electric boiler in which steam is used as the medium for transferring heat from the store to the secondary circuit. The general arrangement has a core of material capable of storing heat, electrically heated elements in the core, a primary heating circuit, in which water is supplied to the base of boiling tubes which pass steam upwards through the core to a heat exchanger, from which condensed water is returned to the base of the boiling tubes, losses being made up from a header tank, and a secondary heating circuit portion which passes through the heat exchanger and which is connected, in use, to a secondary heating circuit in a conventional way. Such an electric boiler will be referred to hereinafter as "an electric boiler of the kind described".

According to the present invention there is provided an electric boiler comprising a core, at least one

electric element for heating the core, a heat exchanger for transferring the stored core heat to a secondary system, an auxiliary heater adapted to heat the secondary system directly,and control means for controlling heat supply to the core and supply of heat from the core and the auxiliary heater to the secondary system.

Preferably the control means allows electricity supply to the boiler to be accepted at different tariff rates, the control means being capable of operating the boiler differently during times of different tariff rates.

Preferably during a high tariff electrical cost period, the control means operates the boiler such that a demand from the secondary system is satisfied by the core first and, if necessary, by the auxiliary heater.

It is also preferable that during a low tarriff electrical cost period, the control means operates the boiler such that a demand from the secondary system is satisfied first by the auxiliary heater.

Preferably the auxiliary heater is activated only if the core cannot satisfy the electric heat requirement. In this instance, it is preferable that a predetermined time delay must pass before the auxiliary heater can be

activated.

The control means may also include a manual override to actuate the auxiliary heater.

Preferably the auxiliary heater is a flow immersion heater.

According to another aspect of the present

invention, there is provided an electric boiler comprising a core, electric element means for heating the core, a heat exchanger for transferring the stored heat to a secondary system and control means for controlling charging of the core with heat and transfer of heat from the core to the secondary system, the control means operating the boiler in either a first mode in which heat supply to the core is terminated when the core temperature reaches a first temperature, and a second mode in which heat supply to the core is terminated when the core temperature reaches a second temperature, lower than the first temperature.

The boiler preferably includes sensing means for controlling the temperature of the core.

Preferably the core is formed from a plurality of unit cores, each unit core having at least one electric element therein. In this instance, the electric elements may be connected in either series or parallel, or in any other suitable fashion.

If the core is formed from many unit cores, it is preferable that it is possible to heat only one or more unit core. Further, it is preferable that the maximum temperature of the or each core does not exceed 750°C.

Preferably, the maximum heat stored by the core can be varied manually to suit the users needs.

Preferably, the temperature of the secondary system can be held between two predetermined values. These predetermined values may, of course, preferably be

variable.

The heat exchanger may include a water supply and a condensing chamber, the water being evaporated by the heat of the core and passing along pipes to condense in the condensing chamber as the heat from the steam is absorbed by the secondary system. If this type of heat exchanger is used, the transfer of stored core heat is preferably controlled by regulating the supply of water to the pipes. To ensure that a realistic temperature of the secondary system is sensed, the temperature of the

secondary system is preferably measured only after a predetermined time delay, such as 15 seconds.

Preferably the core is heated in two stages by the or each heater element. The first stage preferably being at a fast heating rate and the second stage at a slower heating rate. During the first stage the

temperature of the or each heater element is preferably about 200°C above the temperature of the core and during the second stage the temperature of the or each heater element is preferably about 80ºC above the core

temperature.

Preferably the temperature of the core is measured by means of a sensor mounted on an electric element.

The electric boiler of the present invention is preferably controlled by an electronic device which may be configured by means of one or more manual switches. This electronic device preferably includes a clock.

With regard to a boiler controlled in the described manner, a major problem may arise when the core, which may be divided into

three banks of bricks/heating elements, and the auxiliary flow heater are activated at the same time, since the load drawn by the whole appliance is dangerously high (i.e. in the region of 15.5kW). This problem could be overcome by only incorporating a 3kw flow heater in the appliance, but this would cause the appliance to be less efficient.

According to the present invention, there is provided a control means for controlling the heating of and discharge from a core, the core being divided into three independent banks of bricks and heating elements, and the activation and deactivation of an auxiliary flow heater, the control means including sensor means for sensing the input to each of the three banks of heater elements and to the flow heater, said sensor means de-activating the power supply to one of the banks of heater elements whenever power is demanded by all three banks of heater elements and the auxiliary flow heater at the same time. Preferably, if the flow heater comprises two 3kW elements, a bank of core heater elements is not deactivated if only one of the 3kW flow elements is engaged.

Preferably the boiler appliance never draws a load greater than 12.5kW.

Preferably, when low tariff electricity is available and heat is required by the secondary system serving the radiators and the like, the auxiliary flow heater is activated for a preset time to preheat the secondary system before stored heat is taken from the core itself. In this way, efficient running of the boiler can be effected by ensuring that a large amount of stored energy is not taken from the core during a low tariff period when the core is being re-charged. Alternatively, the control means may be arranged such that during a low tariff period the core never provides heat to the secondary system, thus ensuring that the core is always fully charged at the end of the low tariff period.

Preferably, a third program setting is provided by the control means, the third setting being a frost protection mode wherein only a small amount of energy is stored in the core. The stored energy is utilised only to ensure that the fluid in the secondary system and boiler itself are kept sufficiently warm to prevent freezing thereof. In this mode of operation, preferably only one of the three banks of heater elements is activated, and the auxiliary flow heater manual boost is not available.

Preferably the winter setting provided by the control means enables the core to be heated to a high temperature, in the region of 750ºC, and the boost facility provided by the auxiliary flow heater is engaged automatically. The spring/autumn setting provided by the control means enables the core to be heated to a second temperature, lower than , the highest temperature, and the auxiliary flow heater may be activated manually to provide a boost to the heating capability of the boiler as a whole.

When the boiler is first asked to provide heat to the secondary system from the core, there is often in boilers of the kind described a delay as the steam formed in the boiling tubes rises to the heat exchanger due to a pressure build up in the heat exchanger. This delay can be reduced if the pressure build up in the heat exchanger is prevented. Therefore, according to a further aspect of the present invention, a solenoid valve is provided between the heat exchanger and the header tank which opens for a predetermined time when the room thermostat is activated. By opening the solenoid valve for a short period of time, such as one minute, the pressure of cool air in the heat exchanger is removed to enable steam to enter the heat exchanger to provide heat to the secondary system. When such steam is in the heat exchanger, and the boiler is running efficiently, the solenoid valve should be closed to prevent steam passing from the heat exchanger to the header tank. The inclusion of this time dependent solenoid valve provides a marked improvement to the running efficiency of the boiler. Specific embodiments of the present invention are now described by way of example only with reference to the accompanying drawings, in which:

FIG. 1 is a schematic drawing showing the basic arrangement of the electric boiler, but with many of the details removed;

FIG. 2 is a schematic electrical control circuit for an electric boiler according to the invention;

FIG. 3 is a schematic exploded view of certain parts of a boiler appliance according to the present invention;

FIG. 4 is a schematic view of pipework incorporated in a boiler appliance according to the present invention; and

FIG. 5 is a perspective view of the front of a boiler appliance according to the present invention with the front panel removed to show the arrangement of the heater elements and thermostatic sensors.

As a general introduction to different control aspects of the boiler, there now follows a general description of the possible control arrangements:-

INTRODUCTION

The device is required to control two prime functions: A) core temperature and B) output heating requirements. The user operated controls, at the discretion of the purchaser, will be either an "automatic" or "manual" system, providing essentially the same requirements and the electronic control is required to accommodate both types. Via internal wiring and selection of control panel types, the individual system can be established at the time of manufacture. Additionally, it is preferred to incorporate a flow boiler and the device will be required to control its operation

2. CONTROL OF CORE TEMPERATURE

(I) AUTOMATIC SYSTEM

THE MAXIMUM NOMINAL INPUT OF 10KW IS ACHIEVED WITH 24 ELEMENTS OF EQUAL VALUE. THIS ALLOWS SUBDIVISION OF THE ELEMENT INTO BANKS OF 2 , 3, 4 , 6 , 8 AND 12 , TO FACILITATE DIFFERENT FORMS OF SWITCHING AND LIMITING OF FOWER.

THE USER WILL HAVE THE CHOICE OF TWO SETTINGS (LIKELY TO BE REFERRED TO AS WINTER AND AUTUMN/SFRING) AND THESE WILL EQUATE TO TWO PREDETERMINED AND PRESET CORE TEMFΕRATURES, THE UPPΕR TEMPERATURE BEING BETWEEN 700 AND 750 DEGREES C.

BEYOND 600 D EGREES C. , IT IS INADVISABLZ TO OPERATE THE ELEMENTS AT THEIR MAXIMUM RATING AND THE CONTROL IS REQUIRED TO FROVIDE A REDUCTION TO APPROXIMATELY HAIF THE TOTAL LOADING.

*** This can be achieved by pulsing the elements on a mark space basis, incorporation of diodes or sub-dividing the element into banks and employing a series/parallel arrangement . Should the elements be pulsed, the time base must be short, as the main requirements is to limit the element sheath temperature.

(II ) MANUAL SYSTEM

THIS DIFFERS FROM THE "AUTOMATIC" SYSTEM IN ONLY ONE RESPECT. INSIEAD OF TWO PRESET TEMPERATURES, THE USER OPERATES A ROTARY CONTROL, WHICH VARIES THE CORE TEMPERATURE IN THE RANGE BETWEEN THE TWO PRESET VALUES OF THE "AUTOMATIC" SYSTEM.

3. HEAT OUTPUT CONTROL

( I) AUTOMATIC SYSTEM WITHOUT FLOW BOILER

ENIRY OF WATER INTO THE PRIMARY SYSIΕM OF THE BOILER , IS CONTROLIED BY A SOLENOID VALVE, WHICH IS ENERGISED WHEN THE EXIERNALLY MOUNTED PROGRAMMER/ROOM THERMOSTAT INDICATE THAT HEAT IS REQUIRED. IT IS NECESSARY TO INCLUDE IN THE ELEECTRONIC CONTROL TEMPERATURE LIMITATION OF THE SECONDARY WATER BETWEEN TWO PREDETERMINED VALUES, THE CHOICE BEING MADE AT THE POINT OF INSTALLATION. WHEN THE TΕMFERATURE IS REACHED, THE SOLENOID VALVE IS DE-ENERGISED. TO OVERCOME THE IMPACT OF CONDUCTION, WHICH COULD ELEVATE THE SECONDARY SYSTEM PIPE WORK BEYOND THE PRESET VALUES WHEN WATER IS NOT FLOWING, A PRESET DELAY IS

NECESSARY ON EVERY CYCLE OF THE SECONDARY SYSTEM. THIS WILL ALLOW THE INITIAL FLOW OF WATER TO BRING THE POINT OF MEASUREMENT ON THE PIPEWORK TO A VALUE IN KEEPING WITH THE WATER TEMPERATURE.

( I I ) MANUAL SYSTEM WITHOUT FLOW BOILER

THIS DIFFERS FROM THE "AUTOMATIC" SYSTEM IN ONLY ONE RESFECT.

INSTEAD, THE USER OPERATES A ROTARY CONTROL, WHICH VARIES THE WATER TEMPERATURE.

( I ll ) AUTOMATIC SYSTEM WITH FLOW BOILER

THE CONTROL OF THE SECONDARY WATER TEMPERATURE IS AS DESCRIBED IN 3 ( I ) ABOVE. INCORPORATION OF A FLOW BOILER OF A 7KW MAXIMUM, PERMITS SUPPLEMENTARY HEAT TO BE DELIVERED TO THE SECONDARY SYSTEM, SHOULD THE HEAT FROM THE CORE BE INSUFFICIENT. TO AVOID INDISCRIMINATE USE, A

DELAY OF__________ MINUIES IS REQUIRED TO BE INCORPORATED ON EACH CYCLE OF

THE THERMOSTAT, PROGRAMMER OR WATER TEMPERATURE LIMITER. THIS GIVES THE CORE THE OPPORTUNITY TO MEET THE HEAT DEMAND. IN THE CASE OF THE "WINTER" SETTING, INTRODUCTION OF THE FLOW BOILER IS AUTOMATIC AND IN THE "AUTUMN/SPRING" MODE, THE USER CAN SELECT 'BOOST' . THE BOOST WILL REMAIN EFFECTIVE UNTIL TH E NEXT OFF-PEAK CHARGE IS INITIATED. A CANCEL OPTION IS REQUIRED, TO DELETE INADVERTENT SELECTION OR

SUBSEQUENT UNNECESSARY USE OF THE BOOST.

IN ADDITION, PROVISION IS REQUIRED TO ENABLE THE FLOW BOILER TO BE USED DURINS THE "OFF-PEAK" CHARGE PERIOD, WITHOUT THE CORE SERVING THE SECONDARY SYSTEM. THIS OPTION WILL DEPEND ON THE AFFLICATICN

CONCERNED AND WILL BE SET VIA THE INIERNAL WIRING AT THE POINT OF MANUFACTURE OR INSTALLATION.

( IV) MANUAL SYSTΕM WITH FLOW BOILER

THE CONTROL OF THE SECONDARY WATER TEMPERATURE IS AS DESCRIBED IN

3 ( II ) ABOVE. UTILISATION OF THE FLOW BOILER IS BY USER SELEC TED

OPTIONS OF 1 HOUR, 2 HOUR OR A SELECTION WHICH RETAINS THE BOOST UNTIL THE NEXT "OFF-PEAK" CYCLE IS INITIATED. THE CANCEL OPTION IS ALSO REQUIRED.

With reference to Fig. 1 of the drawings, the electric boiler comprises a core (2) surrounding four boiling tubes (4) which are in communication with an evaporation chamber (6). The boiling tubes and evaporation chamber form part of a heat exchanger which transfers heat from the core (2) to a secondary system (8). It will of course be

appreciated that the core may be of any size, and that the number of boiling tubes may vary.

The core (2) is heated by means of electric elements (10) which pass through the core between the boiler tubes (4). The secondary system (8) includes an auxiliary heater (12) which provides heat directly to the system avoiding the need to use the core (2).

The whole electric boiler, including the auxiliary heater (12), is controlled by a control unit (14). The control unit which is hereinafter described in more detail, has a major function of using off-peak and peak rate electrical energy most efficiently. Accordingly, the control unit (14) ensures that as little peak rate

electricity is consumed by the electric boiler as is possible, whilst ensuring the demands on the heating system are satisfied.

A manual switch (A) sets the maximum temperature of the core for use in either Spring/Autumn or Winter, depending on the stored energy required to heat the secondary system (8). For example, the Spring/Autumn setting allows the core to be heated to a maximum

temperature of 550ºC, whereas the Winter setting allows the core to be heated to a temperature of 750°C. The temperature of the core is monitored by a core thermostat (B) which is mounted on an electric element (10), but alternatively may be positioned elsewhere within the core (2).

The control unit (14) regulates the electrical energy to the electric elements (10) which may be either in parallel or in a series arrangement. Below 600°C, groups of electric elements (10) are continuously cycled by means of switch (C), and above 600°C switches (D) effect a series arrangement of the electric elements (10). This set-up provides efficient heating of the core (2) without overheating any particular electric element (10) and hence causing breakdown of that element.

An off-peak detector relay (designated R2) is sensitive to the change from high cost electricity to low cost and back again. The relay R2 has one normally open single pole contact R2,1 and two normally closed single pole contacts R2,2 and R2,3. Contact R2, 1 which is normally open closes when off-peak electricity is

available; this closure of R2, 1 completes part of the circuit to the auxiliary heater (12). The remainder of the circuit to the auxiliary heater (12) is controlled by a relay R1 which has a normally open contact R1,1. This contact R1,1 is closed when it is desirable to utilize the flow heater (12).

A boost facility is available whereby on-peak electricity is used to supply the auxiliary heater (12). Switches J and K activated manually by a user provide a 1 hour or 2 hour respectively supply of electricity to auxiliary heater (12). Further, an automatic boost for winter-time is provided via switch L and an overriding manual boost is activated by switch M. These latter two boost switches L and M are pre-empted by a delay switch N which only closes after a preset time delay during which it is ascertained whether the boost is actually necessary (i.e. whether the core can supply sufficient energy to satisfy the need of the secondary circuit).

A switch G is triggered when contact R2,1 is closed (and hence off-peak electricity is available), thus resulting in the cancellation of the manual boost

activated by switch M. Contact R2,2 of off-peak detector relay R2 is

normally closed to connect switches J, K, L and M to the gate relay R1 which controls the auxiliary heater (12).

Whenever a boost is selected, an L.E.D. is triggered on the control display panel. The L.E.D. remains energised until the cycle ends or "cancel" is initiated.

All the manually operated boost switches can be cancelled (i.e. opened) by means of a manual switch R.

This facility allows the user greater control over the use of on-peak electricity.

A programmer (16) includes a clock and a series of times (preset by the user) for the activation and

de-activation of the heating of the secondary system.

Accordingly, the programmer (16) sends out an activation signal when the secondary system should be hot. This activation signal is controlled by a room thermostat

(18). If the room thermostat (18) is closed, the pump (20) runs.

The secondary system water temperature is sensed by means of a thermostat (T). Hence, if the room temperature is below a predetermined value, the room thermostat is closed, and if the secondary system water temperature is also below a pre-determined value , that switch also is closed thus triggering both the auxiliary heater relay R1 and a solenoid water valve V which allows water to enter the boiling tubes (4) within the core (2).

A short circuit is provided which overrides the secondary system water temperature sensor (T) for a predetermined time to allow the temperature of the water in the secondary system to reach an accurate value, rather than a value dictated by the close proximity of the core (2).

In use, switch (A) is set for either Spring/Autumn or Winter running. This dictates the temperature at which the core thermostat (B) opens. When an off-peak supply of electricity is available, switch (C) allows electricity to reach groups of

heater elements (10) in cycles. Up to a core teπperature of 600 °C it is intended to have all elements continuously energised, but above 600°C, switches (C) and switches (D) may be activated to allow a series/ parallel arrangement and/or cycling to become operable. When the core reaches the predetermined temperature controlled by thermostat (B), the heater elements (10) are disconnected from the supply of electricity.

If it is desired to boost the temperature of the secondary system (8) quickly, or if there is a demand on the electric boiler which cannot be met by the core or if it is preferably that it is not met by the core, the

auxiliary heater (12) is activated. In this instance, the relay contact R1,1 is closed and the electricity is

preferable taken from the off-peak supply by closing

contact R2,1. However, if the off-peak supply is not available, the electricity supply must come from the

on-peak supply.

The on-peak supply may be used by either selecting the one hour (switch J), the two hour (switch K), the manual override (switch M) or the automatic winter boost (switch L). When any one of these booster switches have been activated, and hence the on-peak electricity is being used, the L.E.D.(s) on the display panel is activated.

The electric boiler is designed to store energy in the core during the off-peak time, such that the core is fully charged at the end of the off-peak period. The stored energy may then be released as dictated by the programmer (16) during the on-peak period. The core heat is supplied to the secondary system by way of the heat exchanger.

If there is a demand for heat in the secondary

system which the core cannot satisfy, it is more desirable to use the auxiliary heater (12) since this provides

direct heat to the system rather than via the core (2). An aim of the invention is for the core to provide all the energy during the on-peak period, since this heat energy is stored energy collected during the off-peak period, and for the auxiliary heater (12) to provide the required energy during the off-peak period or alternatively only after the core has exhausted its store of energy during the on-peak period. In this way, the electricity available to the heater can be used particularly efficiently.

With reference to Figures 3, 4 and 5 of the drawings, a boiler appliance includes a core (21) (made up of a plurality of refractory bricks (22)), insulating panels

(24), boiling tubes (26), core heating elements (28), a heat exchanger (30), an auxiliary flow heater (32), a heater tank

(34) and control means (36). The arrangement of the components is described in our co-filed PCT application mentioned earlier and will therefore not be described in detail here.

With specific reference to Fig.4., the return pipe (38) extends from the primary heating circuit to the header tank (34). The top of the return pipe is above the water level in the header tank at all times so that water cannot pass from the header tank down the return pipe (38).

When the secondary heating circuit is switched off, it is advantageous if there is no water or steam left in the primary heating circuit since if there were, this would consume energy from the core due to continual evaporation and condensation of the water/steam. Accordingly, in order to remove the water and steam from the primary heating circuit the motor valve (40) is closed to prevent water entering the primary heating circuit from the header tank (34) and the solenoid valve (42) between the heat exchanger and the header tank (34) is also closed. Pressure within the primary heating circuit then forces the water/steam in the primary circuit up the return pipe (38) and into the header tank (34). No water from the header tank can pass down the return pipe (38) since the upper end of the pipe is above the water level in the header tank (34).

The header tank (34) includes a sensor (44) to determine the water level in the tank (34) and hence when additional water should be added via the inlet (46). When the water level in the header tank (34) has fallen below a predetermined level, the sensor (44) is triggered and a light illuminates on the control panel of the boiler appliance.

It is considered dangerous to have a load of greater than 12.5kW being drawn by the boiler appliance at any one time, and since each of the three banks of core heater elements (28) consume approximately 3.15kW in the preferred embodiment of the invention, the auxiliary flow heater (32) must not consume more than 3kW when all three banks of heater elements (28) in the core are activated.

In a preferred embodiment of the invention the auxiliary flow heater (32) incorporates two 3kW heating elements. When this is the case, sensor means (not shown) is included in the boiler appliance to sense the input into each of the banks of core heater elements (28) and also into each of the 3kW heater elements of the auxiliary flow heater (32). Whenever the sensor means senses that all of the heater elements are to be activated, e.g. when the core is being re-charged and the secondary heating circuit is being boosted by activation of the auxiliary flow heater, the sensor means automatically cuts out the supply to one of the banks of the core heater elements (28). In this way, the overall load taken by the boiler appliance is reduced from a possible 15.5kW to 12.5kW. The arrangement would usually be that the upper of the three blocks of core heater elements (28) would be de-activated, but other options are obviously also available. For example, rather than one of the core blocks being de-activated, one of the two flow heater elements could alternatively be de-activated.

In the preferred embodiment of the invention the header tank is made of plastics material which enables it to withstand changes in internal pressure and temperature by virtue of its suitable properties. Further, the heat exchanger is more efficient if it is made from copper rather than steel, and water can be drained from the heat exchanger more easily if the exchanger is angled slightly with the drain pipe (48) extending from its lower end.

It will of course be understood that the present invention has been described above purely by way of example and that modifications of detail can be made within the scope of the invention.

Claims

1. An electric boiler comprising a core, at least one electric element for heating the core, a heat

exchanger for transferring the stored core heat to a secondary system, an auxiliary heater adapted to heat the secondary system directly, and control means for

controlling heat supply to the core and supply of heat from the core and the auxiliary heater to the secondary system.

2. A boiler according to claim 1 wherein the control means allows electricity supply to the boiler to be accepted at different tariff rates, the control means being capable of operating the boiler differently during times of different tariff rates.

3. A boiler according to claim 2 wherein, during a high tariff electrical cost period, the control means operates the boiler such that a demand from the secondary system is satisfied by the core first and, if necessary, by the auxiliary heater.

4. A boiler according to claim 2 or claim 3 wherein, during a low tariff electrical cost period, the control means operates the boiler such that a demand from the secondary system is satisfied first by the auxiliary heater.

5. A boiler according to any preceding claim wherein the auxiliary heater is activated only if the core cannot satisfy the expected heat requirement.

6. A boiler according to claim 5 wherein a

predetermined time delay must pass before the auxiliary heater can be activated.

7. A boiler according to any preceding claim wherein the control means includes a manual override to activate the auxiliary heater.

8. A boiler as claimed in any preceding claim wherein the auxiliary heater is a flow immersion heater.

9. An electric boiler comprising a core, at least electric element means for heating the core, a heat exchanger for transferring the stored heat to a secondary system and control means for controlling charging of the core with heat and transfer of heat from the core to the secondary system, the control means operating the boiler in either a first mode in which the heat supply to the core is terminated when the core temperature reaches a first temperature and a second mode in which heat supply to the core is terminated when the core temperature reaches a second temperature, lower than the first

temperature.

10. A boiler according to any preceding claim comprising sensing means for controlling the temperature of the core.

11. A boiler according to any preceding claim wherein the core is formed from a plurality of unit cores, each unit core having at least one electric element.

12. A boiler according to claim 11 wherein the electric elements are connected in series or parallel.

13. A boiler according to claim 11 or 12 wherein one or more unit core is heated.

14. A boiler according to any preceding claim wherein the maximum temperature of each unit core is variable.

15. A boiler according to claim 14 wherein the highest maximum temperature is about 750°C.

16. A boiler according to any preceding claim wherein the temperature of the secondary system is held between two predetermined values.

17. A boiler according to claim 16 including means for varying the two predetermined values.

18. A boiler according to any preceding claim wherein the heat exchanger includes a water supply, one or more boiling tubes and a condensing chamber, the water being evaporated by the heat of the core, passing up the or each boiling tube and condensing in the condensing chamber as the heat from the evaporated water passes into the secondary system.

19. A boiler according to claim 18 wherein the transfer of stored core heat is controlled by regulating the supply of water in the heat exchanger.

20. A boiler according to any preceding claim wherein the temperature of the secondary system is

measured only after a predetermined time delay.

21. A boiler according to any preceding claim wherein the core is heated in two stages by the or each heater element.

22. A boiler according to claim 21 wherein the first stage is at a fast heating rate and the second stage is at a slower heating rate.

23. A boiler according to claim 21 wherein during the first stage the temperature of the or each beater element is about 200°C above the temperature of the core and during the second stage the temperature of the or each heater element is about 80°C above the temperature of the core.

24. A boiler according to any preceding claim wherein the temperature of the core is measured by means of a sensor mounted on an electric element.

25. A boiler according to any preceding claim which is controlled by an electronic device including one or more manual switches.

26. A boiler according to claim 25 which includes a clock.

27. A boiler as claimed in any preceding claim, wherein a solenoid valve is provided between the heat exchanger and a header tank which opens for a predetermined time when a room thermostat connected to the control means is activated.

28. A control means for controlling the heating of and discharge from a core, the core being divided into three independent banks of bricks and heating elements, and the activation and deactivation of an auxiliary flow heater, the control means including sensor means for sensing the input to each of the three banks of heater elements and to the flow heater, said sensor means de-activating the power supply to one of the banks of heater elements whenever power is demanded by all three banks of heater elements and the auxiliary flow heater at the same time.

29. A control means as claimed in claim 28, wherein said sensor means prevents a load greater than 12.5kW from being drawn by the core and the auxiliary flow heater.

30. A control means as claimed in claim 28 or claim 29, wherein when low tariff electricity is available and heat is required by a secondary system serving radiators and the like, the auxiliary flow heater is activated for a preset time to preheat the secondary system before stored heat is taken from the core itself.

31. A control means as claimed in 28 or claim 29, which ensures that during a low tariff period the core never provides heat to a secondary system.