GB2220087A - Control of a heating system having an electric boiler - Google Patents

Abstract

An Electric Boiler for a heating system for a building has a plurality of electric heating elements for heating water. An electrically driven pump pumps water through the boiler and a control means responsive to the water temperature at the input end of the boiler is provided to connect and disconnect the power supply to individual heating elements in a predetermined sequence and at a predetermined temperature.

Description

ELECTRIC BOILER This invention relates to a system for electrically heating buildings. It is applicable to, but not exclusively to, relatively small buildings such as private dwellings.

Conventional central heating systems using water circulation generally comprise a series of radiators distributed around the building to be heated fed with hot water from a pair of pipes, one for supply of hot water and the other for recirculation of water discharged from the radiators, the pipes running around the building. Alternatively a single pipe may be used for feeding and discharging from the radiators. A pump is normally provided to propel water around the system and the water is heated by means of a single boiler which may be electrically powered or burn solid fuel, gas or oil. The boiler is normally located at a point in or outside the building, such as in the basement, where it will not be obtrusive.

A system of this type has the disadvantage that when it is switched on the heat provided, originating from a single point, is not conducted immediately to all parts of the building but is initially delivered only to the radiators adjacent to the boiler, which are first supplied with hot water from the supply pipe. The radiators in the building may be provided with their own thermostats so that the feed of hot water to them is stopped when the temperature of the room heated by that radiator reaches a desired value, but even then it can take a very long time for heat to be delivered to rooms of the building which are remote from the boiler. It is then necessary to turn the system on a long time before the heat is actually required, increasing the cost of heating the building considerably.

When a large number of radiators are fed with water from a single boiler it is also necessary to provide relatively large-bore pipes to conduct the necessary volume of hot water from the boiler to all the radiators. Difficulties arise if the rate of transfer of heat is increased by increasing the temperature of the water or by increasing the flow velocity of the water through the pipe beyond a certain value. The use of large-bore pipes increases the volume of water circulating through the system and so increases its thermal inertia.

Boilers for electrically heating shower water are known.

See for example GB2102164B which describes a boiler for a shower which has temperature control according to the output temperature of the system i.e. the temperature of the shower. This is necessary for safety reasons to prevent the user suffering from variation in water temperature but also because such electrically heated shower units are supplied with cold water. Thus such boilers are not suitable for the demands of a heating system.

According to the present invention there is provided an electric boiler for a heating system for a building having a plurality of electric heating elements for heating water, an electrically driven pump for pumping water through the boiler, and control means responsive to the water temperature arranged to connect and disconnect the power supply to the individual heating elements in a predetermined sequence and at predetermined temperature.

The predetermined temperature intervals are proportional to the rate of change of inlet temperature of the electric boiler in order to give a controlled response to the outlet temperature. The present invention attempts to provide a constant output temperature, preferably about 800C, although this output temperature may vary according to requirements of the system. Each of the heating elements is activated by the inlet temperature sensor. Each of the heating elements may have its own thermostatic control such that the temperature of the water in the vicinity of that heating element is detected and causing the heating element to be switched off. These may be switched off progressively, in the range of for example, 700C to 85 0C, such that all of the heating elements are not switched out together.Temperature control in this manner allows an even and progressively temperature control rather than long periods of hot and cold with rapid changes in electricity demand.

Control of the electric boiler according to the present invention, allows the output temperature of the boiler to be matched to the demand of the heating system in which it is installed, allowing progressive and slow reaction to the temperature of the return water reaching the boiler i.e.

the inlet temperature of the water to the boiler and at the same time allowing water to leave the outlet of the boiler, i.e. the supply of the water to the heating system, at a constant demand temperature generally around 80 C as previously indicated.

The power output of the individual heating elements may be 2 KW or less. The heating elements may be used in multiples and although a 2KW could be used for a small heating system, it is preferred to use heaters in groups, optionally three, six, nine or twelve heating elements. The numbers of heating elements need not be so limited if a larger system is desired and may include more than twelve elements.

As relatively small amounts of water may be used in the boiler there is no need to use large diameter pipes for conducting water to and from the boiler. The pipe may be of standard 15, 22 or 28 mm diameter and the volume of the water in the radiators in each area of the building need only be a few litres and radiators are available of this design. The overall thermal inertia of the system may then be very low and the system is capable of very rapid response when turned on or off. This arrangement reduces the apparent bulk of the system to a minimum and makes it as unobtrusive as possible. The boiler may be located inside a generally rectangular enclosure of which the appearance may resemble a wall-hung kitchen unit.

The installation cost of the system will generally be very low as only narrow-borepipe is required for supplying water to the radiators.

The boiler may be a low water content unit containing only sufficient amounts of water to cover the electrical heating elements and may therefore be very small. High density element construction of low-watt density element material and a low volume high water pressure pump ensures quick hot water displacement resulting in a rapid response to a request for heating without over heating water leaving the electric boiler, i.e. a temperature of about 800Cpreferably being maintained.

A mechanical, electro-mechanical or electronic device or any combination thereof controls the pump and heating elements. In general the controller responds to the water temperature of the inlet to the boiler i.e. to the return water temperature to the boiler and connects or disconnects heating elements with a predetermined sequence and delayed time interval to present a relatively small electrical load increase or decrease to the electric mains power supply as the elements are turned on and off. A high temperature automatic electrical cut-out is used to safeguard against electrical malfunction of the control unit. The electrical circuitry in the boiler is protected by circuit breakers and/or fuses. Each heating element preferably has its own thermostatic control.

A pump overrun on switch-off may be included to dissipate any residual heat from the boiler. An electrically.

operated timing device such as-a clock may be incorporated to make the operation of the system automatic. Indicators in the form of bar-graph displays, neon indicators and other indicating devices enhance the front panel and/or control panel, to assist in the operation of the system.

Controls may be added to control the average boiler temperature in conjunction with external control devices such as remote thermostats. Integration of the control system may be provided to integrate the controls with controls of other types of boilers, such as solid fuel fires with back boilers, gas boilers, oil boilers and/or off-peak electric storage boilers when the system is used in conjunction with other types of boiler.

The boiler may be fed by single-phase alternating current or it may use a three-phase supply, different elements being powered by different phases.

Heating systems according to embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 shows schematically the control system for a boiler according to the present invention.

The heating system comprises a boiler of low water capacity, of 3 litres or less, having a plurality of electric heating elements each of not more than 2 KW power for heating the water, piping of external diameter not exceeding 28 mm for conducting water to and from the boiler, one or more radiators for receiving the hot water and an electrically driven pump for pumping the water through the boiler and around the piping circuit. The heating elements and pump are controlled by a control unit as shown in either Figure 1.

The control system shown in Figure 1 is applied to a heating system as described above with the control device operating electronically together with relays and thermostates. Power is fed from mains line 1 through thermal cut-out 2 to a number of contact devices 13 for feeding power to the respective heating elements, via a thermal switch 12 for each heating element the thermal switch being set in the range 7cPC to 85 C to provide general temperature output of about 800C. However in this instance the analogue signals received by the temperature sensor-or sensors associated with the boiler or pipework are amplified by unit 14, converted to digital form by analogue/digital converter 15 and compared with preset values by means of a comparator.

These preset values may be varied by boiler temperature control unit 16. Depending on the magnitudes of the signals received from the sensors, contact devices 13 are operated to turn the requisite number of elements on and off. A triac or thyristor may be included if desired. The triac or thyristor 13a may be installed before or after the heating element although in Figure 1 it is shown in a position before the heating element. The triac is connected to the analogue/digital 15 converter and Units 14 and 15 are arranged so that the elements are again turned on and off in sequence at predetermined time intervals. In this embodiment the pump is operated by contact device 17 with a delay device 18 to allow over-run when the elements are switched off. The temperature sensor 19 detects the temperature of the inlet water to the boiler [return water from the heating system] which is compared with the control unit 16 on the boiler as described to provide an output temperature according to the demand.

The front of the boiler is of a size allowing convenient installation as a wall unit and contains the heating elements, pump and control system. The clock, push-button controls and indicating devices are accessible from the front of the boiler unit.

Claims (7)

1. An electric boiler for a heating system for a building having a plurality of electric heating elements for heating water, an electrically driven pump for pumping water through the boiler, and control means responsive to the water inlet temperature arranged to connect and disconnect the power supply to the individual heating elements in a predetermined sequence and at a predetermined temperature.

2. A boiler according to claim 1, in which each heating element has a power of 2 KW or less.

3. A boiler according to claim 1 or 2, in which the heating elements are sequentially operated by electronic and electromechanical devices.

4. A boiler according any one of the preceding claims wherein the rate of temperature changed is sensed by the inlet temperature sensor.

5. An electric boiler, substantially as hereinbefore described with reference to the accompanying drawing.

6. A heating system for a building, comprising a boiler according to any preceding claim.

7. A heating system according to claim 7, integrated with a further heating system which is solid fuel, gas or oil fired or which is adapted to operate using off-peak electricity.