IL44014A - Space heating systems - Google Patents

Description

Ββ» B1B»R¾> fc»3pnt13 B» ¾>3© IMPROVEMENTS IN A'T5 RRLATIMC* TO SPAtJB HKATIRO SYSTEMS Introduction This invention concerns space heating systems, such as may be used for heating domestic dwellings, shops, offices, factories and the like* in which the demand for heat varies in a cyclic manner during each ≥ hour period from a period of demand to a period of quiescence (during which there is zero or very low demand) and in particular to a thermal accumulator for such a heating system (hereinafter referred to as a cyclic demand heating system) for storing heat during quiescent periods and delivering heat during demand periods.

It is an object of the invention to provide an improved form of thermal accumulator which makes the least possible demands on high cost peak period electricity when topping-up is required.

Background and statement of prior art British Patent Specification 1,2^7,525 describes a thermal storage system in which heat is stored in a pressurized liquid at a very high temperature and is transferred by a heat exchanger to a liquid at a lower temperature, in which a single heater is provided at a low level in the storage reservoir to produce a high temperature zone above the heater and a lower temperature zone below the heater* The temperature of the lower temperature zone is sensed and maintained above a minimum design value by recirculating high temperature liquid from the top of the reservoir to the low temperature zone. The high temperature zone is maintained at its design temperature by a thermostat situated in the high temperature zone and set to bring into operation the heater when the temperature falls.

This type of thermal storage system suffers from the disadvantage that the whole of the high temperature zone must be heated up in order to maintain the design temperature at the upper thermostat and this is very wasteful of electricity. Inevitably this demand for additional heating will occur during high cost periods, when the usage of electricity for "topping up'1 should be reduced to the minimum possible.

Distinguishing feature of the present invention The thermal storage system proposed by the present invention does not suffer from this disadvantage in that a second heater is provided, which operates completely separately from the main heater and which is situated at a high level in the reservoir so that when topping-up heating is required, only the upper levels of liquid in the reservoir are heated up and the temperature gradient in the main body of liquid in the reservoir remains unaffected (i.e. stratification of temperature levels is maintained). The main body of liquid is thus only heated up by the primary (low-level) heater, using cheap electricity during off-peak periods.

According to one aspect therefore of the present invention a thermal accumulator for a cyclic demand heating system comprises a thermally insulated reservoir of fluid medium} first heating means situated at or near the bottom of the reservoir, temperature controlled switch means responsive to the temperature of the fluid at or near the bottom of the reservoir for cutting off the first heating means when a first temperature is reached, fluid outlet means situated at the top of the reservoir whereby heated fluid can be drawn off to deliver heat to the space to be heated, fluid inlet means for conveying fluid to the bottom of the reservoir, baffle means intermediate the first heating means and the top of the reservoir to maintain stratification of the fluid in the reservoir as heated fluid is drawn off from the top and cooler fluid is returned to the bottom thereof, and second heating means at or near the top of the reservoir and second temperature operated switch means adjacent thereto for activating the second heating means if the temperature of the fluid at or near the top of the reservoir drops below a second temperature, lees than said first temperature.

Preferably the reservoir is a closed vessel and the fluid therein is maintained at a pressure in excess of atmospheric pressure. Conveniently this is achieved by providing a so-called header or expansion tank above the reservoir which communicates with a fluid inlet at the bottom of the reservoir and itself contains fluid, an expansion or overflow pipe extending from the top of the reservoir to a point above the header tank.

Conveniently the reservoir is a metal tank the walls of which are braced internally by struts which extend between opposite internal faces of the tank and are secured at their ends thereto.

According to a third preferred feature of the invention at least some of the struts comprise the stratifying baffles.

Conveniently the first and second heating means comprise electrical resistance heaters of the total immersion type, said temperature controlled switches serving to control the supply of electricity thereto. Typically each of the heaters comprises a plurality of separate immersion heaters all located at the same depth but spaced apart so as to provide heat over substantially the whole cross-sectional area of the reservoir.

Preferably separate electrical supplies are employed for the two heaters to obtain the advantage of so-called off-peak electricity tariffs, where such rates obtain. In this event the first heater is supplied from a clock-controlled electricity meter only during so-called off-peak periods and the second heater is supplied from an ordinary meter so as to be available for heating at any time.

Preferably an electrical or mechanical or electro-mechanical interlock is provided to prevent both heaters being supplied with electricity simultaneously.

According to a fourth preferred feature of the invention the interlock gives precedence to a demand for heat from the upper, second heating means.

Preferably a third temperature operated switch is situated at or near the top of the reservoir to cut off the supply of electricity to each of the heaters in the event that the fluid temperature in the reservoir exceeds a third temperature, greater than either of said first and second temperatures. - k - The fluid may, for example, be water or a hydrocarbon or petroleum based oil or a so-called natural oil.

According to a second aspect of the present invention in a thermal accumulator as previously described a heat exchanger may be situated within the reservoir of fluid media through which unheated water can be caused to pass to be heated and provide domestic hot water.

According to a preferred feature of this second aspect of the invention valve means may be provided for mixing a controlled quantity of heated water which has passed through the heat exchanger with cooler water to maintain the temperature of the domestic hot water at a controlled level.

According to another preferred feature of this second aspect of the present invention two or more heating coils connected in series may be located within the reservoir, spaced apart such that one of the heating coils is located in an upper strata of the reservoir in a region in which the fluid rarely drops below a certain elevated temperature and the second heat exchanger coil is situated in a lower strata where the temperature can drop typically to one half that of the temperature of the upper strata, and valve means is provided to adjust the contribution of each respective heat exchanger coil to the heating of cold water supplied to one end of the lower coil and which, after passing through both coils constitutes the domestic hot water supply.

In an alternative design of thermal accumulator having two heat exchanger coils situated as described and connected in series* all the cold water is caused to pass through the lower heat exchanger coil and a single valve means is employed for mixing heated water from the lower heat exchanger coil with the hotter water from the upper heat exchanger coil to produce domestic hot water of the desired temperature* The setting of the valve means is preferably controlled by an electrical signal from a temperature sensitive probe situated downstream from the heat exchanger.

In a further alternative design of thermal accumulator a single heat exchanger coil is mounted vertically or substantially so within the reservoir such that certain of the turns of the coil are situated in a lower temperature region of water stratification within the reservoir and other turns of the coil are situated in the upper hotter strata of fluid within the reservoir. In order to produce the correct balance of thermal take-off from the two regions of the reservoir, the turns of the coil may be arranged to be closer together at the lower end of the coil relative to the spacing between turns at the upper end of the coil. As before valve means may be provided for mixing the proportion of heated water with the cold water under mains pressure to produce any desired temperature of domestic hot water. Again as previously mentioned, the setting of the valve means is preferably controlled by a signal from a probe mounted downstream from the hot output of the heat exchanger coil.

Thus a cyclic demand space heating system may comprise a thermal accumulator incorporating one or both of the said first and second aspects of the invention, radiator means for heating the space, means for conveying heated fluid from the accumulator outlet means to the radiator means and returning fluid from the radiator means to the accumulator inlet means* pump means for causing said fluid flow, and temperature controlled valve means which mixes in varying proportions depending on the temperature thereof, the fluid returning from the radiator means with the hot fluid supplied from the top of the reservoir to the radiator means.

Preferably the valve means is infinitely variable over a range of fluid temperature between one extreme condition in which no returning fluid from the radiator means is mixed with the hot outgoing fluid and the opposite extreme condition in which no fluid from the top of the reservoir is supplied to the radiator and fluid simply circulates through the radiator means and the pump means. Preferably further temperature operated switch means is provided, responsive to the temperature to which the space is heated, which cuts off the supply of power to the pump means when the space temperature exceeds a selected value. Alternatively the further temperature operated switch means may control the condition of said temperature controlled valve means to reduce the rate of flow of hot fluid from the accumulator to the radiator means when the selected temperature is reached in said space.

Preferably the temperature range between the two extreme conditions of the temperature controlled valve is adjustable. Adjustment may be made manually or in response to operation of the space temperature sensitive switch means or in response to operation of a still further temperature sensitive switch means located externally of the heated space and set to be responsive to external changes of temperature.

By way of example, the invention is illustrated in the accompanying drawings, in which;- Fig. 1 is a diagram of a cyclic demand space heating system incorporating a thermal accumulator embodying the first aspect of the invention, Fig. 2 is a front elevation of the thermal accumulator employed in the system of Fig. 1, with the insulating casing partly cut away, Figs. 3(a) and 3(b) illustrate an alternative fluid inlet for returning the fluid from the radiators to the accumulator of Fig.2, Fig. k is a side view thereof, Fig* 5 is a top plan view thereof, Fig.6 is a circuit diagram showing the electrical connections for the heating elements employed in the accumulator of Figs.2-5, Fig.7 is a graph illustrating temperatures obtained at different levels in the thermal accumulator shown in Figs. 2-5 during a typical 2k hour period, Fig. 8 is a side elevation of an alternative design of a thermal accumulator incorporating in accordance with the second aspect of the invention, two domestic hot water heat exchangers and partly cut away to reveal the interior of the reservoir, Fig. is a similar view of a thermal accumulator also having two domestic hot water heat exchangers but only a single mixing valve and Fig. 10 is a similar view of a thermal accumulator having a single domestic hot water heat exchanger coil and a single mixing valve. For clarity the baffles illustrated in the accumulator of Figs. 1-7 are not shown in Figs. 8-10.

Fig. 1 of the drawings illustrates diagraramatically a complete space heating system incorporating a thermal accumulator 10 in accordance with the invention. The accumulator comprises a reservoir of water fed from an expansion tank 12 via a feed pipe 1t and having an overflow-expansion pipe 16 terminating in an outlet just above the tank 12. Cold water is supplied to the tank 12 in known manner (not shown) via a float controlled tap so that a constant level of water is maintained in tank 12. An overflow outlet (not shown) is also provided in tank 12 above the level of the normal water in the tank.

A set of four resistance heaters of the immersion type 18 are mounted near the bottom of the accumulator 10 and supplied with electricity via a clock-controlled electricity flow meter 20, set to supply electricity thereto during the so-called off-peak periods of the normal 24-hour cycle - typically between 23.00 hours and 07.00 hours. The electricity supply to the heaters 18 is cut off by a temperature controlled switch (not shown) mounted at the same level as the heaters 18, and set to operate when a first design temperature is exceeded. A hot water take-off pipe 22 conveys hot water from the top of the reservoir to one input of a temperature controlled flow-mixing valve 2k. The output of the valve 2k is connected via feed pipe 26 to a water circulating pump 28 and thence to the radiators 30 forming the heat dissipating part of the heating system, only two of which are shown in the drawing.

The water is returned to the bottom of the reservoir via return pipe 32.

Valve 2k has a second input which is connected to a T-junction in the return pipe 32 via & pipe 3 .

The action of valve 2k is to close the inlet supplied by pipe 22 when the water flowing through the valve exceeds a selected temperature, so that the water is simply circulated through the closed circuit formed by pipe 26, pump 28, radiators 30, return pipe 2, pipe 3k and valve 2k. As the water cools, the valve closure member (shown diagrammatically at 36) moves and begins to open the inlet from pipe 22. The operation of valve 2k is thus to admit just sufficient 'hot* water from pipe 22 so as to maintain the temperature of the water flowing through the valve at the selected temperature. One temperature controlled valve which has been found to be suitable is that manufactured by TOUR AGENTOHEB AB of Stockholm, Sweden. This is a rotating disc 3-wa valve (Type s VTB), in which the temperature is selected manually.

The return flow of water to the reservoir is, in accordance with the invention, controlled so as not to cause turbulence or undue movement of the water in the reservoir. If the circulation occurs while electricity is available for immersion heaters 18, the v latter will heat the returning water to the selected design temperature as it is slowly displaced upwards through the reservoir. If the circulation occurs after the end of an 'off-peak' period, the 'cool' water from pipe 32 accumulates in the bottom of the reservoir and the cold - hot interface (not shown) begins to rise slowly up the reservoir, as more and more hot water is drawn off from the top, to maintain the temperature of the water through the valve 2*f.

A second set of three resistance heaters 38 also of the immersion type are mounted near the top of the reservoir. Electricity therefor can be obtained at any time from a conventional electricity flow-meter *tO connected to the electricity supply main k≥. The supply of electricity from meter 0 to heaters 38 is controlled by a second temperature operated switch (not shown) located at the same level as the heaters 38. This second switch prevents current flow to the heaters 38 unless the temperature of the water at the level of these heaters drops below a second design temperature* The action of the second heaters 3 is therefore to maintain the temperature of the outgoing water always above the second design temperature. Typically the latter is equal to or Just greater than that set by the temperature selector of mixing valve Zk. They will do so even during a non-off-peak period - and in so doing will of course use 'expensive' electricity. However, in a correctly designed system in which the first design temperature is considerably in excess of that set by the mixing valve, the second heaters 38 should only be required for an hour or so during the non-off-peak period on the coldest days during the year, when the external temperature is at or near freezing.

The accumulator 10 of Fig. 1 is shown in more detail in Figs. 2, and 5· The water reservoir is a rectangular steel tank k , the front panel ½ of which carries the two sets cf immersion heaters 38 and 18. Two thermostats 8 are mounted in between the heaters 38 and connected in parallel to constitute the temperature controlled switch for the upper set of heaters 38. A single thermostat 50 is mounted centrally of the lower heaters 18 and constitutes the temperature controlled switch therefor.

Although not shown, one or more additional thermostats may be mounted at the same level as thermostat 50 and connected in parallel therewith.

The elements 52, 5^ of heaters 18 and 38 respectively can be seen in Fig. . Above and in vertical alignment with the elements 52 are located parallel baffles 56.

A further baffle 58 is located between the heaters 18 and baffles 56» parallel to the elements 52 and aligned vertically above the thermostat element (not shown).

Two further baffles 60, also parallel to the elements 52 are located above the baffles 56 and are vertically aligned with the centres of the spaces between the outer pairs of heater elements 52.

Each baffle is of inverted-V cross section and is welded or otherwise secured to the internal faces of the front and back panels of the tank.

A transverse baffle 62, also of inverted-V cross section is located between the two sets of baffles 56 and 60 and is welded at opposite ends to the mid-points of the inside faces of the two side panels of the tank, one of which, Skt is shown in Fig. .

The transverse baffle 62 forms a tie-strut which gives lateral strength to the tank, as do the baffles 56, 58 and 60 between the front and back panels of the tank. Additional rigidity is given to the side panels by bending to provide intersecting creases, 66 (see Fig. 4).

The heads of the immersion heaters 18, 38 are located within two control boxes 68 and 70 respectively as also are the heads of the thermostats 50 and 8 respectively. The control boxes also serve to house the wiring to the heaters and thermostats in addition to relays and contactors (not shown).

The top panel 72 of the tank (see Fig. 5) is also strengthened by bending, to produce a central transverse crease 7 and end creases 76.

Expansion pipe 16 and hot water flow pipe 22 extend through the top panel 72 as shown in Fig. 5· In the accumulator shown in Fig. 1 the feed and return pipes 1 and J respectively are shown entering the bottom of the tank. However in a more preferred arrangement (which simplifies the thermal insulation of the tank) these two pipes also extend through the top panel 72 and terminate just above the floor of the tank as shown in Figs. 2 and k.

Where the alternative pipe feeds shown in Fig. 1 are employed, the pipe entrances must be shrouded to prevent turbulence. In Figs. ¾ and 5b this shrouding is shown for pipe 32 in Fig. 1. Immediately above the pipe entrance a bracket 7& is welded onto the tank wall. The bracket is in the form of an inverted, shallow 'V .

The same arrangement can be employed for pipe 1*, on the opposite side wall of the tank.

The heaters end thermostats of the accumulator 10 are connected to the electricity supply main as shown in the circuit diagram of Fig. 6.

Dealing first with the lower set of heaters 18, these are supplied from the live bus bar 2 via a clock-controlled meter 20 which meters the flow of electricity and includes a time-controlled switch 80 which is only closed during the so-called off-peak periods. The meter output is supplied to a relay winding A and via normally open contacts 82 of a contactor (not shown) to the elements of the four heaters 18, connected in parallel. Fusee 84 protect each element individually. The circuit is completed via connections to the neutral bus bar 86.

The contactor winding 88 is supplied with current from the ordinary supply meter kO via normally closed switch contacts 90, 92 and 9^ and normally open relay contacts 96 on relay A. Thus assuming 90, 92 and 9 are closed, contactor winding 88 is energised when relay A operates, i.e. during an off-peak period.

The contacts 90 comprise the normally closed contact pair of a thermostat (not shown) located at the top of the tank and set to open the contacts 90 when the temperature exceeds a temperature T3 (greater than the first design temperature, hereinafter referred to - 1* - as T1). Thermostat 90 is thus a safeguard against overheating, since it not only causes contactor winding 88 to de-energise (so isolating heaters 18) but also causes a second contactor (hereinafter to be described) feeding the upper heaters 38, also to drop out.

The contacts ** comprise the normally closed contact pair of the thermostat 50 associated with heaters 18. These are set to open when the first design temperature, T1, is reached, also causing contactor winding 88 to de-energise so removing the supply from heaters 18.

The upper heaters 38 are supplied from ordinary meter * via normally open contacts 8 of a second contactor (not shown) and are individually fused by fuses 100. The winding 102 of the second contactor and the winding of a second relay B are connected in parallel and supplied from ordinary meter *K) via thermostat contacts 90 (previously described) and normally closed contacts 10**, the circuits being completed by connections to the neutral bus bar 86.

The contacts 10*t comprise the normally closed contact pairs of the thermostats ½8, connected in parallel. These are set to open when the water temperature around the upper heaters/thermostats exceeds the second design temperature (hereinafter referred to as T2) .

The normally closed contacts 2 are a contact pair on relay B. Thus when relay winding B is energised, contacts 92 open, and cause the contactor winding 88 to be de-energised and the supply to be removed from heaters 18.

Thus it is impossible for both sets of heaters to be operating simultaneously. The nature of the interlock so provided, is to give preference to the upper set of heaters, since even during an off-peak period, if the temperature at the upper thermostats 48 drops below T2, contacts 10 will close and cause the winding of relay B to become energised, thereby cutting off the supply to heaters 18.

The graph of Fig. 7 illustrates how by employing the baffles hereinbefore described, and ensuring lack of turbulence at the return pipe junction with the tank, stratification within the water in the tank can be established so preserving a level of very hot water in the tank for as long as possible, having regard to the heat demands on the system during the non-off-peak period. The six curves were obtained by plotting the temperature at six di ferent levels in a 300 gallon tank during a 2½-hour period, from 07.00 hours to the same time the next morning. This starting/finishing point was chosen as it represented the end of the off-peak period operated by the Electricity Supply Board, and at that point the pump was arranged to come into operation to start delivering hot water to the radiators. The curves are numbered I to VI and the following table shows the depths in the tank to which tb.2y relate: Curve Depth (from top of tank) I , II 12» III 2*t» IV 36» V 48» VI 60" Aieo shown on the graph is the point (marked X) at which the temperature at the top of the tank (sensed by thermostats 48 having normally closed contacts 10 ) first drops below T2. This occurred approximately one hour before the beginning of the next off-peak period (marked Y), so that during that time the tipper heaters 38 will have been operated due to the energising of contactor winding 102.

In order to preserve the heat stored in the tank, the latter is thermally insulated by cladding. Typically the cladding pieces 106-114 (see figs. 2-5) are of foamed plastics insulating material. The tank is mounted on a base 116 of load bearing thermally insulating board.

It will be appreciated that the head of water produced by virtue of the expansion tank being raised relative to the accumulator tank raises the pressure of the water in the accumulator above atmospheric pressure. This in turn allows the design temperature T1 to be above the normal boiling point for water, thereby increasing the amount of heat which can be stored in the accumulator for a given volume of water.

Although described for water, it will be appreciated that other liquids, preferably having a higher boiling point may be used, so further increasing the quantity of heat which can be stored for a given size of accumulator - or allowing the latter to be reduced in size for a given heat output requirement. Thus fluids marketed under the trade names Eaergol HK65* Theresso -3, Therrainol FR1 and Voluta 27 may be employed. Such fluids can be heated to temperatures in the range 300° - 350° Centigrade, and if employed, a given heat output requirement can be met by a thermal accumulator some ¥ > of the size of the accumulator employing water. Due to the elevated temperatures greater thermal insulation will be required for the thermal accumulator.

A preferred material for the thermal insulating cladding is ROCKSIL LR h as manufactured by Cape Insulation Ltd. of Stirling, Scotland. This material is formed from fibres of silica formed by extruding, and woven to form a sheetblanket. If slabs of ROCKSIL 25 mo thick are employed, the heat loss from an accumulator at 230°F is approximately 3 68 B.Th.U/Hour (overall).

Referring to Fig. 8, a thermal accumulator generally designated 11 includes lower heating elements 116 and upper heating elements 118 together with associated thermostats. Approximately one third up from the bottom of the reservoir is situated a lower heat exchanger coil 120 having an inlet 122 and outlet 1 which are connected to the input and mixing ports of a mixing valve generally designated 126. Cold water via a tap 128 is supplied at mains pressure to the inlet side of the mixing valve and heated water is supplied from the output of the valve 126 along line 130 to an upper horizontal heat exchanger coil generally designated 132 and situated near the top of the reservoir. The input of the upper heat exchanger coil 132 is connected to the line 1 and also to the input of a second mixing valve 13^ and the output connected to the mixing port of the same mixing valve. The output from mixing valve 13^ supplies hot water at a temperature dependent on the setting of the two mixing valves 126 and 13 to the domestic hot water system.

In Fig. an alternative arrangement is shown in which all of the cold water at mains pressure from tap 128 is caused to flow through the lower heat exchanger coil 120 before proceeding via line 13 to the upper heat exchanger coil 132 and a single mixing valve 136 is employed for controlling the temperature of the water along domestic hot water supply output line 138. Mixing valve 136 is connected with its input connected to the input to upper heat exchanger coil 132 and lime 130 and with its mixing port to the output of heat exchanger coil 132. A probe 1*) sensing the temperature of the water available at the output of mixing valve 136 provides an electrical signal for controlling the setting of the mixing valve 136 to maintain the temperature in the output no greater than a designed temperature.

A further arrangement is shown in Fig. 10 which utilises a single vertical heat exchanger coil generally designated 1^2 in place of the two heat exchanger coils of the two previous embodiments. In order to balance the heat drawn o f from the cooler lower regions of the reservoir and the upper hotter regions of the reservoir, the spacing between the turns in the vertical heat exchanger coil varies from top to bottom. Since the fluid in the lower regions of the reservoir will generally be ccoler than that at the top, the spacing between the turns at the lower end of the heat exchanger con is smaller than the spacing between the turns at the top of the coil.

Cold water from tap 128 at mains pressure is supplied to the input to a mixing valve l¥f and also to the input to the vertical heat exchanger coil 1½2 and the output from the coil is supplied to the mixing port of the valve The output port of the valve serves to supply domestic hot water along line 1j58 and a probe 10 situated downstream from the valve serves to generate an electrical signal for controlling the setting of the valve ikh to maintain the temperature of the hot water no greater than a design temperature. 44014

Claims (3)

1. CLAIMS 1. A thermal accumulator for a cyclic demand heating system comprising a thermally insulated reservoir of fluid medium, first heating means situated at or near the bottom of the reservoir, temperature controlled switch means responsive to the temperature of the fluid at or near the bottom of the reservoir for cutting off the first heating means when a first temperature is reached, fluid outlet means situated at the top of the reservoir whereby heated fluid can be drawn off to deliver heat to the space to be heated, fluid inlet means for conveying fluid to the bottom of the reservoir, baffle means intermediate the first heating means and the top of the reservoir to maintain stratification of the fluid in the reservoir as heated fluid is drawn off from the top and cooler fluid is returned to the bottom thereof, and second heating means at or near the top of the reservoir and second temperature operated switch means adjacent thereto for activating the second heating means if the temperature of the fluid at or near the top of the reservoir drops below a second temperature, less than said first temperature.

2. A thermal accumulator as claimed in any of Claims 1 to 3 ia which the reservoir is a closed vessel and the fluid therein is maintained at a pressure in excess of atmospheric pressure.

3. A thermal accumulator as claimed in either of claims 1 or 2 in which the reservoir is a metal tank the walls of which are braced internally by struts which extend between opposite internal faces of the tank and are secured at their ends thereto. . A thermal accumulator as claimed in claim 3, in which at 2 least some of the struts comprise the stratifying baffles. 5. A thermal accumulator as claimed in any of the preceding claims in which the first and second heating means comprise electrical resistance heaters of the total immersion type, said temperature controlled switches serving to control the supply of electricity thereto. 6. A thermal accumulator as claimed in claim 5 in which each of the heaters comprises a plurality of separate immersion heaters all located at the same depth but spaced apart so as to provide heat over substantially the whole cross-sectional area of the reservoir. 7· A thermal accumulator as claimed in any of claims 1 to 6 in which separate electrical supplies are employed for the two heaters. 8. A thermal accumulator as claimed in claim 7 in which the first heater is supplied from a clock-controlled electricity meter only during so-called off-peak periods and the second heater is supplied from an ordinary meter so as to be available for heating at any time. 9. A thermal accumulator as claimed in any of claims 1 to 8 in which an electrical or mechanical or electro-mechanical interlock is provided to prevent both heaters being supplied with electricity simultaneously. 10. A thermal accumulator as claimed in claim 9 in which the interlock gives precedence to a demand for heat from the upper, second heating means. 11. A thermal accumulator as claimed in any of claims 1 to 10 in which a temperature operated switch is situated at or near the top of the reservoir to cut off the supply of electricity to each of the heaters in the event that the fluid temperature in the reservoir exceeds a third temperature, greater than either of said first and second temperatures. 12. A thermal accumulator as claimed in any of the preceding claims in which the fluid is water. 13* A thermal accumulator as claimed in any of the preceding claims in which the fluid is a liquid having a boiling point higher than that of water, at the same temperature and pressure. 1½. A thermal accumulator as claimed in any of claims 1 to 13 further comprising a heat exchanger situated within the reservoir of fluid media through which unheated water can be caused to pass to be treated and provide domestic hot water. 15. A thermal accumulator as claimed in claim "\k further comprising valve means adapted to mix a controlled quantity of heated water which has passed through the heat exchanger with cooler water to maintain the temperature of the domestic hot water at a controlled level. 16. A thermal accumulator as claimed in any of claims 1 to 13 further comprising at least two heating coils connected in series and located within the reservoir, spaced apart so that one is located in an upper strata of the reservoir in a region in which the fluid rarely drops below a certain elevated temperature and the second is situated in a lower strata where the temperature can drop typically to one half that of the temperature of the upper strata and valve means is provided to adjust the contribution of each respect heat exchanger coil to the heating of cold water supplied to one end of the lower coil and which after passing through both coils constitutes the domestic hot water supply. 17. A thermal accumulator as claimed in claim 16 in which all the cold water is caused to pass through the lower coil and a single valve means is employed for mixing a controlled quantity of heated water from the lower coil with the hotter water from the upper coil to control the temperature of the domestic hot water. 18. A thermal accumulator as claimed in claim 1 or 15 ia which the heat exchanger is a single coil device mounted vertically or substantially so within the reservoir such that some of the turns at the lower end thereof are situated in a lower temperature region and other turns at the upper end thereof are situated in a higher temperature region of the fluid in the reservoir and the spacing between the turns of the coil at the lower end thereof is smaller than that between the turns at the upper end thereof. 19. A thermal accumulator as claimed in any of claims to 18 further comprising a temperature sensitive switch operated by the temperature of the water downstream from the heat exchanger. 20. A cyclic demand space heating system comprising a thermal accumulator as claimed in any of the preceding claims, radiator means for heating the space, means for conveying heated fluid from the accumulator outlet means to the radiator means and returning fluid from the radiator means to the accumulator inlet means, pump means for causing said fluid flow, and temperature controlled valve means which mixes in varying proportions depending on the temperature 4401 /^ thereof, the fluid returning from the radiator means with the hot fluid supplied from the top of the reservoir to the radiator means. 21. A cyclic demand space heating system as claimed in claim 20 in which the valve means is infinitely variable over a range of fluid temperatures between one extreme condition in which no returning fluid from the radiator means is mixed with the hot outgoing fluid and the opposite extreme condition in which no fluid from the top of the reservoir is supplied to the radiator and fluid simply circulates through the radiator means and the pump means. 22. A cyclic demand space heating system as claimed in claim 21 in which further temperature operated switch means is provided, responsive to the temperature to which the space is heated, which cuts off the supply of power to the pump means when the space temperature exceeds a selected value. 23· A cyclic demand space heating system as claimed in claim 21 having further temperature operated switch means which control the condition of said temperature controlled valve means to reduce the rate of flow of hot fluid from the accumulator to the radiator means when the selected temperature is readied in said space. 2k. A thermal accumulator for a cyclic demand space heating system constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in Figs. 2 to 6 of the accompanying drawings. 25. A thermal accumulator for a cyclic demand space heating system constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in Figs. 8 to 10 of the accompanying drawings. 26. A cyclic demand space heating system incorporating a thermal accumulator as claimed in claim 24 or 25, constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in Figs. 1 to 7 of the accompanying drawings.