GB2254407A - Domestic hot water heating apparatus - Google Patents

Abstract

A thermal storage water heating installation comprises a storage tank 110. Heat for a domestic heating system is provided by circulatory water from the store to radiators by means of a circulating pump 144. Heat is supplied to the storage tank from a boiler 12 by circulating water from the tank through the boiler by means of a boiler pump 118. Water at mains pressure and for domestic consumption is heated by the heat stored in the storage tank by heat exchange means external of the storage tank 10; run at a lower temperature than the tank; and in the form of parallel heat exchange units 124, 126 of which one is arranged to have storage tank water pumped therethrough by pump 144 and the other has hot water pumped therethrough by the boiler pump 118, when there is a demand for hot water for domestic consumption. <IMAGE>

Description

Improvements Relating to Water Heating Apparatus This invention relates to water heating apparatus, and in particular concerns a water heating apparatus and a method of heating water wherein a relatively large body of water is held in a thermal storage means1 and the heat in that water is utilised, by heat exchange, in order to heat mains pressure secondary water by heat exchange in a vessel or other means which holds only a relatively small quantity of the primary thermal storage water or water heated thereby.

In one known example of such an arrangement, the primary thermal storage water is heated for example by being circulated through a heating means such as a boiler.

Alternatively, the primary thermal storage water may be heated by means of an electric immersion element. Whatever the heating means, the primary thermal storage water is used, in a domestic situation, as the space heating means and is circulated to for example central heating radiators.

In presently adopted thermal storage systems, it is usual for the mains pressure secondary water to be passed through a finned heat exchanger coil or several coils contained in the primary thermal storage tank and surrounded by the primary hot water. Also in such systems, it is usual to maintain the primary hot water at a temperature in the region of say 80 to 850 in order to enable the thermal storage tank which may be of a size in the order of 120 litres to 200 litres, to meet normal domestic requirements.

However, the running of the thermal storage tank in the indicated temperature range leads to the deposit in the bore of the finned heat exchange coil or coils of scale deposits which at best cause inefficiency of the heat exchange, and at worst can lead to failure of the system. The scaled-up coil or coils has or have to be periodically de-scaled, and when the coil or coils are contained inside the primary thermal storage tank, then this remedial work causes great inconvenience and considerable expense to the householder.

There are advantages, in accordance with this invention, in utilising what might be termed an external heat exchanger in that it is external to the thermal storage tank and as indicated above, heat exchange takes place in the external heat exchanger by pumping the hot primary water or water heated thereby to the external heat exchanger for heat exchange with mains pressure secondary water which passes through a heat exchange element in the external heat exchanger, in that it is easier to de-scale the heat exchanger element in the external heat exchanger than it is to remove a heat exchange coil from the thermal storage tank.

The present invention adopts such a system, but provides in a preferred arrangement yet further advantage in its construction and operation.

This advantage arises in that in the invention, the external heat exchanger is designed to run at a temperature which is lower than that in the thermal storage tank so that the extent of scaling up will be much reduced and perhaps in some case eliminated.

In one embodiment, the hot water from the thermal storage tank is circulated through the external heat exchanger and is mixed with incoming colder water in order to control the temperature of the primary water passing through the external heat exchanger. The colder water may be the primary water from the thermal storage tank after it has been passed through the heat exchange element and supply of the colder returning water may be under automatic control dependent upon the temperature of the hot primary water from the thermal store and the temperature of the returning colder water. In any given system, the heat exchange element may be of the same surface area as would normally be used for the same thermal storage tank when the heat exchange element is located inside the thermal storage tank.Although the heat exchange vessel will be considerably smaller in volume than the thermal storage tank, and as the water passing over the heat exchange element in the external heat exchanger will normally be at a lower temperature than that of the water in the primary storage tank, any potential loss of heat exchange is compensated for in that the heating water is pumped through the external heat exchanger and therefore there is faster heat exchange than would occur in the case where the heat exchange coil is located inside the thermal storage tank.

The thermal storage tank may have an outlet to which a circulating pump is connected, said pump serving to circulate hot water from the thermal storage tank around, for example, a space heating circuit on the one hand, or in the alternative through a diverter valve and then the external heat exchanger, depending upon the condition of a flow switch operated when mains water passes through the heat exchange element in the external heat exchanger.

Thus, if there is a demand for secondary hot water, for example by the turning on of a tap, the flow of cold water through the flow switch will actuate the diverter valve on the downstream side of the circulating pump, to cause the hot water from the thermal storage tank to be circulated through the external heat exchanger providing heat for the mains water passing through the heat exchanger element.The hot water flowing to the external heat exchanger from the thermal storage tank will be mixed automatically with colder water returning from the heat exchanger to the extent required to control the temperature of the hot primary water flowing through the external heat exchanger in accordance with a preset value, preferably to avoid the said scaling By virtue of running the external heat exchanger at a lower temperature than the thermal store, the extent of scaling of the heating element in the external heat exchange is much reduced or terminated. Even if a small amount of scale does build up in the external heat exchanger, it is a much simpler matter to dismantle the heat exchanger and to remove the scale, compared with removing heat exchange coils from the thermal storage tank.Also, the small heat exchanger can be returned to the factory for reconditioning, servicing and repair, whereas when the heat exchange coils are in the thermal storage tank, it is often or usually necessary to perform the reconditioning on site.

Additionally, because the heat exchange coils are external of the thermal storage tank, it is possible to make the storage tank of different geometrical configurations. For example the tank could be made in a "slimline" version to fit into an appropriate domestic location.

In the external heat exchanger, any suitable heat exchange element can be used such as a heat exchange tube which is preferably finned, or a plate heat exchange element of a proprietory or custom designed construction.

In integrated thermal storage systems wherein the storage tank water is heated by a boiler which is connected to the tank by a boiler circuit including a boiler pump, the boiler basically operates depending upon the demand from the storage tank. If the temperature of the water in the tank for example falls, then thermostatically the boiler will be signalled to fire, and its pump runs in order to circulate water from the tank through the boiler circuit for the heating of same. When the tank is satisfied the boiler is cut off.

As explained, the heat of the water stored in the tank is used for two purposes. Firstly, it can be circulated on demand through a heating circuit such as a central heating circuit which has a heating circuit pump. When there is a demand for example from a room thermostat that the heating circuit should be operated, the pump is activated to circulate the water. It may draw the water from the tank directly through the heating circuit or it may be indirect by some heat exchange means.

As also explained above, the heat in the stored water is used for heating the secondary domestic hot water by heat exchange. The domestic hot water is supplied from the mains at mains pressure and passes through heat exchange means to which heat is supplied from the water stored in the tank, and eventually emerges at the taps as heated secondary water. In the more commonly used integrated thermal storage systems, the heat exchange means comprises a heat exchange coil or coils located inside the storage tank.

It is desirable, obviously, to endeavour to achieve the most efficient heat transfer between the stored water in the thermal storage tank and the cold mains water supplied to form the secondary domestic hot water, and the general feature of the present invention as indicated above is concerned with a system for rendering the heat exchange efficient. The optimising of heat exchange can mean smaller fuel bills, resulting from the more efficient utilisation of the stored heat in the thermal storage tank, but further advantages in using heat exchange external to the tank can be gained by appropriate heat exchanger design and layout.

According to a further preferred feature of the present invention further improvement in heat exchange and utilisation of the stored heat in the thermal storage tank is achieved.

According to this preferred feature as applied to an integrated thermal storage water heating system, parallel external heat exchangers are used the respective heat exchangers being connected in flow circuits including pump means on the one hand for circulating heating flow water from the thermal storage tank and/or boiler therethrough, and on the other hand in cold mains water supply means so that the cold water can be heated in passing through the heat exchangers. Preferably the heating circuit circulating pump is the pump means of the flow circuit in which one of the heat exchangers is located, and the boiler pump is the pump means in the flow circuit in which the other heat exchanger is located. The flow circuit or any of them may include an appropriate diverter valve or valves.

Preferably, the said heat exchangers include internal heat exchange coils through which the cold mains water flows, said heat exchange coils of the respective heat exchangers being connected in parallel.

Each of the heat exchangers may be of any suitable construction, but it is preferred that they be of the form set out in our co-pending British Application No 9113249.8 filed on even date herewith or British Patent Application No 9125547.1 The system as indicated above preferably also includes a flow switch in the cold mains water supply so that any flow diverters utilised will be operated when there is a demand for hot water to cause the flows from the heating circuit pump and/or from the boiler pump to be circulated through the heat exchangers.

The preferred arrangement maximises heat transfer by using separate pumps for the separate heat exchangers. This means that the heat exchangers can each be smaller than a single equivalent heat exchanger and resistance to flow of the cold water is reduced. The arrangement further increases the hot water potential where the only limiting factor is the cold water supply to the property in question. Utilisation of the store is maximised in that when there is a demand for hot water, both system pumps are used for delivering the hot water in the system for heat exchange. The supply of heat to the heat exchangers is as concentrated as possible.

In a particularly preferred arrangement using the parallel heat exchangers, one of the heat exchangers is arranged to receive hot water by circulation from the main storage tank through an appropriate diverter valve, whereas the other heat exchanger is adapted to receive heat from water circulated through the boiler and the storage tank by the boiler circulation pump avoiding the use of diverter valves.

The use of separate pumps for separate heat exchangers can be adopted in other water heating systems, for example a conventional system wherein the secondary water tank is used as the heat storage medium and the heat exchangers are connected in parallel thereto. A further pump may be required for circulating hot water through the first heat exchanger, the boiler/circulating pump having to circulate hot water through the other heat exchanger.

Although it is not necessary to arrange for the boiler to fire each time there is a demand for hot water, this can be done if desired and there will be the added advantage that direct heat input may be added as heat is being withdrawn at the heat exchangers. Normally, however, the boiler firing will be controlled by the conditions of the store, as is usual in integrated thermal storage and conventional systems.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, wherein: Fig. 1 is an illustration of a conventional thermal storage system; and Fig. 2 shows a thermal storage system according to an embodiemtn of the present invention; Fig. 3 shows the elements of an integrated thermal storage system according to another embodiment of the invention, when in a first receiving state; Figs 4 and 5 are similar to Fig 3 but show the system in two other receiving states; Fig. 6 shows a water heating system according to a further embodiment of the invention; and Fig. 7 shows a water heating system according to a further embodiment of the invention.

Referring to the drawings, the conventional system shown in Fig. 1 comprises a thermal storage tank 10 of copper or any other suitable material, which typically will be thermally insulated. Water is supplied to the storage tank through input line 12 by gravity feed or by direct mains pressure feed as required, and the water in the tank 10 is heated by means of a power unit such as boiler 14 through which the water is circulated. Alternative heating means such as electric heating means may be adopted if required.

The water in tank 10 is primary water in that it is circulated by means of a pump 16 through a central heating system 18 when it is required to heat the domestic dwelling in which the system is located. Circuit 18 is shown as having a plurality of radiators 20.

To provide the secondary hot water for domestic consumption at taps and the like, cold water is supplied from a mains line 22 and it is circulated through a heat exchange element in the form of a finned coil or several coils 24 located inside tank 10. As a result the cold water supplied on line 22 issues as hot water on output line 26 which leads to the domestic consumption points.

The thermal storage system of Fig. 1 may be as constructed and/or modified as set forth in or according to any feature in any of our prior published U.K. patents or patent applications.

The system of Fig. 1 operates in that the larger body of water in the tank 10 is heated for example during periods of little demand such as during the night, and the heat stored in the tank 10 is gradually utilised during the day.

The system works extremely well, but a difficulty has been noticed in relation to the finned tube heat exchanger coil 24 in that in areas of hard water, the inside of the tube scales up over a period of time, and the system shown in Fig. 2 is one embodiment of the present invention which seeks to overcome this difficulty.

In the arrangement of Fig. 2 similar reference numerals have been used for parts already described in relation to Fig. 1, and operating temperatures have also been indicated as these are material to this preferred form of the invention, and the major difference of the arrangement in Fig. 2 compared with that of Fig. 1 is that the heat exchange coil 24 is located in an external heat exchanger 30. That is to say, the coil 24 has been removed to a position outside the tank 10. The heat for the mains water supplied on line 22 is still derived from the hot water in tank 10, but by means of an additional circuit comprising a branch line 32 which leads from line 18, which has a diverter valve 34 therein.Heat exchanger 30 has a return line 36 which connects with line 37 which leads from the heating circuit return line 38 to a mixer valve 39 so that line 32 and line 37 lead to the mixer valve 39, the output of which is connected to the external heat exchanger 30 along line 40.

Finally, the input line 22 is provided with a flow switch 40 which controls the operation of the diverter valve 34.

In the arrangement described in Fig. 2, when there is a demand for hot water at the domestic consumption taps, for example by the turning on of a tap, the result of flow of cold water through line 22 actuates flow switch 40 which in turn sets the diverter valve 34 into a position where output from the tank 10 via pump 16 is diverted into line 32, and if necessary it causes the switching on of pump 16 if the pump is not already running. The hot water from the tank 10 is therefore pumped through line 32, and it enters the external heat exchanger 30 through mixer valve 39. Diluting colder water is supplied to mixing valve 39 through line 36 and line 37 so that the water entering the heat exchanger 30 through line 40 is at a required temperature less than that in the tank 10.The thus diluted hot water is forced across the heat exchanger coil 24 and returns to the mixer valve 39 via line 36 and line 37. Some of the water flowing from line 36 is returned to the tank 10 via line 38 to make up the water being drawn from the tank 10 by pump 16. At the same time, the cold water passing through the coil 24 emerges as secondary hot water in line 26.

Thus, the primary water from the store with diluting colder water is pumped over the coil 24 whilst the cold water flows through the coil 24 and this approximately doubles the heat exchange rate per metre of tube 24 compared with simply locating the tube 24 in the thermal storage tank 10 as indicated in Fig. 1.

By ensuring however that the water forced over the tube 24 in the Fig. 2 arrangement is at a lower temperature, for example 200 lower than that in the store 10, then the degree of scaling in tube 24 is much reduced or possibly eliminated, and even if a small amount of scaling does take place in tube 24, it is a simple matter when the system is to be serviced, to exchange the heat exchanger 30 and to return the used exchanger for reconditioning in factory circumstances, whereas with the arrangement in Fig. 1, when the tube 24 is to be descaled, there is a considerable amount of work required on site.

By removing the heat exchanger coil 24 from the tank 10, the geometrical configuration of the tank can be varied to a much greater extent, and for example it can be made of a relatively slim construction so as to fit into a narrow space in a domestic situation.

Any suitable type of heat exchanger can be used and it may be one with a finned tube or finned tubes, or it may be one having a plate heat exchanger. Both types are known in connection with the passing of liquids therethrough under pressure.

Even although there may be no reduction in the size of the heat exchange surface in the arrangement of Fig. 1 as compared with that in Fig. 2 since reduction in the temperature of the water flowing therethrough is offset by higher efficiency of heat exchange gained in the Fig. 2 arrangement,, nevertheless the sales benefit in having an external heat exchanger are considerable as maintenance costs due to scale formation can be considerable.

As indicated in Fig. 2, in a typical domestic installation, the store 10 may be designed to operate at a temperature in the region of 800C, and therefore if the water passing through the heat exchanger 30 is to be for example in the region of 600C, then sufficient colder water must be mixed with the incoming hot water at the mixer valve 38 to achieve this operational temperature. Typically, the colder water will be at a temperature in the region of 300C. The mixing valve 39 may be thermostatically controlled in order to allow sufficient of the hot and colder water to produce output water at a temperature as set by the valve 39.

Across the heat exchanger 30, the cold water may be raised for example from 100C to 500C, whilst the hot water may drop from 600C to 300C.

The diverter 34 preferably is spring loaded so as normally to remain in a position to meet demand from the heating circuit 18, but when the flow switch 40 is actuated, the diverter is thrown to the alternative position causing water to be circulated to the external heat exchanger.

Another advantage of the arrangement of Fig. 2 is that the water in the thermal store 10 is kept in a highly stratified state as the cooler water is returned at the bottom and no heat exchange takes place within the store. The maintenance of the store water in a highly stratified state i.e. high temperature at the top and low temperature at the bottom is highly desirable for thermal efficiency.

The embodiment of the invention envisaged in Fig. 2 makes use of the features that the heat exchanger means at a temperature which is lower than that of the store. In the following embodiments whilst this condition can apply and all or any of the features of embodiment of Fig 2 can be included, such requirement is not necessary and the systems are described without any mixing valves although these could be included if required.

Referring now to Fig 3, the basic element of the system comprise a thermal storage tank 110 which may have a volume in the order of 120 litres, which is filled with the primary water of the system which is heated to a predetermined temperature by means of a boiler 112 connected to the thermal store by means of the heating circuit including lines 114 and 116, line 116 embodying a boiler pump 118 and a diverter valve 120.

When the store 10, by thermostatic sensing, indicates a demand for heat, the boiler 12 and pump 18 are fired, and water is circulated from the storage tank 10 through lines 14 and 16 by the pump 18 as indicated in Fig. 3 by arrows 122.

When the demand ceases, the boiler firing is terminated and the pump 118 is stopped.

Fig. 3 shows the system in a quiescent state i.e. when there is no demand from the heating circuit which is supplied by the system, and when there is no demand for hot water at the consumption points e.g. the taps in the building.

Included within the system of Fig. 1 are two heat exchangers 124 and 126 which preferably are of identical construction, but need not be.

Each heat exchanger 124, 126 includes an outer casing 128, 130 and inside the casing is a heat exchanger coil 132, 134.

Each of the heat exchangers 124 and 126 preferably is constructed in the manner described in our co-pending British patent application No 9113249.8 filed on even date herewith.

With regard to heat exchanger 124, the interior of the outer casing is connected to the storage tank 110 by pipe 136, and the other end of the casing is connected by the pipe 138 to the diverter valve 120. The diverter valve 120 has two positions the first of which as shown in Fig. 1 allows flow along line 116 from the tank 110 to the boiler 112 and a second position which is indicated in and will be described in relation to Fig. 5.

The other heat exchanger 126 is connected between the flow and return lines 140, 142 of the heating circuit which is connected to the system, and line 140 is provided with a heating circuit pump 144 which ensures the circulation of the water from the tank 110 around the heating circuit. The pump 144 and the heating circuit are activated depending upon the thermostatic demand from a room thermostat in the building in which the system is located. When the demand is present, hot water is circulated through lines 140 and 142 and the heating circuit.

Line 140 includes a further diverter valve 146 which is connected by a line 148 to one end of the casing 130 of heat exchanger 126, and the other end of heat exchanger 126 is connected by a line 150 to the return line 142 of the heating circuit. Diverter 146 also has two positions which will be described in relation to Figs. 4 and 5.

The cold water supply circuit includes an input line 152 leading from the cold water supply mains, and line 152 includes a flow switch 154, the function of which will be explained hereinafter. Line 152 splits at point 154 so as to be connected in parallel to the heat exchanger coils 132 and 134, and the heat exchange coils at their outlet ends are coupled at point 156 to a single outlet line 158 for hot water. Line 158 may include a mixer valve 160 which is thermostatically controlled in order to bleed cold water from line 152 along branch line 162 to the mixer valve 160 so that the water eventually issuing from the outlets e.g. the taps will be at a temperature which is attenuated compared to the temperature to which the water is heated in passing through the heat exchange coils 132 and 134, should such temperature be too great.

As mentioned above, Fig. 3 shows the system in a relatively quiescent state i.e. when there is no demand, while the figure does show that the water is being circulated from the tank 10 through the boiler 12 for the heating of the water in the store up to the predetermined value. When it reaches the predetermined value, the boiler will be switched off, and the pump 18 will be stopped.

If, from this quiescent condition, there is a demand for hot water to be passed through the heating circuit, then conditions as indicated in Fig. 4 prevail. Pump 144 is made operational, and diverter valve 146 is positioned to allow hot water to be circulated from the storage tank 110 through lines 140 and 142 and the accompanying heating circuit as indicated by arrows 164 in Fig. 4. The diverter valve 146 has a first or home position to which it will revert, and in such position flow takes place as indicated in Fig. 4.

Diverter valve 20 also has a similar home position which is the position it will take as shown in Fig. 3 to allow water to be circulated through the boiler 112.

Reverting now to Fig. 5, should there be a demand from a hot water tap, established by opening of the tap, the flow switch 154 is activated by the flowing water, and this causes diverter valves 120 and 146 to be positioned to their second positions, and the pumps 118 and 144 (if not already running) are started. These actions result in the flow of hot and cold water as indicated by the arrows in Fig. 5.

Considering firstly heat exchanger 124 cold water as indicated by arrows 166 and 168 flows from the mains through the heat exchanger coil 132 and outlet pipe 158 as hot water flows from the tank 110, as indicted by arrows 170 through the heat exchanger casing 128 via line 136 and line 138, the repositioned diverter valve 120, pump 118 boiler 112 and back to the tank 110 through line 114.

At the same time, by the repositioning of the diverter valve 146, hot water is circulated from the tank 10 as indicated by arrows 172 through the heat exchanger casing 130 via lines 148 and 150 to the return line 142 of the heating circuit and back to the tank 110 whilst cold water is simultaneously passed through the heat exchange coil 134 as indicated by arrows 174 so as to be heated and to mix with the hot water coming from the parallel heat exchanger 124 so that the heated water emerges from the outlet line 158.

By utilising the two system pumps 118 and 144, an extremely effective heat exchange method is provided. The heat exchangers can be made smaller, and by adopting the modifications as in Fig. 2, the heat exchangers can be run at a lower temperature than the temperature of the store.

The resistance to the flow of cold water is reduced by providing the heat exchangers in parallel and in fact the only limiting factor is the cold water supply 152.

As described, the mixer arrangement 160, 162 can be provided or other mixer arrangements can be adapted in order to attenuate the temperature of the water issuing from the taps and the temperature at which the heat exchangers run.

Although the boiler will normally be set to fire only when there is a demand from the thermal store, it may be possible automatically to fire the boiler at the same time as there is a demand for hot secondary water so that heat will be being added to the thermal store as it is being withdrawn in the form of hot secondary water.

It is invisaged that the flow switch 154 will be a priority component insofar as when there is a demand for secondary hot water and the flow switch is actuated, this will have priority over the replenishment of the store and the heating demand.

It is noted in the arrangement described that the return flow line 14 from the boiler is connected to the thermal store at the bottom of the tank so as as far as possible not to destratify the store during hot water demand.

Referring now to Fig 6 of the drawings, a domestic hot water system comprises a storage tank 210 containing a body of hot water which is heated by circulation of the water through a boiler 212 and circulation lines 214 and 216. The water is circulated by means of a boiler pump 218 in line 214, the pump 218 and boiler 212 being activated when the thermostat control means in the tank 210 indicates a demand for heat.

In the circulation line 216 is embodied a heat exchanger 220 and the water circulating through lines 214 and 216 and the boiler constitutes a first fluid passing through the heat exchanger, and the second fluid which passes through the heat exchanger in contraflow direction to receive heat exchange from the first fluid is in fact cold water supplied from a cold water mains supply line 222 which is at mains pressure.

Line 222 includes a flow switch 224 and it then branches into lines 222A and 222B so as to pass through respective heat exchangers 220 as described, and 224 to be described hereinafter.

The outlet lines from the exchangers 220 and 224 are indicated by reference numerals 226 and 228 and the now heated water enters an outlet pipe 230 from the said lines 226 and 228 and if required passes through a mixing valve 232 before passing to the dispensing point such as the hot water taps, showers and washing machines, indicated by reference 234.

The heat exchanger 224 is embodied in the central heating circuit in the same manner as described in relation to previous embodiments. That is to say, hot water is drawn on demand from the tank 210 through go and return lines 236 and 238 for a heating system for example made up of domestic radiators, and line 236 includes a circulating pump 240 and a diverter valve 242. The diverter valve 242 has a normal rest position in which water can be circulated through the heeating circuit, but if there is a demand for hot water at a consumption point e.g. point 234, then the diverter valve is temporarily positioned to cause the hot water issuing from the tank 210 through line 236 to be diverted through line 244 and through the heat exchanger 224 to transfer heat to the cold water flowing through lines 222B and 228 and the heat exchanger 224.

As regards the boiler circuit 214 and 216, it will be noticed that there is no diverter valve in this arrangement, and this is achieved by circulating the boiler water directly through the heat exchanger 220. In the arrangement of Figs 3 to 5 the boiler circuit is shown as being coupled directly to the thermal store. It is a significant aspect of the embodiment that the boiler circuit is connected directly to the heat exchanger 220.

As regards the construction of the heat exchanger, this may be as disclosed in our co-pending British Application No 9113249.8, or as described in our co-pending British Application No 9125547.1.

Should the boiler 212 tend towards run-away condition, the danger of explosion is much reduced, as the water can continue to be circulated freely back to the tank 210. It should be noticed that the provision of the pump 218 does not cause a blockage in the same way that a closed diverter valve would.

The mixing valve 232 also has the function of allowing incoming cold water to be mixed with the issuing hot water in line 230 so that the water which is delivered to the taps is of the required temperature. To this end there may be a branch connection in line 222 either before or after the flow switch and connected to the mixing valve 232, in much the same fashion as described in relation to Fig 3.

In use, when there is a demand for hot water at a consumption point for example by the opening of a tap or the turning on of a shower, cold water flows through switch 224 which actuates the pumps 218 and 240 if not already in operation, and positions the diverter valve 242 to cause the water delivered by pump 240 to pass through heat exchanger 224. The pumps 218 and 240 are also under the control, in the case of pump 218, of the store thermostat and in the case of pump 240, the demand for space heating for example by means of a room thermostat. The arrangement described provides an increase in heat extraction capability, and it is less dependent upon the boiler giving maximum temperature output.

High standing losses are avoided by virtue of the heat exchangers being located outside the thermal store, and because the heat exchange fluids are pumped through the heat exchangers in contraflow improved results are achieved.

When the heat exchanger coil is located inside the tank, the pump 240 for the central heating system typically will be down from April/May until September/October of the same year, and often the pump will cease up during this down period.

With the arrangement described, the pump 240 will be functioning at all times when hot water is demanded.

Siezures are likely to be less frequent.

The arrangement of the invention shown in Fig. 7 is essentially similar to the Fig. 6 embodiment (and therefore the same reference numerals are used) with a modification in the circuit containing the boiler 212. With the Fig. 6 embodiment, if there is a long period of demand on the central heating, the boiler 212 remains operational and the heat exchanger 220 heats up which can lead to undesirable scaling. The arrangement of Fig. 7 avoids this problem in that the boiler input has a diverter valve 300 having two input lines 302 and 304. Line 302 connects directly with the water in tank 210, whilst line 304 connects with tank 210 through the heat exchanger 220.

When there is a demand for hot water by for example opening of a tap, the flow switch 224 detects this and sits the two divider valves 242 coupled to exchanger 224 and 300 and if they are not running pumps 240 and 218 are operated, to circulate hot water through the heat exchangers 224 and 220.

In the case of exchanger 220 the hot water from tank 210 is circulated through line 304 and flow through line 302 is blocked. If there is a simultaneous demand from the tank 210 for heat, the boiler 212 will be operational and some additional heat will be added to the water passing through the exchanger 220 but only for the time that the demand for the hot water as the tap is present.

When there is no demand for hot water, the diverter valves 242 and 300 revert to their alternative positions and, in the case of the diverter valve 300, a flow path through line 302 and the diverter valve 300 to boiler 212 is established. This means that if there is a demand for heat from the store 210, the water is circulated through the boiler via line 302, diverter 300, pump 218, and the return line 306, by passing the heat exchanger 220 so that it will not heat up if the demand for heat continues for a long time.

Scaling is less of a problem because the heat exchangers are located outside the tank, and the system can operate at a lower temperature.

The invention, or at least embodiments thereof can also be used in water heating apparatus such as combi-boilers, where there is no large thermal store.

The embodiments of the invention of Figs. 3 to 6 utilise the capabilities of both system pumps at times when there is a hot water demand, and considerably improved efficiency can be achieved.

The use of separate pumps for the separate heat exchangers can be applied to any known form of water heating system and will be viable unless the cost of the heat exchangers, pumps and pipework exceed greatly the increased performance advantages.

Claims (14)

1. A thermal storage installation for heating water, wherein the installation has a heat exchanger which is external of the thermal storage tank, wherein heat exchange takes place in the external heat exchanger by pumping the hot primary water or water heated thereby to the external heat exchanger for heat exchange with mains pressure secondary water which passes through a heat exchange element in the external heat exchanger.

2. An installation according to claim 1, wherein the heat exchanger is designed to run at a temperature which is lower than that in the thermal storage tank so that the extent of scaling up will be much reduced and perhaps in some case eliminated.

3. An installation according to claim 2, wherein the arrangement is that the hot water from the thermal storage tank is circulated through the external heat exchanger and is mixed with incoming colder water in order to control the temperature of the primary water passing through the external heat exchanger.

4. An installation according to claim 3, wherein the arrangement is that the colder water is the primary water from the thermal storage tank after it has been passed through the heat exchange element.

5. An installation according to any preceding claim, wherein the thermal storage tank has an outlet to which a circulating pump is connected, said pump serving to circulate hot water from the thermal storage tank around, for example, a space heating circuit on the one hand, or in the alternative through a diverter valve and then the external heat exchanger, depending upon the condition of a flow switch operated when mains water passes through the heat exchange element in the external heat exchanger.

6. An installation according to any of claims 1 to 5, wherein the storage tank is of a 'slendair' design in that it is narrower in width in one direction than in the other.

7. An installation according to any preceding claim, wherein the heat exchanger comprises a casing in which is provided a coiled, finned heat exchange tube.

8. An installation according to any preceding claim, wherein there are two of said heat exchangers, arranged in parallel.

9. An installation according to claim 8, wherein the respective heat exchangers are connected in flow circuits including pump means on the one hand for circulating heating flow water from the thermal storage tank and/or boiler therethrough, and on the other hand in cold mains water supply means so that the cold water can be heated in passing through the heat exchangers.

10. An installation according to claim 9, wherein the heating circuit circulating pump is the pump means of the flow circuit in which one of the heat exchangers is located, and the boiler pump is the pump means in the flow circuit in which the other heat exchanger is located.

11. An installation according to Claim 10, wherein the flow circuit including the boiler including a diverter valve for diverting the water heated by the boiler from passing through the associated heat exchanger except when there is a demand for hot water.

12. An installation according to claim 10, wherein the flow circuit for circulating heating flow water includes an appropriate diverter valve, the other flow current including the boiler being devoid of diverter valves.

13. An installation according to Claim 11, wherein said flow current containing the said other heat exchanger comprises a boiler and circulation pipes enabling circulation of the storage tank water through the boiler for the heating of same.

14. A thermal storage installation substantially as hereinbefore described with reference to the accompanying drawings.