GB2069667A - A hot water system - Google Patents

Abstract

The invention relates to a hot water system in which there are primary and secondary circuits with a heat exchanger 14 active between them. The system includes a header tank 12 providing a reservoir of water for the secondary circuit and a feed can 32 with a fluid flow control valve 34 active between the header tank and the feed can, the feed can providing a means of supplying topping-up water to the primary circuit. An expansion and vent pipe 42 from the primary circuit has an outlet arranged over the feed can and a separate feed pipe leads from the feed can to a position in the primary circuit below that at which the expansion/vent pipe is connected to the primary circuit. The separation between the feed pipe and expansion and vent pipe ensures that water can always flow though the feed pipe even when the primary circuit is boiling. <IMAGE>

Description

SPECIFICATION A hot water system The present invention relates to a hot water system which incorporates an indirect hot water cistern and a header tank. An indirect cistern comprises a water storage container and a heat exchanger. As a rule, the heat exchanger includes an element of a primary hot water circuit within the cistern and the cistern itself provides the water storage container, forming part of the secondary circuit.

Usually, the primary circuit will include a boiler and in some cases also the radiators of a central heating system in addition to the element of the heat exchanger, and the secondary circuit supplies the hot water taps of a hot water system. It is necessary to supply water to the secondary circuit (generally by feeding cold water from a header tank direct into the lower part of the cistern) and there has to be provision for topping up the primary circuit to make up for any water losses in that circuit. It is also necessary to make provision for expansion of the water and venting of air and/or steam from the system. Further, it is necessary to ensure complete separation between the primary and secondary circuits, so that water circulating through the central heating system and/or boiler cannot become mixed with the secondary hot water.

The invention is particularly applicable to arrangements in which there is an automatic valve (such as a hydrostatic valve) between the cold water reservoir in a header tank and the primary circuit. Further, the invention is particularly applicable to arrangements of the kind in which the header tank is immediately above the cistern, and is secured to the cistern to provide a combined cistern and header tank unit, but it is to be understood that the invention can also be applied in situations where the header tank is remote from the cistern.

Operational probiems have arisen in connection with a known type of combined cistern and header tank, in which there is a feed and expansion can in the header tank, with a supply pipe from the can to the primary circuit and with a hydrostatic valve between the cold water reservoir in the header tank and the interior of the feed can. In this known arrangement, a primary circuit combined feed, expansion and vent pipe leads from the feed can to the primary element of the heater exchanger in the cistern, and besides allowing flow of topping-up water into the primary circuit, this feed pipe also acts as the expansion and vent pipe for the primary circuit. As the temperature of the water in the primary circuit rises, the water in that circuit expands, and in normal operating conditions, this expansion is accommodated in the feed can, the size of which is adequate to accept normal expansion.In the case of boiling of the water in the boiler, due for example to failure of the boiler thermostat, the emission of steam from the feed/vent pipe hinders the free flow of water down the feed/vent pipe. Boiling can produce such a vigorous action that the water and steam can rush up the feed/vent pipe into the feed can, causing water to bubble over out of the feed can into the header tank and hence causing mixing of the primary water and the cold feed water available for the secondary circuit. Moreover, this upward flow of steam and/or hot water prevents flow of compensatory cold water into the primary circuit via the feed/vent pipe and as a result, the boiler may boil dry, in which case, parts of the boiler may distort or even melt.This is particularly serious if it causes distortion of elements associated with a gas supply to the boiler, as this can permit escape of gas with the consequent risk of explosion or fire. Moreover, when the hot water has all boiled out of the feed/vent pipe, water may suddenly flow down that pipe and flash off the hot surfaces of the boiler with a risk of explosion. Besides the damage which can ensue to the system, there is then a release of large quantities of steam into the building in which the system is located.

According to one aspect of this invention a hot water system comprises: a primary hot water circuit and a secondary hot water circuit; a heat exchanger arranged to transmit heat from the water in the primary circuit to water in the secondary circuit; a header tank; a hot water cistern connected into the secondary circuit; a feed can and a fuid flow valve arranged so that the feed can is adapted to receive water from the header tank through the valve; an expansion and vent pipe connected to the primary circuit and having an outlet so arranged that any water flowing out of it falls into the feed can, and a separate feed pipe between the feed can and a position in the primary circuit below that at which the expansion and vent pipe is connected to the primary circuit. Preferably the separate feed pipe is connected to the primary circuit on the output side of the heat exchanger.

Thus, instead of the single feed/expansion/vent pipe of the known apparatus, the invention provides two completely separate pipes, one acting primarily as a feed pipe supplying water from the feed can to the primary circuit, and the other acting as an expansion/vent pipe, which directs any water overflowing from the primary circuit as a result of excessive expansion in that circuit, into the feed can. The feed pipe can also act as an expansion pipe for the primary circuit, but any venting of steam from the primary circuit should take place through the expansion/vent pipe.Thus, there is complete separation between the primary circuit and the secondary circuit, and the feed can is able to operate normally even during boiling conditions in the primary circuit, because any outflow from the primary circuit as a result of expansion in that circuit is through the expansion/vent pipe, rather than through the feed pipe.

In a preferred arrangement, the primary circuit includes a boiler, and a control valve is arranged in a return pipe of the primary circuit between the heat exchanger and the boiler, and the feed pipe is connected into the return pipe between the control valve and the boiler.

A control valve fitted in this position can be employed to shut off the flow through the primary heat exchange element. This cut-off can be in response to any predetermined signal, for example, after a predetermined time interval, or if the temperature in the primary or secondary circuit rises or falls above or below a threshold temperature or it can be operated manually. Providing the feed pipe is connected to the primary circuit in accordance with this preferred feature of the invention, it is possible for topping-up of the primary circuit to continue by operation of the feed can and fluid flow valve, despite closure of the control valve, and even in boiling conditions.

Part or all of the heat exchanger may be in the cistern. Thus, the cistern may be similar to a conventional indirect hot water cistern, the primary heating element being located within the cistern and the cistern itself being connected in the secondary circuit. It is preferred to provide a hydrostatic valve between the header tank and the feed can, and in the preferred arrangement, the feed can is located within the header tank. It is further preferred to provide a secondary expansion pipe connected to the upper part of the cistern and having an outlet disposed so that water flowing out of it falls into the header tank.

According to another aspect of the invention, a hot water cistern and header tank arrangement includes a primary heat exchange element, some or all of which is in the cistern; a feed can and fluid flow valve arranged so that the feed can is adapted to receive water from the header tank through the valve; an expansion and vent pipe connected to the primary heat exchange element and having an outlet arranged so that any water flowing out of it falls into the feed can, and a separate feed pipe connected between the feed can and the primary heat exchange element at a position below that at which the expansion and vent pipe is connected to the primary heat exchange element. Preferably the feed pipe is connected to the output side of that part of the element which is within the cistern.The arrangement may include various preferred feature of the hot water system described above, according to the first aspect of the invention.

A combined hot water cistern and header tank unit for use in a domestic central heating and .host water arrangement, and an alternative central heating and hot water arranged both in accordance with the invention, will now be described by way of examples only, with reference to the accompanying drawings, in which Figure 1 is a partly diagrammatic representation of a combined cistern and header tank unit, Figure 2 is a detail cross-section to a larger scale through a header tank, and Figure 3 is a diagram showing part of a domestic hot water and central heating system.

Referring to Figs. 1 and 2, a copper cistern 10 of conventional construction is provided, and a cold water header tank 1 2 is positioned above the cistern 10, and structurally connected to the cistern by framework not shown in the drawing, so that the cistern and header tank form a unit, which can be fitted as a unit to a house.

The cistern 10 is of the so-called indirect type, that is to say it includes a primary circuit pipe 1 4 formed into a coil within the cistern, this coil providing the heat exchange element of the primary circuit, there being a return connection 1 6 where this pipe 14 leaves the cistern at a lower position, and a flow connection 18, where the pipe 14 enters the cistern at a higher position. The primary circuit pipe 1 4 is connected to a boiler (not shown) the return pipe 20 to the boiler being connected to the return connection 1 6 through a thermostatically controlled valve 22, and the flow pipe 21 from the boiler being connected to the flow connection 1 8.

The primary circuit includes the boiler, possibly a series of radiators, and probably a circulator or circulators, assisting the water in the primary circuit through the coil formed by the primary pipe 14 and through the boiler and radiators. The thermostatically controlled valve 22 is preset, so that it will close when the temperature of the water in the return pipe 20 between the valve and the pipe 14 reaches a predetermined maximum value, so that the water in the primary circuit will cease to be circulated through the coil within the cistern 10, if the temperature of the water in the primary circuit rises above the preset threshold level.

A secondary circuit cold feed pipe 24 leads from the bottom of the header tank 1 2 to the lower end of the cistern 10, and a secondary expansion and vent pipe 26 leads from the top of the cistern 10, to an outlet 28 within the header tank 12, and above the maximum water line 30 in the header tank.

A cold water supply feeds into the header tank, through the conventional ball float valve (not shown) whereby a reservoir of cold water is normally maintained within the header tank 1 2. Water flows from the header tank through the secondary cold feed pipe 24 into the cistern 10, and normally the cistern is maintained full of water, which is referred to as the secondary water. An outlet pipe (not shown) leads from the upper end of the cistern 10 or out of the expansion and vent pipe 26, to the taps of a domestic hot water supply system, and hence, those taps can be supplied with hot water from the secondary circuit provided by the cistern 10, the reservoir of cold water in the header tank 12, the secondary cold feed pipe 24 and the secondary expansion pipe 26.This secondary water is heated by conduction from the primary circuit coil within the cistern and by convection within the secondary water in the cistern. As water is drawn off from the secondary circuit through the hot water taps, it is replaced by cold water flowing from the reservoir within the header tank 12, through the secondary cold feed pipe 24 into the cistern 1 0.

A feed and expansion can 32 is located within the header tank 12, and provides a means of supplying water to the primary circuit, for the purpose of topping-up that circuit to make good any losses in the primary circuit-principally occasioned by evaporation. A hydrostatic valve 34 (see Fig. 2) is fitted to the feed can 32, and essentially comprises a U-tube 36 having one leg 33 inside the feed can 32 and the other leg 35 in the reservoir within the header tank 12, and an outer tube 38 fitted around the leg 33 of the tube 36 within the feed can, the tube 38 being closed at the top, and open at the bottom.Such hydrostatic valves are known, and function to allow water to flow from the reservoir within the header tank 1 2 into the feed can, until the water rising within the feed can 32, and hence inside the outer tube 38, creates an airlock 39 in the top end of the tube 38. As water is drawn off from the interior of the feed can 32 into the primary circuit, the level of water within the can falls, and when it reaches a threshold level determined by the characteristics of the hydrostatic valve 34, water is able to flow through that valve (passing through the leg 35 and up through the leg 33) into the can 32. As the level of water rises in the can 32, it also rises in the tube 38, and eventually the balance position is established where the airlock 39 again prevents flow of water through the valve 34.In this way, the hydrostatic valve 34 maintains a reservoir of water within the feed can 32 for the purpose of topping-up the primary circuit. Since there will always be an airlock within the tube 38, water cannot syphon back from the feed can 32 into the header tank 12, and the arrangement thus provides a non-return valve. The reservoir of water in the feed can is available for maintaining the level of water in the primary circuit even when the secondary water is drained from the header tank. Further, as will hereinafter appear, the feed can also accommodates expansion in the primary circuit.

A primary cold feed and expansion pipe 40 leads from the bottom of the feed can 32, to the return pipe 20 of the primary circuit, and it is to be noted, that this pipe 40 is connected to the return pipe -20, on the opposite side of the thermostatic valve 22, to the cistern 1 0. Whenever there is an inadequacy of water within the primary circuit, cold water flows from the feed can 32 through the cold feed pipe 40 into the primary circuit, and this happens irrespective of whether or not the valve 22 is open. Hence, topping up of the primary circuit can continue from the feed can 32, even though the valve 22 may have shut off circulation through the primary coil within the cistern 10.

A separate expansion/vent pipe 42 is connected to the top end of the coil 14 within the cistern, and leads up through the header tank 1 2, and is bent over at its top end, to provide an outlet 44 positioned over the inside of the feed can 32. Hence, any water flowing out from the outlet 44 will fall into the feed can 32, thus preventing mixing of the primary and secondary water.

If there is expansion of water within the primary circuit, due to overheating of that water, the water level in the can 32 will rise because of expansion, through the feed pipe 40. In addition, the water level in the expansion/vent pipe 42 will also rise. Continued expansion will cause water to fall from the outlet 44 of the expansion/vent pipe 42 into the can 32. If the water in the primary circuit begins to boil, the steam will be given off through the expansion/vent pipe 42, because that pipe is connected to a higher position in the primary circuit than the feed pipe 40.

Consequently, there is no blowing back of water and/or steam from the feed pipe 40 into the feed can 32 (as can occur with the known system) and hence there is no danger of the water in the primary circuit flowing into the cold water in the header tank 1 2. Furthermore, there is virtually no possibility of the primary circuit boiling dry, as circulation of the feed water to the primary circuit is unimpeded by steam. If there is loss of water in the primary circuit due to evaporation, this will be made up by water flowing from the feed can 32 through the feed pipe 40, allowing the hydrostatic valve 34 to function in the ordinary way.

It will be appreciated, that the combined hot water cistern and header tank unit shown in Fig. 1, represents only one application of the invention. In fact, most hot water supply systems have the header tank and hot water cistern separated from each other, in some cases by an appreciable distance. The manner in which the invention can be applied to a system in which the header tank and cistern are separated from each other, is illustrated in Fig. 3. In that drawing, a boiler is shown at 50, an indirect hot water cistern at 52, and a cold water header tank at 54. A primary circuit includes a flow pipe 56 leading from the boiler 50 to an upper region of the cistern 52, a coil similar to the coil shown at 14 in Fig. 1 within the cistern 52, and a return pipe 58 from a lower region of the cistern 52 to the boiler 50.The boiler is also provided with a flow pipe 60 leading to central heating radiators (not shown) and a return pipe 62 from the radiators. Although the radiator part of the system does not require description, because it is not directly concerned with the present invention, the flow and return pipes 60 and 62, with their radiators, can be regarded as part of the primary circuit. For present purposes, however, the essential parts of the primary circuit are the flow pipe 56, the coil within the cistern 52, and the return pipe 58.

As with the arrangement described with reference to Fig. 1, there is a thermostatically controlled valve 64 provided in the return pipe 58 and the purpose of this valve is to shut off flow through the primary coil of the heat exchanger, whenever the temperature of the hot water flowing through the primary circuit exceeds a threshold value.

A secondary circuit comprises a cold water feed pipe 66 leading from the header tank 54, to a lower region of the cistern 52, the cistern itself, and an expansion pipe 68 leading from the top of the cistern 52 to an outlet 70 disposed above the header tank 54, so that any water flowing out of the expansion pipe 68 falls into the header tank 54. In addition, there is a hot water supply pipe 72 leading from the expansion pipe 68, to the hot water taps (not shown) of the domestic hot water system. Whilst it is convenient to tap off the supply pipe 72 from the expansion pipe 68, it is to be understood, that the supply pipe 72 could be taken direct from an upper region of the cistern 52.

A feed and expansion can 76 is provided within the header tank 54, and there is also a hydrostatic valve 78 similar to the hydrostatic valve 34 described with reference to Figs. 1 and 2 of the drawings. The purpose the valve 78, is to allow topping-up water to flow from the header tank 54 into the feed can 76, but to prevent return flow of water from the feed can 76 into the header tank 54. Since the operation of the valve 78 is exactly the same as that of the valve 34, no further description of the valve 78 is required.

A feed pipe 74 leads from the feed and expansion can 76, to the return pipe 58, and it is to be noted, that the feed pipe 74 is connected into the return pipe 58 on the opposite side of the thermostatically controlled valve 64 to the primary coil within the cistern 52. In other words, the essential arrangement of the feed pipe 74 is the same as that of the feed pipe 40 shown in Fig. 1.

There is also a separate combined expansion and vent pipe 80 leading from an elevated part of the feed pipe 56, to an outlet 82 located over the interior of the feed can 76. The functioning of the system illustrated in Fig. 3 is exactly the same as that shown in Figs. 1 and 2, the main difference being that the cistern 52 and header tank 54 are separated from each other. When hot water is drawn off through the hot water taps, cold water feeds through the pipe 66 from the header tank 54 into the cistern 52, and this provides the water supply for the secondary circuit. If there is a loss of water in the primary circuit due for example to evaporation, then topping-up water for that circuit flows through the feed and expansion pipe 74 into the primary circuit. As has been previously explained, this can happen, even though the thermostatically controlled valve 64 might be shut.When the water level in the feed can 76 falls below a threshold level, additional topping-up water flows through the hydrostatic valve 78 into the feed can to make up for losses due to evaporation.

When there is expansion of water in the primary circuit, the water level in the feed can 76 will rise, and if there is excessive expansion, some water may flow out through the outlet 82, but in that case it will be collected in the feed can, and will not be allowed to mix with the water in the header tank 54 which is required for the secondary circuit. If the water in the primary circuit boils, venting of steam will take place through the vent pipe 80, rather than through the feed pipe 74, and hence topping-up water is still able to flow unimpeded through the feed pipe 74 into the primary circuit.

In both the arrangements illustrated in the drawings, the heat exchanger includes a primary circuit coil within the cistern 10 or 52.

It will be appreciated, that instead of a coil, there could be a hollow sleeve within the cistern, the interior of the sleeve being connected to the flow and return pipes for the primary circuit. Alternatively, a completely separate heat exchanger could be provided, n which case, the flow and return pipes of the primary circuit would enter the heat exchanger, and there would then be additional' flow and return pipes connecting the secondary water in the heat exchanger, with the cistern.

Claims (14)

1. A hot water system comprising: a primary hot water circuit and a secondary hot water circuit; a heat exchanger arranged to transmit heat from water in the primary circuit to water in the secondary circuit; a header tank; a hot water cistern connected into the secondary circuit; a feed can and fluid flow valve arranged so that the feed can is adapted to receive water from the header tank through the valve; an expansion and vent pipe connected to the primary circuit and having an outlet so arranged that any water flowing out of it falls into the feed can, and a separate feed pipe connected between the feed can and a position in the primary circuit below that at which the expansion and vent pipe is connected to the primary circuit.

2. A hot water system as claimed in Claim 1, in which the separate feed pipe is connected to the primary circuit on the output side of the heat exchanger.

3. A hot water system as claimed in Claim 1 or Claim 2, in which part or all of the heat exchanger is in the cistern.

4. A hot water system as claimed in any one of claims 1 to 3, in which the primary circuit includes a boiler, a control valve is arranged in a return pipe of the primary circuit between the heat exchanger and the boiler, and the feed pipe is connected into the return pipe between the control valve and the boiler.

5. A hot water system as claimed in any one of Claims 1 to 4, in which the valve is a hydrostatic valve.

6. A hot water system as claimed in any one of Claims 1 to 5, in which the feed can is located within the header tank.

7. A hot water system as claimed in any one of Claims 1 to 6, in which a secondary expansion pipe is connected to the upper part of the cistern and has its outlet disposed so that any water flowing out of it falls into the header tank.

8. A hot water cistern and header tank arrangement including a primary heat exchange element some or all of which is in the cistern; a feed can and fluid flow valve arranged so that the feed can is adapted to receive water from the header tank through the valve; an expansion and vent pipe connected to the primary heat exchange element and having an outlet arranged so that any water flowing out of it falls into the feed can, and a separate feed pipe connected between the feed can and the primary heat exchange element at a position below that at which the expansion and vent pipe is connected to the primary heat exchange element.

9. A hot water cistern and header tank arrangement as claimed in Claim 8, in which the feed pipe is connected to the output side of that part of the element which is within the cistern.

1 0. A hot water cistern and header tank arrangement as claimed in Claim 8 or Claim 9, in which the valve is a hydrostatic valve.

11. A hot water cistern and header tank arrangement as claimed in any one of Claims 8 to 10, in which the feed can is located within the header tank.

1 2. A hot water cistern and header tank arrangement as claimed in any one of Claims 8 to 11, in which a secondary expansion pipe is connected to the upper part of the cistern and has its outlet disposed so that any water flowing out of it falls into the header tank.

1 3. A hot water system constructed and arranged substantially as herein described with reference to Figs. 1 and 2 or Fig. 3 of the accompanying drawings.

14. A hot water cistern and header tank arrangement constructed and arranged substantially as herein described with reference to Figs. 1 and 2 of the accompanying drawings.