GB2382124A

GB2382124A - Heat exchange system with pre-heater

Info

Publication number: GB2382124A
Application number: GB0226501A
Authority: GB
Inventors: Henrik Moore
Original assignee: Individual
Current assignee: Individual
Priority date: 2001-11-14
Filing date: 2002-11-13
Publication date: 2003-05-21
Anticipated expiration: 2022-11-13
Also published as: GB0127288D0; GB2382124B; GB0226501D0

Abstract

The system 2 is up-stream from a water supply conduit 3 and has a conduit portion 14 in the form of a heat exchange element 15. The element extends through hot water 16 held in a cylinder 17 which acts as a heat reservoir. The element may be in the form of U-shaped parallel tubes 18 extending between an inlet 20 and an outlet 21. Cold water is pre-heated by the element before it is supplied to the conduit 3 for subsequent use such as in a shower unit 1, or for on-demand hot water or central heating. The tubes of the heat exchange element may alternatively be straight, zig-zag, or right-angled between the inlet and outlet. The pre-heat exchange element 15 may be thermally coupled to a remote heat reservoir instead.

Description

1 Heat Exchange System The present invention relates to systems for dispensing a flow of heated water from an on-demand water heater system and especially, but not exclusively, to systems for dispensing heated water from mains water fed electric showers or hot water run from combination boilers.

Appliances such as electric showers are generally supplied with cold water from the mains supply or a cold water storage tank, which must then be heated instantly by the heater element when there is a demand for hot water e. g. a hot water tap is opened or a shower is switched on to a"hot"setting.

Such appliances have the advantage that energy is not wasted in heating any more water than is necessary or required, which is often the case when the whole contents of a large tank or cylinder of water is heated and kept hot. They are also convenient when hot water is required instantly because it is not necessary to wait for a hot water storage cylinder to be filled or replenished with hot water, which might take some hours.

However, a major disadvantage of these appliances is the restricted rate of flow of sufficiently hot water which is dispensed. This is particularly apparent during the cold

winter months because of the significantly lower temperature of the cold water supply and hence the larger rise in temperature needed to bring the temperature of the water up to a satisfactory level. Since the rate at which heat is supplied remains the same, this results in a longer time being needed to heat the water and thus a reduced flow rate.

It is an object of the present invention to avoid or minimize one or more of the above disadvantages.

In one aspect the present invention provides a heat exchange system for an on-demand water heating system comprising a water supply conduit and a heater element activated when water is flowing through said conduit for heating said flowing water in a water-heater portion of said conduit, in use of the system, said heat exchange system comprising a pre-heating conduit portion of said water supply conduit, upstream of said water-heater conduit portion, thermally coupled to a heat reservoir so as to transfer heat from said heat reservoir to water flowing through said pre-heating conduit portion, in use of said heat exchange system.

The pre-heating conduit portion may be thermally coupled to said heat reservoir in any convenient manner. In the simplest form of the invention, an, essentially unmodified, section of the water supply conduit constituting the pre-heating conduit

portion, is routed directly through said heat reservoir.

Whilst such an unmodified conduit will provide some heat transfer from the heat reservoir to the pre-heating conduit portion and the water passing through it, preferably there is used a pre-heating conduit portion formed and arranged as a heat exchange element device with extended heat exchange surfaces for providing a relatively efficient thermal transfer from said heat reservoir. Alternatively the pre-heating conduit portion is thermally coupled to a more or less remotely disposed heat reservoir via a heat exchange element device having first and second portions thermally coupled to respective ones of the pre-heating conduit portion, and the heat reservoir.

In use of the invention water to be heated to a predetermined temperature flows into said water supply conduit and through the pre-heating conduit portion of said water supply conduit where the temperature of the flowing water is raised, by transfer of heat from the heat reservoir, to the water in the pre-heating conduit portion. The pre-heated water then flows into the water-heater conduit portion of the water supply conduit where the temperature of the flowing water is raised further to the predetermined temperature by means of a conventional heater element which may be activated by the flowing water. The heated water is then dispensed at a significantly greater rate of flow than would occur without

the water having had its temperature partially raised in the pre-heating conduit portion.

It will be appreciated that the heat reservoir of the present invention may in principle be any convenient suitably proximate heat reservoir. One particularly convenient form of heat reservoir may be in the form of a hot water reservoir, such as a hot water storage cylinder. In another embodiment of the present invention the hot water heat reservoir may be included within the by-pass conduit of a combination boiler central heating system and/or any other by-pass conduit provided in a central heating system. Thus, for example, in a fully pumped central heating system the heat reservoir may be included within the pipework feeding a hot water storage cylinder heat exchanger and may be connected across the domestic hot water pumped primary flow and return pipes.

Alternatively the heat reservoir may be the expansion vessel of a combination boiler central heating system or a conveniently located radiator or other part of a central heating system.

It will be appreciated that for the heat exchange system of the present invention to work efficiently the heat reservoir must be at a sufficiently elevated temperature relative to the temperature of water flowing in the water supply conduit to be able to transfer sufficient heat to the cold water at a

reasonable rate so that an adequate degree of pre-heating can be obtained.

Typically a cold water supply in a temperate climate would be at around 15 C in the summer months and would drop to around 0 to 50e in the winter. The pre-heating should, therefore, desirably be sufficient to raise the temperature of the cold water supply by at least 10 C when using a reasonable water flow rate-typically in the range from 3. 5 to 5. 00 litres/min for an electric shower and generally around 10 litres/min for a combination boiler hot water supply. It will be appreciated, though, that by increasing the temperature of the pre-heated water by a greater amount i. e. to above normal summer temperature, a faster flow rate can be obtained than would normally be available, which may be found advantageous by some users.

Where the heat reservoir is a body of hot fluid, the preheating conduit portion of the respective part of the heat exchange element device will generally be immersed in said hot fluid. Those skilled in the art will appreciate that the heat exchange element device or pre-heating conduit portion can be formed from a variety of materials which have good thermal conductivity properties, preferably of a metal with good thermal conductivity, for example, copper.

The heat exchange element device is preferably of a shape with a high surface area to volume ratio to maximise transfer of heat to said heat exchange element device. For example said heat exchange element may comprise a plurality of small diameter tubes connected in parallel, which may additionally be provided with fin elements, which could, for example, be arranged to extend axially along or circumferentially, for example helically, around the tubes in order to increase the heat exchange surface area in generally known manner.

In the case where the pre-heating conduit portion is in the form of a heat exchange element device comprising a multi-tube heat exchanger, the tubes are generally interconnected by means of a manifold at each end.

In a particularly advantageous embodiment of the present invention, the heat reservoir is a vessel supplied with water by-passing the heat exchanger of a hot water storage cylinder in a fully pumped central heating system. Typically the vessel would have an inlet for heated water flowing in the pumped primary flow pipe (i. e. water flowing from the boiler towards the heat exchanger of the hot water storage cylinder) and an outlet feeding to the return pipes to the boiler from the heat exchanger of the hot water storage cylinder. This arrangement is advantageous because when the mains supply rate remains stable it provides a sustained flow of heated water

through the heat reservoir vessel whilst the heating boiler is activated, maximising the heat transfer to the pre-heating conduit portion thereby stabilising the temperature of preheated mains water supplied to the electric shower unit.

In a further aspect, the present invention provides a heat exchange system for use in a fully pumped central heating system for an on-demand electric shower heating system comprising a water supply conduit and a heater element activated when water is flowing through said conduit for heating said flowing water in a water-heater portion of said conduit, in use of the system, said heat exchange system comprising a pre-heating conduit portion of said water supply conduit, upstream of said water heater conduit portion, thermally coupled to a heat reservoir connected across the hot water pumped primary flow and return pipes of said fully pumped central heating system so as to transfer heat from hot water flowing through said heat reservoir vessel to water flowing through said pre-heating conduit portion in use of said heat exchange system.

In an alternative advantageous embodiment of the present invention the heat reservoir is a domestic hot water storage cylinder and the pre-heating conduit portion is in the form of a heat exchange element device comprising a multi-tube heat exchanger immersed therein. Advantageously such a pre-heating

conduit portion heat exchange element device is formed and arranged to be for retro-fitting in an existing hot water storage cylinder. Conveniently it is provided with a threaded flange formed and arranged for releasably engaging with and securing in an immersion heater tapping of the hot water storage cylinder.

Thus, in another preferred aspect the present invention provides a heat exchange system for an on-demand electric shower heating system comprising a water supply conduit and a heater element activated when water is flowing through said conduit for heating water flowing in a water-heater portion of said conduit, in use of the system, said heat exchange system comprising a pre-heating conduit portion of said water supply conduit, upstream of said water-heater conduit portion, thermally coupled to a hot water storage cylinder so as to transfer heat from said hot water storage cylinder to water flowing through said pre-heating conduit portion in use of said heat exchange system.

It will be appreciated that such a pre-heating conduit portion heat exchange element device suitable for retro-fitting in a hot water cylinder may be of a variety of configurations. In one preferred form it comprises a conduit with a first end portion, and a second end portion, and a threaded flange formed and arranged for releasably securing said heat exchange

element device in a hot water storage cylinder threaded tapping for an immersion heater, said first end portion and said second end portion extending through said flange, said first and second end portions having first and second end manifolds respectively, and a body portion comprising a plurality of small diameter tubes extending from said first end manifold to said second end manifold, said conduit being formed and arranged for insertion inside a hot water storage cylinder through said tapping.

The body portion of said pre-heating conduit portion heat exchange element device may comprise a parallel array of elongate small diameter tubes suitably configured into a form which includes an extended length of the tubing and which can at the same time be more or less readily inserted through the tapping in a hot water storage cylinder.

In a further aspect, the present invention provides a heat exchange system for use in a combination boiler system for providing on-demand hot water comprising a water supply conduit and a heater element activated when water is flowing through said conduit for heating said flowing water in a water-heater portion of said conduit, in use of the system, said heat exchange system comprising a pre-heating conduit portion of said water supply conduit, upstream of said waterheater conduit portion, thermally coupled to a heat reservoir

included within a by-pass conduit of said combination boiler system so as to transfer heat from hot water flowing through said by-pass conduit of said combination boiler system to water flowing through said pre-heating conduit portion in use of said heat exchange system.

Preferably the pre-heating conduit portion extends through said by-pass heat reservoir to maximise heat transfer thereto.

Most preferably the pre-heating conduit portion is formed and arranged as a heat exchange element device.

The by-pass conduit is normally provided in a combination boiler heating system as a heat loss circuit to circulate hot central heating water, by means of a pump, to dissipate residual heat within the boiler after the burner has been switched off. This prevents over-heating of the water within the boiler heat exchanger which is no longer being pumped through the radiators.

It will be appreciated that by utilising this energy source, which would otherwise be wasted, to pre-heat water to be dispensed from a hot water outlet, the present invention provides a highly efficient system with cost and environmental advantages, as well as improving the rate of flow from the hot water outlet.

Preferably the heat reservoir is provided with insulated walls. This limits heat loss to the exterior surroundings thereby maximising the heat energy available for transfer to the pre-heating conduit portion. The pre-heating conduit portion heat exchange element device preferably comprises a series of small diameter tubes passing through said heat reservoir. More preferably said tubes are finned to further increase transfer of heat energy to water flowing in the preheating conduit heat exchange element device.

It will be appreciated by those skilled in the art that in use of the device when demand for hot water ceases, static water in the pre-heating conduit portion of said water supply conduit will continue to absorb available heat from said heat reservoir so that the temperature thereof is elevated well above that which would be obtained when water is flowing through the pre-heating conduit portion. Preferably said static heated water in said pre-heating conduit portion of said water supply is purged from said heat exchange system when demand for hot water resumes, before said heater element is activated, in order to prevent over-heating of the water when the water begins to flow through the system and tripping of safety cut-out switches and the like. This can be achieved in various ways. Thus for example in a domestic electric shower system said heated static water may be purged by simply turning the shower on initially with the temperature set to

"cold"mode until this pre-heated water is dispensed. It will be appreciated that the length of time required to purge the heated water will depend on the volume of heated water in the pre-heating conduit portion of the water supply conduit and the rate of flow of said water through the conduit. Typically the hot water may be purged within around 30 seconds or less.

Alternatively a delay switch may be incorporated in the heat exchange system so as to delay activation of said heater element when demand for hot water is made until the static heated water is purged from the system. Such a delay switch may be a simple timer switch or could possibly use a temperature sensor and be formed and arranged so as to prevent activation of the heater element until the water supply temperature thereto has dropped below a predetermined maximum threshold. Such a delay switch has the advantage that it does not rely on the memory of a user of said heat exchange system to actively purge the hot water.

In yet a further aspect the present invention provides an ondemand water heating system provided with a heat exchange system of the present invention.

Further preferred features and advantages of the invention will appear from the following examples and detailed description provided for the purposes of illustration and

illustrated with reference to the accompanying drawings in which : Fig. 1 is a schematic part sectional view showing the principal parts of an electric shower system provided with a heat exchange system in accordance with present invention ; Fig. 2 A, B and C are sectional views of heat exchange element devices suitable for use with the heat exchange system of Fig. l ; Fig. 3 is a schematic diagram of a combination boiler system provided with a heat exchange system of the present invention ; and Fig. 4 is a detail perspective view of the heat reservoir vessel and heat exchange element device of the combination boiler system of Fig. 3.

Fig. 5 is a schematic diagram of a fully pumped central heating system provided with a heat exchange system of the present invention.

Fig. l shows schematically an electric shower unit 1 provided with a heat exchange system 2 in accordance with the present invention. The shower unit 1 is of generally conventional

form with a water supply conduit 3 provided with a solenoid valve 4 and a flow regulator valve 5, and leading into a heating block 6 which is provided with an electrical heater element 7. A showerhead supply conduit 8 extends from the heating block 6 with a distal flexible hose portion 9 terminating in a showerhead 10. The shower unit 1 is provided with an electrical sector control 11 which typically is switchable between"off","cold", and"hot"positions.

This electrical sector control 11 interconnects 12,13 to the solenoid valve 4 and the electrical heater element 7 so that the solenoid valve 4 is opened in the"hot"and"cold" positions, and the heater element 7 is activated in the"hot" position.

The water supply conduit 3 has an upstream, pre-heating, conduit portion 14 in the form of a heat exchange element device 15, which extends through the hot water 16 held inside a hot water cylinder 17 which acts as a heat reservoir.

In more detail, the heat exchange element device 15 is in the form of a more or less closely spaced apart generally U-shaped bundle 18 of parallel tubes of small diameter 19 extending between an inlet manifold 20 and an outlet manifold 21 mounted inside a screwthreaded plug 22 which is formed and arranged for securing in an immersion heater tapping 23. The plug 22

also has an inlet connection 24 for coupling to a cold water supply 25 and an outlet connection 26 for coupling to the shower unit water supply conduit 3. Apart from mounting of heat exchange element device 15 in the hot water cylinder 17 as explained above and coupling thereof to a cold water supply 25 and to the shower unit water supply conduit 3, the hot water cylinder 17 is substantially unmodified and works in the normal way with the water 16 therein being heated by means of a hot water heat exchange coil 27 supplying heat from a boiler unit (not shown), with hot water being drawn off via an outlet 28 and replaced by colder water via an inlet 29.

As cold water flows through the tubes 19 of the heat exchange element device 15, heat is absorbed from the hot water 16 in the cylinder 17, so that water emerges from the heat exchange element device outlet connection 26 at a higher temperature than water flowing into said heat exchange element device inlet connection 24. Hence, water flowing into the shower unit water supply conduit 3 is at a greater temperature than if it had not previously passed through said heat exchange element device 15 and, therefore, requires a shorter duration of time in said heating block 6 for said electrical heater element 7 to raise the temperature of the water to the predetermined selected shower water temperature. This results in a greater flow rate of heated water being dispensed from said showerhead 10.

The shower unit 1 is fitted with a timer delay switch 30 to delay activation of the heater element 7 when the control switch 11 is turned on. This allows"static"pre-heated water residing in said heat exchange element device 15 to be purged to downstream of the heating block 6 before the heater element 7 is activated, thereby preventing automatic cut-out of the heater element 7 due to overheating of the water thereby.

Alternatively this pre-heated water could be purged from the system by simply turning the shower on to"cold"setting initially before selecting"hot"mode and activating the heater elements.

It will be appreciated that the heat exchange element device 15 could be secured in the hot water storage cylinder 17 by other means. For example by using a dedicated connection point, eg, in order to allow for an immersion heater still to be used.

It will be appreciated that various different geometric configurations of the heat exchange element device may be used. Fig. 2A shows a"straight through"configuration in which the inlet and outlet manifolds 20, 21 are provided at opposite ends of a substantially rectilinear tube bundle 31.

Alternatively (Fig. 2B) the tubes 19 could adopt a zig-zag configuration 32 disposed between the inlet manifold 20 and outlet manifold 21, to increase the surface area of said heat exchange element device over which heat transfer may occur.

Where it is desired for water flowing in the heat exchanger element device to change direction (Fig. 2C) the bundle of tubes 33 may be cranked so that the inlet and outlet manifolds 20, 21 are in different areas.

Fig. 3 shows a schematic representation of a combination boiler system with a heat exchange system 38 of the present invention, indicating the direction of flow of water through the combination boiler system by arrowheads. The combination boiler system 37 shown is a typical example of such a heating system which provides hot water for central heating and domestic hot water on demand. When demand is made for central heating, water, which circulates within the central heating circuit 39, enters the boiler 37 through return conduit 40 and is circulated by pump 41. Water passes through a combination boiler heat exchanger 42 where it is heated and flows out of the combination boiler heat exchanger 42 through outflow pipe 43 and into the central heating circuit 39 via supply pipe 44, then returning to the boiler for re-heating via return conduit 40. The cycle continues until the water temperature stabilises to the boiler thermostat setting when the boiler

heater burner B switches off, although the pump 41 still remains active. When demand for (room) heating ceases, heating water circulating through the boiler heat exchanger 42 in the combination boiler system 37 by-passes the central heating supply pipe 44 and flows through a by-pass conduit 45, linking outflow pipe 43 with return conduit 40 permitting heated water to circulate within the boiler unit 37. Without this by-pass feature there would be no water movement and the pump 41 would burn out. This also acts as a heat loss circuit to prevent over-heating of said water within boiler heat exchanger 42.

When demand is made for domestic hot water, for example a tap 46 is opened, cold water from the domestic cold supply pipe 47 flows through a flow switch 48 which activates the boiler 37, and circulating pump 41, followed by burner B ignition.

Demand for central heating (if in use) is overridden. Water enters the combination boiler heat exchanger 42 where it is heated to a predetermined temperature regulated by the water flow rate through heat exchanger 42 which ultimately depends on the boiler thermostat setting. Heated water exits the combination boiler heat exchanger 42 and enters a hot water supply pipe 49 to be dispensed from hot water tap 46. When demand for hot water ceases, water flow ceases and flow switch 48 signals for the burner B to switch off or resume central heating mode (if in use). During the period when the burner B

is activated the pump 41 is activated to circulate central heating water through the outflow pipe 43 and by-pass conduit 45 to prevent said water, which would otherwise be static, over-heating. The pump 41 has a run-on time beyond the burner B switch-off time, to dissipate residual heat in the combination boiler system 37.

In accordance with the present invention, a heat reservoir vessel 50 (shown in more detail in Fig. 4) is incorporated inline with the by-pass conduit 45 and a heat exchange element device 51 immersed therein. The heat exchange element device 51 is connected at one end 52 to the domestic cold supply pipe 47 downstream of the flow switch 48 and at the other end 53 to the domestic cold supply pipe 47 upstream of the combination boiler heat exchanger 42.

A timer delay switch or water temperature sensor controlled switch is included in the combination boiler electrical control circuit (not shown) to prevent over-heating of water in the combination boiler heat exchanger 42 and burner B shut down, should existing pre-heated water, previously static in the by-pass heat exchange element device 51 over-heat when heated further by the combination boiler system burner B.

As shown in Fig. 4, the heat reservoir vessel 50 comprises a first vessel inlet pipe 54 through which central heating water

flowing through by-pass conduit 45 enters, and a vessel outlet pipe 55 returning said flowing central heating water to the boiler heat exchanger 42, to be reheated prior to re-entering the heat reservoir through by-pass conduit 45. The vessel is parallelepiped with urethane foam insulated walls 56 to limit loss of heat from central heating water 57 residing therein.

(Suitable urethane foam insulation is available from RSA Waterheating Ltd of Stalybridge, UK under the Trade Name HERCULAG .) The heat exchange element device 51 accommodated in the vessel 50 has a heat exchange element device inlet 58 in one wall 56 of the vessel 50 through which cold water flows after flowing through domestic water flow switch 48. The heat exchange element device inlet 58 connects to a heat exchange element device inlet manifold 59 which separates out into a plurality of finned tubes 60 of small diameter (for clarity only four tubes are shown and only one of these is shown to be finned).

The tubes 60 follow a tortuous path 61 through the interior of the vessel 50 to further increase the surface area of the tubes 60 in contact with heated central heating water 57 in the vessel 50, thereby maximising transfer of heat from the heated central heating water 57 to water flowing through the tubes 60. The tubes 61 rejoin at heat exchange element device outlet manifold 62 which connects to a heat exchange element device outlet 63, from which pre-heated water flows towards

the combination boiler heat exchanger 42, where the temperature of the flowing water is then further raised to the predetermined temperature in the normal way by the burner B, before being dispensed from the hot water tap 46 with a greater flow rate than had the water not been pre-heated in the heat exchange element device 51 prior to entering the combination boiler heat exchanger 42.

The combination boiler system 37 is provided with a further time delay switch or a water temperature sensor controlled switch (not shown) to delay activation of the burner B when the hot water tap 46 is opened until static heated water residing in the heat exchange element device 51 is purged from the system to prevent over heating and boiler shut down.

Fig. 5 shows a schematic representation of a fully pumped central heating system indicated generally by reference number 66 with a heat exchange system 67 of the present invention.

The direction of flow of the water is indicated by arrowheads.

The fully pumped central heating system 66 provides hot water for central heating and hot water to heat water 68 in a hot water storage cylinder 69 by means of a hot water heat exchange coil 70.

When the fully pumped central heating system 66 is activated pump 71 circulates water through return pipe 72 to the boiler

B which is activated, and water flowing through the boiler B is heated. Heated water flows from the boiler B into boiler outflow pipe 73 from where is flows to radiators 74 (only one shown) via heating supply pipe 75. Water is returned to return pipe 72 from the radiators through heating return pipe 76.

Heated water from the boiler B to heat water 68 in the hot water storage cylinder 69 flows into boiler outflow pipe 73 and into pumped primary flow pipe 77 to the hot water heat exchange coil 70 from where it is returned to the boiler B via return pipe 72. When demand for hot water only mode is selected motorised valve 78 checks flow of water to the radiators 74 via heating supply pipe 75. When the required domestic hot water temperature of water 68 in the hot water storage cylinder 69 is reached thermostat controlled valve 79 checks flow of hot water to the hot water heat exchange coil 70 from pumped primary flow pipe 77.

Fully pumped central heating systems are generally provided with a by-pass circuit, typically in the form of a pipe, through which water, checked by valve 79, continues to flow as pump 71 continues to pump water in the system. Such a by-pass is located at any suitable position in the system to allow check valve 79 and the hot water heat exchange coil 70 to be by-passed. In the embodiment shown in Fig. 5 a by-pass

connected across the primary flow pipe 77 and return pipe 72 is modified in accordance with the present invention to provide a fully pumped central heating system 66 with a exchange system 67 which comprises a heat reservoir vessel 80 which is substantially similar to that shown in Fig. 4 and is incorporated as a by-pass of the hot water heat exchange coil 70, downstream of thermostat controlled valve 79. The heat reservoir vessel 80 has a vessel inlet pipe 81, through which water flows from the pumped primary flow pipe 77, and a vessel outlet pipe 82 returning water flowing through the heat reservoir vessel 80 to return pipe 72 to be reheated by boiler B. A heat exchange element device 83 is accommodated within the heat reservoir vessel 80. The heat exchange element device 83 has a heat exchange element device inlet 84 through which cold water flows, indicated by the broken flow line with arrow heads, from the mains cold water supply 85 when demand for a shower is made. The cold water is preheated in the heat exchange element device 83 and flows out of a heat exchange element device outlet 86 from where the preheated water flows to electric shower 87 via a shower water supply pipe 88 and is further heated as described hereinabove for the electric shower of Fig. 1.

It will be appreciated that while the hereinabove described embodiment modifies an existing by-pass circuit of the fully pumped central heating system by incorporating the heat

reservoir vessel therein, the heat reservoir vessel in accordance with the present invention could be connected across the primary flow pipe 77 and return pipe 72 to form an additional separate by-pass circuit.

This heat exchange system is advantageous because while the boiler B and the pump 71 are active a sustained flow of heated water flows through the heat reservoir vessel 80 enhancing transfer of heat from the heat reservoir to the heat exchange element device 83.

It will be understood that if the water present in the vessel 50 or 80 or in the hot water storage cylinder 17 in the abovedescribed embodiments happens to be cold, then the heat exchange system of the present invention does not affect the normal operation of the heating systems or electric shower system, and cold water will simply be heated in the normal way.

It will be appreciated that various modifications may be made to the above described embodiments without departing form the scope of the present invention. Thus, for example, in the combination boiler system embodiment of Fig. 3 in order to increase the heat supplied to the heat reservoir vessel 50, and hence available for preheating purposes, provision is made for providing an increased "by-pass" flow rate through the

vessel 50. Thus in addition the normally relatively small diameter by-pass conduit 45, there is provided a second larger diameter reservoir inlet pipe 64, in parallel with the first reservoir inlet pipe 54. The second reservoir inlet pipe 64 includes a diverter valve 65, by which central heating water, is checked and prevented from entering the heat reservoir vessel 50 when there is a central heating demand (which has not been temporarily over-ridden by a hot water demand). In addition the, normally small diameter heat reservoir vessel outlet pipe 55, would be replaced by a larger diameter pipe to allow a greater flow rate through the vessel 50. When the combination boiler system is switched on in hot water only mode, diverter valve 65 is actuated (opened) allowing heated water to surge through heat reservoir vessel 50, thereby maximising the heat transfer to cold water flowing through heat exchange element device 51, at the same time closing water supply to outflow pipe 44, immobilising the central heating circuit.

It will be understood that various other standard components of water heating systems which have not been specified hereinabove, such as a pressure relief valve 90 in the combination boiler system, can be included in the above described embodiments according to the present invention.

CLAIMS 1. A heat exchange system for an on-demand water heating system comprising a water supply conduit and a heater element activated when water is flowing through said conduit for heating said flowing water in a water-heater portion of said conduit, in use of the system, said heat exchange system comprising a pre-heating conduit portion of said water supply conduit, upstream of said water-heater conduit portion, thermally coupled to a heat reservoir so as to transfer heat from said heat reservoir to water flowing through said preheating conduit portion, in use of said heat exchange system.

2. A heat exchange system according to claim 1 wherein said heat reservoir is a hot water reservoir.

3. A heat exchange system according to claim 2 wherein said hot water reservoir is a hot water storage cylinder.

4. A heat exchange system according to claim 1 or claim 2 wherein the heat reservoir is a by-pass in a central heating system through which water heated by said central heating system can flow.

5. A heat exchange system for an on-demand electric shower heating system comprising a water supply conduit and a heater element activated when water is flowing through said conduit

Claims

for heating water flowing in a water-heater portion of said conduit, in use of the system, said heat exchange system comprising a pre-heating conduit portion of said water supply conduit, upstream of said water-heater conduit portion, thermally coupled to a hot water storage cylinder so as to transfer heat from said hot water storage cylinder to water flowing through said pre-heating conduit portion in use of said heat exchange system.

6. A heat exchange system for use in a fully pumped central heating system for an on-demand electric shower heating system comprising a water supply conduit and a heater element activated when water is flowing through said conduit for heating said flowing water in a water-heater portion of said conduit, in use of the system, said heat exchange system comprising a pre-heating conduit portion of said water supply conduit, upstream of said water heater conduit portion, thermally coupled to a heat reservoir connected across the hot water pumped primary flow and return pipes of said fully pumped central heating system so as to transfer heat from hot water flowing through said heat reservoir vessel to water flowing through said pre-heating conduit portion in use of said heat exchange system.

7. A heat exchange system for use in a combination boiler system for providing on-demand hot water comprising a water

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supply conduit and a heater element activated when water is flowing through said conduit for heating said flowing water in a water-heater portion of said conduit, in use of the system, said heat exchange system comprising a pre-heating conduit portion of said water supply conduit, upstream of said waterheater conduit portion, thermally coupled to a heat reservoir included within a by-pass conduit of said combination boiler system so as to transfer heat from hot water flowing through said by-pass conduit of said combination boiler system to water flowing through said pre-heating conduit portion in use of said heat exchange system.

8. A heat exchange system according to any one of claims 1 to 7 wherein the pre-heating conduit portion, is routed directly through said heat reservoir.

9. A heat exchange system according to any one of claims 1 to 7 wherein said pre-heating conduit portion is formed and arranged as a heat exchange element device with extended heat exchange surfaces.

10. A heat exchange system according to any one of claims 1 to 7 or claim 9 wherein the pre-heating conduit portion is thermally coupled to a remotely disposed heat reservoir via a heat exchange element device having first and second portions

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thermally coupled to respective ones of the pre-heating conduit portion, and the heat reservoir.

11. A heat exchange system according to any one of claims 1 to 7, or claims 9 and 10, wherein the pre-heating conduit portion is in the form of a heat exchange element device comprising a plurality of small diameter tubes connected in parallel.

12. A heat exchange system according to claim 11 wherein said small diameter tubes are provided with fin elements.

13. A heat exchange system according to any one of claims 1 to 5, or claims 9, 11 or 12 when not dependent on claims 6, 7, 8 or 10 wherein the heat reservoir is a domestic hot water storage cylinder and the pre-heating conduit portion is in the form of a heat exchange element device comprising a multi-tube heat exchanger immersed therein.

14. A heat exchange system according to claim 13 wherein the pre-heating conduit portion heat exchange element device is formed and arranged to be for retro-fitting in an existing hot water storage cylinder.

15. A heat exchange system according to claim 14 wherein said pre-heating conduit portion heat exchange element device

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comprises a conduit with a first end portion, and a second end portion, and a threaded flange formed and arranged for releasably securing said heat exchange element device in a hot water storage cylinder threaded tapping for an immersion heater, said first end portion and said second end portion extending through said flange, said first and second end portions having first and second end manifolds respectively, and a body portion comprising a plurality of small diameter tubes extending from said first end manifold to said second end manifold, said conduit being formed and arranged for insertion inside a hot water storage cylinder through said tapping.

16. A heat exchange system according to any one of claims 1 to 15 wherein said heat reservoir is provided with insulated walls.

17. A heat exchange system according to any one of claims 1 to 16 wherein purging means is provided to purge water from said pre-heating conduit portion prior to activation of said heater element.

18. An on-demand water heating system provided with a heat exchange system according to any one of claims 1 to 17.

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19. A heat exchange element device for use in a system according to claim 15 said heat exchange element device comprising a conduit with a first end portion, and a second end portion, and a threaded flange formed and arranged for releasably securing said heat exchange element device in a hot water storage cylinder threaded tapping for an immersion heater, said first end portion and said second end portion extending through said flange, said first and second end portions having first and second end manifolds respectively, and a body portion comprising a plurality of small diameter tubes extending from said first end manifold to said second end manifold, said conduit being formed and arranged for insertion inside a hot water storage cylinder through said tapping.

20. A heat exchange system substantially as described hereinbefore with reference to Figs. 1 and 2, Figs. 3 and 4 or Figs. 4 and 5 of the accompanying drawings.

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