GB2495905A

GB2495905A - Water heating system arranged to heat mains pressure water using a thermal store and a heat exchanger

Info

Publication number: GB2495905A
Application number: GB1116914.1A
Authority: GB
Inventors: Richard Hanson-Graville
Original assignee: Thermal Integration Ltd
Current assignee: Thermal Integration Ltd
Priority date: 2011-09-30
Filing date: 2011-09-30
Publication date: 2013-05-01
Also published as: GB201116914D0

Abstract

The water heating system 1 comprises a thermal store 3 containing a fluid such as water; a first heat exchanger 5 to heat mains water 19 using the fluid; a first pump 7 to circulate the fluid through the first heat exchanger; and a pump speed controller 9. The pump speed controller progressively controls the speed of the pump in response to a flow rate of mains water passing through the heat exchanger, which may be measured by a flow sensor 11. Ideally, the pump speed is proportional to the flow rate of the mains water and the pump speed is additionally controlled based upon the temperature of the heated mains water 21 as measured by a temperature sensor 13, possibly using pulse-width modulation (PWM). The heated mains water can be used for domestic hot water applications such as taps or showers. The thermal store fluid can be indirectly heated by a second heat exchanger 29 having connections 37,39 to a boiler. The system is quicker to respond to pressure variations in the mains water than similar systems that only use temperature measurements to control pump speed.

Description

Water-Heating System This invention relates to a water-heating system, a method for controlling water heating and a pump-speed controller.

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Background to the Invention

Water heating systems typically contain a thermal store for storing heated water. The heat from the thermal store is transferred to mains-pressure water io through a heat exchanger which can be, for example, a coil within the thermal store itself or a heat exchanger external to the thermal store, such as a plate heat exchanger. An advantage of these systems is that the pressure of the thermal store water is independent of the pressure of the mains-pressure water.

It is desirable for mains-pressure water not to exceed a predetermined temperature, to reduce the risk of scalding. In order to achieve this, some existing systems employ controls. A known method of controlling mains-pressure water temperature is to use a thermostatic mixing valve which senses the temperature of mains-pressure water exiting the heat exchanger. If the temperature exceeds a predetermined value, the mixing valve is able to introduce cold mains-pressure water into the hot water exiting the heat exchanger. However, inclusion of valves in hot water systems can be costly.

Systems exist in which mains-pressure water temperature is controlled by varying the flow rate at which thermal store water is circulated through the heat exchanger. This may be achieved by using valves in the thermal store water circuit (which couples the thermal store to the heat exchanger) which are responsive to the temperature of mains-pressure water exiting the heat exchanger.

In GB 2293438 A, a system is described in which a pump speed is variable in response to the temperature of mains-pressure water. The rate at which thermal store water circulates through the heat exchanger is varied to keep the mains-pressure water output temperature within predetermined limits. If the temperature of mains-pressure water becomes too high, the pump speed can be slowed accordingly.

s However, measuring the temperature of mains-pressure water and modulating pump speed disadvantageously leads to time delays and inaccuracies in temperature correction and does not eradicate the risk of scalding.

Summary of the Invention

The invention relates to a water-heating system, a method for controlling water heating, a pump-speed controller and kits as defined in the appended independent claims to which reference should now be made. Advantageous or preferred features are set out in dependent claims.

The invention may thus provide a water-heating system comprising a thermal store containing thermal store fluid; a heat exchanger external to the thermal store for heating mains-pressure water (secondary fluid) using thermal store fluid (primary fluid); and a pump-speed which, in use, progressively modulates or controls the pump speed or output in response to a flow rate of mains-pressure water through the heat exchanger.

The term pump speed" as used herein may be interchanged with the term "pump output" because, in practice, a variable pump speed corresponds to a variable pump output which, in turn, results in a variable rate of supply of thermal store fluid to the heat exchanger and back to the thermal store.

A progressive control of the pump-speed may mean that the pump-speed controller operates to increase the pump speed it the flow rate of mains-pressure water increases, having the effect of increasing the flow rate of thermal store fluid flowing through the heat exchanger. Similarly, as the mains-pressure water flow rate decreases, the pump-speed controller may operate to decrease the pump speed, decreasing the flow rate of the thermal store fluid flowing through the heat exchanger. For example, small changes in flow late may be accompanied by small changes in pump speed.

In a preferred example, the pump speed may vary in proportion to the flow rate s of mains-pressure water through the heat exchanger.

A progressive control of pump speed may enable variation of a flow-rate of thermal store fluid through the heat exchanger in response to the flow-rate of mains-pressure water, such that the temperature of mains-pressure water io leaving the heat exchanger is maintained within a pre-determined target temperature range. This level of control could not be achieved if, for example, a fixed-speed pump were simply switched on if a mains-pressure water flow is initiated and switched off if the mains-pressure water flow is stopped. For example, in a fixed-speed pump system, if mains-pressure water flow-rate is low, the thermal store fluid is delivered to the heat exchanger at a greater rate than is required to heat the mains-pressure water to its desired temperature.

The temperature of mains-pressure water exiting the heat exchanger may then rise to a dangerous level and lead to scalding. Progressively varying the tlow-rate of the thermal store fluid in response to the flow-rate of mains-pressure water may help to maintain at all times a rate of heat exchange between the thermal store fluid and the mains-pressure water which is optimal to heat the mains-pressure water to the desired target temperature.

By responding progressively to the flow rate of the mains-pressure water, the pump may respond very rapidly without any time-lag caused by waiting for a temperature change. For example, a more rapid increase in mains-pressure water temperature, obtained by responding to mains-pressure water flow rate, is illustrated in Figure 4. Measuring only the temperature may lead to a slower increase in mains-pressure water temperature.

Measuring and responding to the flow-rate of mains-pressure water may provide a simple and effective mechanism by which scalding can be avoided, especially if heated mains-pressure water is domestic hot water. For example, if mains-pressure water flow rate suddenly decreases, the flow rate of thermal store fluid may rapidly decrease in response to prevent more heat being transferred to the mains-pressure water than is necessary.

In a preferred example, the system has a flow sensor arranged to sense the s flow rate of mains-pressure water, the pump-speed controller being responsive to the flow rate measured by the flow sensor. The pump-speed controller may be linked with or connected to the flow sensor and the pump such that the pump-speed controller can respond to the flow rate and modulate a progressive change in pump speed. The link may be an electrical connection established by an electrical wire.

For example, the flow sensor may generate an output representative of the flow rate of mains-pressure water, which is input to the pump-speed controller.

The pump-speed controller may then calculate and generate an output to control the pump.

The pump-speed controller may act by providing a modified power supply, such as a modified mains power supply, to a fixed-speed pump. Alternatively, a variable-speed pump may be used, such that the pump varies its speed in response to a signal generated by the pump-speed controller, such as a O-5V, 0-by, PWM (pulse-width modulated) signal or a 4-2OmA signal.

In a particularly preferred example, the flow rate of thermal store fluid through the heat exchanger is controlled in response to a temperature of mains-pressure water exiting the heat exchanger. Preferably, the pump-speed controller is responsive to the temperature of the mains-pressure water exiting the heat exchanger, the pump-speed controller being able to vary the pump speed in response to mains-pressure water temperature. Thus, the pump-speed controller may additionally be connected to a temperature sensor and to the pump.

The temperature sensor may generate an output representative of the temperature of mains-pressure water, which is input to the pump-speed controller. The pump-speed controller may then calculate and generate an output to control the pump.

The output to the pump may be adjusted depending on deviation from a target S temperature. The target temperature may be set using, for example, a dial, which provides a target temperature input to the pump-speed controller.

In such a system, the flow rate of mains-pressure water may provide a control parameter, with the temperature of the mains-pressure water providing a io feedback parameter. The pump may initially be configured to operate at a specific speed at a specific mains-pressure water flow-rate. If the temperature of mains-pressure water then deviates from the target temperature, the specific pump speed at the specific flow rate may be adjusted. For example, if the actual temperature of mains-pressure water exiting the heat exchanger is above the target temperature, the specific pump speed at the specific flow rate may be reduced and if the actual temperature of mains-pressure water exiting the heat exchanger is below target temperature, the specific pump speed at the specific flow rate may be increased. This advantageous effect is demonstrated in Figure 5.

Consequently, substantially instantaneous adjustments to pump speed may be made in response to the flow rate of the mains-pressure water. These very rapid adjustments in response to the flow rate of mains-pressure water may allow the temperature of the mains-pressure water exiting the heat exchanger to quickly achieve a steady temperature which, in accordance with an initial setting or calibration of the controller, may advantageously be close to the target temperature. Fine-tuning of the pump speed may then be made using the measured temperature of the mains-pressure water exiting the heat exchanger as a feedback parameter.

Alternatively or additionally, a temperature sensor may monitor temperature at another area of the system to provide an additional feedback parameter. For example, there may be a temperature sensor arranged to monitor a temperature of thermal store fluid upstream of the heat exchanger and/or a temperature sensor arranged to measure the temperature of thermal store fluid in the thermal store, such as at the top of the thermal store. For example, if the temperature of thermal store ftuid is above a pie-determined value, such as fiom 75 to 85°C, preferably about 80°C, pump speed may be reduced.

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There may be a temperature sensor monitoring temperature of mains-pressure water entering the heat exchanger. For example, if the temperature of the mains-pressure water is above a pie-determined value, the pump speed may be reduced and if the temperature is below a pre-determined value, the pump io speed may be increased.

Each temperature sensor may be used individually or in combination with one or more other temperature sensors which input to the pump-speed controller.

Using additional temperature sensors may provide more feedback to the pump-speed controller and provide a greater degree of control of the mains-pressure water temperature.

Temperature sensors are typically silicon-based, providing digital signals to the controls, but may also be thermocouples, thermistors or any other suitable sensor.

A combined sensor, comprising both a flow-rate sensor and a temperature sensor may be used. This is particularly advantageous if a flow rate and a temperature are being monitored in the same location, for example in a mains-pressure water outlet conveying mains-pressure water away from the heat exchanger.

The deviation of a sensed temperature from a target temperature of the mains-pressure water may generate or adjust an offset factor which can be used to calibrate the pump-speed controller. For example, at a particular mains-pressure water flow rate, the pump may be run at a pre-determined speed.

After a short delay of a few seconds (such as ito 10 seconds, or 3 to 6 seconds) the measured mains-pressure water temperature is compared to the target temperature, and if there is a difference, the offset factor is generated or adjusted. The offset factor may be stored in the memory of the pump-speed controller and used to modify the pump-speed controller output.

An advantageous effect of systems embodying the invention as described above is demonstrated in Figure 4. This shows that progressively controlling the flow of primary fluid from the heat store through the heat exchanger in response to mains-pressure water flow rate may lead to a more rapid response than controlling the flow of primary fluid in response to the temperature of the mains-pressure water.

Figure 4 also shows that sensing both the flow rate (as the control parameter) and the temperature (as the feedback parameter) of the mains-pressure water may lead to a similarly rapid response but may also allow a more accurate attainment of the target temperature.

Preferably, the optimum temperature for mains-pressure water output is 45 to 60°C, most preferably to 50 to 55°C.

In a preferred embodiment of the invention, the heat exchanger may be a plate heat exchanger. Preferably, the heat exchanger is external to the thermal store i.e. located outside of the thermal store, but it may be located inside of the thermal store to, for example, hide it from view. If the heat exchanger is located on the inside of the thermal store, it is preferable for the heat exchanger to be located nearer to the bottom of the thermal store, to reduce scaling.

In a preferred aspect of the invention, thermal store fluid may be circulated through the heat exchanger by the pump from a higher section of the thermal store to a lower section of the thermal store. It is most preferred that thermal store fluid is circulated through the heat exchanger from substantially the top of the thermal store to substantially the bottom, such that cooler thermal store fluid re-enters the bottom of the thermal store.

When a thermal store of this type is used, a further advantage of a water-heating control system embodying the invention is to minimise the flow of hot water from the top of the thermal store to the bottom, as follows.

s The system may comprise a second heat exchanger external for heating thermal store fluid, in addition to the heat exchanger heating mains-pressure water (i.e. the first heat exchanger). The second heat exchanger may transfer heat from water or other fluid heated by a boiler or any other heating means, to thermal store fluid. Preferably, thermal store fluid is circulated through the io second heat exchanger by a pump from a lower section of the thermal store to a higher section of the thermal store, most preferably from substantially the bottom of the thermal store to substantially the top.

The second heat exchanger is preferably a plate heat exchanger. The second heat exchanger is preferably external to the thermal store.

The inventor has recognised advantages associated with maintaining a temperature differential in the thermal store. Keeping hotter water at the top of the thermal store and colder water at the bottom, may greatly improve efficiency of heat exchange at the second heat exchanger, by minimising the temperature of thermal-store water entering the second heat exchanger.

Embodiments of the invention may maintain the desired temperature difference in the thermal store as follows.

At high mains-pressure water flow rates, the thermal store fluid exiting the mains-pressure water (first) heat exchanger and returning to the base of the thermal store may be at a relatively low temperature, as much of its heat will be drawn away from the thermal store by the mains-pressure water. However, at low mains-pressure water flow rates, if the flow rate of thermal store fluid through the second heat exchanger is maintained, the temperature of the thermal store fluid exiting the first heat exchanger will increase as less heat is drawn away by the mains-pressure water. This leads to a significant problem because warm water is delivered to the base of the thermal store which increases thermal store fluid temperature at the bottom or lower section of the thermal store. When water is then circulated from the bottom of the thermal store to the boiler heat exchanger for receiving heat from the boiler, the temperature difference at the boiler heat exchanger is disadvantageousty reduced. This may be particularly disadvantageous for water heaters which s are constitutively active, such as in some combined heat and power systems where, for example, heat from hot flue gases is recovered.

The pump-speed control mechanism may advantageously minimise the flow of thermal store fluid through the mains-pressure water heat exchanger at all io times, while providing sufficient flow to heat the mains-pressure water to the required temperature, and so ensure that thermal store fluid returning to the base of the thermal store is as cool as possible. Consequently, the temperature of water circulated to the second heat exchanger is likewise kept low.

Preferably, flow rate of thermal store fluid circulating through the first heat exchanger is not controlled by a valve. By using a pump which is controlled in response to flow rate and, optionally, temperature, a valve may not be necessary. This may advantageously save costs.

In a further aspect, the invention may also provide a method of controlling water heating, comprising sensing a flow rate of mains-pressure water passing through a heat exchanger and, in response to the flow rate, progressively varying a speed or output of a pump circulating thermal store fluid from a thermal store through the heat exchanger.

Preferably, the method comprises additionally sensing the temperature of mains-pressure water exiting the heat exchanger and using the sensed temperature as a feedback parameter, so as to vary the speed or output of the pump in response to the mains-pressure water flow rate and mains-pressure water temperature exiting the heat exchanger.

The method may comprise sensing the temperature of thermal store fluid entering the heat exchanger from the thermal store, sensing the temperature of mains-pressure water entering the heat exchanger and/or sensing the temperature of thermal store fluid in the thermal store. Pump speed may be varied in response to the or each temperature.

s In a still further aspect, the invention may also provide a pump-speed controller for use in a system or method as described above. The pump-speed controller may thus be configured to receive an input representative of a flow-rate of mains-pressure water through a heat exchanger and to generate an output for controlling a pumps speed in proportion to the flow rate of mains-pressure water.

The pump-speed controller may, for example, provide a modified mains power supply to a fixed-speed pump. Alternatively, a variable-speed pump may be used, such that the pump varies its speed in response to a signal generated by the pump-speed controller, such as a 0-5V, 0-1 OV, PWM (pulse-width modulated) signal or a 4-2OmA signal.

The pump-speed controller may also be configured to receive an input representative of a water temperature and to generate an output for controlling the pump's speed in response to the water temperature, as described above.

The pump-speed controller may be configurable or programmable to vary the temperature of mains-pressure water exiting the heat exchanger. This may be achieved by, for example, adjusting how the pump responds to a particular flow rate of mains-pressure water.

In a preferred embodiment, the pump-speed controller may be configured to achieve a desired target mains-water temperature. The pump-speed controller may be configured to receive a target temperature input and adjust its output depending on how much the target temperature deviates from a measured temperature. The pump-speed controller may also be configured to calculate and respond to an offset factor, as described above.

In a further aspect, the invention may provide a kit comprising a pump-speed controller as described above and a set of instructions directing a user to link the pump-speed controtter to a mains-pressure water ftow sensor, to a pump circulating thermal store fluid from a thermal store through a heat exchanger, s and optionally to a mains-pressure water temperature sensor.

In a still further aspect, the invention may also provide a kit for assembling a water-heating system as described above comprising a pump-speed controller, a flow rate sensor for sensing a flow of mains-pressure water, a pump responsive to an output of the pump-speed controller, a heat exchanger, a thermal store and, optionally, a temperature sensorfor sensing a temperature of mains-pressure water exiting the heat exchanger. The kit may comprise instructions to place the flow sensor in a mains-pressure water circuit circulating water through the heat exchanger, to place the pump in a thermal store fluid circuit circulating water from the thermal store through the heat exchanger, to link the pump-speed controller to the flow sensor and to the pump and optionally, to place the temperature sensor of the mains-pressure water circuit such that it measures a temperature of mains-pressure water exiting the thermal store and to link it to the pump controller.

The kit may optionally comprise a temperature sensor for measuring a temperature of thermal store fluid entering the heat exchanger from the thermal store and instructions to place it in position to measure the temperature of the thermal store fluid entering the heat exchanger from the thermal store, and to link it to the pump controller.

The kit may optionally comprise a temperature sensor for measuring a temperature of mains-pressure water entering the heat exchanger and instructions to place it in position to measure the temperature of the mains-pressure water entering the heat exchanger, and to link it to the pump controller.

The kit may optionally comprise a temperature sensor for measuring a temperature of thermal store fluid in the thermal store and instructions to place it in position to measure the temperature of the thermal store fluid in the thermal store, and to link it to the pump controller.

The kit may comprise a thermal store or it may comprise only components s listed above as required to install a system embodying the invention in combination with a pre-existing thermal store.

Description of Specific Embodiments of the Invention Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a water-heating control system according to an embodiment of the invention; Figure 2 is a schematic diagram of a water-heating control system according to another embodiment of the invention; Figure 3 is a schematic diagram of a water-heating control system according to yet another embodiment of the invention; Figure 4 is a graph illustrating improved control of mains-pressure water using embodiments of the invention; and Figure 5 is graph illustrating control of pump speed in response to mains-pressure water flow rate and mains-pressure water temperature according to an embodiment of the invention.

A water-heating control system 1 as shown in Figure 1 comprises a thermal store 3 and a plate heat exchanger 5 external to the thermal store.

A primary fluid circuit 6, in the form of a thermal store water circuit, is arranged between a thermal store outlet 15 and a thermal store inlet 17, passing through the heat exchanger 5 and a pump 7. The thermal store outlet is at the top of the thermal store 3 and the thermal store inlet is at the bottom of the thermal store.

A secondary fluid circuit 8 in the form of a mains-pressure water circuit is s arranged between a mains-pressure water inlet 19 and a mains-pressure water outlet 21. The secondary fluid circuit passes through the heat exchanger 5.

A flow sensor 11 and a temperature sensor 13 are situated in the secondary water circuit. The flow sensor and temperature sensor are each electronically io linked to a pump-speed controller 9. The pump-speed controller is also electronically linked to the pump 7.

Primary fluid, in the form of thermal store water, is stratified into layers at different temperatures within the thermal store 3. Water at the hottest, target temperature 23 is at the top and cooler water 25 is at the bottom. There is a separation layer 27 between the top and bottom layers.

In use, thermal store water is heated using a boiler or any other suitable heating means such as a solar heater or wood burner (not shown). Water heated by a boiler or other such means may be conveyed directly into the thermal store itself. Alternatively, heated water may be used to heat thermal store water using a second heat exchanger (not shown).

Thermal store water is circulated around the primary fluid circuit. Hot water 23 at the top of the thermal store 3 exits the thermal store through the thermal store outlet 15 and passes through the heat exchanger 5. As heat exchange occurs, thermal store water is cooled and is delivered to the bottom of the thermal store at the thermal store inlet 17. Thermal store water is circulated around the primary fluid circuit 6, through the heat exchanger 5, by the pump 7.

Mains-pressure cold water circulates through the secondary water circuit from the mains-pressure water inlet 19 to the mains-pressure water outlet 21, passing through the heat exchanger 5. In the heat exchanger, heat from the thermal store water is transferred to the mains-pressure water such that heated mains-pressure water exits the heat exchanger through the mains-pressure water outlet 21 to provide a supply of domestic hot water. Heated mains-pressure water may be delivered to, for example, domestic hot water appliances such as taps or showers (not shown).

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Water exiting the heat exchanger 5 through the mains-pressure water outlet 21 is monitored for its flow rate and its temperature by the flow rate sensor 11 and the temperature sensor 13. The sensors generate two voltage signals, typically in the range of 0 -3.5V. Signals from the sensors are transmitted to io the pump-speed controller 9. The pump-speed controller, in response to the signals, calculates the required pump speed and transmits a signal to the pump 7. The pump speed is set in response to the flow rate and to the temperature. The flow rate is used as the primary control parameter and the temperature signal, which provides a measured temperature difference between the mains-pressure water temperature and a target temperature, is used as a feedback parameter to fine-tune the pump speed and to minimise the extent to which the actual temperature deviates from the target temperature.

The pump-speed controller 9 is pre-programmed with data that allows it to calculate the required pump speed from the input signals. The pump-speed controller is configured to receive an input signal indicative of the target temperature (typically 50-55°C) of mains-pressure water exiting the heat exchanger 5. A control dial (not shown) allows selection of a desired target temperature.

If the pump speed initially set by the pump-speed controller 9 in response to the flow rate of the mains-pressure water does not result in attainment of a target temperature, a feedback control is initiated. If, for example, the temperature sensed by temperature sensor 13 is below target temperature, the pump speed of the pump 7 will be increased.

Deviation from the target temperature is monitored by the pump-speed controller 9 which generates an offset factor and stores the offset factor in its internal memory. Initially, there is no offset factor or the offset factor is set to zero. When water flow is detected by the flow rate sensor 11, the pump 7 runs at a catculated speed. After a detay of a typicatty 2 to 5 seconds, the temperature sensed by the temperature sensor 13 is checked against the s target temperature, and if there is a difference, the offset factor is generated or adjusted. This process is repeated at predetermined intervals whenever mains-pressure water is flowing. The offset factor is taken into account for further control of the pump speed, and is stored for use the next time mains-pressure water flow is initiated.

The pump-speed controller 9 can control the speed of a fixed-speed pump by providing a modified mains a/c power supply to the pump. The pump-speed controller operates by clipping the supply to remove a number of cycles. The pump speed will reduce as more cycles are clipped out. At very low speeds, the pump will need pulsing to avoid stalling.

However, variable pump speed may be achieved in a number of ways. For example, an alternative method is to use a variable-speed pump that adjusts its speed in response to an input signal from the pump-speed controller 9. This will typically be a O-5V, 0-WV, 4-20m or a pulse-width modulated (PWM) signal.

A water-heating control system 1 as shown in Figure 2 contains the same elements as described for the system of Figure 1. Similar features have been given like reference numerals. However, the system of Figure 2 also contains some additional features.

A first primary fluid circuit 6 passes through a first heat exchanger Sand a first pump 7 between a first thermal store outlet 15 and a first thermal store inlet 17.

A second plate heat exchanger 29 external to the thermal store 3 is arranged in a second primary fluid circuit 30, between a second thermal store outlet 35 and a second thermal store inlet 33. The second thermal store outlet is at the bottom of the thermal store whereas the second thermal store inlet is at the top of the thermal store. The second thermal store inlet 33 is connected to the thermal store at substantially the same height as the first thermal store outlet 15. Similarly, the second thermal store outlet 35 is connected to the thermal store at substantially the same height as the first thermal store inlet 17. A s second pump 31 is connected to the second primary fluid circuit.

A boiler or any other suitable heating source (not shown) is connected to the heat exchanger by a boiler water inlet 37 and a boiler water outlet 39.

io In operation, cooler thermal store water 25 at the bottom of the thermal store 3 exits the thermal store through the second thermal store outlet 35 and is conveyed, with the aid of the second pump 31, through the second heat exchanger 29. Heated water exits the second heat exchanger 29 and is delivered to the top of the thermal store by the second thermal store inlet 33.

Hot water or any other suitable fluid is delivered to the second heat exchanger from the boiler through the boiler water inlet 37 and cooler water is delivered from the heat exchanger back to the boiler through the boiler water outlet 39.

Heat from the boiler fluid is transferred to thermal store water at the second heat exchanger 29, such that hot water 23 is fed to the top of the thermal store 3. During operation, the temperature of water at the top of the thermal store is about 80°C or higher and temperature of water at the bottom of the thermal store is about 50°C or lower.

As the speed of the pump 9 driving thermal store water through the first heat exchangers is progressively controlled in response to the flow rate and the temperature of mains-pressure water exiting the heat exchanger, the volume of the thermal-store water flowing through the first heat exchanger is minimised and the temperature differential in the thermal store 3 is advantageously maintained as far as possible, regardless of the demand for hot mains-pressure water.

For example, if mains-pressure flow rate decreases, the pump 9 may rapidly respond to this and progressively decrease the flow of thermal store water through the first heat exchanger 5. This prevents hot water in excess of 50°C, from entering the bottom of the thermal store. Cooler thermal store water is s thus consistently delivered to the second heat exchanger 29, maintaining efficiency of heat exchange.

A water-heating control system 1, as shown in Figure 3, contains many similar features as described above for the embodiments shown in Figure 1 and io Figure 2. Similar features have been given like reference numerals.

A plate heat exchanger 5, is situated on the inside of the thermal store 3. A first pipe 115 is also situated within the thermal store. The first pipe is connected to the heat exchanger and has an open end positioned in hot water 23 at the top of the thermal store.

A second pipe 117 connects the heat exchanger with the pump 7, the pump being situated on the outside of the thermal store 5. A thermal store inlet 17 is at the bottom of the thermal store. Therefore, a primary fluid circuit 6 is arranged between the open end of the first pipe 115 and the thermal store inlet 17. The primary fluid circuit is located partially on the inside and partially on the outside of the thermal store.

A secondary fluid circuit 8 is arranged between a mains-pressure water inlet 19 and a mains-pressure water outlet 21. The secondary fluid circuit is thus also located partially on the inside and partially on the outside of the thermal store 3.

A flow sensor 11 and temperature sensor 13 are located in the secondary fluid circuit, on the outside of the thermal store 5. A pump-speed controller 9, electronically linked to the flow sensor, the temperature sensor and the pump 7, is also located on the outside of the thermal store.

In use, the thermal store water enters the first pipe 115 at the top of the thermal store Sand is delivered to the heat exchanger 5. The second pipe 117 conveys water from the heat exchanger to the outside of the thermal store and to a pump. The thermal store water is delivered from the pump to the bottom of the thermal store through the thermal store inlet 17.

The flow rate of the thermal store water in the primary fluid circuit is controlled by the pump-speed controller 9 in response to a flow rate of mains-pressure water and a temperature of mains-pressure water, as described above for the embodiments of Figures 1 and 2.

Claims

<claim-text>Claims 1. A water-heating system comprising: a thermal store containing a thermal store fluid; a heat exchanger for heating mains-pressure water using the thermal store fluid; a pump arranged to circulate the thermal store fluid through the heat exchanger; and a pump-speed controller which, in use, progressively controls a pump speed or output in response to a flow rate of the mains-pressure water through the heat exchanger.</claim-text> <claim-text>2. A system according to claim 1 comprising a flow sensor arranged to measure the flow rate of the mains-pressure water, the pump-speed controller is being responsive to the flow rate measured by the flow sensor.</claim-text> <claim-text>3. A system according to claim 1 or claim 2, in which, in use, the pump-speed controller additionally controls the pump speed or output in response to a temperature of the mains-pressure water or the thermal store fluid.</claim-text> <claim-text>4. A system according to claim 3, comprising a temperature sensor arranged to measure a temperature of the mains-pressure water exiting the heat exchanger, the pump-speed controller being responsive to the temperature measured by the temperature sensor.</claim-text> <claim-text>5. A system according to claim 3 or claim 4 comprising: a temperature sensor arranged to measure a temperature of the thermal store fluid upstream of the heat exchanger; and/or a temperature sensor arranged to measure a temperature of the thermal store fluid within the thermal store; and/or a temperature sensor arranged to measure a temperature of the mains-pressure water upstream of the heat exchanger, in which the pump-speed controller is responsive to temperature measurements at the or each sensor.</claim-text> <claim-text>6. A system according to any preceding claim in which the thermal store ftuid can be circulated through the heat exchanger by the pump from a higher section of the thermal store to a lower section of the thermal store.</claim-text> <claim-text>7. A system according to claim 6 in which the thermal store fluid can be circulated through the heat exchanger from substantially the top of the thermal store to substantially the bottom of the thermal store.</claim-text> <claim-text>8. A system according to any preceding claim which comprises a heat exchanger for heating the thermal store fluid.</claim-text> <claim-text>9. A system according to claim 8 in which the thermal store fluid can be circulated through the heat exchanger for heating the thermal store fluid, from is a lower section of the thermal store to a higher section of the thermal store.</claim-text> <claim-text>A system according to claim 9 in which the thermal store fluid can be circulated through the heat exchanger for heating the thermal store fluid, from substantially the bottom of the thermal store to substantially the top of the thermal store.</claim-text> <claim-text>11. A method of controlling water heating comprising: sensing a flow rate of mains-pressure water passing through a heat exchanger; and progressively varying a speed or output of a pump arranged to circulate a thermal store fluid from a thermal store through the heat exchanger, in response to the flow rate of the mains-pressure water.</claim-text> <claim-text>12. A method according to claim 11, comprising sensing a temperature of the mains-pressure water exiting the heat exchanger; and varying the speed or output of the pump in response to the temperature of the mains-pressure water.</claim-text> <claim-text>13. A method according to claim 11 or claim 12 comprising: sensing a temperature of the thermal stole fluid upstream of the heat exchanger; and/or sensing a temperature of the thermal store fluid in the thermal store; and/or sensing a temperature of the mains-pressure water upstream of the heat exchanger; and varying the speed or output of the pump in response to the or each temperature.</claim-text> <claim-text>14. A pump-speed controller for a system according to any of claims 1 to 10, configured to receive an input representative of a mains-pressure water flow rate and to produce an output for progressively controlling a pump's speed or output in response to the mains-pressure water flow rate.is 15. The pump-speed controller according to claim 14, configured to receive an input representative of a water temperature and to transmit an output for controlling the pump's speed in response to the water temperature.16. A kit comprising: a pump-speed controller configured to receive an input representative of a mains-pressure water flow rate and to produce an output for progressively controlling a pump's speed or output in response to the mains-pressure water flow rate; and a set of instructions to connect the pump-speed controller to a mains-pressure water flow sensor and to a pump circulating a thermal store fluid from a thermal store through a heat exchanger 17. A kit according to claim 16, in which the pump-speed controller is configured to receive an input representative of a water temperature and to produce an output for controlling the pump's speed or output in response to the water temperature, and in which the kit comprises a set of instructions to connect the pump speed controller with a water temperature sensor.18. A kit for assembling a water-heating system of any of claims 1-10 comprising: a pump-speed controller configured to receive an input representative of a water flow rate and to produce an output for progressively controlling a pump's speed or output in response to the water flow rate; a flow rate sensor for sensing a flow rate of mains-pressure water; a pump responsive to an output signal operated by the pump-speed controller; a heat exchanger; and optionally, a water temperature sensor.19. A water-heating control system substantially as hereinbefore described with reference to the drawings.20. A method substantially as hereinbefore described with reference to the drawings.21. A pump-speed controller substantially as hereinbetore described with reference to the drawings.22. A kit substantially as hereinbefore described with reference to the drawings.</claim-text>