GB2576327A

GB2576327A - A heater and a method of operating such a heater

Info

Publication number: GB2576327A
Application number: GB1813239.9A
Authority: GB
Inventors: Forrest Justin; Farrell Christopher
Original assignee: Canetis Metering Ltd
Current assignee: Canetis Metering Ltd
Priority date: 2018-08-14
Filing date: 2018-08-14
Publication date: 2020-02-19
Anticipated expiration: 2038-08-14
Also published as: GB201813239D0; GB2576327B

Abstract

A heater 2, such as a gas-fuelled boiler, used in combination with a thermal store 4 that is used to heat liquids such as water prior to entering the heater. The heater has a controller 52 and flow control means 50 such as a valve that are used to control a heat flow path from the thermal store to the liquid to be heated, with the controller being adapted to control the flow control device based on an expected hot liquid demand and energy price as a function of time. The thermal store can be a flue gas heat recovery device with a heat exchanger 20 to define a water flow path for heating the water. A method of operating a heater in combination with a thermal store based on a use pattern of hot water and a fuel tariff, wherein taking heat from the thermal store is prevented if a cost of an expected demand for hot water is expected to be reduced by conserving heat in the thermal store is also claimed

Description

A HEATER AND A METHOD OF OPERATING SUCH A HEATER

Field

The present disclosure relates to a heater or a heating system and to a method of operating a heater or a heating system.

Background

With the advent of combination boilers the hot water demand for washing and the like is serviced instantaneously, i.e. by burning gas at the time of use. However operation in this hot water mode typically takes the boiler outside of its condensing mode and hence the boiler is not as efficient as when it is providing water for space heating purposes where the rate of gas burn at the boiler is generally much lower and the flue temperature is controlled to be around the dew point.

More recently flue gas heat recovery devices have become more readily available. These allow the exiting flue gas to warm a thermal store which is held in the exhaust flue gas heat recovery device. The thermal store can give its heat up to water passing to the cold water inlet of the boiler when the boiler is operated in a hot water mode, i.e. for generating hot water for washing and the like (as opposed to operating in a space heating mode).

With the roll out of so called smart meters the energy companies have the ability to monitor gas usage as a function of time and in particular are able to apply different tariffs at different times of the day. This contrasts with the current range of mechanical meters which can merely sum the volume of gas used but have no knowledge about when the gas was used, either in terms of the day on which it was used or the time at which the gas was used. Thus the energy companies have to apply a uniform tariff to gas being monitored by one of the conventional non-smart meters. The same comments apply to electrically powered hot water and space heating systems.

The inventor realised that there could be financial advantages to modifying the operation of the boiler or heater and the thermal store to take account of the usage pattern of the boiler or heater and the tariff information.

Summary

According to a first aspect of the present disclosure there is provided a heater operable to warm water, the heater being in combination with a thermal store where water to be heated can receive heat from the thermal store prior to being admitted into the heater for further heating. The heater also comprises a controller and a flow control device responsive to the controller. The flow control device is adapted to control a heat flow path from the thermal store to the water to be heated. The controller is adapted to control the flow device based on an expected hot water demand and an energy price as a function of time.

It is thus possible for the controller to inhibit extraction of heat from the thermal store for a hot water use, such as a hot water draw, (e.g. extracting for water at a tap / faucet, a shower of the like) in preference to saving it for an expected hot water use at a later time, where the hot water use occurs at a higher tariff.

The heater may be an electric heater or the heater may be a gas fuelled heater, such as a boiler.

Preferably the controller is responsive to a temperature sensor associated with the thermal store such that the controller can estimate the amount of usable energy that can be extracted from the thermal store. The controller may use this information to modify the rate at which the energy is extracted from the thermal store.

In some embodiments a thermal store heater may be provided and/or the gas fuelled heater may be operable to heat the thermal store.

Preferably where the heater is a gas fuelled boiler the thermal store is part of a flue gas heat recovery device.

Advantageously the controller may be operable to modify the operation of a gas fuelled heater, for example to change the gas burn rate and thereby modify the flue gas temperature so as to precharge the thermal store with thermal energy in advance of a tariff change from a lower tariff to a higher tariff. Such a modification may be done during a heating operation relating to the drawing of hot water, or during operation of the heater in a space heating mode.

According to a second aspect of the present disclosure there is provided a method of operating a heater having or being in combination with a thermal store, the method comprising, in response to a demand for hot water, examining a use pattern and the fuel tariff and selectively inhibiting withdrawal of heat from the thermal store if an estimate of cost to service an expected future demand in conjunction with the present water demand is likely to be reduced by conserving heat in the thermal store.

The hot water may be domestic hot water for washing and cleaning. However, the same approach can also be applied to the production of hot water for space heating.

Brief Description of the Drawings

Embodiments of the present disclosure will now be described, by way of non-limiting example only with reference to the accompanying Figures, in which:

Figure 1 is a schematic diagram of a combination boiler and flue gas heat recovery unit in accordance with the teachings of the present disclosure;

Figures 2A to 2C represent the temperature of the thermal store, the expected hot water demand, the actual hot water demand, and the gas tariff as a function of time in order to exemplify operation of an embodiment of the present disclosure;

Figure 3A to 3D show the thermal store temperature, the expected hot water demand, the actual hot water demand and the gas tariff for a boiler operating with a thermal store, but without implementing the additional control steps constituting embodiments of the present disclosure;

Figures 4A to 4D show the thermal store temperature, expected hot water demand, actual hot water demand and gas tariff price for a boiler operating in accordance with the teachings of the present disclosure;

Figure 5 shows operation of an embodiment of the present disclosure where the tariff changes during a water drawing operation;

Figure 6 shows a further boiler and flue gas heat recovery combination in accordance with teachings of the present disclosure;

Figure 7 shows a further embodiment of the present disclosure;

Figure 8 shows an embodiment where the heat store recovers heat from a space heating circuit; and

Figure 9 is a flowchart of a method of operating a boiler or a water-heater in accordance with the teachings of this disclosure.

Description of some Embodiments of this Disclosure

Figure 1 diagrammatically illustrates a gas fuelled boiler 2 in combination with a flue gas heat recovery device 4. Typically, though not necessarily, the boiler 2 is a combination gas fuelled boiler operable to provide warmed water for washing and the like, herein referred to as domestic hot water, together with warmed water for space heating purposes, herein referred to as space heating water. For simplicity, the space heating circuit has not been shown in Figure 1. The boiler 2, which is a conventional device available from many manufacturers has a cold water inlet 10 connected to a cold water main, a hot water outlet 12, a gas inlet 14 and the flue gas outlet 16. In response to a demand for domestic hot water, such a demand being detectable by water flow commencing through the boiler, the boiler 2 opens its gas valve to admit gas into a combustion chamber of the boiler, ignites the gas and uses the heat from the gas to warm water in a primary heat exchanger. The warmed water from the primary heat exchanger is then passed through a secondary heat exchanger, generally in the form of a plate heat exchanger, such that it gives up heat to water passing through the plate heat exchanger. The plate heat exchanger has one of its flow paths connected between the cold water inlet 10 and the hot water outlet 12. In this way water flowing through the primary heat exchanger becomes warmed and gives its heat up to the cold water flowing through the secondary heat exchanger in order to form the domestic hot water which exits the boiler at the outlet 12. In such a mode of operation, the hot water is always formed instantaneously and on demand.

In older combination boiler systems the combustion gases exit the flue 16 and pass directly to the atmosphere. However in more modern systems the flue gas from the flue 16 is directed into the flue gas heat recovery device 4 where it gives up heat to a thermal store 18 within the flue gas heat recovery device 4. The thermal store 18 contains a heat storage medium which is typically formed by condensate which has been recovered from the flue gas itself. This provides for a self-replenishing heat storage medium within the thermal store which is open to atmosphere, and hence does not represent any explosion risk in the event of overheating. A suitable flue gas heat recovery device is disclosed in EP1809967B and US7415944. The flue gas heat recovery device 4 includes a heat exchanger 20 which is in a fluid flow path having an inlet, generally designated 22 and an outlet generally designated 24. The inlet 22 is, in the example shown in Figure 1, in direct connection to a cold water main 30 which provides the cold water to be heated. The outlet 24 is connected to a first port of a mixing valve 40. The cold water main is connected to a second port of the mixing valve 40 and an outlet of the mixing valve is provided to the boiler's cold water inlet 10.

In known heating systems having a flue gas heat recovery device 4 as soon as the hot water draw is requested, the mixing valve 40 is operable to blend water from the cold main 30 with water passing through the flue gas recovery device heat exchanger 20, so as to achieve a target temperature of around 25°C for the water arriving at the cold water inlet 10 of the boiler 2. The mixing valve 40 may be a thermostatic valve.

The present disclosure modifies the prior art arrangement by including means, such as flow control valve 50 responsive to a controller 52 for inhibiting the removal of heat from the thermal store 18. In the arrangement shown in Figure 1, this is achieved by placing the valve 50 in series with the heat exchanger 20 such that when the valve 50 is closed no water can flow through the heat exchanger 20 and from there into the mixing valve 40. It should be noted that other valve configurations are possible. For example, a controllable valve may be placed in parallel with the heat exchanger 20 so as to provide a bypass route if so desired. In a further embodiment the mixing valve 40 may be motor controlled so as to determine the degree of blending between fluids at its first and second ports, including the option to close one of the ports, such as the first port, in its entirety.

Advantageously, though not strictly necessarily, a temperature sensor 60 is provided in contact with the thermal store 18 or with the pipework immediately exiting the heat exchanger 20 so as to estimate the temperature of the heat storage medium in the thermal store. An output of the temperature sensor 60 is provided to the controller 52 such that the controller can have knowledge of the effective amount of useful energy contained within the thermal store 18.

It is worthwhile considering the operation of such a boiler and thermal store combination, both when it is operating in accordance with the teachings of the present invention and when it is not, for example because the controller 52 has been disabled or because no usage patterns have been learnt or input into the controller 52, for example at installation.

Suppose that a combination boiler is operable to provide space heating, either for the entirety of the night or, for example, from 05:00 hours onwards such that operation of the space heating system causes the thermal store 18 to be warmed and the heat storage medium therein to have acquired a store temperature T1 as indicated in Figure 2A. Suppose also, that a householder gets up in the morning and regularly showers, commencing the shower at 06:30 hours and finishing the shower at 06:35 hours. In this example, the controller 52 has either learnt or been pre-programmed with a hot water demand schedule and expects the shower to last from 06:30 to 06:35 hours and has learnt how to control the flow rate through the thermal store such that the temperature of the heat storage medium within the thermal store drops from temperature T1 at 06:30 hours to temperature T2 at 06:35 hours as indicated by line 70 where, in this example, T2 represents the effective exhaustion of the energy held within the thermal store. Thus, in the example of Figure 2 the expected hot water demand shown in Figure 2B matches with the actual hot water demand shown in Figure 2C. Figures 2B and 2C have the hot water demand indicated by the regions 80 and 82 respectively where region 80 represents the amount of energy imparted to the water by burning of gas, and region 82 represents the amount of energy imparted into the water by virtue of passing the cold main through the thermal store of the flue gas heat recovery device 4. By way of further information, Figure 2D shows the gas tariff which indicates that some time just before 06:30 hours the gas price rises from a relatively low tariff indicated by TARIFF 1 to a higher tariff indicated by TARIFF 2.

Figure 3 indicates the modified scenario of Figure 2 where two additional hot water draws occur before the shower commences at 06:30 hours. The first additional hot water draw commences at 06:20 hours and lasts for a couple of minutes. A further hot water at a higher flow rate occurs at around 06:25 hours and lasts for about a minute as shown in Figure 3C. For a conventional combination boiler and thermal store arrangement not operating with the teachings of the present disclosure, each of the water draws 90 and 92 causes some water from the cold main to pass through the thermal store. It is to be expected as, from the boiler's point of view, this represents the most efficient use of gas. As a result, the temperature of the heat storage medium in the thermal store takes a different temperature trajectory, as indicated by the chain line 100 in Figure 3A. Specifically, it can be seen that the temperature of the heat storage medium in the thermal store drops as a result of the first draw 90, then stabilises, then drops again as a result of the second draw 92, such that by the time the shower is commenced at 06:30 hours the temperature of the heat storage medium of the thermal store has dropped to a value T3 intermediate TI and T2. As a result, the thermal store becomes depleted of energy before the shower is completed. This is shown in Figure 3C where the region 82 representing the heat contribution from the thermal store ceases before the shower ceases resulting in a period, extending from about 06:33 hours to 06:35 hours where all of the heat for the shower is provided by combusting gas. It can therefore be seen that a proportion of the shower is made by combusting gas at a higher rate tariff, TARIFF 2. Depending on the difference between the tariffs this may have a significant cost impact over an extended period of time.

In contrast, Figure 4A schematically illustrates the operation of the same boiler and flue gas heat recovery system, but in accordance with the teachings of the present disclosure. Figures 4A to 4D represent the same scenario as was discussed with respect to Figure 3C, except now the controller 52 has been provided in association with the valve 50 so as to be able to inhibit water flow through the thermal store 18. Thus, as before the controller is expecting a hot water demand between 06.30 and 06:35 hours corresponding to the user taking a shower. Also as before two unexpected demands 90 and 92 occur around about 06:20 and 06:25 hours. However in this instance the controller 52 analyses the expected hot water demand pattern, as represented in Figure 4B and analyses the gas tariff information as represented in Figure 4D and realises that, as some of the expected gas use is being sold at a greater tariff, it is advantageous not to deplete the thermal store to service the demands 90 and 92 but to save it for the shower occurring between 06:30 and 06:35 hours. Consequently the controller 52 closes valve 50 such that the entirety of the heating demand for the draws 90 and 92 is accomplished by burning gas. Furthermore the controller can modify the target flue temperature of the boiler such that the combustion of gas associated with the water draws 90 and 92 increases the temperature of the thermal store to T4 which is greater than Tl. The evolution of the temperature of the thermal store in this scenario is indicated by chain-dot line 110. Consequently, at 06:30 hours when the expected shower is commenced the thermal store has achieved a higher temperature T4. Indeed, T4 may be above the thermal store's normal operating temperature and the temperature of operation may have been temporarily elevated in preparation for the shower. Thus, additional energy is now available in the thermal store and this manifests itself by a change in the balance of the amount of energy 82 attributable to energy removed from the thermal store and the amount of energy 80 attributable to the combustion process. Thus, it can be seen that the amount of gas that is burnt during the higher tariff period has been reduced by choosing to burn more gas at the lower tariff period to service the water draws 90 and 92. Consequently the user sees a lower gas bill.

In the example considered with respect to Figures 2 to 4, the tariff change between tariff 1 and tariff 2 occurred during a period when no hot water was being drawn. However Figure 5, for completeness, indicates the scenario where the tariff change occurs during the hot water draw. Thus, as shown here, the controller 52 may maintain the valve 50 in a closed state during a time that the gas is at the cheaper tariff, Tariff 1, and then once the tariff increases to the higher value, Tariff 2, may open the valve 50 such that water which has been warmed by from the thermal store increases the temperature of the water at the inlet 10 to the boiler, and hence the amount of gas burnt by the boiler can be decreased during the period corresponding to Tariff 2.

Figure 6 indicates a modification to the embodiment shown in Figure 1 where the thermostatically controlled mixing valve 40 has been dispensed with and the valve 50 is movable in response to a signal from the controller 52 between a plurality of positions between fully open and fully closed so as to control the water flow rate through the heat exchanger 20 within the thermal store 18. In order to facilitate this mode of operation additional temperature sensors 120 and 125 are provided, with temperature sensor 120 measuring the temperature of the water at the inlet to the boiler 2 and temperature sensor 125 measuring the cold water main temperature. This information can be used by the controller to set the positon of the valve 50 so as to cause an appropriate amount of water to flow through the heat exchanger 20 and on to the inlet 10 of the boiler 2. In order to provide an appropriate amount of back pressure a flow restrictor 128 is provided in the fluid flow path in the cold main between the connections with the inlet 22 and the outlet 24 to the heat exchanger 20. The heating system may also be provided with a water flow rate meter 130 in order to inform the controller more accurately about the operating conditions at the boiler 2. Knowledge of the water flow rate may allow the controller 52 to modify the operation of the boiler, for example the to modify a gas use ramp up rate at turn on or the mixing between the water from the cold main and the water from the thermal store so as to try and reduce lukewarm water rejection, which represents water which has been warmed but to a temperature which is not acceptable to the user, and hence resource in terms of water and energy are wasted. Specifically, if the boiler were to fire immediately the hot water draw was requested, the water entrained in the boiler and in the outlet path 24 from the thermal store may be too cold for acceptance by a user, but would be subjected to heating by the boiler. Hence the energy imparted to this volume of water is effectively wasted. If the thermal store is warm, then it is desirable to wait for the water warmed by the thermal store to be admitted into the boiler before igniting the burner within the boiler. The reduced temperature lift required by the boiler means that it can produce water more quickly at the desired temperature and hence the volume rejected water is decreased.

The controller 52 may include a processor and a suitable learning algorithm in order to monitor the hot water usage as a function of time, and build up an expected usage pattern which can then be held in non-volatile memory. Additionally or alternatively a user may program the controller 52 via a user interface 140. The user interface may be provided on the controller or may be provided as a web page on a browser or an APP on a smartphone, tablet, computer of any other appropriate device which is operable to communicate with the controller 52 by way of a data path 142 which might be Bluetooth, Wi-Fi, some other near-field transmission protocol or a wired route. Additionally or alternatively the controller 52 may be associated with a data gateway 150 which could be a wireless modem connected to or integrated within a controller 52 to allow communication over the mobile telephone infrastructure or by way of a local area network such as Wi-Fi, or by way of a wired connection, to a remote service provider operating a server 160. The server 160 may provide a route through which a user using the user interface 140 programs the controller 52 or the service provider 160 may provide additional information, such as tariff updates and weather updates to allow the controller 52 to operate in such a way so as to minimise or at least reduce the financial cost of burning gas as seen by the consumer, even if this means not operating the system in its most economical manner.

In the examples discussed hereinbefore, the thermal store has been a flue gas heat recovery device. In Figure 7 a modification is shown where the thermal store is provided as a tank 200 which may receive heat from the boiler 2 so as to deliberately pre-charge the tank 200 with heat whilst the gas tariff is at a lower rate. Pre-charging of the thermal store 200 can be achieved by diverting water which was intended for supply to space heaters 210 to the thermal store 200 by suitable operation of an electrically operated diverter valve 220 under the control of the controller 52. The arrangement shown in Figure 7 can be used in place of or in addition to the flue gas heat recovery device 4 discussed with respect to Figures 1 and 6. Use of a slightly larger thermal store 200 enables additional heat sources to be utilised in order to reduce the energy cost to the consumer. Thus, for example, where a building has solar photo voltaic panels installed, it is generally necessary for the output of the panels to be deemed stable for a period of time before an inverter can be used to sell electricity back to the grid. However, stability is not required for the purposes of using that electricity to heat water in the thermal store 200. Thus, to some extent, energy from solar PV which would be otherwise wasted can be usefully used to reduce the gas bill associated with the production of hot water for washing, showering and the like. Similarly lower grade heat from a heat pump can be provided to and stored in the thermal store.

Furthermore other sources of heating the thermal store may be used, such as heat pumps, heat recovery from grey water and so on.

The approaches described hereinbefore have been described in the context of gas boilers. However, with the exception of flue gas heat recovery, the approach can also be sued with electric heaters. Thus, a heater can be used to electrically heat water from the cold main or a mixture of water from the cold main and water warmed by a thermal store so as to reduce the financial cost of heating water by favouring use of lower cost electricity to warm a thermal store, and then depending on the thermal store based on knowledge of an expected use pattern and tariff data.

The teachings can also be extended to storing heat from space heating systems.

Figure 8 schematically illustrates a heating system 301 constituting an embodiment of the present invention. The heating system comprises a heater 302, which could be any heating source but typically comprises a fossil fuel boiler. The fossil fuel may be coal, oil or gas.

The boiler heats a fluid, which is almost invariably water, and uses a pump 304 to circulate the water through a delivery network 306 to at least one space heater 308, 310, 312. Space heaters 308 and 310 may, for example, be radiators which are common in European homes. A further space heater 312 may be an under floor loop for providing under floor heating.

Water is cooled as it passes through the space heaters and gives its heat up to the enclosed space within a building. The water then flows along a return path 314 of the delivery network back to the heater 302 for reheating. The system described to this part is a conventional space heating arrangement.

A space heating system is usually operated in a time controlled manner so as to only heat a building when it is occupied and/or the inhabitants are active. In the context of a building such as a school or office, this typically means the heating is run from, say, 07:00 hours until 17:00 hours. The heating is off overnight.

In a domestic dwelling the heating may be on in the morning, off in the afternoon, on in the evening and off at night.

When the heating is initially switched off the delivery network 306, 314 may contain significant volumes of water, as may the space heaters. Volumes in excess of 80 litres are common in domestic systems and many thousands of litres in schools and other large buildings. Thus water may have been heated to 60°C or so. If we take ambient temperature as 20°C and an 80 litre system then it is clear there is (60 - 20) x 80000 x 4.2 = 13.4 MJoules of heat in the water and that the heat is going to be allowed to leak into the building shortly after boiler switch off. The heat does warm the building, but its effect may decay away overnight so its heating benefit is largely lost.

In the arrangement shown in Figure 8 a thermal store 320 is provided which can be selectively switched into fluid flow communication, or heat transfer communication, with the distribution network 306, 314 by a valve 322. The operation of the valve 322 and the boiler/heater 302 may be controlled by a controller 52 which has been shown as being external to the heater 302 but which could be integrated within it. The controller may be responsive to one or more room thermostats (not shown).

The thermal store 320 comprises a tank 330 containing a volume of water 332. The tank 300 may be unvented (as shown) or vented to atmosphere, depending upon the designer's preferences. A heat exchanger 334 is provided within the tank. The heat exchanger might simply be a meandering or coiled pipe. In some embodiments a pressure relief device may be provided to allow fluid exchange to occur between the water 332 in the tank 330 and the water in the heat exchanger 334 such that excess pressure build up can be accommodated via safety features built into the heater 302.

In use, the valve 322 is normally set to the position shown in Figure 8 such that a flow loop is formed from the heater 302, to the space heaters 308, 310 and 312 and back to the heater 302, without flow occurring via the heat exchanger 334 of the thermal store 320.

Initially the water in the thermal store will probably be cool, i.e. at ambient temperature or a little above. The heater can be operated under the control of the controller 52. The controller 52 may implement a very simple control scheme based on switch on times and switch off times. The temperature to which the heater heats the water or other fluid in the space heaters can be controlled by monitoring the fluid temperature at the heater, monitoring the temperature obtained in a zone having a thermostat, or a combination of these approaches.

When the heater comes to the end of its heating mode and is going to stop space heating for a significant period of time, the controller cuts the supply of fuel to a burner 340 within the heater. However, the pump continues to run and the valve 322 is operated so as to place the heat exchanger 334 in the fluid flow path. Thus the warm water in the delivery network 306, 314 and in the space heaters can flow through the heat exchanger and give heat up to the thermal store. This can be done for a time determined by the system designer, or the temperature of the thermal store may be monitored by the controller 52 using a temperature sensor (not shown) and the pump 304 run until such time as the temperature in the thermal store stops rising, or the rate of increase drops below a threshold value.

Thus heat is transferred to the thermal store 320, which is insulated, and the heat is retained there.

In Figure 8, the store is shown as being downstream of the space heating circuit, and as being insertable in the return path to the boiler. This is however, not the only position that the store could be inserted at. The store could be inserted anywhere practical in the fluid flow path. The thermal store is generally physically large and is external.

At heating start up the heater can place or keep the thermal store in the fluid flow loop and can run the pump without igniting the burner so as to recover the heat in the thermal store to pre-warm the water in the space heaters and in the delivery network. Once a pre-warm period has elapsed, the controller operates the valve 322 so as to remove the heat exchanger 334 of the thermal store from the fluid flow path. The normal space heating mode can then be resumed by igniting the burner within the heater. Alternatively the water from the store could be passed into the boiler with the boiler burning fuel so as to provide an aggressive restart to the heating system. However if the gas price is going to change, for example from a lower tariff to a higher tariff the controller may choose to keep the warmed water in the thermal store for the supply of heat to a further heat exchanger 200, as was described with respect to figure 7, so as to make the heat available for other purposes, such as the supply of domestic hot water. Additionally or alternatively, it may be more cost effective to save the heat in the thermal store for introduction into the space heating circuit of the energy price increase is sufficient to make this worthwhile.

As a further alternative the valve 322 may be a blending valve such that during start up cold water in the space heating circuit can be mixed with warm water from the thermal store, so as to bring the water at the return inlet to the boiler to an increased temperature, thereby enabling the water exiting the boiler to be warmer. This approach means that the thermal store does not become depleted so quickly or a smaller thermal store can be used.

The store may be selectively connectable to receive warmed water from the boiler 302 without that water having passed through the space heating circuit so as to allow the controller 52 to keep the store warm - or alternatively to deliberately warm the store. The store 300 may include an electric heating element 336 which can be energised to warm the water in the store. Thus the controller 324 may energise the heating element 336 to keep the store temperature at a target value, or may use the heating element 36 to pre-warm the store. Thus the controller may act to increase the amount of energy in the thermal store during times of lower energy cost to make that energy available for release at times of higher energy cost.

For completeness a method of operating a boiler or hot water heater in accordance with the present disclosure will now be described with reference to Figure 9. The method commences at step 400 where a test is made to see if a hot water demand has commenced. This can be achieved by monitoring a flow rate sensor provided at part of a boiler. If a demand, such as a domestic hot water draw, is detected control passes to step 410, otherwise control loops back to step 400. At step 410 the expected use data is retrieved and control is passed to step 420 where the tariff data is read. Then a test is performed at step 430. Step 430 looks to see if there is expected use within a threshold time limit Time 0 and a change in the tariff is expected within a time period TIME1. If there is expected use within the period Time 0 and a tariff change within the period TIME1, then a further test is made to see if a tariff change occurs it also checks to see if the increase in the fuel cost is greater that COSTI, which represents a value where it is worthwhile modifying the normal operation of the boiler and thermal store combination so as to conserve heat in the thermal store. Time 1 may be related to the heat retention properties of the thermal store, such as the time it takes to lose X degrees of heat starting at temperature Tl. If the test as step 430 finds all the conditions are satisfied, such that the operation of the combination of the boiler and the thermal store should be modified to conserve stored energy for an expected use at an increased gas tariff, then control passes to step 440 where the valve 50 is closed, otherwise control passes to step 450 where the valve 50 is opened. From steps 440 and 450, control is passed to step 460 where a test is made to see if the hot water draw has finished. If the draw is still in progress control loops back to step 460. Once step 460 determined that the demand has finished control is returned to step 400.

The values of TIMEO and TIME1 may be programmed by a user or leant by the controller by monitoring for usage patterns and the evolution of condensate temperature in the thermal store.

It is thus possible to provide an improved apparatus and a method for modifying the operation of a condensing boiler so as to reduce its cost of operation in an environment where variable gas tariffs are applied.

Claims

1. A heater operable to warm water, the heater being in combination with: a thermal store where liquid to be heated can receive heat from the thermal store prior to being admitted into the heater for heating; a controller; and a flow control device responsive to the controller for controlling a heat flow path from the thermal store to the liquid to be heated; wherein the controller is adapted to control the flow control device based on an expected hot liquid demand and the energy price as a function of time.

2. A heater as claimed in claim 1, in which the heater is a gas fuelled heater.

3. A heater as claimed in claim 1 or 2, in which the liquid is water.

4. A heater as claimed in claim 3 when dependent of claim 2, in which the thermal store is part of or in association with a flue gas heat recovery device and a heat exchanger defining a water flow path having an inlet and an outlet, and the flow control device is a valve operable to open or close the water flow path, or to open or close a bypass path around the water flow path.

5. A heater as claimed in any preceding claim, wherein the controller is adapted to control a target temperature of the heater.

6. A heater as claimed in any preceding claim, further comprising a temperature sensor for measuring the temperature of the thermal store, an output of the temperature sensor being provided to the controller

7. A heater as claimed in any preceding claim in which the heater comprises a boiler operable to provide warmed water to at least one space heater, and the boiler warms the thermal store while providing space heating.

8. A heater as claimed in any preceding claim, where the thermal store is adapted to receive energy derived from at least one of a space heating circuit, a heat pump, and solar panels.

9. A heater as claimed in any preceding claim, further comprising a water flow rate meter for providing water flow rate information to the controller.

10. A method of operating a heater having or being in combination with a thermal store, the method comprising in response to a demand for hot water, examining a use pattern and a fuel tariff, and selectively inhibiting withdrawal of heat from the thermal store if an estimate of cost to service an expected future demand in conjunction with the water demand is likely to be reduced by conserving heat in the thermal store.

11. A method as claimed in claim 10, in which the method comprises inhibiting the withdrawal of heat from the thermal store to supply heat to cold water entering the boiler for water heating if an expected demand occurs within a first time threshold TIMEO.

12. A method as claimed in claim 10 or 11, in which the method comprises inhibiting the withdrawal of heat from the thermal store to supply heat to cold water entering the boiler for water heating if an expected demand occurs within a first time threshold TIME1.

13. A method as claimed in claim 10, 11 or 12, in which the method comprises inhibiting the withdrawal of heat from the thermal store to supply heat to cold water entering the boiler for water heating if a tariff increase is less than a threshold COSTI.

14. A method as claimed in any of claims 10 to 13, further comprising increasing the temperature of the thermal store prior at an expected draw of hot water.

15. A method as claimed in any of claims 10 to 14 in which a user enters an expected use pattern into a use pattern memory.

16. A method as claimed in any of claims 10 to 15, further comprising learning a use pattern by monitoring operation of the water heater as a function of time.