US 201102.71948A1 (19) United States (12) Patent Application Publication (10) Pub. No.: US 2011/0271948 A1 Redford (43) Pub. Date: Nov. 10, 2011

(54) APPARATUS FOR CAPTURING HEAT FROM Publication Classification A STOVE (51) Int. Cl. (76) Inventor: Simon Redford, Mold Flintshire F24B I/00 (2006.01) (GB) (52) U.S. Cl...... 126/67 (21) Appl. No.: 13/143,472 (57) ABSTRACT Apparatus for capturing heat from a stove to enable it to be (22) PCT Filed: Jan. 6, 2010 transferred to a central is disclosed. The appa (86). PCT No.: PCT/GB2O1 O/OSOO13 ratus comprises an air-water , the heat exchanger having an air side and a water side and being S371 (c)(1), operative to transfer heat between air on the air side and water (2), (4) Date: Jul. 6, 2011 flowing in channels in the water side. The apparatus includes an air through which air heated by a stove can pass by (30) Foreign Application Priority Data to contact the air side of the heat exchanger and control means operable to control the amount of airflowing to Jan. 9, 2009 (GB) ...... O900259.3 the heat exchanger. The control means may include a that Mar. 19, 2009 (GB) ...... O904718.4 can be operated to direct the flow of air to the heat exchanger.

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APPARATUS FOR CAPTURING HEAT FROM 0007. It is, in principle, possible to heat a whole house A STOVE from a large stove with a gravity-fed system. How ever, most stoves do not have sufficient output to heat a whole CROSS REFERENCE TO RELATED house, and few people would want to rely on manual feeding APPLICATION of logs or coal to maintain heat and hot water in the home. The majority of households with a stove will also have gas, oil or 0001. This application is a national stage entry of PCT/ electric , such that the stove provides an addi GB2010/050013 filed Jan. 6, 2010, under the International tional heat source to Supplement the main heating systemand, Convention claiming priority over GB Patent Application No. to some extent, its fuel consumption; this is likely to become 0900259.3 filed Jan. 9, 2009 and GB Patent Application No. far more prevalent as the costs of fossil fuels have escalated O904718.4 filed Mar. 19, 2009. dramatically in recent times. 0008 Systems are available to link the output from a grav BACKGROUND TO THE INVENTION ity-fed stove circuit with a central heating system that include either neutraliser tanks, heat exchangers or combined grav 0002 1. Field of the Invention ity and pumped systems. Neutralisers offer a comprehensive 0003. This invention relates to apparatus for capturing Solution, but are complex, expensive, cannot readily be ret heat from a stove. In particular, it relates to apparatus for rofitted into an existing central heating system, and are capturing heat from a stove whereby that heat can be trans incompatible with modern, sealed central heating systems. ferred to a central heating system. In the context of this They have various different circuits, whether pumped or natu invention, the term stove' is used as a generic term for appa rally circulated, connected together at a single (hydraulically ratus to allow enclosed burning of solid fuels. It does not neutral) point, whereby each circuit will take and return the necessarily imply a stove that can be used for cooking, same flow of water and cannot hydraulically interfere with although some do offer cooking or warming plates. More one another, although thermally the water streams mix in a over, the invention is described principally in connection with hot water tank. wood-burning Stoves, because this is the context in which the greatest potential benefit can be obtained. However, it can SUMMARY OF THE INVENTION also find application to stoves intended to burn solid fossil 0009. An aim of this invention is to provide apparatus by fuels (such as coal) or to multifuel stoves. means of which heat from a stove can be used to Supplement 0004 2. Summary of the Prior Art heat input into a central heating System, the apparatus being 0005 Burning wood on open fires and in stoves plays a compatible with an existing central heating system, including Small but increasing role as a practical source of renewable, sealed systems, and being fail-safe in the event of an electrical biomass residential heat. Heat produced by burning wood can power failure. displace heat produced by burningfossil fuels in central heat 0010 A particular, but not exclusive, aim of this invention ing systems, thereby saving costs for the householder and is recovery of heat from outer Surfaces of Stoves, especially reducing carbon emissions. modern, efficient stoves. There is little merit in trying to 0006 To maximise the benefit of burning wood in a mod recoverheat from the chimney gasses of efficient Stoves since ern stove, some of the heat produced should preferably be only a small part of the total heat output of Such stoves is dissipated throughout a building in which it is installed to carried in the chimney gasses. Taking heat from the chimney offset the amount of heat required from fossil-fuel based gasses may cause problems: cooler gasses tend to form con heating systems. One way in which this has traditionally been densates in the chimney, which can cause tars and creosotes to achieved is by incorporating a back into a solid-fuel accumulate in the chimney, and an insufficiently warm chim stove. As a stove has a relatively high and ney can dramatically reduce the convective draw required limited burn control, back need to be connected to a for good combustion. Heat exchangers that are exposed to gravity fed circuit to ensure heat can be dissipated without combustion gasses are likely to Suffer from fouling by com having to rely on a pumped water circuit. Failure to circulate bustion products. Moreover, if a heat exchanger is located in water through a back boiler might lead to potentially explo a chimney, it is not an easy matter to divertheat from the heat sive boiling of the water in the jacket. The term gravity fed exchanger Such that it can be discharged into a room Sur refers to natural circulation caused by the lower density of hot rounding the stove. water leaving the top of the back boiler being displaced 0011 To this end, from a first aspect, the invention pro upwards by the higher density of cooler water entering the vides apparatus for capturing heat from a stove, the apparatus bottom of the back boiler, so forming a natural thermal cir comprising an air-water heat exchanger, the heat exchanger culation. As the thermal circulation force is quite weak com having an air side and a water side and being operative to pared to a pumped system, large-bore pipes must be used transfer heat between air on the air side and water flowing in (typically 28 mm) to reduce pressure losses associated with channels in the water side, the apparatus including an air duct circulation and the primary heat sink (a radiator or a hot water through which air heated by a stove can pass and control tank) must be relatively close to and above the stove, and means operable to control the flow of air within the apparatus; cannot be turned off. A typical arrangement for a stove 10 wherein in a first mode, the control means allows air (heated with a back boiler is shown in FIG.1. In the system of FIG. 1, by the stove) to flow from the air duct to the surroundings of the radiators 12 and the indirect hot-water cylinder 14 must be the stove at least partly by natural convection and in a second able to dissipate all of the heat from the back boiler to avoid mode, the control means induces air (heated by the stove) to boiling of the water. The pipes of the main circuit 16 are of flow from the air duct to the air side of the heat exchanger (as large (28 mm) diameter. Most back boiler systems must also by ). be open vented with a header tank 17 in the loft. Installation 0012. During operation of the stove, operation in the first of Such a large-bore system is inconvenient and expensive. mode minimises the hot airflowing in the heat exchanger, and US 2011/027 1948 A1 Nov. 10, 2011

hot air carrying a maximum amount of heat is passed to the 0022. Embodiments may further include an outer cover Surroundings of the Stove. Therefore, a minimum of heat is that creates a second air duct between itself and the cover. transferred to the water and the stove acts as a room heater. This can be disposed to increase the efficiency of the appara However, during operation in the second mode, heat is trans tus. In Such embodiments, air can be caused to flow in the ferred from the heated air to water flowing within the water second air duct by natural convection to reduce the outer side of the heat exchanger. This later condition allows heat casing temperature of the apparatus. from the stove to be transferred to another heating system, hot 0023 Preferably the control means is fail-safe, such that it water system, etc. diverts air from the heat exchanger in the event of there being 0013. In the context of this specification, “natural convec insufficient flow of water within its water side. This ensures tion' and “forced convection' should be taken as having their normal, technical meaning. Put simply, natural convection is that the heat exchanger is protected from damage by over the flow of air that occurs due to buoyancy pressure created by heating. the heating of air, and forced convection is the flow of air due 0024. The apparatus for capturing heat may be adapted to to pressure created by an active device, such as a fan. capture heat from outer Surfaces of the stove (such as one or 0014 Air within the air duct typically flows in opposite more of a side Surface, a top Surface or a rear Surface). directions in the first and second modes. Most typically, in the 0025. From a second aspect, the invention provides a stove first mode it flows generally upward (although it may be that that incorporates apparatus for capturing heat from a stove it does not flow upward throughout its entire flow path), being according to the first aspect of the invention. driven by natural convection. Such an arrangement has been 0026. The apparatus for capturing heat may be adapted to found to offer a system in which the flow of air and transfer of capture heat from outer Surfaces of the stove (such as one or heat can be controlled predictably and easily. more of a side Surface, a top Surface or a rear Surface). 0.015. In embodiments of the invention, the control means may include a fan that is operable to induce a flow of air in the 0027. From a third aspect, the invention provides aheating apparatus during operation in the second mode, Such a flow installation that includes a stove according to the first aspect typically being in a direction opposite to that in which it of the invention and a central heating system that includes a would flow due to natural convection. More particularly, the pumped water circulation circuit that operates to transport fan may be operable to draw air from the Surroundings into heat from a boiler to one or more radiators, the water side of the air duct for heating by the stove and then to the heat the heat exchanger being connected to the circulation circuit exchanger. Thus, when the fan is operating, heat is transferred such that water from the circulation circuit is caused to flow to the heat exchanger. Moreover, when the fan is not operat through the heat exchanger while the pump of the circulation ing, air may be drawn from the Surroundings for heating by circuit is operational. the stove, and Subsequently returned to the Surroundings by 0028. In embodiments of this aspect of the invention, the natural convection. control means may operate to minimise the flow of heated air 0016. In embodiments that include a fan, the state of the from the stove to the heat exchanger when the pump of the control means (and therefore the mode of operation of the circulation system is not operational. apparatus) is primarily determined by whether or not the fan 0029. The stove may be connected in series with or in is running. Automatic control of a fan is a straightforward parallel with radiators in the heating system. Embodiments in matter, so this provides a convenient control system. which the stove is connected in parallel with the radiators may 0017 Embodiments of the invention may include bypass further include a pump connected in series with the heat means that includes a closable opening to Surrounding air in exchanger of the stove that can be operated independently of the duct between the stove and the heat exchanger. The bypass the central heating system pump to transfer heat from the can provide a low-resistance path for air to flow under natural stove to the radiators. A non-return valve may additionally be convection when the apparatus is operating in the first mode. provided to prevent flow through the stove heat exchanger The bypass means includes a closable opening to Surrounding when the central heating system is running but the Stove is not air in the duct between the stove and the heat exchanger. For example, a flap may be provided to close the opening. lit. 0018. In embodiments of the invention, the air duct may include a plate, a first portion of the air duct being defined as BRIEF DESCRIPTION OF THE DRAWINGS a chamber that is formed between the plate and a heating Surface of a stove. 0030 FIG. 1 is a diagram of a conventional back boiler system, and has already been discussed; 0019. A second portion of the air duct may connect the lower part of the first portion of the air duct to the air side of 0031 FIG. 2 is a diagram of a stove upon which an the heat exchanger. At least part of the control means may be embodiment of the invention can be based; located within the second portion of the air duct. 0032 FIGS. 3a and 3b illustrate an embodiment of the 0020. In typical embodiments, the air duct causes air to invention operating, respectively, in a first and a second flow over one or more external heating Surfaces of a stove. mode; These external heating Surfaces may include heat transfer 0033 FIGS. 4a and 4b show the stove of FIGS. 3a and 3b fins. The external heating Surfaces typically include one or incorporated into a central heating system in two alternative more sides of the stove, and may further include the rear of the configurations; StOVe. 0034 FIG.5 illustrates the heat output that can be obtained 0021 Various types of heat exchangers may be used. For from the embodiment of FIGS. 3a and 3b, example, the heat exchanger may include a fin-and-tube heat 0035 FIG. 6 shows a central heating system that is a exchanger element. modification of the system of FIGS. 3a and 3b; and US 2011/027 1948 A1 Nov. 10, 2011

0036 FIG.7 shows an alternative form of a stove embody as explained below, some heat is transferred to the central ing the invention. heating system, whereas in natural convection all of the heat goes to the room. DETAILED DESCRIPTION OF THE PREFERRED 0046. In this embodiment, a conventional stove 18 is sur EMBODIMENT rounded, typically on two sides 20 and a rear wall, by an 0037. An embodiment of the invention will now be enclosure 80 formed of thin sheet metal. The enclosure forms described in detail, by way of example, and with reference to a vertically-extending air chamber that partly Surrounds the the accompanying drawings. stove, the chamber being C-shaped, in this embodiment, 0038. As an introduction to the embodiment, operation of when viewed in plan. Vertical heat-convective fins 32 project a modern wood-burning Stove will now be discussed. from the stove into the chamber. 0039 Heat transfer from a stove to its surroundings (typi 0047 Below the stove 18, the enclosure 80 is connected cally, a room in a building) occurs by way of radiation (A) and into a vertical air duct 82. A heat exchanger 84 is located convection (B) from the surfaces of the stove to air within a within the air duct 82 vertically above a motor-driven fan 86. room in which the stove is located and there is also heat lost The heat exchanger 84 is connected into a central heating (C) up the chimney 28. For a well-designed stove, the chim system, as will be described below. This embodiment ney loss should be less that 30%, and may be less than 20% for includes a heat exchanger 84 of the tube-and-fin type Some of the most efficient new designs. The proportions of although other variants may be deployed. radiative and convective heat transfer can vary depending on 0048. When the fan 86 is off, air is drawn upwards (as the design and temperature of the stove sides. indicated in FIG.3a) by natural convection through the duct 0040. For a stove 18 with finned sides 20, as shown in FIG. 82 into the air chamber within the enclosure 80, from which 2, the majority of the heat generated by the burning fuel is it passes to the room. This is the embodiment operating in a transferred by convection from the stove sides 20, with a first mode to transfer a maximum amount of heat to a Sur higher proportion of radiantheat transfer from a glass front 22 rounding space. of the stove 18, where such a front is fitted, as in this example. 0049. When the electric fan 86 is on, it draws heated air Bigger and cooler Surface areas are provided in Stoves in within the duct 82 downwards (as indicated in FIG. 3b). which it is intended that convection should be maximised, countering the natural convection. The air is heated within the with typical operating Surface temperatures in the region of chamber and drawn over the heat exchanger 84 to heat the 200-250° C. If a greater radiative output is required, higher central heating system water. surface temperatures and flat sides (less roughness and/or no 0050. The control of this embodiment can be very simple, fins) are generally preferred. since it is inherently fail-safe. A signal from the central heat 0041 Convective heat transfer also takes place from the ing system turns on the fan 86 to transfer heat when required. top of the stove 18, but more heat is transferred from external Alternatively, the fan 86 may be controlled locally by sensing vertical surfaces than horizontal surfaces due to the induced the flow and temperature of water entering the heat exchanger convective airflow. Little heat is transferred from the bottom 84, to ensure the fan only operates when water is flowing and of the stove, as combustion air is usually drawn in at the the water temperature is not too high. Additional functionality bottom of the Stove, and the grate and ash-pan act as heat can easily be incorporated. For example, there may be a shields. (Some stoves also incorporate a separate, protective thermostatic control that allows heat to be sent to the central external heat shield.) heating system (by operating the fan) only when a room 0042. There is little merit in trying to recoverheat from the temperature selected by a user is achieved and subject to the chimney gasses of a modern, efficient stove, as these carry a same flow and temperature conditions described above. To minority of the heat output. Taking heat from the chimney ensure fail-safe operation, the fan 86 may be wired in series gasses can cause problems, as cooler gasses tend to form with a simple low-cost overheat Switch sensing the water condensates in the chimney, which can deposit tars and creo temperature at the top of the heat exchanger Such as those Sotes, and a cool chimney can dramatically reduce the con often found in central heating boilers (typically bi-metallic, vective draw, which is required for good combustion. (This manually reset type). This operates to stop the fan 86 if the is not the case for open fires, where the majority of the temperature of water within the heat exchanger 84 becomes heat—85% or more is lost up the chimney and some heat excessive. recovery is possible without affecting the draw.) 0051. When the fan is on, the air leaving the duct 82 to the 0043. The stove 18 illustrated in FIG. 2 does not have a room will still carry Some heat; the exiting air temperature back boiler. Many modern stoves that operate without a back cannot be lower than the central heating water temperature boiler incorporate a refractory lining 24 that ensures an even entering the heat exchanger. Therefore, not all of the heat distribution of the heat from the fire 26 to external surfaces of from the sides 20 of the stove 18 can be transferred to the the stove 18 to avoid localised overheating. central heating system. However, by Suitable arrangements of 0044) Given the basic configuration of a stove described the chamber adjacent to the sides 20 of the stove 18 and the above, the aim of the invention is to transfer some of the heat top of the heat exchanger, some heat will also be drawn in from external surfaces 20 of the stove 18 in a manner that can from the top of the stove and the outside of the . Alterna be controlled between maximising the heat going direct to the tively, a configuration is possible in which the exhaust from room, or some of the heat going to a water circuit feeding a the heat exchanger 84 is ducted back to the top of the stove central heating system. With reference now to FIGS. 3a and sides such that the warm air exiting the ducting is recuperated 3b, an embodiment of the invention is constituted by a modi to the chamber 80. fication of the stove 18 shown in FIG. 2. 0.052 The water heat exchanger 84 is shown below the 0045. In FIGS. 3a and 3b, first and second operating stove 18, but could be located behind the stove 18, provided a modes are shown; natural convection in FIG. 3a, and forced free air path is available for natural convection when the fan convection FIG. 3b. When forced convection is in operation 86 is off. US 2011/027 1948 A1 Nov. 10, 2011

0053 When the central heating is on but the stove is not lit, connected to the central heating system in the same manner as heat will be lost from the water heat exchanger 84 unless a a radiator, such that some of the flow from pipe 122 within the thermally or electrically operated valve is incorporated in the system passes into the bottom of the heat exchanger 84, and central heating circuit. This is also true when the stove oper exits from the top of the heat exchanger 84 to be returned to ates in the first (fan off) mode. Therefore, an electrically the system through the return pipe 124. This manner of con operated water valve connected to operate with the fan may be nection makes it a straightforward matter to connect the heat beneficial. Valves to control the flow of water in the heat exchanger assembly to an existing heating system, since the exchanger 84 are shown in FIGS. 4a and 4b and are described pipes leaving the boiler need not be disturbed. Connection is below no more complex than the addition of a new radiator and can 0054 The fan 86 may also be operated when the stove is be made at any part of the circuit by convenient small-bore not lit to provide central heating output, such that the stove pipes. could replace a radiator. In this mode, the stove acts like a 0062) A valve 129 may be connected between the flow fan-assisted room radiator. In Such a mode, it may be advan pipe 122 and the heat exchanger 84 to allow it to be isolated tageous to operate the fan 86 at a higher speed to increase the from the heating system. This valve may be electrically oper air flow over the heat exchanger 84 because the heat ated and is normally only opened if the fire is lit and it is exchanger will be sized for high air temperatures and low air desirable to divert heat to the central heating system (see flow rates when transferring heat from the fire in the second comment regarding thermostatic control above). When the mode described above. valve 129 is open, the control system can then sense whether 0055 Although this embodiment is described for use with there is flow in the central heating system to allow heat to be a stove that has distinct sides and rear Surfaces, an analogous transferred from the stove by operating the fan 86. In this same design might be applied to stoves with curved surfaces. arrangement, if the fan 86 is operating and water is flowing in In such embodiments, the sheet metal enclosure follows the the heat exchanger 84, the fire 26 will add heat to the central contour of the stove in question. heating system. If the fire 26 is not burning, the valve 129 will 0056. In yet further embodiments, the enclosure may be normally be closed unless it is desirable to operate the heat formed from non-metallic materials and may also be insu exchanger to output heat from the central heating system as lated on the outer surface. described above. 0057. In the embodiment as described above with refer 0063. The water that is heated by the heat exchanger 84 ence to FIGS. 3a and 3b, the driving pressure created by returns to the boiler through the return pipe 124, and mixes natural convection is quite weak, so care must be taken to with water returning from other radiators 126 before reaching ensure that there is a reasonably free path for airflow. The heat the boiler 120. As the temperature of the water leaving the exchanger design must be of Sufficiently low pressure drop at heat exchanger 84 is higher than that in the flow pipe 122, the the air flow rate induced by natural convection to ensure that heated water entering the returnpipe 124 adds heat energy to air flow is not substantially impeded. Likewise, ducting 82 the return stream entering the boiler 120 by mixing with the must be designed to ensure a low pressure drop and even cooler water returning from the radiators 126. distribution of air to the chamber 80. 0064. In an alternative application of an embodiment of 0058. Therefore, in an alternative arrangement of the the invention, as shown in FIG. 4b, the heat exchanger 84 is embodiment there is incorporated a bypass that includes a connected to the central heating system, such that all water flap 88 which is opened to allow free flow of air into the air returning to the central heating boiler 120 through the return duct to partly or completely bypass the heat exchanger 84 pipe 124 from the radiators passes into the bottom of the heat when the fan is off. When the fan is on, the flap 88 is closed to exchanger 84, and the top of the heat exchanger 84 is con ensure the air passing over the heat exchanger 84 comes only nected to the boiler return. Such connection is more appro from the chamber 80 and its temperature is therefore a maxi priate for a new heating system installation where the layout U of the central heating system can be arranged to accommo 0059. The flap 88 may be caused to close when the fan is date Such a connection. Configured in this manner, water running either by exploitation of the air pressure difference entering the heat exchanger 84 will typically be cooler than at when the fan is operating or by active means, such as an other parts of the flow circuit, such that greater heat transfer electrical actuator, that operates when the fan is on. may take place from the fire 26 to the central heating circuit 0060 Typical applications of an embodiment of the inven when the fire is lit, and less heat is transferred to the room tion are shown in FIGS. 4a and 4b. Both examples provide a from the outlet of duct 82 when the fan 86 is operating. heating system that includes a conventional central heating 0065 Clearly, it is not acceptable to provide a simple valve boiler 120 that will typically be powered by gas, oil, solid fuel to isolate the heat exchanger 84 from the central heating or electricity. Hot water is pumped from a flow outlet of the circuit in the configuration of 4b, since this would prevent any boiler into a flow pipe 122 of a heating circuit. Water is flow within the circuit. Therefore, it may be beneficial to returned to a return inlet of the boiler 120 from a return pipe incorporate a controlled bypass circuit that includes two 124 of the heating circuit. Several radiators 126 and a hot valves 130, 132 to divert water flow through the heat water cylinder 128 are connected between the flow and return exchanger 84 to a bypass pipe 134 in parallel with the heat pipes 122, 124, each of which can be provided with a portion exchanger84 when the stove is not lit. The valves 130 and 132 of the flow within the circuitas a whole. The radiators 126 and act in a mutually exclusive manner. Such that when the valve the hot water cylinder 128 may incorporate thermostatically 130 is open, the valve 132 is closed, and vice versa. This may operated valves to control local heat distribution. This far, the be usefully implemented by the valve 132 being of the nor heating system described is entirely conventional, and need mally-open type and the valve 130 being of the normally not be further elaborated. closed type. Such that both can be operated from a single 0061. In the embodiment of FIG.4a, the heat exchanger 84 electrical source which when activated allows flow through of a stove 18 equipped with an assembly described above is the heat exchanger 84 and when deactivated diverts flow US 2011/027 1948 A1 Nov. 10, 2011 through the bypass 134. Alternatively, a single divert valve ber 218 joins air drawn in through the apertures 214 by may be incorporated to send flow to either the heat exchanger operation of the fan and is further heated by the stove side 20. or the bypass, which may have the advantage of being more Operating in this mode, the outer air chamber 218 described versatile in application as the bypass 134 could be excluded acts to recuperate losses that would otherwise occur from the (blanked off or isolated by a manually operated valve) for the hot outer side of cover 80 and has the benefit of increasing the configuration of 4a and connected for the configuration of 4b. temperature of the air reaching the heat exchanger 84. 0066. A modification to the system of Figures and 4a is (0071. In the configuration of FIG. 7, if the inlet of ducted shown in FIG. 6. Components of this embodiment that are in air travelling thorough the outer chamber 218 is arranged to common to the system of FIG. 4a will be given the same be close to the exiting warm air from the heat exchanger 84. reference signs as in those figures and will not be described further recuperation may be achieved through recovery of further. warm air leaving the heat exchanger which would otherwise 0067. This embodiment incorporates an alternative con be lost to the room. The overall effect of this further embodi trol mechanism. In this embodiment, a Subsidiary water pump ment is to increase the amount of heat that can be recovered to 200 is connected in series with the heat exchanger 84. The the central heating water when operating in the second mode. pump 200 is configured in parallel to the central heating pump 0072 The practical effect of the embodiments described to take water from the cooler return circuit 124 and deliver it, above is to provide a switchable control of heat transfer through the heat exchanger, to the warmer flow circuit 122. between the stove and the central heating system. At one When the fan 86 is operated, the subsidiary water pump 200 setting, Substantially all heat is passed directly to the room. At is also operated to circulate central heating water through the the other setting, much of the convective heat passing to the heat exchanger 84 irrespective of whether or not the central heat exchanger 84 and thereby to the central heating system. heating circuit pump is running. In this embodiment, the stove In the latter condition, radiative and convective heat from the can Supply heat to the central heating circuit independent of bottom, front and top of the stove still passes to the room. the operating status of the central heating system pump. A Results of an experimental investigation of the configuration non-return valve 202 is connected in series with the subsid described in FIGS. 3a and 3b will now be described with iary water pump 200. The purpose of the non-return valve 202 reference to FIG. 5. is to prevent flow of water through the heat exchanger when 0073. The heat exchanger arrangement described above the system is operating in the first operating mode (that is, has been investigated experimentally. One side of a stove was with the fan off). This avoids the need to provide electrically simulated by electrically heating a finned surface to maintain operated valves to support operation of the heat exchanger. it at approximately 200° C. A plate was added to form a fire 0068. The embodiments of the invention described above side chamber as described above and ducting was added to can be further enhanced by addition of a second outer cover connect the chamber to a tube-and-fin heat exchanger and a 210 as shown in FIG. 7. The second outer cover 210 substan fan, in the configuration of FIGS. 3a and 3b. The graph of tially completely encloses the cover 80 and defines an outer FIG. 5 shows the effect of operating the fan to reverse the air chamber between itself and the cover 80. The second outer natural convection and divertheated air to a tube-and-fin heat cover 210 has a top panel 212 through which several apertures exchanger. 214 (which may be in the form of slots or a ) are formed 0074 Before the fan is operated, hot air leaves the top of to connect the Stove 20 to Surrounding atmosphere to allow air the fire side chamber by natural convection as shown by the to flow. curve described as “Air out fire side' on the chart, and the air 0069. In FIG. 7, on the right hand side, arrows indicate the entering the chamber from the bottom is at ambient tempera direction of air flow in the first mode of operation (natural ture as shown by the curve described as “Air in fire side' (air convection, fan off). On the left hand side, arrows indicate air temperature in the duct). When the fan is operated to over flow the second mode of operation (forced convection, fan on) come the natural convection and reverse the air flow, the to transfer heat to the heat exchanger 84. These two modes of temperatures of these two streams reverse, showing that operation are mutually exclusive and are shown on one dia ambient air enters the top of the fire side chamber and heated gram for illustration purposes only. As with the embodiments air passes downwards through the duct. As soon as the fan of FIGS. 3a and 3b, the action of turning the fan on is to operates, heat starts transferring to the central heating water, reverse the natural, upward flow of air over hot surfaces 20 of rising to a maximum overa couple of minutes as shown by the the stove in the duct formed between the stove sides 20 and curve described as “Water power (%)’. When the fan is turned off, natural convection is quickly re-established so that heat is the cover 80. When operating in the first mode with the fan dissipated to the room from the top of the fire side chamber as off the outer air chamber acts as a second convective space shown by the Air out fire side curve. Heat transferred to the that operates to draw ambient air in at the bottom of the duct water drops away rapidly as the heat exchanger components 216. This air is heated by contact with cover 80, rises and joins cool in the ambient air stream induced by natural convection. the higher temperature airstream inside the cover 80 previ 0075. The experimental evaluation demonstrates how ously described to exit at the top of the stove through the controllable the above embodiment is, and that no heat trans apertures 214. In this mode of operation, the second outer fers to the heat exchanger when the fan is not operative. cover 210 has the effect of further reducing the outer skin Provided operation of the fan is linked to the water tempera temperature which may be advantageous for reducing the ture in the heat exchanger as described above (potentially radiant heat transfer from the stove surfaces. including overheat protection), the water in the heat 0070. In the second mode of operation, in which the fan exchanger cannot boil and is therefore inherently safe. draws hot air from the stove sides 20 to the heat exchanger 84, air continues to flow upwards through the outer air chamber 1. Apparatus for capturing heat from a stove, the apparatus 218 by natural convection and due to reduced pressure at the comprising an air-water heat exchanger, the heat exchanger upper part of this chamber 218 caused by operation of the fan. having an air side and a water side and being operative to In this mode, heated air leaving the top of the outer air cham transfer heat between air on the air side and water flowing in US 2011/027 1948 A1 Nov. 10, 2011 channels in the water side, the apparatus including an air duct 13. Apparatus according to any preceding claim in which through which air heated by a stove can pass and control the heat exchanger includes a fin-and-tube heat exchanger means operable to control the flow of air within the apparatus; element. wherein in a first mode, the control means allows air to flow 14. Apparatus according to any preceding claim that fur from the air duct to a space Surrounding the stove at least ther includes an outer cover that creates a second air duct partly by natural convection and in a second mode, the control between itself and the cover. means induces air to flow from the air duct to the air side of the 15. Apparatus according to claim 14 in which air can be caused to flow in the second air duct by natural convection to heat exchanger, such that air within the duct flows in opposite reduce an outer casing temperature of the apparatus. directions in the first and second modes. 16. Apparatus according to any preceding claim in which 2. Apparatus according to claim 1 in which, in the first the control means is fail-safe such that it diverts air from the mode, air flows generally upward within the air duct. heat exchanger in the event of there being insufficient flow of 3. Apparatus according to claim 1 or claim 2 in which the water within its water side. control means includes a fan that is operable to induce a flow 17. A stove that incorporates apparatus for capturing heat of air in the apparatus during operation in the second mode. from a stove according to any preceding claim. 4. Apparatus according to claim 3 in which the fan is 18. A stove according to claim 17 in which the apparatus operable to draw air from a space Surrounding the stove into for capturing heat can capture heat from outer Surfaces of the the air duct for heating by the stove and then to the heat StOVe. exchanger. 19. A stove according to claim 18 in which the apparatus 5. Apparatus according to claim 3 or claim 4 in which, for capturing heat can capture heat from one or more of a side Surface, a rear Surface or a top surface of the stove. when the fan is not operating, air can be drawn from a space 20. A heating installation that includes a stove according to Surrounding the stove for heating by the stove, and Subse any one of claims 16 to 19 and a central heating system that quently returned to the space Surrounding the stove by natural includes a pumped water circulation circuit that operates to convection. transportheat from a boiler to one or more radiators, the water 6. Apparatus according to any one of claims 3 to 5 in which side of the heat exchanger being connected to the circulation the state of the control means is primarily determined by circuit Such that water from the circulation circuit is caused to whether or not the fan is running. flow through the heat exchanger while the pump of the circu 7. Apparatus according to any preceding claim that further lation circuit is operational. includes a bypass means that allows air to flow through the 21. A heating system according to claim 20 in which the duct to receive air transferred from the stove without passing control means operates to minimise the flow of heated air through the heat exchanger in the first mode. from the stove to the heat exchanger when the pump of the 8. Apparatus according to claim 7 in which the bypass circulation system is not operational. means includes a closable opening to Surrounding air in the 22. A heating system according to claim 20 or claim 21 in duct between the Stove and the heat exchanger. which the water side of the heat exchanger is connected between a flow and a returnpipe of the central heating system 9. Apparatus according to any preceding claim in which the in parallel with radiators of the central heating system. air duct includes a plate, a first portion of the air duct being 23. A heating system according to claim 22 further includ defined as a chamber that is formed between the plate and a inga water pump connected in series with the heat exchanger. heating Surface of a stove. 24. A heating system according to claim 23 further includ 10. Apparatus according to any preceding claim in which ing a non-return valve connected in series with the heat the air duct has a second portion that connects the lower part exchanger. of the first portion of the air duct to the air side of the heat 25. A heating system according to claim 20 or claim 21 in exchanger. which the water side of the heat exchanger is connected 11. Apparatus according to any preceding claim in which within a return pipe of the central heating system such that the air duct causes air to flow over one or more external water returning to the boiler passes through the heat heating Surfaces of a stove. exchanger. 12. Apparatus according to claim 11 in which the external heating Surfaces include heat transfer fins.