Abstract:
Apparatus for capturing heat from a stove to enable it to be transferred to a central heating system is disclosed. The apparatus comprises an air-water heat exchanger, the heat exchanger having an air side and a water side and being operative to transfer heat between air on the air side and water flowing in channels in the water side. The apparatus includes an air duct through which air heated by a stove can pass by convection to contact the air side of the heat exchanger and control means operable to control the amount of air flowing to the heat exchanger. The control means may include a fan that can be operated to direct the flow of air to the heat exchanger.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is Continuation application of pending prior patent application Ser. No. 13/143,472 filed Jul. 6, 2011, pending, entitled APPARATUS FOR CAPTURING HEAT FROM A STOVE, which is a national stage entry of PCT/GB2010/050013 filed Jan. 06, 2010, under the International Convention claiming priority over GB Patent Application No. 0900259.3 filed Jan. 9, 2009 and GB Patent Application No. 0904718.4 filed Mar. 19, 2009. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to apparatus for capturing heat from a stove. In particular, it relates to apparatus for capturing heat from a stove whereby that heat can be transferred to a central heating system. In the context of this invention, the term ‘stove’ is used as a generic term for apparatus to allow enclosed burning of solid fuels. It does not necessarily imply a stove that can be used for cooking, although some do offer cooking or warming plates. Moreover, the invention is described principally in connection with wood-burning stoves, because this is the context in which the greatest potential benefit can be obtained. However, it can also find application to stoves intended to burn solid fossil fuels (such as coal) or to multifuel stoves. 
       BACKGROUND OF THE INVENTION 
       [0003]    Burning wood on open fires and in stoves plays a small but increasing role as a practical source of renewable, biomass residential heat. Heat produced by burning wood can displace heat produced by burning fossil fuels in central heating systems, thereby saving costs for the householder and reducing carbon emissions. 
         [0004]    To maximise the benefit of burning wood in a modern stove, some of the heat produced should preferably be dissipated throughout a building in which it is installed to offset the amount of heat required from fossil-fuel based heating systems. One way in which this has traditionally been achieved is by incorporating a back boiler into a solid-fuel stove. As a stove has a relatively high thermal mass and limited burn control, back boilers need to be connected to a ‘gravity fed’ circuit to ensure heat can be dissipated without having to rely on a pumped water circuit. Failure to circulate water through a back boiler might lead to potentially explosive boiling of the water in the jacket. The term ‘gravity fed’ refers to natural circulation caused by the lower density of hot water leaving the top of the back boiler being displaced upwards by the higher density of cooler water entering the bottom of the back boiler, so forming a natural thermal circulation. As the thermal circulation force is quite weak compared to a pumped system, large-bore pipes must be used (typically 28 mm) to reduce pressure losses associated with circulation and the primary heat sink (a radiator or a hot water tank) must be relatively close to and above the stove, and cannot be turned off. A typical arrangement for a stove  10  with a back boiler is shown in  FIG. 1 . In the system of  FIG. 1 , the radiators  12  and the indirect hot-water cylinder  14  must be able to dissipate all of the heat from the back boiler to avoid boiling of the water. The pipes of the main circuit  16  are of large (28 mm) diameter. Most back boiler systems must also be open vented with a header tank  17  in the loft. Installation of such a large-bore system is inconvenient and expensive. 
         [0005]    It is, in principle, possible to heat a whole house from a large stove with a gravity-fed radiator system. However, most stoves do not have sufficient output to heat a whole house, and few people would want to rely on manual feeding 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 electric central heating, such that the stove provides an additional heat source to supplement the main heating system and, to some extent, its fuel consumption; this is likely to become far more prevalent as the costs of fossil fuels have escalated dramatically in recent times. 
         [0006]    Systems are available to link the output from a gravity-fed stove circuit with a central heating system that include either ‘neutraliser’ tanks, heat exchangers or combined gravity and pumped systems. Neutralisers offer a comprehensive solution, but are complex, expensive, cannot readily be retrofitted into an existing central heating system, and are incompatible with modern, sealed central heating systems. They have various different circuits, whether pumped or naturally circulated, connected together at a single (hydraulically neutral) point, whereby each circuit will take and return the same flow of water and cannot hydraulically interfere with one another, although thermally the water streams mix in a hot water tank. 
       SUMMARY OF THE INVENTION 
       [0007]    An aim of this invention is to provide apparatus by means of which heat from a stove can be used to supplement heat input into a central heating system, the apparatus being compatible with an existing central heating system, including sealed systems, and being fail-safe in the event of an electrical power failure. 
         [0008]    A particular, but not exclusive, aim of this invention is recovery of heat from outer surfaces of stoves, especially modern, efficient stoves. There is little merit in trying to recover heat from the chimney gasses of efficient stoves since only a small part of the total heat output of such stoves is carried in the chimney gasses. Taking heat from the chimney gasses may cause problems: cooler gasses tend to form condensates in the chimney, which can cause tars and creosotes to accumulate in the chimney, and an insufficiently warm chimney can dramatically reduce the convective ‘draw’ required for good combustion. Heat exchangers that are exposed to combustion gasses are likely to suffer from fouling by combustion products. Moreover, if a heat exchanger is located in a chimney, it is not an easy matter to divert heat from the heat exchanger such that it can be discharged into a room surrounding the stove. 
         [0009]    To this end, from a first aspect, the invention provides apparatus for capturing heat from a stove, the apparatus comprising an air-water heat exchanger, the heat exchanger having an air side and a water side and being operative to transfer heat between air on the air side and water flowing in channels in the water side, the apparatus including an air duct through which air heated by a stove can pass and control means operable to control the flow of air within the apparatus; wherein in a first mode, the control means allows air (heated by the stove)to flow from the air duct to the surroundings of the stove at least partly by natural convection and in a second mode, the control means induces air (heated by the stove)to flow from the air duct to the air side of the heat exchanger (as by forced convection). 
         [0010]    During operation of the stove, operation in the first mode minimises the hot air flowing in the heat exchanger, and hot air carrying a maximum amount of heat is passed to the surroundings of the stove. Therefore, a minimum of heat is transferred to the water and the stove acts as a room heater. However, during operation in the second mode, heat is transferred from the heated air to water flowing within the water side of the heat exchanger. This later condition allows heat from the stove to be transferred to another heating system, hot water system, etc. 
         [0011]    In the context of this specification, “natural convection” and “forced convection” should be taken as having their normal, technical meaning. Put simply, natural convection is the flow of air that occurs due to buoyancy pressure created by the heating of air, and forced convection is the flow of air due to pressure created by an active device, such as a fan. 
         [0012]    Air within the air duct typically flows in opposite directions in the first and second modes. Most typically, in the first mode it flows generally upward (although it may be that it does not flow upward throughout its entire flow path), being driven by natural convection. Such an arrangement has been found to offer a system in which the flow of air and transfer of heat can be controlled predictably and easily. 
         [0013]    In embodiments of the invention, the control means may include a fan that is operable to induce a flow of air in the apparatus during operation in the second mode, such a flow typically being in a direction opposite to that in which it would flow due to natural convection. More particularly, the fan may be operable to draw air from the surroundings into the air duct for heating by the stove and then to the heat exchanger. Thus, when the fan is operating, heat is transferred to the heat exchanger. Moreover, when the fan is not operating, air may be drawn from the surroundings for heating by the stove, and subsequently returned to the surroundings by natural convection. 
         [0014]    In embodiments that include a fan, the state of the control means (and therefore the mode of operation of the apparatus) is primarily determined by whether or not the fan is running. Automatic control of a fan is a straightforward matter, so this provides a convenient control system. 
         [0015]    Embodiments of the invention may include bypass means that includes a closable opening to surrounding air in the duct between the stove and the heat exchanger. The bypass can provide a low-resistance path for air to flow under natural convection when the apparatus is operating in the first mode. The bypass means includes a closable opening to surrounding air in the duct between the stove and the heat exchanger. For example, a flap may be provided to close the opening. 
         [0016]    In embodiments of the invention, the air duct may include a plate, a first portion of the air duct being defined as a chamber that is formed between the plate and a heating surface of a stove. 
         [0017]    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 the heat exchanger. At least part of the control means may be located within the second portion of the air duct. 
         [0018]    In typical embodiments, the air duct causes air to flow over one or more external heating surfaces of a stove. These external heating surfaces may include heat transfer fins. The external heating surfaces typically include one or more sides of the stove, and may further include the rear of the stove. 
         [0019]    Various types of heat exchangers may be used. For example, the heat exchanger may include a fin-and-tube heat exchanger element. 
         [0020]    Embodiments may further include an outer cover that creates a second air duct between itself and the cover. This can be disposed to increase the efficiency of the apparatus. In such embodiments, air can be caused to flow in the second air duct by natural convection to reduce the outer casing temperature of the apparatus. 
         [0021]    Preferably the control means is fail-safe, such that it diverts air from the heat exchanger in the event of there being insufficient flow of water within its water side. This ensures that the heat exchanger is protected from damage by overheating. 
         [0022]    The apparatus for capturing heat may be adapted to capture heat from outer surfaces of the stove (such as one or more of a side surface, a top surface or a rear surface). 
         [0023]    From a second aspect, the invention provides a stove that incorporates apparatus for capturing heat from a stove according to the first aspect of the invention. 
         [0024]    The apparatus for capturing heat may be adapted to capture heat from outer surfaces of the stove (such as one or more of a side surface, a top surface or a rear surface). 
         [0025]    From a third aspect, the invention provides a heating installation that includes a stove according to the first aspect of the invention and a central heating system that includes a pumped water circulation circuit that operates to transport heat from a boiler to one or more radiators, the water side of the heat exchanger being connected to the circulation circuit such that water from the circulation circuit is caused to flow through the heat exchanger while the pump of the circulation circuit is operational. 
         [0026]    In embodiments of this aspect of the invention, the control means may operate to minimise the flow of heated air from the stove to the heat exchanger when the pump of the circulation system is not operational. 
         [0027]    The stove may be connected in series with or in parallel with radiators in the heating system. Embodiments in which the stove is connected in parallel with the radiators may further include a pump connected in series with the heat exchanger of the stove that can be operated independently of the central heating system pump to transfer heat from the stove to the radiators. A non-return valve may additionally be provided to prevent flow through the stove heat exchanger when the central heating system is running but the stove is not lit. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a diagram of a conventional back boiler system, and has already been discussed; 
           [0029]      FIG. 2  is a diagram of a stove upon which an embodiment of the invention can be based; 
           [0030]      FIGS. 3   a  and  3   b  illustrate an embodiment of the invention operating, respectively, in a first and a second mode; 
           [0031]      FIGS. 4   a  and  4   b  show the stove of  FIGS. 3   a  and  3   b  incorporated into a central heating system in two alternative configurations; 
           [0032]      FIG. 5  illustrates the heat output that can be obtained from the embodiment of  FIGS. 3   a  and  3   b;    
           [0033]      FIG. 6  shows a central heating system that is a modification of the system of  FIGS. 3   a  and  3   b ; and 
           [0034]      FIG. 7  shows an alternative form of a stove embodying the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0035]    An embodiment of the invention will now be described in detail, by way of example, and with reference to the accompanying drawings. 
         [0036]    As an introduction to the embodiment, operation of a modern wood-burning stove will now be discussed. 
         [0037]    Heat transfer from a stove to its surroundings (typically, a room in a building) occurs by way of radiation (A) and convection (B) from the surfaces of the stove to air within a room in which the stove is located and there is also heat lost (C) up the chimney  28 . For a well-designed stove, the chimney loss should be less that 30%, and may be less than 20% for some of the most efficient new designs. The proportions of radiative and convective heat transfer can vary depending on the design and temperature of the stove sides. 
         [0038]    For a stove  18  with finned sides  20 , as shown in  FIG. 2 , the majority of the heat generated by the burning fuel is transferred by convection from the stove sides  20 , with a higher proportion of radiant heat transfer from a glass front  22  of the stove  18 , where such a front is fitted, as in this example. Bigger and cooler surface areas are provided in stoves in which it is intended that convection should be maximised, with typical operating surface temperatures in the region of 200-250° C. If a greater radiative output is required, higher surface temperatures and flat sides (less roughness and/or no fins) are generally preferred. 
         [0039]    Convective heat transfer also takes place from the top of the stove  18 , but more heat is transferred from external vertical surfaces than horizontal surfaces due to the induced convective airflow. Little heat is transferred from the bottom of the stove, as combustion air is usually drawn in at the bottom of the stove, and the grate and ash-pan act as heat shields. (Some stoves also incorporate a separate, protective external heat shield.) 
         [0040]    There is little merit in trying to recover heat from the chimney gasses of a modern, efficient stove, as these carry a minority of the heat output. Taking heat from the chimney gasses can cause problems, as cooler gasses tend to form condensates in the chimney, which can deposit tars and creosotes, and a cool chimney can dramatically reduce the convective ‘draw’, which is required for good combustion. (This is not the case for open fires, where the majority of the heat—85% or more—is lost up the chimney and some heat recovery is possible without affecting the draw.) 
         [0041]    The stove  18  illustrated in  FIG. 2  does not have a back boiler. Many modern stoves that operate without a back boiler incorporate a refractory lining  24  that ensures an even distribution of the heat from the fire  26  to external surfaces of the stove  18  to avoid localised overheating. 
         [0042]    Given the basic configuration of a stove described above, the aim of the invention is to transfer some of the heat from external surfaces  20  of the stove  18  in a manner that can be controlled between maximising the heat going direct to the room, or some of the heat going to a water circuit feeding a central heating system. With reference now to  FIGS. 3   a  and  3   b , an embodiment of the invention is constituted by a modification of the stove  18  shown in  FIG. 2 . 
         [0043]    In  FIGS. 3   a  and  3   b , first and second operating modes are shown; natural convection in  FIG. 3   a , and forced convection  FIG. 3   b . When forced convection is in operation as explained below, some heat is transferred to the central heating system, whereas in natural convection all of the heat goes to the room. 
         [0044]    In this embodiment, a conventional stove  18  is surrounded, typically on two sides  20  and a rear wall, by an enclosure  80  formed of thin sheet metal. The enclosure forms a vertically-extending air chamber that partly surrounds the stove, the chamber being C-shaped, in this embodiment, when viewed in plan. Vertical heat-convective fins  32  project from the stove into the chamber. 
         [0045]    Below the stove  18 , the enclosure  80  is connected into a vertical air duct  82 . A heat exchanger  84  is located within the air duct  82  vertically above a motor-driven fan  86 . The heat exchanger  84  is connected into a central heating system, as will be described below. This embodiment includes a heat exchanger  84  of the tube-and-fin type although other variants may be deployed. 
         [0046]    When the fan  86  is off, air is drawn upwards (as indicated in  FIG. 3   a ) by natural convection through the duct  82  into the air chamber within the enclosure  80 , from which it passes to the room. This is the embodiment operating in a first mode to transfer a maximum amount of heat to a surrounding space. 
         [0047]    When the electric fan  86  is on, it draws heated air within the duct  82  downwards (as indicated in  FIG. 3   b ), countering the natural convection. The air is heated within the chamber and drawn over the heat exchanger  84  to heat the central heating system water. 
         [0048]    The control of this embodiment can be very simple, since it is inherently fail-safe. A signal from the central heating system turns on the fan  86  to transfer heat when required. Alternatively, the fan  86  may be controlled locally by sensing the flow and temperature of water entering the heat exchanger  84 , to ensure the fan only operates when water is flowing and the water temperature is not too high. Additional functionality can easily be incorporated. For example, there may be a thermostatic control that allows heat to be sent to the central heating system (by operating the fan) only when a room temperature selected by a user is achieved and subject to the same flow and temperature conditions described above. To ensure fail-safe operation, the fan  86  may be wired in series with a simple low-cost overheat switch sensing the water temperature at the top of the heat exchanger such as those often found in central heating boilers (typically bi-metallic, manually reset type). This operates to stop the fan  86  if the temperature of water within the heat exchanger  84  becomes excessive. 
         [0049]    When the fan is on, the air leaving the duct  82  to the room will still carry some heat; the exiting air temperature cannot be lower than the central heating water temperature entering the heat exchanger. Therefore, not all of the heat from the sides  20  of the stove  18  can be transferred to the central heating system. However, by suitable arrangements of the chamber adjacent to the sides  20  of the stove  18  and the top of the heat exchanger, some heat will also be drawn in from the top of the stove and the outside of the flue. Alternatively, a configuration is possible in which the exhaust from the heat exchanger  84  is ducted back to the top of the stove sides such that the warm air exiting the ducting is recuperated to the chamber  80 . 
         [0050]    The water heat exchanger  84  is shown below the stove  18 , but could be located behind the stove  18 , provided a free air path is available for natural convection when the fan  86  is off. 
         [0051]    When the central heating is on but the stove is not lit, heat will be lost from the water heat exchanger  84  unless a thermally or electrically operated valve is incorporated in the central heating circuit. This is also true when the stove operates in the first (fan off) mode. Therefore, an electrically operated water valve connected to operate with the fan may be beneficial. Valves to control the flow of water in the heat exchanger  84  are shown in  FIGS. 4   a  and  4   b  and are described below 
         [0052]    The fan  86  may also be operated when the stove is not lit to provide central heating output, such that the stove could replace a radiator. In this mode, the stove acts like a fan-assisted room radiator. In such a mode, it may be advantageous to operate the fan  86  at a higher speed to increase the air flow over the heat exchanger  84  because the heat exchanger will be sized for high air temperatures and low air flow rates when transferring heat from the fire in the second mode described above. 
         [0053]    Although this embodiment is described for use with a stove that has distinct sides and rear surfaces, an analogous same design might be applied to stoves with curved surfaces. In such embodiments, the sheet metal enclosure follows the contour of the stove in question. 
         [0054]    In yet further embodiments, the enclosure may be formed from non-metallic materials and may also be insulated on the outer surface. 
         [0055]    In the embodiment as described above with reference to  FIGS. 3   a  and  3   b , the driving pressure created by natural convection is quite weak, so care must be taken to ensure that there is a reasonably free path for air flow. The heat exchanger design must be of sufficiently low pressure drop at the air flow rate induced by natural convection to ensure that air flow is not substantially impeded. Likewise, ducting  82  must be designed to ensure a low pressure drop and even distribution of air to the chamber  80 . 
         [0056]    Therefore, in an alternative arrangement of the embodiment there is incorporated a bypass that includes a flap  88  which is opened to allow free flow of air into the air duct to partly or completely bypass the heat exchanger  84  when the fan is off. When the fan is on, the flap  88  is closed to ensure the air passing over the heat exchanger  84  comes only from the chamber  80  and its temperature is therefore a maximum. 
         [0057]    The flap  88  may be caused to close when the fan is running either by exploitation of the air pressure difference when the fan is operating or by active means, such as an electrical actuator, that operates when the fan is on. 
         [0058]    Typical applications of an embodiment of the invention are shown in  FIGS. 4   a  and  4   b . Both examples provide a heating system that includes a conventional central heating boiler  120  that will typically be powered by gas, oil, solid fuel or electricity. Hot water is pumped from a flow outlet of the boiler into a flow pipe  122  of a heating circuit. Water is returned to a return inlet of the boiler  120  from a return pipe  124  of the heating circuit. Several radiators  126  and a hot water cylinder  128  are connected between the flow and return pipes  122 ,  124 , each of which can be provided with a portion of the flow within the circuit as a whole. The radiators  126  and the hot water cylinder  128  may incorporate thermostatically operated valves to control local heat distribution. This far, the heating system described is entirely conventional, and need not be further elaborated. 
         [0059]    In the embodiment of  FIG. 4   a , the heat exchanger  84  of a stove  18  equipped with an assembly described above is connected to the central heating system in the same manner as a radiator, such that some of the flow from pipe  122  within the system passes into the bottom of the heat exchanger  84 , and exits from the top of the heat exchanger  84  to be returned to the system through the return pipe  124 . This manner of connection makes it a straightforward matter to connect the heat exchanger assembly to an existing heating system, since the pipes leaving the boiler need not be disturbed. Connection is no more complex than the addition of a new radiator and can be made at any part of the circuit by convenient small-bore pipes. 
         [0060]    A valve  129  may be connected between the flow pipe  122  and the heat exchanger  84  to allow it to be isolated from the heating system. This valve may be electrically operated and is normally only opened if the fire is lit and it is desirable to divert heat to the central heating system (see comment regarding thermostatic control above). When the valve  129  is open, the control system can then sense whether there is flow in the central heating system to allow heat to be transferred from the stove by operating the fan  86 . In this arrangement, if the fan  86  is operating and water is flowing in the heat exchanger  84 , the fire  26  will add heat to the central heating system. If the fire  26  is not burning, the valve  129  will normally be closed unless it is desirable to operate the heat exchanger to output heat from the central heating system as described above. 
         [0061]    The water that is heated by the heat exchanger  84  returns to the boiler through the return pipe  124 , and mixes with water returning from other radiators  126  before reaching the boiler  120 . As the temperature of the water leaving the heat exchanger  84  is higher than that in the flow pipe  122 , the heated water entering the return pipe  124  adds heat energy to the return stream entering the boiler  120  by mixing with the cooler water returning from the radiators  126 . 
         [0062]    In an alternative application of an embodiment of the invention, as shown in  FIG. 4   b , the heat exchanger  84  is connected to the central heating system, such that all water returning to the central heating boiler  120  through the return pipe  124  from the radiators passes into the bottom of the heat exchanger  84 , and the top of the heat exchanger  84  is connected to the boiler return. Such connection is more appropriate for a new heating system installation where the layout of the central heating system can be arranged to accommodate such a connection. Configured in this manner, water entering the heat exchanger  84  will typically be cooler than at other parts of the flow circuit, such that greater heat transfer may take place from the fire  26  to the central heating circuit when the fire is lit, and less heat is transferred to the room from the outlet of duct  82  when the fan  86  is operating. 
         [0063]    Clearly, it is not acceptable to provide a simple valve to isolate the heat exchanger  84  from the central heating circuit in the configuration of  4   b,  since this would prevent any flow within the circuit. Therefore, it may be beneficial to incorporate a controlled bypass circuit that includes two valves  130 ,  132  to divert water flow through the heat exchanger  84  to a bypass pipe  134  in parallel with the heat exchanger  84  when the stove is not lit. The valves  130  and  132  act in a mutually exclusive manner, such that when the valve  130  is open, the valve  132  is closed, and vice versa. This may be usefully implemented by the valve  132  being of the normally-open type and the valve  130  being of the normally-closed type, such that both can be operated from a single electrical source which when activated allows flow through the heat exchanger  84  and when deactivated diverts flow through the bypass  134 . Alternatively, a single ‘divert’ valve may be incorporated to send flow to either the heat exchanger or the bypass, which may have the advantage of being more versatile in application as the bypass  134  could be excluded (blanked off or isolated by a manually operated valve) for the configuration of  4   a  and connected for the configuration of  4   b.    
         [0064]    A modification to the system of Figures and  4   a  is shown in  FIG. 6 . Components of this embodiment that are in common to the system of  FIG. 4   a  will be given the same reference signs as in those figures and will not be described further. 
         [0065]    This embodiment incorporates an alternative control mechanism. In this embodiment, a subsidiary water pump  200  is connected in series with the heat exchanger  84 . The pump  200  is configured in parallel to the central heating pump to take water from the cooler return circuit  124  and deliver it, through the heat exchanger, to the warmer flow circuit  122 . When the fan  86  is operated, the subsidiary water pump  200  is also operated to circulate central heating water through the heat exchanger  84  irrespective of whether or not the central heating circuit pump is running. In this embodiment, the stove can supply heat to the central heating circuit independent of the operating status of the central heating system pump. A non-return valve  202  is connected in series with the subsidiary water pump  200 . The purpose of the non-return valve  202  is to prevent flow of water through the heat exchanger when the system is operating in the first operating mode (that is, with the fan off). This avoids the need to provide electrically-operated valves to support operation of the heat exchanger. 
         [0066]    The embodiments of the invention described above can be further enhanced by addition of a second outer cover  210  as shown in  FIG. 7 . The second outer cover  210  substantially completely encloses the cover  80  and defines an outer air chamber between itself and the cover  80 . The second outer cover  210  has a top panel  212  through which several apertures  214  (which may be in the form of slots or a grille) are formed to connect the stove  20  to surrounding atmosphere to allow air to flow. 
         [0067]    In  FIG. 7 , on the right hand side, arrows indicate the direction of air flow in the first mode of operation (natural convection, fan off). On the left hand side, arrows indicate air flow the second mode of operation (forced convection, fan on) to transfer heat to the heat exchanger  84 . These two modes of operation are mutually exclusive and are shown on one diagram for illustration purposes only. As with the embodiments of  FIGS. 3   a  and  3   b , the action of turning the fan on is to reverse the natural, upward flow of air over hot surfaces  20  of the stove in the duct formed between the stove sides  20  and the cover  80 . When operating in the first mode with the fan off, the outer air chamber acts as a second convective space that operates to draw ambient air in at the bottom of the duct  216 . This air is heated by contact with cover  80 , rises and joins the higher temperature airstream inside the cover  80  previously described to exit at the top of the stove through the apertures  214 . In this mode of operation, the second outer cover  210  has the effect of further reducing the outer skin temperature which may be advantageous for reducing the radiant heat transfer from the stove surfaces. 
         [0068]    In the second mode of operation, in which the fan draws hot air from the stove sides  20  to the heat exchanger  84 , air continues to flow upwards through the outer air chamber  218  by natural convection and due to reduced pressure at the upper part of this chamber  218  caused by operation of the fan. In this mode, heated air leaving the top of the outer air chamber  218  joins air drawn in through the apertures  214  by operation of the fan and is further heated by the stove side  20 . Operating in this mode, the outer air chamber  218  described acts to recuperate losses that would otherwise occur from the hot outer side of cover  80  and has the benefit of increasing the temperature of the air reaching the heat exchanger  84 . 
         [0069]    In the configuration of  FIG. 7 , if the inlet of ducted air travelling thorough the outer chamber  218  is arranged to be close to the exiting warm air from the heat exchanger  84 , further recuperation may be achieved through recovery of warm air leaving the heat exchanger which would otherwise be lost to the room. The overall effect of this further embodiment is to increase the amount of heat that can be recovered to the central heating water when operating in the second mode. 
         [0070]    The practical effect of the embodiments described above is to provide a switchable control of heat transfer between the stove and the central heating system. At one setting, substantially all heat is passed directly to the room. At the other setting, much of the convective heat passing to the heat exchanger  84  and thereby to the central heating system. In the latter condition, radiative and convective heat from the bottom, front and top of the stove still passes to the room. Results of an experimental investigation of the configuration described in  FIGS. 3   a  and  3   b  will now be described with reference to  FIG. 5 . 
         [0071]    The heat exchanger arrangement described above has been investigated experimentally. One side of a stove was simulated by electrically heating a finned surface to maintain it at approximately 200° C. A plate was added to form a fire side chamber as described above and ducting was added to connect the chamber to a tube-and-fin heat exchanger and a fan, in the configuration of  FIGS. 3   a  and  3   b.  The graph of  FIG. 5  shows the effect of operating the fan to reverse the natural convection and divert heated air to a tube-and-fin heat exchanger. 
         [0072]    Before the fan is operated, hot air leaves the top of the fire side chamber by natural convection as shown by the curve described as “Air out fire side” on the chart, and the air entering the chamber from the bottom is at ambient temperature as shown by the curve described as “Air in fire side” (air temperature in the duct). When the fan is operated to overcome the natural convection and reverse the air flow, the temperatures of these two streams reverse, showing that ambient air enters the top of the fire side chamber and heated air passes downwards through the duct. As soon as the fan operates, heat starts transferring to the central heating water, rising to a maximum over a couple of minutes as shown by the curve described as “Water power (%)”. When the fan is turned off, natural convection is quickly re-established so that heat is dissipated to the room from the top of the fire side chamber as shown by the “Air out fire side” curve. Heat transferred to the water drops away rapidly as the heat exchanger components cool in the ambient air stream induced by natural convection. 
         [0073]    The experimental evaluation demonstrates how controllable the above embodiment is, and that no heat transfers to the heat exchanger when the fan is not operative. Provided operation of the fan is linked to the water temperature in the heat exchanger as described above (potentially including overheat protection), the water in the heat exchanger cannot boil and is therefore inherently safe.