WO2009093073A2

WO2009093073A2 - Heat exchanger apparatus, system and method

Info

Publication number: WO2009093073A2
Application number: PCT/GB2009/050056
Authority: WO
Inventors: Stephen Taylor
Original assignee: Enertek International Limited
Priority date: 2008-01-25
Filing date: 2009-01-23
Publication date: 2009-07-30
Also published as: GB2456819A; WO2009093073A3; GB0801383D0; EP2242958A2

Abstract

A subsidiary stage heat exchanger apparatus for a conventional condensing heater such as an oil or gas fuelled condensing boiler, comprising a vessel to receive and reserve flue gas condensate from a primary stage condensing heat exchanger of thecondensing heater and a flue gas conduit having an inlet to receive flue gas from such a primary stage heat exchanger and an outlet, the flue gas conduit defining a flow path for such flue gas; wherein the outlet of the conduit is disposed to cause flue gas to pass into the condensate reserved in the vessel in use and a heat transfer means is provided in association with, and at least partly within, the vessel for transferring heat from the condensate into the said heat transfer means. The apparatus thereby advantageously transfers heat, which is otherwise wasted, from the flue gas leaving the primary stage heat exchanger to a heat transfer medium for increased efficiency of the primary stage heat exchanger.

Description

HEAT EXCHANGERAPPARATUS, SYSTEM AND METHOD

The invention relates generally to heat exchanger apparatus, to heating systems including such apparatus and to methods of heat exchange. The invention in particular relates to heat exchanger apparatus and methods for use in association with high efficiency condensing heating systems such as "boilers'Vwater heaters that operate by combustion of fuel, for example oil or gas, to produce hot gaseous combustion products. The invention specifically relates to the provision of a secondary stage heat exchanger, and to a method for the recovery of heat from combustion product gases at a secondary stage, locatable fluidly downstream of a primary heat exchanger apparatus which may be of conventional design.

Conventional boilers/water heaters are typically fuelled by a suitable combustible fuel, such as oil or gas. A conventional apparatus typically includes a burner for burning a mixture of fuel and air to generate hot combustion gases, typically predominantly carbon dioxide, water and excess air, and a heat exchanger. The purpose of the heat exchanger is to extract the resultant thermal energy from the combustion gases.

In a typical conventional apparatus, a burner combusts a mixture of fuel and air and the gaseous products of combustion and excess air (hereinafter referred to as flue gas) are passed across a suitable heat transfer apparatus, by a suitable impeller for example, before being exhausted to the atmosphere by suitable ducts or similar. The heat transfer apparatus transfers heat energy from the flue gas to a heat transfer medium. Typically the heat transfer medium is a fluid, such as water, and the heat transfer apparatus comprises a conduit for such fluid having a fluid input and a fluid output and suitable heat transfer means, for example in the form of heat transfer surfaces, to facilitate the transfer of heat from the flue gases to the heat transfer fluid.

The heat transfer fluid may be a working fluid which it is primarily intended to heat, for example being water for use directly in domestic or commercial hot water or central heating systems or other industrial processes, or may be used indirectly to transfer heat to such a primary working fluid for example via an indirect hot water system.

Such a heat exchanger apparatus may be of tubular, monoblock or sectional construction. In all such arrangements, heat transfer is effected by radiation in the immediate vicinity of the burner and by convection/ conduction thereafter through the suitable heat transfer surfaces from the hotter flue gas to cooler heat transfer medium in accordance with the temperature gradient at the relevant point in the heat exchanger.

In a condensing boiler/ water heater, as the temperature of the flue gas falls within the heat exchanger to a level where the dew point of water vapour within the flue gas is reached the water vapour begins to condense. Further latent heat is recovered and the efficiency of the boiler/ water heater can be enhanced over non-condensing systems, as will be familiar in the art.

Condensate is formed in that part of the heat exchanger, away from the burner, where the temperature of the flue gas falls below the water dew point. The condensate typically flows under gravity through suitable flow channels and exits the bottom of the heat exchanger into a suitable collection vessel. The condensate is typically discarded. That part of the heat exchanger where condensation occurs is sometimes referred to as the secondary part of the exchanger or condensing section, and may be of different construction to a primary part where heat is recovered directly and only from hot flue gas. Such a primary and secondary part may be of separate construction, particularly in heat exchangers of sectional construction. Multiple exchanger apparatus or sections may be provided successively. However, in the context of the present invention, all such heat exchanger arrangements are referred to as, and considered to be, primary stage heat exchangers.

Although conventional condensing boilers/water heaters offer potential efficiency advantages, and potentially recover more of the energy from the combustion products than non-condensing designs, there can still be a considerable amount of residual heat in the flue gas when vented to atmosphere following the condensing stage. In particular, although water condenses at least in part from the flue gas, a significant degree of residual heat may be retained in carbon dioxide, excess air and other combustion gases, which is simply vented to atmosphere and wasted.

Moreover, in a conventional condensing boiler/ water heater, exhaust gases may still include a substantial quantity of residual water vapour. It is known that this can produce a visual effect referred to as "pluming" which can impact on a surface in the vicinity of an external vent, for example on an external wall. This pluming effect is unsightly and undesirable, particularly in domestic environments.

The exhaust gases vented from a conventional condensing boiler/ water heater may also include various minor combustion products, such as oxides of sulphur and nitrogen, which, even when present in relatively low quantities, can still constitute an undesirable pollutant. It is generally desirable to develop a heat exchanger apparatus and method that mitigates one or more of the above problems and/or provides one or more of the following additional features: the potential for recovery of additional heat from combustion gases; - the reduction of an undesirable tendency to produce pluming; and the reduction of otherwise undesirable and, for example, pollutant combustion product levels in exhaust gases vented to the atmosphere.

In accordance with the invention in a first aspect, a subsidiary stage heat exchanger apparatus for a condensing heater such as a condensing boiler/ water heater comprises: a vessel to receive and reserve flue gas condensate from a primary stage condensing heat exchanger of a condensing heater; and - a flue gas conduit having an inlet to receive flue gas from such a primary stage heat exchanger and an outlet, the flue gas conduit defining a flow path for such flue gas; wherein the outlet of the conduit is disposed to cause flue gas to pass into the condensate reserved in the vessel in use and a heat transfer means is provided in association with, and for example at least partly within, the vessel for transferring heat from the condensate into the said heat transfer means.

The invention thus comprises a "secondary stage" heat exchanger for a condensing heater apparatus such as that generally referred to as a condensing boiler/ water heater that operates by combustion of fuel, for example oil or gas, to produce hot gaseous combustion products and uses a condensing principle to improve efficiency of operation, for example in known manner. The heat exchanger comprises a "secondary stage" heat exchanger in the sense that it is located fluidly downstream (as regards flue gas flow) of a primary heat exchanger apparatus which recovers heat initially from the hot flue gas and produces condensate in generally conventional manner.

The secondary apparatus of the invention is fluidly in line with such a primary heat exchanger in use in two respects. Firstly, it includes a containment vessel which acts, at least in part, as a reservoir for condensate liquid produced in the primary stage, and is therefore so positioned and/or adapted as to enable condensate to flow into the interior of the vessel. Secondly, a flue gas conduit is provided for defining a flow path for the flue gases from such a primary stage into the secondary stage apparatus of the invention.

The invention is distinctly characterised in that the flue gas conduit directs flue gas into the condensate liquid itself during use, in that it is provided with an outlet which lies in use below a designed condensate level. The vessel is suitably configured, for example by provision of condensate outlet means at an appropriate height, to maintain a designed condensate level above the outlet of the flue gas conduit.

Therefore, the flue gases conveyed from the primary stage heat exchanger are caused to pass through condensate in the vessel, and to give up further residual thermal energy directly to the condensate liquid. The condensate liquid is heated, and this heat is retrieved by the heat transfer means associate with (e.g. around or within) the vessel such as to enable heat transfer to be effected from the heated condensate liquid. Preferably, the heat transfer means are at least partly located within the vessel so as to pass below the designed condensate level in use. Preferably, the heat transfer means comprises a heat transfer conduit defining a flow path for a heat transfer medium therethrough. Preferably the heat transfer medium is a heat transfer fluid and more preferably a heat transfer liquid. Suitably, the heat transfer liquid may be water.

Preferably, the conduit is provided with or comprises appropriate heat transfer surfaces to effect efficient transfer of heat from the condensate liquid to the heat transfer medium. Additional heat energy still present in the flue gases after the primary stage of heat exchange is therefore recovered. This recovery can be particularly efficient where the heat transfer medium is a liquid, so that the transfer within the secondary heat exchanger is essentially a liquid to liquid transfer.

Condensate liquid is put to use as part of the heat exchange apparatus rather than simply being discarded.

Preferably, the vessel defining a reservoir for condensate is considerably deeper than a conventional condensate discard sump in a conventional condensing boiler/ water heater apparatus. Preferably, the vessel is configured to define in use a partially or completely enclosed volume comprising a lower portion in which condensate liquid is retained and an upper portion which is vented to atmosphere. In this way, flue gases conveyed via the conduit bubble through the condensate liquid in the lower portion and rise to the upper portion to be vented.

Thus, during operation, additional heat is recovered from flue gas by bubbling the flue gas through the condensate liquid retained in the containment vessel. This can also have secondary advantages in some modes of operation. Additional water vapour may be removed from the flue gas, recovering further heat and reducing the pluming effect when the flue gas is finally exhausted. The action of bubbling the flue gas through the condensate liquid may also have some effect in scrubbing secondary combustion products from the flue gas and taking them into solution in the condensate liquid, which can reduce pollutant levels, such as NOx.

As described above, the vessel defines at least a partial enclosure containing condensate liquid in use, and allows flue gas which rises to the surface of the condensate liquid to be vented, ultimately to atmosphere, while at the same time providing for maintenance of generally constant condensate liquid levels. Preferably, the vessel includes at least one condensate outlet and at least one aperture serving as an outlet vent for flue gas and drain point. The condensate outlet defines a condensate liquid level in use. The outlet vent for flue gas is disposed above this level in use.

For example, the vessel preferably comprises one or more condensate outlets disposed in a wall thereof to define a condensate liquid level in use. During use, a steady-state operation will be set up whereby as condensate constantly replenishes the condensate liquid retained in the vessel, excessive condensate may overflow via the condensate outlet. This serves two purposes in particular. Firstly, it helps to maintain a constant condensate liquid level in use and thus ensures that the flue gas conduit outlet and the secondary heat transfer means remain below the condensate liquid level during use. Secondly, it tends to assist in the maintenance of a chemical steady state with respect to composition in the condensate liquid, preventing excessive build up of acidity from dissolved combustion products.

Preferably the vessel should be of a material that is chemically and electrically inert when in contact with condensate. Preferably, at least one aperture is provided in the vessel as an entry point for at least one flue gas conduit to deliver flue gas from a primary stage heat exchanger into the vessel. An aperture is also provided to allow condensate from the flue gas generated in the primary stage to enter the vessel and replenish the condensate liquid contained in the vessel. In one convenient embodiment, the flue gas conduit also serves as a conduit for condensate and the aperture provided for the flue gas conduit serves as an aperture for both flue gas and condensate.

In one possible embodiment, the vessel comprises a complete, substantially fluid-tight enclosure, save for such necessary apertures, inlets and outlets. That is to say, the vessel comprises a complete enclosure, save for at least one flue gas aperture, at least one condensate aperture which may be coextensive therewith, preferably at least one condensate overflow outlet, and at least one flue gas outlet to allow flue gas to be vented, and preferably also a drain point for shipping and service.

Preferably, the heat transfer means provided with the vessel comprises a fluid conduit having an inlet and an outlet external to the vessel to define a flow path for a heat transfer medium around or more preferably through the vessel (and at least in part through the condensate in use). Such a conduit comprises, for example, an arrangement of pipes. Suitably, the pipes may be coiled or finned. Preferably, a spiral pipe arrangement is provided.

Preferably, the conduit walls comprise heat transfer surfaces to allow heat to be transferred from the condensate liquid to the heat transfer medium. These may be suitably configured and/or include suitable additional structures to enhance thermal transfer. In the preferred embodiment, with a liquid heat transfer medium, liquid to liquid transfer can be particularly efficient, and simple pipe walls may be sufficient to serve as a heat transfer surface without the requirement for more complex structures, such as are typical for gas to liquid transfer. The conduit needs to be chemically and electrically inert when in contact with condensate.

The heat exchanger apparatus of the invention is thus a "secondary stage" heat exchanger located in use downstream of a conventional "primary stage" condensing heat exchanger. The primary stage heat exchanger can be of any suitable condensing design. In particular, reference herein to "primary stage" and "secondary stage" should not be read as in any way limiting the heat exchanger structure at the primary stage to monoblock alternatives, for example. A primary stage heat exchanger apparatus for use in conjunction with the heat exchanger apparatus of the invention may still be sectioned, for example into a primary part and a secondary part. More than one heat exchanger may be provided in a primary stage apparatus, for example disposed serially or in parallel.

Preferably, in a more complete aspect of the invention there is provided a heat exchange system comprising a secondary stage heat exchanger as hereinbefore described in serial connection with, and fluidly downstream of, a primary stage principal heat exchanger of suitable condensing design in a condensing heater apparatus.

For example, the present invention provides a secondary stage heat exchanger as hereinbefore described disposed serially in conjunction with a primary condensing heater and heat exchanger comprising a heat transfer medium conduit defining a flow path for a heat transfer medium through the primary heat exchanger, a gas flow path means for defining a flow path for hot flue gas from a burner through the primary heat exchanger to a flue gas outlet, and heat transfer surfaces associated with the conduit for transferring heat from the gas in the gas flow path to a heat transfer medium in the conduit of the primary heat exchanger.

Preferably, the flue gas conduit in the secondary stage apparatus is so disposed that the inlet thereof is in fluid communication with an outlet of the flue gas flow path in the primary stage heat exchanger, either directly or via a flow path defined by a transfer conduit. Preferably, the primary stage heat exchanger includes means to collect condensate, and condensate flow path means defining a flow path from the primary heat exchanger to the containment vessel of the secondary heat exchanger.

Conveniently, the secondary heat exchanger may be located below the flue outlet of the primary heat exchanger to allow condensate from the flue gas to enter the vessel under the action of gravity.

In accordance with the invention in a further aspect there is provided a method of recovering heat in a subsidiary stage in a condensing heater such as a condensing boiler/ water heater system comprising the steps of:

collecting flue gas after passage through a primary stage condensing heat exchanger; reserving condensate from said primary stage heat exchanger in a retaining vessel; causing flue gas to pass into the reserved condensate liquid; and operating a heat exchanger to extract heat from the condensate liquid, and in particular causing a heat transfer medium to flow into or around the vessel, and preferably through the condensate liquid, within a suitable heat transfer means defining a flow path for the heat transfer medium, whereby heat is transferred via the heat transfer means from the condensate liquid to the heat transfer medium.

In accordance with the method of the invention, flue gas from a conventional "primary" condensing heat exchanger including gaseous combustion products from burning of fuel such as oil or gas and excess air is caused to pass through recovered condensate liquid. This can produce efficient and effective recovery of residual heat from the flue gas and/or the condensate. This recovered heat is drawn off via the "secondary" heat exchanger which is configured as described above. Flue gas bubbles to the surface of the condensate liquid and is collected within an upper volume of the vessel above the condensate surface where a suitable vent can be provided from which the gas can be conveyed to the atmosphere.

Preferably, the method comprises maintaining a generally steady-state level of collected condensate liquid within the retaining vessel by means of operation of the suitably positioned overflow outlet in a wall of the vessel. A secondary advantage of setting up a steady state mode of operation, whereby the condensate level is constantly replenished by condensate recovered from the primary condensing heat exchanger, and is constantly discarded by an overflow outlet, is that a relatively steady chemical composition state may also be maintained and acidity build up reduced.

In a more complete embodiment of the method, the method comprises the additional steps of:

burning fuel such as oil or gas to produce hot combustion product gases; causing the hot combustion gases to pass through a primary condensing heat exchanger so that heat is recovered to a suitable heat transfer means from the hot gases and preferably further so that at least some of the water vapour present in the hot combustion gases is caused to condense to recover further heat; and carrying the gases so passed and collecting and conveying the condensate so generated to a secondary heat exchange apparatus to perform the method steps in accordance with the foregoing aspect.

Preferably the heating apparatus is operated in condensing mode at all times. As will be familiar however this is dependent on temperature. A condensing boiler does not condense if maximum temperature is above dew point.

The invention will now be described in one aspect by way of example only with reference to figures 1 and 2 of the accompanying drawings in which: figure 1 illustrates a possible embodiment of a heat exchanger apparatus operating in accordance with the principles of the invention and suitable for putting into practice the principles of the method of the invention; figure 2 illustrates an alternative arrangement of flue gas conduit outlet for fitment with the apparatus of figure 1.

The figure illustrates a secondary or subsidiary stage heat exchanger in accordance with an embodiment of the invention, designated generally by reference 1. In a practical system, this will be used with a primary condensing heating system of the type generally referred to as a condensing boiler/ water heater with suitable heat exchanger apparatus (not shown). Such a condensing heating system will familiarly consist of a burner for burning a suitable fuel, for example gas or fuel oil, to produce hot gaseous combustion products, and an impeller to cause the hot gaseous combustion products to pass through a suitable flow path in at least one heat exchanger to extract some of the heat therefrom and, at least in part, cause water vapour to condense from the combustion product. The apparatus 1 is used as a secondary heat exchanger fluidly downstream of such a primary in use.

The apparatus 1 includes a fluid containment vessel 2 which provides a reservoir for condensate liquid formed in the primary stage condensing heat exchanger. The vessel 2 defines a lower volume 4 for retaining collected condensate liquid in use and an upper volume 6 above the lower volume 6 for gaseous containment. The upper volume 6 is vented to exhaust via the vent 8. The lower volume 4 is defined, and the condensate level 10 maintained, via the drain 12. The vessel 2 thus serves as a relatively deep condensate sump.

During operation, flue gases (shown by arrow 14) pass from a suitable primary stage condensing heat exchanger via the flue gas conduit 16. The flue gas conduit 16 enters the vessel 2 via a suitable aperture 18.

Condensate produced in the primary stage is also passed into the vessel 2 to replenish the reservoir 4 either via the flue gas conduit 16 or a separate 20. In the example, the vessel 2 forms an almost complete enclosure, and a discrete conduit 20 are provided to allow condensate to pass into the volume 4,6 defined by the vessel 2. Alternatively, condensate may pass via the conduit 4. Alternative arrangements could, of course, be envisaged, for example where the vessel 2 was open at the top and condensate dropped into the vessel 2 through the open top or at least one further aperture was provided for condensate to enter the vessel 2. Although a substantial quantity of heat has typically been removed from the flue gas by the primary heat exchanger, and a quantity of the water vapour contained therein has been condensed, the depleted combustion products'! 4 and any residual air passing into the conduit 16 still typically retain considerable residual heat and moisture. The depleted combustion products 14 and any residual air are then passed into the apparatus of the invention via conduit 16, which has an open outlet 22 below the level of condensate liquid 10 in the vessel 2, causing the flue gas 14 to pass directly into the condensate 4 in the vessel 2 in the direction of and shown by arrows 24. Flue gas 24 is thus passed directly through the condensate liquid 4 retained in the vessel 2, thereby raising the condensate 4 temperature but extracting heat (sensible and latent) from the flue gas 24. The flue gas 24 bubbles through the condensate liquid 4 and rises to the surface of the condensate volume 4 into the gaseous volume 6 above it and is vented via the aperture 8.

In accordance with the operation of this embodiment, condensate 4 is therefore used in an appliance for the direct extraction of heat from the flue gas 14, 24 and transfer of this heat to a primary or secondary water circuit via a suitable secondary stage heat transfer surface or heat exchanger. In the embodiment, a coil 26 is passed through the condensate vessel 2 thereby extracting heat from the condensate 4. Different arrangements involving an alternative coil, pipe or similar heat exchanger arrangement passing through or around the condensate vessel 2 can be envisaged. The heat transfer medium in the coil 20 may be sanitary water, central heating water or a combination of both.

A layer of mesh 28is provided above the conduit 22 but below the surface 10 of the condensate 4 to reduce the formation of large bubbles as the flue gas 24 rises to the surface 10 of the condensate 4, and thus to reduce the noise in the system 1.

The system 1 recovers further residual heat from the flue gas 14, 24 which would otherwise be lost by a conventional primary stage apparatus alone. Flue gas 14, 24 passes through relatively cool condensate 4 directly exchanging sensible heat and absorbing latent heat through quenching. This direct heat exchange offers potential efficiency advantages over conventional gas/water heat transfer through the medium of a heat exchanger. The secondary coil 26 becomes a water to water heat exchanger, which can generally be expected to simplify design, for example generally requiring less complex heat transfer surface arrangements, than a gas/water heat exchanger.

Secondary advantages can also be identified that might result from passing the flue gas 14, 24 directly through the condensate volume 4, rather than through a heat exchanger apparatus that keep the flue gas, heat transfer surface and condensate separate. Additional uncondensed water vapour may be extracted from the flue gas 14, 24 as the flue gas 14, 24 passes through the condensate volume 4, reducing moisture levels, and, for example, reducing pluming effects when the residual gas is eventually conveyed to the atmosphere. Some pollutants which might be present in the flue gas 14 might be at least partially "scrubbed" from the flue gas 14 by going into solution in the condensate 4 thus reducing NOx levels.

Sensing of a blocked condensation outlet 12 may be achieved by a suitably placed pressure sensing point positioned in the flue gas conduit 16. In the embodiment the sensing tube 32 is provided for this purpose. This could also act as a sensor for a blocked flue outlet 22, provided that the water trap/ siphon on the condensate outlet 12 was of sufficient depth to prevent combustion products escaping via that route.

A drain point 34 can also be envisaged to allow any condensate 4 to be removed after the appliance has been tested and is to be shipped. The suitable drain point 34 can also be used for servicing.

An alternative arrangement for the outlet 22 of the flue gas conduit 16 of figure 1 is illustrated in figure 2.

In figure 1 the flue gas conduit 16 is showing as a simple cylindrical arrangement. Figure 2 modifies this arrangement in the vicinity of the outlet 22, where the cylindrical conduit 16 is provided with an outlet region which flares in a first direction and narrows in a second direction to produce an outlet aperture 30 which is of generally the same cross sectional area as the cross sectional area of the bore in the main part of the conduit 4, but which is considerably longer in the first direction and narrower in the second. Providing an outlet 30 having this aspect ratio increases the effective surface area of bubbles in contact with condensate 4 in the vessel 2, in particular in a manner which tends to reduce the formation of the large bubbles as the flue gas 24 rises to the surface 10 of the condensate 4. This can complement the effect of the layer of mesh 28, or even allow it to be dispensed with.

In figure 2, the flue gas conduit 16 is shown extending from a generally cylindrical portion 16 to a flared portion 34 defining a rectangular outlet 30 of generally the same cross sectional area, but the same principle could apply if the entire flue gas conduit 16 defined a rectangular, or generally rectangular, flow bore.

Claims

1. A subsidiary stage heat exchanger apparatus for a condensing heater, comprising: - a vessel to receive and reserve flue gas condensate from a primary stage condensing heat exchanger of a condensing heater; and - a flue gas conduit having an inlet to receive flue gas from such a primary stage heat exchanger and an outlet, the flue gas conduit defining a flow path for such flue gas; wherein the outlet of the conduit is disposed to cause flue gas to pass into the condensate reserved in the vessel in use and a heat transfer means is provided in association with, and at least partly within, the vessel for transferring heat from the condensate into the said heat transfer means.

2. Apparatus according to claim 1 , wherein the outlet of the conduit is disposed below a designed condensate level in use.

3. Apparatus according to claim 1 or 2, wherein the heat transfer means is disposed below a designed condensate level in use.

4. Apparatus according to any preceding claim, wherein the heat transfer means comprises a heat transfer conduit defining a flow path for a heat transfer medium therethrough.

5. Apparatus according to claim 4, wherein the conduit comprises appropriate heat transfer surfaces to affect efficient transfer of heat from the condensate liquid to the heat transfer medium.

6. Apparatus according to any preceding claim, wherein the vessel is configured to define an enclosed volume comprising a lower portion in which condensate liquid is retained in use and an upper portion which is vented to atmosphere.

7. Apparatus according to claim 6, wherein the vessel comprises at least one condensate outlet and at least one aperture serving as an outlet vent for flue gas and drain point.

8. Apparatus according to any preceding claim, wherein at least one aperture is provided in the vessel as an entry point for at least one flue gas conduit to deliver flue gas from the primary stage heat exchanger into the vessel.

9. Apparatus according to any preceding claim, wherein the heat transfer means comprises a fluid conduit having an inlet and an outlet external to the vessel to define a flow path for a heat transfer medium around or through the vessel and at least in part through the condensate in use.

10. Apparatus according to claim 9, wherein the heat transfer means comprises a spiral pipe arrangement.

11. A heat exchange system comprising a subsidiary stage heat exchanger apparatus according to any preceding claim in serial connection with, and fluidly downstream of, a primary stage principal condensing heat exchanger.

12. A heat exchange system according to claim 11 , wherein the flue gas conduit in the subsidiary stage heat exchanger is so disposed that the inlet thereof is in fluid communication with an outlet of a flue gas flow path in the primary stage heat exchanger.

13. A heat exchange system according to claim 11 or 12, wherein the primary stage principal heat exchanger comprises means to collect condensate, and condensate flow path means defining a flow path from the primary stage principal heat exchanger to the vessel of the subsidiary stage heat exchanger.

14. A method of recovering heat in a subsidiary stage in a condensing heater, comprising the steps of:

- collecting flue gas after passage through a primary stage condensing heat exchanger;

- reserving condensate liquid from said primary stage heat exchanger in a retaining vessel;

- causing flue gas to pass into the reserved condensate liquid; and

- operating a heat exchanger to extract heat from the condensate liquid, and in particular causing a heat transfer medium to flow into or around the vessel, and through the condensate liquid, within a heat transfer means which defines a flow path for the heat transfer medium, whereby heat is transferred via the heat transfer means from the condensate liquid to the heat transfer medium.

15. A method according to claim 14, further comprising the step of: - maintaining a generally steady-state level of collected condensate liquid within the retaining vessel by means of operation of a suitably positioned overflow outlet in a wall of the vessel.

16. A method according to claim 14 or 15, comprising the steps of: - burning fuel to produce hot combustion product gases; - causing the hot combustion gases to pass through the primary condensing heat exchanger so that heat is recovered to the heat transfer means from the hot gases and so that at least an amount of water vapour present in the hot combustion gases is caused to condense to recover further heat; and

- carrying the gases so passed and collecting and conveying the condensate so generated to a subsidiary stage heat exchanger apparatus.