GB2569865A

GB2569865A - Boiler assembly and flue gas non-return valve

Info

Publication number: GB2569865A
Application number: GB1817930.9A
Authority: GB
Inventors: Alan Allwood Nicholas
Original assignee: Ideal Boilers Ltd
Current assignee: Ideal Boilers Ltd
Priority date: 2017-11-03
Filing date: 2018-11-02
Publication date: 2019-07-03
Anticipated expiration: 2038-11-02
Also published as: GB201718232D0; GB2569865B; GB201817930D0

Abstract

A boiler assembly 10 defining a flue gas train has a flue gas Non-Return Valve (NRV) 32 located within the flue gas train. In an embodiment, the boiler assembly comprises a flue gas manifold 24 connectable to a flue system and the NRV is located in the manifold upstream of a flue gas outlet of the manifold. The NRV can be in the form of a cassette removably mountable in the boiler assembly. The NRV may have a rigid valve body which is slidably mountable in the boiler assembly and a rubber valve member mounted to the valve body. An outer perimeter region of the valve body and a mating surface in the boiler assembly may be profiled so as to define a gas tight seal. Alternatively the NRV cassette may be mounted within the manifold to avoid the requirement for an external seal. The manifold may be integral to a boiler sump. The NRV may be used to prevent reverse flow of flue gas when cascaded boilers are connected to a common flue.

Description

Boiler Assembly and Flue Gas Non-Return Valve

Technical Field of the Invention

The present invention relates to boilers. The present invention relates in particular to a boiler assembly incorporating a non-retum valve within the flue gas train of the boiler assembly itself. The present invention also relates to a non-retum valve for incorporation in a boiler assembly.

Background to the Invention

Heating and domestic hot water requirements in large apartment complexes or commercial buildings are frequently satisfied by fitting multiple smaller boilers cascaded together in the plant room or basement of the building. The cascaded boilers share common water and gas connections and are usually connected with a cascade flue system in which each boiler is connected to a common flue branch though an individual boiler flue branch. The use of common water and gas connections and a common flue branch simplifies installation and modern control systems can modulate the cascaded boilers to satisfy the heating or domestic hot water demand so that not all the boilers are fired when the demand is low.

In order to prevent flue products from the common flue branch recirculating and entering boilers which are not firing, a non-retum valve (NRV) is located in each individual boiler flue branch (i.e. before the individual boiler flue branch joins to the common flue branch). This arrangement is illustrated schematically in Figure 1 which shows a number of boilers la-lf cascaded together. Each boiler has a flue outlet manifold 2a-2f connected to a common flue branch or header 3 by means of an individual boiler flue branch 4a-4f. A NRV 5a-5f is located in each of the individual boiler flue branches 4a-4f. Without such NRVs, recirculating flue products could potentially enter the living/working space through boiler(s) which are not firing.

In known cascading arrangements, the NRVs are typically provided as component parts of the flue system by the installer and assembled on site as part of the cascade boiler installation. Whilst this is a perfectly robust and repeatable procedure, it is reliant on the competence of the installer to fit all the equipment to a satisfactory standard. Furthermore, supplying NRVs as part of a proprietary flue system can be quite an expensive solution.

It would be desirable to provide an alternative boiler assembly suitable for use in a cascaded boiler arrangement which overcomes, or at least mitigates, some or all of the drawbacks of known boiler systems.

It would also be desirable to provide an alternative NRV suitable for use in boiler assembly (especially a boiler assembly for use in a cascaded boiler arrangement) which overcomes, or at least mitigates some or all of the drawbacks of the known NRVs.

Summary of the Invention

According to a first aspect of the invention, there is provided a boiler assembly defining a flue gas train, wherein a flue gas NRV is located within the flue gas train of the boiler assembly.

In an embodiment the boiler assembly comprises a flue manifold connectable to a flue system, the flue manifold defining a flue gas outlet and the NRV is located upstream of said flue gas outlet.

In an embodiment, the boiler assembly comprises a boiler having a casing, the flue manifold being fluidly connected with the interior of the boiler through the casing such that when the boiler is running, flue gas passes from the boiler into the manifold as part of the boiler assembly flue gas train, the NRV being located to control the flow of flue gas between the boiler and the flue manifold. The boiler assembly may comprise a sump, the flue manifold being fluidly connected with the sump such that when the boiler is running, flue gas passes through the sump into the manifold as part of the boiler assembly flue gas train, the NRV being located to control the flow of flue gas between the sump and the flue manifold. The sump and the flue manifold may be formed as a single integral component and fluidly connected by means of an aperture in a common wall.

The NRV may be removably mounted in the boiler assembly. The NRV may be mounted within the flue manifold such that it can be removed from the boiler assembly through the flue manifold.

The NRV may be slidably and removably mountable in the boiler assembly and may be in the form of a cassette.

The NRV may comprise a rigid valve body to which is mounted a valve member having a movable valve flap. An outer perimeter region of the valve body may slidingly engage with a mating surface of boiler assembly. The outer perimeter region of the valve body and the mating surface of the boiler assembly may be profiled so as to define a substantially gas tight seal. One of the perimeter region of the valve body and the mating surface may be provided with at least one land and the other of the perimeter region of the valve body and the mating surface provided with at least one corresponding groove, the, or each, land being received in a respective groove to form said substantially gas tight seal. There may be two lands spaced apart and two corresponding grooves. Where the boiler assembly has a flue manifold, the mating surface may be defined within the flue manifold. Where the boiler assembly comprises a sump and a flue manifold fluidly connected by an aperture in a common wall, the mating surface may be defined on the manifold side of the common wall such that in use, the NRV allows flue gas to pass from sump in to the manifold but prevents a reverse flow of flue gas from the manifold into the sump.

The boiler assembly may form part of a cascade boiler arrangement in which a plurality of boilers are connected to a common flue branch.

The boiler assembly may be a premix boiler assembly. As the skilled person will appreciate, this means that the or each boiler of the boiler assembly may utilise a premix burner.

“Flue gas” may be considered as exhaust gas from combustion by the or each boiler of the boiler assembly.

In accordance with a second aspect of the invention, there is provided a flue gas NRV for a boiler assembly, wherein the NRV is removably mountable in a boiler assembly. The NRV may be slidably mountable in a boiler assembly.

The NRV may comprise a rigid valve body to which is mounted a valve member comprising a movable valve flap. An outer perimeter region of the valve body may be configured to slidingly engage with a mating surface in a boiler assembly. At least one land may extend about the perimeter region of the valve body. A pair of spaced lands may extend about the perimeter region of the valve body.

In an embodiment, the valve body defines a valve opening, the movable valve flap being movable between a valve closed position in which it engages with the valve body on one side so as to close the valve opening and an open position in which it is at least partially displaced away from the valve body.

The valve member may have a mounting portion fixedly attached to the valve body and connected with the movable valve flap by a hinge. The mounting portion may comprise a buffer configured to engage the movable flap member in use to limit its movement. The mounting portion may be clamped between the valve body and a clamping member secured to the valve body. The clamping member may be secured to the valve body by means of releasable fasteners or by means of spring clips. The clamping member may have a buffer portion configured to engage the movable flap member in use to limit its movement.

The valve body may be made of a composite material.

The valve member may be made from an elastomeric material, such as rubber.

The NRV in the boiler assembly according to the first aspect of the invention may include any of the features of the NRV in accordance with the second aspect of the invention.

In accordance with a further aspect of the invention, there is provided a boiler assembly comprising a boiler and a flue manifold connectable to a flue system, a flu gas NRV being removably mounted within the manifold and operative in use prevent a reverse flow of flue gas through the manifold into the boiler.

Detailed Description of the Invention

In order that the invention may be more clearly understood embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a schematic illustration of a known cascaded boiler flue arrangement;

Figure 2 is a view from the rear of an embodiment of a boiler assembly in accordance with an aspect of the invention;

Figure 3 is a cross sectional view through the boiler assembly of Figure 2 taken on line B-B;

Figure 4 is an exploded perspective view of a first embodiment of an integral sump and flue manifold and a NRV in accordance with an aspect of the invention forming part of the boiler assembly of Figures 2 and 3;

Figure 5 is a further exploded perspective view of the sump/flue manifold and the NRV of Figure 4 but taken from a different angle;

Figure 6 is an exploded perspective view of the NRV shown in Figures 4 and 5 but on an enlarged scale;

Figure 7 is a perspective view of an alternative embodiment of an integral sump and flue manifold which can be adopted in the boiler assembly of Figures 2 and 3;

Figure 8 is a further perspective view of part of the sump/flue manifold of Figure 7 but taken from a different angle and on a slightly larger scale;

Figure 9 is a perspective view from the front of an alternative NRV cassette for use with the integral sump and flue manifold of Figure 7 and 8;

Figure 10 is a perspective view from the rear of the alternative NRV cassette of Figure 9; and,

Figure 11 is a view similar to that of Figure 10 but partially exploded.

A boiler assembly 10 in accordance with an aspect of the invention is illustrated in Figures 2 and 3. The boiler assembly 10 comprises a condensing boiler 12. The condensing boiler 12 is constructed and operated in a manner well known in the art and so only those aspects of the boiler 12 and boiler assembly 10 relevant to the present invention will be described in detail.

The boiler 12 comprises an outer casing 14 within which are located a burner (not shown) and a heat-exchanger 16. The products of combustion from the burner in the form of hot flue gasses are passed through the heat-exchanger 16 as indicated by the arrows Z. Below the heat-exchanger 16 is a sump 18 into which the flue gas enters after it has passed through the heat-exchanger and in which condensate from the heat-exchanger is collected. The sump 18 forms part of the outer flue gas train 14 of the condensing boiler and has a base 20 which slopes downwardly towards the rear 22 of the boiler. Mounted to sump 18 at its rear end is a flue manifold 24 to which can be attached a flue pipe 26. The interior of the sump 18 and the interior of the flue manifold 24 are fluidly connected by means of an aperture 28 in a rear wall 30 of the sump. In use, flue gasses pass from the sump 18 into the flue manifold 24 through the aperture 28. The flue manifold 24 defines a flue gas outlet 25 which denotes the end of the flue gas train within the boiler assembly 10. The flue pipe 26 is mounted to the flue manifold 24 and in a cascaded boiler arrangement forms at least part of the individual boiler flue branch connected to the common flue branch.

The boiler sump 18 and the flue manifold 24 are formed as a single integral component 31 and the aperture 28 is defined in a common wall of the sump and manifold. The sump and manifold can be manufactured from any suitable materials, such as polymeric and/or composite materials. In an alternative embodiment, the sump 18 and flue manifold 24 are separate components mounted together, though in this case care would need to be taken to ensure a gas tight connection.

In accordance with an aspect of the invention, a NRV 32 is incorporated into the flue gas train within the boiler to prevent flue gases flowing in the reverse direction (i.e. from the common flue branch into the boiler 12) when the boiler 12 is not firing but other boilers connected to the common flue branch are.

The NRV 32 is mounted inside the flue manifold 24 upstream from the manifold outlet 25 and is used to control the flow of flue gases between the manifold and the sump 18. The NRV 32 is in the form of a releasably mounted cassette 33 having a rigid valve body 34 which is slidably mountable in the flue manifold 24 and a valve member 36 attached to the body 34 by means of a rigid clamping member 38. Details of the NRV 32 can be seen best from Figure 6.

Terms such as “upstream” and “downstream” as used herein in relation to the flue gas train should be understood with reference to the direction of flow of the flue gas when the boiler is running, that is to say in a direction from the burner to the flue gas manifold outlet 25.

In this first embodiment, the rigid valve body 34 has a generally planar rectangular portion 40 defining a valve opening 42. The outer perimeter 44 of valve body 34 is slidably received in a corresponding mating surface 46 defined with the flue manifold. The arrangement is such that when the valve body 34 is mounted in the flue manifold, the valve opening 42 aligns with the aperture 28 between the flue manifold and the sump to define a flue outlet though which the flue gasses must flow to pass between the sump and the flue manifold. The profile of the outer perimeter 44 of the valve body 34 and the profile of the mating surface 46 are configured to key together to provide a substantially gas tight seal whilst still allowing the valve cassette 32 to be removed easily from the manifold. To this end, a pair of spaced apart lands or ridges 48, 50 extend about the outer periphery of the valve body 34 on three sides. The lands 48, 50 slidably engage in corresponding grooves 52, 54 defined about the mating surface 46 to form a labyrinth seal. When the valve cassette 33 is mounted in the in the flue manifold, the gas tight seal substantially prevents flue gasses from passing between the valve body 34 and the wall of the flue manifold, so that the flue gases are constrained to pass through the valve opening 42. The valve body 34 is a tight, though still sliding, fit in the flue manifold 24 so that when installed, a forward or inlet side face 56 of the valve body 34 engages the inner surface of the manifold wall about the aperture 28 and valve opening 42 so that flue gas is not able to pass between them.

It will be appreciated that the outer perimeter 44 of the valve body 34 and the mating surface 46 can be configured in other ways to provide a substantially gas tight seal. For example, the lands could be provided on the mating surface and the grooves on the outer perimeter 44 of the valve body 34. In some arrangements, a single land and groove may be sufficient to provide a substantially gas tight seal or more than two lands and grooves could be used. In a further alternative, a seal member (e.g. an elastomeric seal) may be located between the outer perimeter 44 of the valve body 34 and the mating surface 46 and could be attached to either.

In practice a perfect gas tight seal between the valve body 34 and the mating surface 46 in the manifold may not be achievable. Accordingly, references herein to a “gas tight seal” should be understood as encompassing seal arrangements in which the amount of leakage falls within acceptable limits for the type of boiler assembly concerned to comply with statutory requirements.

The valve member 36 has a mounting portion 36a and a valve flap portion 36b interconnected by a live hinge 36c. The valve member 36 can be made from any suitable materials in any manner but can conveniently be made of a resiliently deformable material such as rubber or other elastomeric materials and may be moulded.

The mounting portion 36a has a generally planar region 58 which abuts a rear (outlet side) face 60 of the valve body 34 above the valve opening 42 and is clamped to the valve body 34 by the clamping member 38. The clamping member 38 is L shaped, having an upright portion 62 which engages a rear face of the planar region 58 of the mounting portion 36a. The clamping member 38 has a length which is greater than the width of the mounting portion 36a and has threaded apertures 64 at either end which locate either side of the mounting portion 36a. Corresponding counter sunk holes 66 are provided in the valve body through which respective threaded fasteners (not shown) are inserted to engage with the threaded apertures 64 in the clamping member 38. The fasteners are tightened to firmly clamp the mounting portion 36a to the valve body 34, with the heads of the fasteners located in the counter sunk holes 66.

The clamping member 38 has a buffer plate portion 68 which projects rearwardly from a lower edge of the upright portion 62 in a direction perpendicular to the upright portion (e.g. generally horizontally). The buffer plate portion 68 is located above a tapered buffer stop 70 which projects rearwardly from the planar region 58 of the mounting portion 36a of the valve member 36. The buffer stop 70 and the buffer plate portion 68 of the clamping member from a stop for limiting opening movement of the valve flap portion 36b when the boiler 12 is firing.

The valve flap portion 36b has a front face 72, the perimeter of which defines a ridge 74 configured to engage with a valve seat 76 defined in the rear face 60 of the valve body surrounding the valve opening 42. The valve cassette 33 is mounted in the flue manifold 24 with the valve member 36 on the rear (outlet) side of the valve body 34, that is to say on the side which faces into the centre of the flue manifold 24. When the boiler 12 is running, flue gases flowing through the sump 18 impinge on the front face 72 of the valve flap portion 36b and move the valve flap portion rearwardly about the hinge 36c away from the valve seat 76 to open the valve and allow the flue gases from the boiler to pass through the valve opening 42 to enter the flue manifold 24 and the flue system via the flue pipe 26. The opening movement of the valve flap portion 36b is limited by engagement of its rear face 78 with the buffer stop 70. When the boiler 12 is not running, the valve flap portion 36b is biased into engagement with the valve seat 76 by the resilience of the hinge 36c tending to close the valve. Flue gases from other boilers entering the flue manifold 24 in the reverse direction will impinge on the rear face 78 of the valve flap portion 36b pressing it into closer engagement with the valve seat 76 so that the valve 32 is closed and the reverse flow flue gases are not able to enter the boiler sump 18.

In the embodiment described above where the NRV valve opening 42 is defined in the rigid valve body 34, it is necessary to form a substantially gas tight fit between the valve body 34 and the flue manifold. Figures 7 to 11 illustrate an alternative NRV arrangement which avoids this issue. In this further embodiment, the valve opening 42’ is defined in the integral sump and flue manifold 31’ as part of the aperture 28’ between the sump and the manifold and the NRV cassette 33’ includes a ridged body 34’ slidably mounted inside the manifold and a movable valve member 36’ which engages with a valve seat 76’ defined about the valve opening 42’ to prevent a reverse flow of flues gasses into the sump 18’ from the manifold 24’.

As illustrated in Figures 7 and 8, the sump 18’ and the flue manifold 24’ are manufactured as a single integral component 31’ which share a common wall 30’ as in the previous embodiment. The opening 28’ in the common wall 30’ which fluidly interconnects the sump and the manifold in this embodiment is profiled to define the NRV valve opening 42’. In particular, a valve seat 76’ is defined about the valve opening 42’ in the manifold 24’, which is to say on the manifold side of the common wall 30’.

With reference to Figures 9 to 11, the valve member 36’ is similar to that of the previous embodiment, having a mounting portion 36a’ and a valve flap portion 36b’ connected to the mounting portion by a hinge 36c’ and can be made of similar materials.

In this embodiment, the rigid body 34’ of the valve cassette 33’ does not surround the movable valve flap portion 36b’ but rather comprises a short header region to which the mounting portion 36a’ of the valve member 36’ is attached and the clamping member 38’ which secures the mounting portion 36a’ to the valve body 34’ is attached to the rigid valve body 34’ using a spring clip arrangement rather than releasable screw type fasteners. The rigid valve body 34’ has a number of pin-like projections 80’ on its rear face 82’ which are received in corresponding apertures 84’ in the mounting portion 36a’ so at to locate the mounting portion 36a’ correctly in a mounting region 86’ on the rigid valve body 24’. A number of resilient clip members 88’, three in this case, project from the rear face 82’ of the valve body about periphery of the mounting region 86’. The clamping member 38’ has an upright portion 62’ in the form of a plate which overlies the mounting portion 36a’ of the valve member 36’. The plate 62’ has a number of circular apertures 90’ which receive end regions of the pin-like projections 80’ and further spring clip apertures 92’ which receive head portions 88a of the resilient clip members. The head portions 88a’ define undercut surfaces 94’ which engage the rear face 96’ of the upright portion 62’ with a snap-fit when the clip members are inserted through the apertures 92’ to hold the clamping member 38’ in place and firmly secure the mounting portion 38a’ to the rigid valve body 34’. A buffer plate portion 68’ projects from the rear face of the upright portion 62’ of the clamping member 38’ to engage with the movable valve flap portion 36b’ and limit its opening movement in use in a similar manner to the buffer arrangement in the previous embodiment.

This spring clip arrangement for securing the mounting portion 36a’ to the rigid valve member 34’ can be adopted in the first embodiment of the NRV described above. Similarly, the clamping arrangement described above in relation to the first embodiment can be used to secure the mounting portion 36a’ to the rigid valve member 34’ in this later embodiment.

Side edge regions 96’ of the rigid body 34’ have splines or ridges 48’ which locate in corresponding grooves 52’ in mating surfaces 46’ in the flue manifold. However, in this embodiment, the ridges 48’ and grooves 52’ do not have to form a gas tight seal.

When the valve member 36’ is mounted to the rigid body 34’, the movable valve flap portion 36b’ depends downwardly below the rigid body 34’ such that when the cassette 33’ is mounted inside the manifold 24’ with the rigid body received in a recess above the valve opening 42’, the movable valve flap portion 36b’ hangs downwardly covering the valve opening 42’ adjacent the valve seat 76’. The front face 72’ of the movable valve flap portion 36b’ opposes the valve seat 76’. Whilst not shown in Figure 9, the front face 72’ could have a ridge about its perimeter for engagement with the valve seat 76’ similar to the ridge 74 in the first embodiment. The movable valve flap portion 36b’ is relatively heavy and the resilience of the hinge portion 36c’ is such that when the NRV cassette 33’ is mounted in the manifold of a boiler in its upright working orientation, the front face 72’ (or the ridge 74 where present) of the valve flap portion 36b’ abuts the valve seat 76’ surrounding the valve opening 42’ so that the valve opening is closed. When the boiler 12 is running, flue gases flowing through the sump 18’ impinge on the front face 72’ of the valve flap portion 36b’ and move the valve flap portion rearwardly about the hinge 36c’ away from the valve seat 76’ to open the valve and allow the flue gases from the boiler to pass through the valve opening 42’ to enter the flue manifold 24’ and the flue system via the flue pipe 26. Opening movement of the valve flap portion 36b’ being limited by the buffer plate portion 68, of the clamping member 38’. When the boiler 12 is not running, the front face 72’ (or at least the ridge 74 where provided) of valve flap portion 36a’ is biased into engagement with the valve seat 76’ by the resilience of the hinge 36c’ and the weight of the flap portion 36b’ tending to close the valve. Flue gases from other boilers entering the flue manifold 24’ in the reverse flow direction will impinge on the rear face 78’ of the valve flap portion 36b’ pressing it into closer engagement with the valve seat 76’ so that the valve 32’ is closed and the reverse flow flue gases are not able to enter the boiler sump 18’.

In both embodiments, the NRV and its mounting in the flue manifold are configured so that condensate can drain from the sump 18, 18’ into the base of the flue manifold 24, 24’ when the boiler is running. From here, the condensate is disposed of through a condensate outlet

Whilst the primary function of the NRV is as a safety device in preventing reverse flow flue gases from other boilers entering the boiler assembly 10, the NRV 32 prevents moisture inherent within the flue gases from re-entering the boiler where it could otherwise start to degrade some of the components within the boiler assembly, which could have an effect on the long term operation and reliability of the appliance.

Providing the NRV 32 as part of the boiler assembly itself means that the NRV is assembled by the boiler manufacture as part of their manufacturing process and can be fully tested. This negates the need to rely on an installer’s competence to construct the flue system on site and to select and fit an appropriate NRV. It also provides a more effective way to manufacture and install a complete cascade boiler system.

Furthermore, as at least part of the NRV 32 is in the form of a cassette 33, 33’ which is in effect a sub-assembly, the design can be configured so that same basic NRV 32 can adopted interchangeably between a number of boiler models within a boiler range. This offers greater flexibility in the commonality of parts when building a number of models within a boiler range and so helps improve manufacturing efficiency, reducing the overall number of components that a boiler manufacture has to produce and/or stock. The basic NRV 32 can be adapted to deal with performance issues for specific boiler models (or specific applications of a boiler model). This may be through modest modifications to the internal profile of the valve body 34 or the profile of the valve member 36, 36’, For example, different models in a boiler range or the operating conditions (i.e. different gas types in different countries) may require a different size valve opening 42, 42’ in order to manage a performance problem encountered (for example resonance). Such modifications or adaptations can be provided for in the cassette type NRV 32, 32’ with a relatively modest tooling modification. This could result in a range of slightly modified valve bodies 34, 34’ being produced which all fit a common valve member 36, 36’, and/or a range of different valve members 36, 36’ which fit a common valve body 34, 34’, for example.

In the embodiments as described, the NRV 32, 32’ is mounted within the flue manifold 24,

24’ where it can be easily accessed by removing the flue pipe 26. The NRV 32, 32’ is therefore easily removed and/or reinstalled once a boiler has been installed, thus offering greater flexibility for servicing in the field. Apart from disconnecting the flue pipe 26, no other disassembly of the boiler assembly 10 is required in order to remove and/or reinstall the 10 NRV 32, 32’.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

1. A boiler assembly defining a flue gas train, wherein a NRV is located within the flue gas train of the boiler assembly.

2. A boiler assembly as claimed in claim 1, wherein the boiler assembly comprises a flue manifold connectable to a flue system, the flue manifold defining a flue gas outlet and wherein the NRV is located upstream of said flue gas outlet.

3. A boiler assembly as claimed in claim 2, wherein the boiler comprises a boiler having a casing, the flue manifold being fluidly connected with the interior of the boiler through the casing such that when the boiler is running, flue gas passes from the boiler into the manifold as part of the boiler assembly flue gas train, the NRV being located to control the flow of flue gas between the boiler and the flue manifold.

4. A boiler assembly as claimed in claim 3, wherein the boiler casing comprises a sump, the flue manifold being fluidly connected with the sump such that when the boiler is running, flue gas passes through the sump into the manifold as part of the boiler assembly flue gas train, the NRV being located to control the flow of flue gas between the sump and the flue manifold.

5. A boiler assembly as claimed in claim 4, wherein the sump and the flue gas manifold are formed as a single integral component and fluidly connected by means of an aperture in a common wall of the sump and the flue manifold.

6. A boiler assembly as claimed in any one of the preceding claims, wherein the NRV is removably mounted in the boiler assembly.

7. A boiler assembly as claimed in claim 6 when dependent on claim 2, wherein the NRV is mounted with the flue manifold such that it can be removed from the boiler assembly through the flue manifold.

8. A boiler assembly as claimed in any one of the preceding claims, wherein the NRV is slidably and removably mountable in the boiler assembly.

9. A boiler assembly as claimed in any one of the preceding claims, wherein the NRV comprises a rigid valve body to which is mounted a valve member having a movable valve flap.

10. A boiler assembly as claimed in claim 9, wherein an outer perimeter region of the valve body slidingly engages with a mating surface of boiler assembly.

11. A boiler assembly as claimed in claim 9 or claim 10, wherein the outer perimeter region of the valve body and the mating surface of the boiler assembly are profiled so as to define a substantially gas tight seal.

12. A boiler assembly as claimed in claim 11, wherein one of the perimeter region of the valve body and the mating surface is provided with at least one land and the other of the perimeter region of the valve body and the mating surface is provided with at least one corresponding groove, the, or each, land being received in a respective groove to form said substantially gas tight seal.

13. A boiler assembly as claimed in any one of claims 10 to 12 when dependent on claim 2, wherein the mating surface is defined within the flue gas manifold.

14. A boiler assembly as claimed in claim 13 when dependent on claim 5, wherein the mating surface is defined on the manifold side of the common wall such that in use, the NRV allows flue gas to pass from sump in to the manifold but prevents a reverse flow of flue gas from the manifold into the sump.

15. A boiler assembly as claimed in any one of claims 9 to 14, wherein the rigid valve body defines a valve opening.

16. A boiler assembly as claimed in any one of claims 9 to 14, wherein a valve opening is defined in the casing of the boiler

17. A boiler assembly as claimed in any one of the preceding claims, the boiler assembly forming part of a cascade boiler arrangement in which a plurality of boilers are connected to a common flue branch.

18. A flue gas NRV for a boiler assembly, wherein the NRV is removably mountable in a boiler assembly.

19. A flue gas NRV for a boiler assembly as claimed in claim 16, wherein the NRV is slidably mountable in a boiler assembly.

20. A flue gas NRV as claimed in claim 16 or claim 17, wherein the NRV comprises a rigid valve body to which is mounted a valve member comprising a movable valve flap.

21. A flue gas NRV as claimed in claim 18, wherein an outer perimeter region of the valve body is configured to slidingly engage with a mating surface in a boiler assembly.

22. A flue gas NRV as claimed in claim 18 or claim 19, wherein at least one land extends about the perimeter region of the valve body.

23. A flue gas NRV as claimed in any one of claims 18 to 20, wherein the valve body defines a valve opening, the movable valve flap being movable between a valve closed position in which it engages with the valve body on one side so as to close the valve opening and an open position in which it is at least partially displaced away from the valve body.

24. A flue gas NRV as claimed in any one of claims 18 to 21, wherein the valve member comprises a mounting portion fixedly attached to the valve body and connected with the movable valve flap by a hinge.

25. A flue gas NRV as claimed in claim 22, wherein the mounting portion comprises a

5 buffer configured to engage the movable flap member in use to limit its movement.

26. A flue gas NRV as claimed in claim 22 or claim 23, wherein the mounting portion is clamped between the valve body and a clamping member secured to the valve body.

27. A flue gas NRV as claimed in any one of claims 18 to 24, wherein the valve body is made of a composite material.

10

28. A flue gas NRV as claimed in any one of claims 18 to 25, wherein the valve member is made from an elastomeric material, such as rubber.