GB2172698A

GB2172698A - Gas-fired appliance adapted to be incorporated in a cooking and heating system

Info

Publication number: GB2172698A
Application number: GB08606050A
Authority: GB
Inventors: Graham Albert John Lake; John Owens
Original assignee: Glynwed Consumer and Building Products Ltd
Current assignee: Glynwed Consumer and Building Products Ltd
Priority date: 1985-03-16
Filing date: 1986-03-12
Publication date: 1986-09-24
Also published as: BE904425A; GB8506901D0; GB8606050D0; GB2172698B; DE3608618A1

Abstract

A gas-fired appliance comprises cooking means which may be a hotplate 3 and/or oven 4 heated by one or more burners 5, 6 from which the products of combustion are ducted to a heat exchanger 8 which serves as, or is connected to, a boiler for central heating and/or hot water. Preferably the products of combustion are contained within the appliance until they leave by way of a flue 9 and so do not enter into the space in which the appliance is installed for use. The hotplate may have separate zones at which it is heated to different extents for boiling and simmering, or it may, provide a temperature gradient across its upper surface. The appliance has particular application in the provision of cooking and heating systems in small homes designed to have low energy requirements. <IMAGE>

Description

SPECIFICATION Gas-fired appliance adapted to be incorporated in a cooking and heating system This invention relates to a gas-fired appliance adapted to be incorporated in a cooking and heating system suitable for, but not exclusively for, domestic use.

Substantial progress has been made in recent years in the development of small, well insulated homes which have low energy requirements, and thereby save on energy costs. One and two bedroom homes based on timber frame constructions, or using materials with low thermal loss characteristics, have been built to meet these requirements, for example as "starter homes", as also have flats and bungalows for retired and elderly people.

Since revision of British Building Regulations in 1982 the designed heat loss in new starter homes, for example, has been substantially reduced and a figure of about 3 kW is accepted as standard.

Whilst gas-fired appliances are known which are able to meet a wide range of domestic heating requirements, known gas-fired cookers present a problem in that typically they produce up to 8 kW output during a normal domestic cooking period and such heat inputs to the small, well insulated homes referred to are greatly in excess of the standard designed heat loss.

It is likely that there will also be requirements in the future to limit the emissions into homes from gas-fired appliances of combustion products containing certain pollutants.

It is an aim of the present invention to at least reduce the aforesaid problem of gas-fired cookers.

The present invention consists in a gas-fired appliance adapted to be incorporated in a cooking and heating system and comprising cooking means and a heat exchanger, the cooking means being heated by a gas burner from which the products of combustion are ducted to the heat exchanger which serves as, or is connected to, -a boiler for central heating and/or hot water purposes, and there being a flue to which the products of combustion are ducted from the heat exchanger.

Preferably the products of combustion from the burner are contained within the appliance as they pass from the burner to the heat exchanger and flue so that in use of the appliance they may be taken away from the appliance without having entered into the space in which the appliance is installed for use.

Heat in the products is extracted by the heat exchanger for the benefit of central heating and/or a hot water supply. Depending upon the requirements of the building served by the installed system, the heat extracted from the products of combustion from the burner by the heat exchanger may alone be sufficient for the central heating and/or hot water supply, or there may be provided in association with the heat exchanger, or boiler to which the heat exchanger is connected, a further burner the products of combustion of which are also ducted to the flue. The further burner would preferably be operable independently of the burner which heats the cooking means.

The cooking means may comprise a hotplate and/or an oven. Preferably the hotplate is indirectly fired, that is to say it is heated by the burner and in turn imparts heat to vessels placed on it to be heated so that the vessels are not heated directly by the burner. There may be more than one burner for heating the cooking means, each burner preferably being independently and selectively operable.

It is desirable for the appliance to be thermally insulated so as to reduce heat loss from it, and store heat at the cooking means when the cooking means is not being heated by the burner or burners. Where the cooking means comprises a hotplate a suitable insulative cover or covers may be provided which can be closed over the hotplate when not in use to reduce heat loss from the hotplate.

Where a hotplate is provided it may have separate zones at which it is heated to different extents by the burner for boiling and simmering purposes, or the hotplate and burner may be arranged to provide a heat gradient across the hotplate to enable boiling and simmering to be carried out. The hotplate may be adapted to serve as a griddle for direct cooking of food on it. The hotplate is ideally of a material and design which combines the ability to store heat with a rapid response to being heated by the burner. Suitable materials may include glass ceramics and cast aluminium alloys. More than one hotplate may be provided.

For the small, well-insulated home to which reference has been made, a heat output of up to approximately 13 kW from the burner or burners should satisfy the normal requirements of cooking, heating and hot water production.

Where an oven is provided it is preferably indirectly fired also and the products of combustion from the burner, or that burner by which the oven.is heated are ducted around the sides of the oven to the heat exchanger.

The oven may possibly be directly fired by a suitable burner or burners, and the products of combustion be ducted away from the oven to the heat exchanger. With either arrangement operating temperatures of the oven may be thermostatically controlled in known manner. For a satisfactory response time in oven cooking together with good control characteristics the heat input of the burner to the oven should generally be of the order of 3.04.0 kW.

The heat exchanger may be constructed of finned tubing, for example the finned copper tubing sold under the trade name INTEGRON.

For the small, well-insulated home, the heat output requirement from the heat exchanger will typically be in the region of 4.0 kW.

There may be more than one heat exchanger.

It is estimated that the energy consumption of a system including an appliance in accordance with the present invention in a home having a designed heat loss of approximately 3 kW, and operating on heat inputs at a hotplate, oven and the heat exchanger, would amount to approximately 159 MJ under arduous conditions.

Whilst the appliance now provided has particular application to small homes as indicated, it may be applied to larger installations and its heat output may be increased accordingly.

Where the cooking means and heat exchanger have separate burners independent controls of the burners may be linked to a selector at which cooking and heating functions of the system may be set manually and/or automatically programmed.

The flue is preferably of the balanced type which may be fan assisted or rely on natural draught. A condensing mode of operation may be required depending upon the requirements of the heat exchanger and limiting conditions considered acceptable for a particular installation of the system.

The appliance may be made as a module of a size, typically 500 to 600mm wide and 800mm deep, which will enable it to be accommodated in standard domestic fitted kitchen units.

Where a hotplate is provided the hotplate and heat exchanger may be combined as a unit. In the unit the gas burner is preferably incorporated in the hotplate and the products of combustion from the burner are taken by a closed duct or ducts to the heat exchanger.

The heat exchanger may itself form the boiler of the appliance or it may be connected to the boiler. In the latter arrangement the boiler is preferably also included in the unit. The boiler may have its own burner or burners independent of the burner at the hotplate. Products of combustion from the boiler burner or burners may be ducted to the heat exchanger for heat recovery. The boiler may be in the form of one or more heat exchangers. Where the cooking means includes an oven in addition to the hotplate and separately heated by one or more burners the products of combustion are also preferably ducted to the heat exchanger. A fan may be included in the unit which operates to encourage products of combustion to pass to the heat exchanger.

In the unit as aforesaid the hotplate may be arranged to provide zones of different temperatures or a temperature gradient across its upper surface for cooking at varying rates. The zones or gradient may be achieved by selection of the position of the burner at the hotplate, the direction of the burner flame or flames relative to the hotplate and the course which products of combustion are caused to take before leaving the hotplate and passing to the heat exchanger. The hotplate may have heat conducting fins or other suitable projections which receive heat from the flame or flames and the products of combustion to assist in achieving the temperature zones or gradient. Two or more suitably positioned, and controlled, burners may be incorporated in the hotplate.

Temperature at the hotplate may be controlled by pulsing of the burner or burners to make them operative and inoperative to variable extents according to temperature requirements. Burners at the oven and/or boiler may be similarly controlled.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a diagrammatic side view, partly broken away, of an appliance for domestic use in accordance with the present invention; Figure 2 is a simplified plan view of an appliance having another hotplate and burner arrangement; Figure 3 is a fragmentary vertical section through the upper part of an appliance which has a hotplate, heat exchanger and boiler combined as a unit; and Figure 4 shows a diagrammatic layout of a domestic cooking and heating system in which any one of the appliances of the preceding Figures may be included.

Referring to Figure 1,, the appliance, generally indicated at 1, comprises a box-like body 2 at the top of which is a hotplate 3 and which accommodates an oven 4 below the hotplate. A fully aerated surface gas burner 5 heats the hotplate 3 indirectly from below, and a further burner 6 of a similar type heats the oven 4, being positioned below a base 7 of the oven. Towards the rear of the body 2 behind the hotplate 3 is a heat exchanger 8 of finned copper, or similar, tubing to which products of combustion from both burners 5, 6 are dueted, the products from the oven burner passing around the sides of the oven and collecting above the oven to pass to the heat exchanger. From the heat exchanger 8 the products of combustion are ducted to a balanced flue 9 at the rear of the body which prevents air change and heat ingress to the dwelling in which the appliance is installed for use. Controls 10 for the burners 5, 6 are positioned at the front of the upper part of the body. A thermostatic control, not shown, which may be of a suitable known kind, is provided in association with the oven burner 6 for controlling the cooking temperature in the oven.

An electric grill may be incorporated in the oven.

The body and oven are fully insulated to minimise heat losses from the appliance.

The hotplate 3 is solid and in two halves which provide separate boiling and simmering zones, not shown. The hotplate burner 5 may be suitably positioned relative to those two zones to. provide the required heating, and/or the hotplate may'have baffles at its underside which assist in controlling the heating at the two zones of the hotplate. The design of the hotplate is such that the heat flux through the plate gives the required temperatures for boiling, whilst the temperature of the underside of the hotplate is maintained below the maximum service temperature of the hotplate material.

Heat reaches the simmering zone of the hotplate either purely by conduction, or by a combination of conduction, convection, and possibly radiation.

The residual heat from the hotplate, the quantity of which will depend upon whether or not the hotplate is in use, is directed to the heat exchanger, and from there to the flue.

A single insulated cover 11, or two insulated covers, hinged on the top of the body 2 closes over the two zones of the hotplate 3 to minimise heat loss from them when they are not in use.

Access can be gained to the heat exchanger 8 for maintenance purposes from the top of the body behind the hotplate.

Typically for domestic use the appliance is 900mm high to the tops of the closed covers 11, 600mm wide and 600mm from front to back of the body. In such an appliance the hotplate is preferably 420mmX300mm equally divided into the boiling and simmering zones, and the hotplate burner provides a heat input of 2.4 kW. Assuming 50% efficiency, the heat output at the boiling zone is 0.95 kW and at the simmering zone it is 0.2 kW. The heat input to the oven from the oven burner is in the order of 2.5-3.0 kW. With good insulation of the appliance to minimise standing heat losses, the heat which the heat exchanger is able to recover from the products of combustion from the two burners providing the stated heat inputs to the hotplate and oven, is able to meet the normally desired heat output requirement of approximately 3 kW for central heating and a domestic hot water supply.

When the appliance is installed for use the flue 9 is connected to a stack or duct which discharges the products of combustion to the exterior of the building in which the appliance is installed. As the products of combustion from the burners leave the appliance by way of the flue the space in which the appliance is positioned is free of pollutants from the burners.

Reference will now be made to Figure 2 of the accompanying drawings which shows another hotplate and burner arrangement. The hotplate, generally indicated at 12, is in one piece, for example a casting of aluminium LM6 alloy, is rectangular and in use has a temperature gradient across its upper surface which enables it to provide boiling and simmering zones. Two burners 13, 14 are provided at, and indirectly fire, the hotplate 12, and a third burner 15 is provided in association with the heat exchanger of the appliance, indicated at 16, which is of finned copper or like tubing.

All three burners are fully aerated surface burners. Ignition of the burners is automatic. A single cover, not shown, closes over the hotplate 12 when the hotplate is not in use so as to minimise heat loss from the hotplate whilst also preventing it from overheating. A further surface combustion burner, not shown, which is thermostatically controlled indirectly fires, at the base, an oven, not shown, of the appliance. Ignition of the oven burner is also automatic, and ignition sensing means is provided.

Controls, not shown, for the burners are all positioned at a fascia at the top of the front of the appliance. Both of the burners 13, 14 at the hotplate 12 can be switched on for "high" level cooking on the hotplate, or just one of them, 13, can be switched on for "low" level cooking on the hotplate. For the production of hot water only the burner 15 associated with the heat exchanger 16 and the adjacent burner 13 at the hotplate are utilised. Thus, if both "high" level cooking at the hotplate and hot water production are required, all three of the burners 13, 14, 15 are operated.

The controls on the fascia panel for controlling the central heating and hot water supply from the appliance may be of a known type programmed by the user according to his requirements and including an electronic clock showing the time of day and the set times on request. The desired functions are: clock set; advance time; retard time; period 1 ON and OFF (one button each); period 2 ON and OFF (one button each), override to advance the next programme setting; OFF; hot water/central heating once; hot water/central heating twice; hot water once; hot water twice; hot water/central heating donstant; hot water constant; minute minder.

Typically for a small well-insulated home the hotplate measures 530mm X 250mm approximately and the temperature gradient across its upper surface ranges from about 220"C in the simmering zone to about 340"C in the boiling zone. The corresponding cooking output requirements for these temperatures are 3.8 kW/m2 and 15.1 kW/m2 respectively, giving an overall heat input requirement of approximately 4.2 kW, assuming a 30% gross efficiency. For a satisfactory full range of central heating, hot water and cooking functions from the three burners 25, 26, 27 the total input requirement from them will typically be of the order of 9.2 kW. Preferably the maximum space between the surface of any one of the burners and the under surface of the hotplate is 132mm.

Typically the oven will be 420mm high, 465mm wide and 420mm deep, and the heat input from the burner heating the oven will be about 3kW.

For the heat exchanger made of INTEGRON finned copper tubing of 19.05mm diameter the heat output should typically be in the region of 3.7kW-4.0kW, and for that a 400mm length of the tubing should suffice.

The appliance should have a balanced flue.

An exhaust fan may be incorporated.

In the further arrangement shown in Figure 3 of the accompanying drawings the appliance has a hotplate 17, heat exchanger 18 and boiler 19 combined as a unit 20 which is accommodated in the appliance over an oven, not shown. The hotplate 17 comprises two co-planar rectangular cast plates 21, 22, for example of aluminium alloy, separated by a thermal break 23 of suitable insulating material and respectively providing boiling and simmering zones. Both plates 21, 22 are supported on and sealed to a thermally insulated sealing panel 24. The plates 21, 22 have hollowed undersides which together with the sealing panel 24 define a combustion chamber 25 which extends under a substantial part-of the area of the supporting surface of the hotplate.

A burner 26 incorporated into the hotplate at one side of the boiling plate 21 remote from the simmering plate 22 fires into the combustion chamber 25 towards the simmering plate.

Finning 27 at the hollowed undersides of the plates provides baffles in the combustion chamber which extend away from the burner and are designed to assist in providing a controlled temperature gradient at the boiling and simmering plates. Gas and air are taken to the burner 26 by a supply pipe 28.

The sealing panel 24 seats on a thermally insulated carrier 29 which houses the heat exchanger 18, boiler 19, a separate burner 30 for the boiler, and a fan 31. A transfer passage 32 defined in the simmering plate 22 and sealing panel 24 remote from the hotplate burner 26 connects the combustion chamber 25 to a duct 33 in the carrier 29 leading to the heat exchanger 18. A gasket 34 at the carrier provides a fluid tight seai between the sealirig panel 24 and the carrier where the transfer passage 32 connects to the duct 33, but the connection can be released to enable the sealing panel and hotplate to be separated from the carrier, for example for servicing the hotplate burner 26 and access to the boiler, boiler burner 30 and the heat exchanger. Gas and air are taken to the boiler burner 30 by a supply pipe 35.The boiler burner fires into a combustion chamber 36 in which the boiler is located and which merges with the duct 33 upstream of the heat exchanger. The fan 31 is housed in the carrier downstream of the heat exchanger and its function is to draw products of combustion from the two combustion chambers 25, 36 and past the heat exchanger.

The heat exchanger and boiler are similar.

They are both cbnstructed of finned copper, or similar, tubing and they are connected so that the boiler benefits from the heat recovered by the heat exchanger from the products of combustion. They may in fact be defined by a single tube, or they may be formed by several tubes, according to operating requirements. A condensate drain may be provided at the boiler.

The thermal insulation in the unit reduces heat loss. As in the previously described embodiments an insulated cover or covers preferably close over the hotplate to minimise heat loss from it when it is not in use.

The oven of the appliance is heated by a separate burner. Products of combustion from the oven are ducted around the oven, within the appliance, and pass through a vent 37 at the carrier 29 of the unit 20 to the merged duct 33 and combustion chamber 36 containing the boiler, and thence to the heat exchanger 18. The fan also encourages the flow of the products of combustion from the oven burner to the heat exchanger.

Having been drawn past the heat exchanger the products of combustion from the various burners are ducted away to a flue, not shown, of the appliance from which they are arranged to be discharged when the appliance is installed for use to the exterior of the building in which the appliance is housed. Again, therefore, the products of combustion do not enter into the space in which the appliance is installed for use.

The various burners are selectively controlled. Thus the hotplate burner may be operated on its own to heat the hotplate, the boiler burner may be operated without the hotplate burner being in use, and the oven burner may heat the oven without the other burners being operative, or all the burners may be operated together or in any desired combination.

Suitable closure devices, for example butterfly valves, are preferably provided to isolate independently of one another, the combustion chambers and vent from the heat exchanger when the respective burners are not in use. In this way cooling air which would reduce the efficiency of the heat exchanger is not drawn to the heat exchanger through the combustion chamber 25 at the hotplate when the hotplate burner is not in use, though the combustion chamber 36 containing the boiler when the boiler burner is not in use, or through the vent 37 from the oven when the oven burner is not in use. The closure devices may conveniently be provided, for example, in the duct 33, the combustion chamber 36 and the vent 37.

Instead of one burner at the hotplate, two burners, independently controllable, may be provided at opposite sides of the hotplate and arranged to fire towards the centre of the hotplate. There may be more than two of the cast plates providing the hotplate, and there may be a burner, or burners, for each plate.

The products of combustion from all of the burners are ducted to the heat exchanger.

Using the unit 20 high operating temperatures have been achieved in trials, for example in excess of 1 2000C at the hotplate, and also a high efficiency in recovery of heat by the heat exchanger and of heat transfer to the boiler has been achieved.

A domestic system will now be described with reference to Figure 4 of the accompanying drawings in which any one of the appliances described may be included. A lagged hot water cylinder 38 connected by a feed pipe 39 to, and having a vent 40 back to, a hot water header tank 41 supplies hot water by a delivery pipe 42 for direct use by the consumer. A mixer valve 43 in the delivery pipe 42 connected to the feed pipe 39 controls the temperature of the hot water supplied. There are lower and upper limit temperature sensors TSI and TS2 respectively at the cylinder. The appliance, indicated at 44, has water supplied from a central heating header tank 45 by way of a feed pipe 46 to its heat exchanger, or boiler and heat exchanger, as the case may be.A supply line 47 from the appliance 44, which has a vent pipe 48 back to the central heating header tank 45, takes heated water through a circulation pump 49, which may be manually or automatically controlled, and water zone valve 50 to radiators, not shown. A heating coil 51 inside the cylinder 38 takes heated water from the supply line 47 between the circulation pump 49 and the water zone valve 50 through a further water zone valve 52. From the heating coil 51 the water is returned to the appliance 44 by a return line 53 running back to the appliance from the radiators. The system may be operated in the following manner.

Hot Water Only When the system is to supply hot water only the circulation pump is operated and the zone heating valve 50 in the supply line 47 to the radiators is closed. The appliance heats the water in the system to a temperature, typically 65"C-70"C, set by the lower limit thermostatic temperature sensor TS1. When the required temperature is reached the sensor TS1 controls the appliance and the circulation pump to maintain the water at that temperature.

Hot Water and Cooking With the addition of the cooking mode the circulation pump operates and heat recovered by the heat exchanger from products of combustion from the hotplate and/6r oven burners of the appliance is taken to the cylinder for storage. This continues until an upper temperature limit is reached when the upper limit thermostatic temperature sensor TS2 operates and opens the zone heating valve 50 to the radiators, thereby dispensing heat into the centrol heating system, until the temperature reduces and the sensor TS2 closes the zone heating valve 50.

Central Heating, Hot Water and Cooking When this mode is selected both of the zone heating valves 50 and 52 are opened and the appliance raises the temperature of water in the cylinder to provide the required central heating temperature set by a thermostatic control at or associated with the appliance. In the case of the appliance which has both the heat exchanger and the boiler, heat recovered by the heat exchanger increases the rating and efficiency of the overall system and reduces the operational time of the boiler in a heating cycle, thereby reducing wear of the boiler.

Cooking Only In this mode the heat recovered by the heat exchanger of the appliance steadily increases the temperature of water stored in the cylinder until the upper limit thermostatic temperature sensor TS2 is operated. Operation of that sensor opens the zone heating valve 50 to the radiators to disperse heat into the central heating system until the temperature falls sufficiently for the sensor TS2 to close the zone heating valve again. The heat output from the central heating system resulting from the heat dispersed into it whilst the appliance is in the cooking mode only will normally be relatively low.

Claims

1. A gas-fired appliance adapted to be incorporated in a cooking and heating system and comprising cooking means and a heat exchanger, the cooking means being heated by a gas burner from which the products of combustion are ducted to the heat exchanger which serves as, or is connected to, a boiler for central heating and/or hot water purposes, and there being a flue to which the products of combustion are ducted from the heat exchanger.

2. A gas-fired appliance according to claim 1 wherein the products of combustion from the burner are contained within the appliance until they reach the flue.

3. A gas-fired appliance according to claim 1 or claim 2 wherein a further burner is provided in association with the heat exchanger the products of which are also ducted to the flue, and which is operable independently of the burner which heats the cooking means.

4. A gas-fired appliance according to any preceding claim wherein the cooking means comprises a hotplate.

5. A gas-fired appliance according to any of claims 1 to 3 wherein the cooking means comprises an oven.

6. A gas-fired appliance according to any of claims 1 to 3 wherein the cooking means comprises a hotplate and an oven heated by respective gas burners the products of combustion from which are ducted to the heat exchanger, and which burners are independently and selectively operable.

7. A gas-fired appliance according to claim 4 or claim 5 wherein the hotplate has a thermally insulative cover or covers which can be closed over the hotplate when it is not in use to reduce heat loss from the hotplate.

8. A gas-fired appliance according to any of claims 4, 5 and 7 wherein the hotplate has separate zones at which it is heated to different extents by the burner for boiling and simmering purposes.

9. A gas-fired appliance according to any of claims 4, 5 and 7 wherein the hotplate and burner are arranged such that the heating of the hotplate by the burner gives a temperature gradient across the hotplate.

10. A gas-fired appliance according to claim 4 or any of claims 6 to 9 wherein a plurality of burners is provided to heat the hotplate, the products of combustion from the burners being ducted from the hotplate to the heat exchanger.

11. A gas-fired appliance according to claim 4 or any of claims 6 to 10 wherein the hotplate has baffles at its underside which assist in controlling the heating of the hotplate by the burner or burners.

12. A gas-fired appliance according to claim 4 or any of claims 6 to 11 wherein the or each burner heating the hotplate fires into a chamber directly below the hotplate from which the products of combustion are ducted to the heat exchanger.

13. A gas-fired appliance according to claim 12 wherein the hotplate has heat conducting projections which extend into the chamber to receive heat from the flame or flames of the or each burner and from the products of combustion.

14. A gas-fired appliance according to claim 4 or any of claims 6 to 13 wherein the or each burner by which the hotplate is heated is incorporated in the hotplate.

15. A gas-fired appliance according to claim 4 or any of claims 6 to 14 wherein the hotplate and heat exchanger are combined as a unit.

16. A gas-fired appliance according to claim 15 wherein the hotplate is mounted on a carrier included in the unit which houses the heat exchanger.

17. A gas-fired appliance according to claim 16 wherein the hotplate is separably mounted on the carrier and ducts at the hotplate and carrier by which the products of combustion from the burner or burners heating the hot plate pass to the heat exchanger, are connected at a releasable sealing joint.

18. A gas-fired appliance according to claim 16 or 17 wherein the hotplate has a hollowed underside, is sealed on a thermally, insulated panel on the carrier and the hollowed underside of the hotplate and the panel together define a combustion chamber into which the burner or burners heating the hotplate fire.

19. A gas-fired appliance according to any of claims 15 to 18 wherein the heat exchanger is connected to a boiler which is included in the unit and heated by its own burner operable independently of the burner or burners heating the hotplate.

20. A gas-fired appliance according to any of claims 15 to 19 a dependent from any of claims 6 to 14 wherein a fan is included in the unit which operates to encourage products of combustion from the respective burners to pass to the heat exchanger.

21. A gas-fired appliance according to claim 20 wherein closure devices are provided operable independently of one another to isolate from the heat exchanger the paths by which products of combustion pass from the respective burners to the heat exchanger, the arrangement being such that when products of combustion are not passing along any one of the paths when the appliance is in use that path can be isolated from the heat exchanger to prevent cooling air being encouraged by the fan to pass along that path to the heat exchanger.

22. A gas-fired appliance according to claim 5 or any of claims 7 to 19 as dependent from claim 6, or either of claims 20 and 21 wherein the oven is indirectly fired by the burner, or its respective burner, and the products of combustion from the. burner are ducted around the oven to the heat exchanger.

23. A gas-fired appliance according to any preceding claim wherein the heat exchangei is constructed of finned tubing.

24. A gas-fired appliance according to claim 19 wherein the boiler is constructed of finned tubing.

25. A gas-fired appliance according to any preceding claim wherein the flue is a balanced flue.

26. A cooking and heating system including a gas-fired appliance as claimed in any preceding claim, the heat exchanger of the appliance recovering heat to be used in heating water for central heating and a hot water supply.

27. A gas-fired appliance substantially as described herein with reference to and as illustrated by Figure 1 of the accompanying drawings.

28. A gas-fired appliance substantially as described herein with reference to and as illustrated by Figure 2 of the accompanying drawings.

29. A gas-fired appliance substantially as described herein with reference to and as illustrated by Figure 3 of the accompanying drawings.

30. A cooking and heating system substantially as described herein with reference to Figure 4 of the accompanying drawings.