GB2296316A

GB2296316A - Destratifying air heater

Info

Publication number: GB2296316A
Application number: GB9523605A
Authority: GB
Inventors: John Andrew Thomason
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-11-25
Filing date: 1995-11-17
Publication date: 1996-06-26
Anticipated expiration: 2015-11-17
Also published as: GB9523605D0; GB2296316B

Abstract

A destratifying fuel fired air heater has a housing 25 containing an impeller 12 for circulating air to be heated through the housing. A heat exchanging flue 21 and air inlet duct 28 arrangement upstanding from the housing 25 can preheat the circulated air. Control means (60 to 65) permit selective circulation or circulation and heating of the air. <IMAGE>

Description

HEATING APPARATUS This invention concerns air-heating units and apparatus.

Air heating units of a kind comprising a forced draught fuel-burning heater having a flue connected thereto, a housing or cabinet in which a combustion chamber of the heater is disposed, and an impeller disposed below said heater to draw air from an inlet of the housing or cabinet and propel it upwardly past the combustion chamber to an outlet of the housing or cabinet, are known from, for example, a leaflet numbered ISH12-1 published by the Myson Group PLC in 1989, a leaflet numbered WMP/DF/05/88 published by WANSON and a leaflet concerning the "CP range" published in November 1991 by Powermatic Limited. Such units have the disadvantage that the heated air is discharged forcefully well above floor level, and as is well known, hot air rises leading to stratification of the air in large volume spaces such as are found in factories and warehouses.

It is also known to install high level down-blowing fans in such spaces to reduce stratification, but such expedients can give rise to discomfort for persons working directly below the fans.

The present invention seeks to reduce the problems of stratification and discomfort.

According to the present invention there is provided an air heating unit comprising a forced draught fuelburning heater having a flue connected thereto, a housing in which a combustion chamber of the heater is disposed, and an impeller disposed below said heater to draw air from an inlet of the housing and propel it past the combustion chamber to an outlet of the housing; wherein a riser portion of the flue extends in direct or indirect thermal contact with air in an air inlet duct connecting an elevated air inlet opening with the inlet so that said air can be drawn down and pre-heated before entering the housing for selective recirculation or recirculation and further heating by the heater.

More particularly, the present invention provides an air heating unit comprising a housing, a fuel burning heater, an impeller to propel air to be heated by the heater from an inlet of the housing to an outlet of the enclosure, and a flue pipe to convey the products of combustion from the heater; wherein the housing is internally partitioned to define a flow path for the air to be heated so that said air descends a descending limb of said path from the inlet (and preferably past said flue pipe) to a lower chamber from which chamber the air is driven to ascend an ascending limb of said path past a combustion chamber of the heater to the outlet; wherein the heater is a forced draught heater having a heat exchanger, which heat exchanger is disposed in said flow path and conveys the products of combustion from the combustion chamber to said flue pipe; and wherein the unit is adapted for fitting of an air inlet duct to the inlet and a flue riser to the flue pipe so that part of the riser extends within the inlet duct.

The invention more particularly provides heating apparatus comprising said fuel-burning unit, wherein a riser is connected to said flue pipe and an air inlet duct is connected to said air inlet to convey said air downwards from an elevated air inlet opening so that the recirculated air absorbs heat from the riser when the heater is operating, and comprising thermally sensitive control means to selectively energise the impeller or both the impeller and the heater.

The air inlet opening is preferably at least one metre above the housing, and is preferably fitted with a screen, e.g. to prevent entry of birds, airborne objects and falling debris.

A condensate collector is preferably disposed in the flue or adjacent a lower part of the flue to intercept condensate before it can enter the combustion chamber.

The thermally responsive control means is preferably responsive when heating is required to maintain a sensed temperature within a predetermined range by selectively causing either the impeller only to run or the impeller to run and the heater to operate for heating the air.

The invention further includes an installation of the heating apparatus in a structure defining a space between a floor and a ceiling (or equivalent elevated barrier spaced above the floor) wherein the apparatus is arranged to collect air from an upper zone of said space, preferably close to the ceiling, and discharge the air into a lower zone of said space for recirculating air within said space.

The flue preferably extends upwards beyond said upper zone to an exhaust outlet external to the structure; or the flue may have a return limb which leads to an exhaust outlet, for example a balanced flue outlet and combustion air inlet device for admitting combustion air for the heater; or the flue may have an external outlet associated with or separate from an external inlet for admitting combustion air for operation of the heater in a room sealed manner.

Condensate drains may be provided for the heater and housing as well as for the flue.

A filter may be provided in the housing for filtering the recirculated air, and, preferably also for absorbing some of the noise generated by the impeller.

The impeller is driven by a motor which preferably has a separate cooling air feed to bring air from outside the housing, which air may be discharged, after cooling the motor, into the lower chamber.

The invention may be applied to heaters of a known form in which the combustion chamber and heat exchanger lie an ascending flow path, so that this path forms the ascending limb in the unit of the invention. The known combustion chambers are fired horizontally by a burner disposed at one end so that the wall at the opposite end of the combustion chamber is usually the hottest wall and is subject to the greatest rate of thermal stress, eventually results in corrosion and erosion. In order to overcome this problem, the descending limb is preferably arranged to cause air to sweep down in contact with that hottest wall of the combustion chamber so that that wall, in effect, forms a partition bounding part of the descending limb in order preferentially to cool that wall.

However, in order to obviate such problems and provide improved combustion, as well as other advantages, the invention further provides and includes an air heating unit in which a forced draught burner fires vertically or substantially vertically downwards in an upstanding combustion chamber from which the products of combustion are led away from the bottom of the combustion chamber through a bottom duct connected to the bottom of an upwardly extending heat exchanger exhausting into a flue pipe, and in which air to be heated is impelled to flow from an inlet past the heat exchanger and then the combustion chamber to an outlet.

Thus, the heat exchanger is effectively in a descending limb of an air flow path of U-shape from the inlet to the outlet to preheat air before it enters the ascending limb in the space around the combustion chamber.

An impeller drive motor is preferably separately cooled by a dedicated air flow.

The combustion chamber may have longitudinal corrugations or finning to increase the effective heat transfer area of the upstanding wall of the combustion chamber.

The ascending limb may be partitioned to provide a hot sub-path proximal to the combustion chamber and a cooler sub-path spaced apart from the combustion chamber.

A fresh air inlet duct may be connected to deliver air into the descending limb.

The burner is preferably a forced draught burner and may be fed, e.g. through a balanced flue, with combustion air drawn from air external to the building, and separate from the flows of air to be heated.

The control means is preferably constructed to sense a plurality of temperatures for controlling the operation of the impeller and the heater to provide either operation of the impeller only when a low heating demand is deemed to exist or operation of the impeller and burner when a higher heating demand is deemed to exist.

Said plurality of temperatures preferably comprise a first or threshold temperature sensed externally of the apparatus below which a heating demand exists, and a second temperature at which said higher demand is deemed to exist.

Said second temperature may be sensed: (a) at physically greater height than the height at which the threshold temperature is sensed, e.g. to respond to the temperature of the air available at about the level of the elevated air inlet opening; (b) within the air inlet duct or enclosure or adjacent the outlet to respond to the temperature of the recirculated air, or (c) at about the same height as that at which the first temperature is sensed, and in this case the second temperature is set lower instead of higher than that at which the first temperature is set.

In (a) and (b) above a low demand may be deemed to exist when the air at high level or recirculated air is at a temperature which is hot enough to provide the heating required at low level.

In (c) above a low demand may be deemed to exist when the air at low level is at a temperature which is less than the first set temperature but greater than the second set temperature.

The invention will be described further, by way of example, with reference to the accompanying diagrammatic drawings, wherein FIGURE 1 is a diagrammatical cross-sectional view of a first embodiment of apparatus of the invention; FIGURE 2 is a view similar to FIGURE 1 of a modified form of the first embodiment; FIGURE 3 is a view similar to FIGURE 1 of a second embodiment; FIGURE 4 shows the second embodiment in front elevation; FIGURE 5 is a view similar to FIGURE 3 showing a modified form of the second embodiment; and FIGURES 6 and 7 are electrical circuit diagrams of control means employed in the apparatus of the invention.

The first embodiment, shown in FIGURE 1, is based on a conventional free-standing warm air heater unit which comprises a cabinet 10 divided into a lower compartment 11 (containing an impeller 12, electric motor 13 and a belt 14 to drive the impeller) and an upper compartment 15 (containing a combustion chamber 16 and a heat exchanger 17) of a heater 18. The heater 18 has a forced draught burner 19, which fires a flame into a front side of the combustion chamber, and the hot flue gases pass from the combustion chamber via the heat exchanger and a flue pipe 20 into a flue riser 21 external to the cabinet to be conveyed out of the building 30 via the outlet terminal 22.

The air to be heated is drawn via vents (not shown) in the cabinet into the lower compartment 11 by the impeller. The air is then forced by the impeller over the combustion chamber and the heat exchanger to be warmed before it is discharged through hot air outlets 23 on top of the cabinet.

In the present invention, the cabinet 10 forms a front part of a housing 25 further including a rear part 26, the known air inlet vents are omitted and the lower compartment 11 opens into the lower end of the rear part 26 to form a lower chamber 11. The flue pipe 20 emerges into the upper end of said rear part 26, and the upper part of a rear panel of the cabinet is retained to serve as a partition 27 between the rear part 26 and the upper compartment 15.

An air inlet duct 28 is located around the flue 21 and is connected to the upper end of the rear part 26 to convey air to be heated from an upper air inlet 29 down to the housing. In use, warm air from an upper part of a space 31 between a ceiling 56 and a floor 57 of the building 30 is drawn down the air duct 28 in thermal contact with the flue before it enters the lower chamber 11.

Flue condensate liquid may be collected either by a condensate drain cap 32 or alternatively an annular condensate collector 33 situated in the flue, from which the condensate drains via a condensate drain pipe 34 to a drain (not shown).

In a modified form of the first embodiment, as shown in FIGURE 2, the rear panel 27 of the cabinet is omitted, so that the descending air stream flowing down the rear part 26 passes in direct thermal contact with a rear wall 24 of the combustion chamber 16. Partitioning 35 is provided to separate the descending and ascending air flows, so that the return air flows over the flue pipe 20 and over the back plate 36 of the heat exchanger 17, as well as the combustion chamber rear wall 24, before entering the lower chamber 11. The rear part 26 of the housing has a thermally insulated lower duct panel 37.

In this modified embodiment the temperature of the air entering the lower chamber 11 will be higher than that in the first embodiment, and so the temperature of the air surrounding the electric motor 13 will be relatively high, e.g. 35"C. The electric motor is protected from overheating by means of a separate cooling air flow delivered via a cooling air duct 38. This duct is connected to the cooling fan of the electric motor at one end, and is open to the building at the other end, so that ambient air from within the building can be drawn along the electric motor cooling duct, and over the motor windings, before being discharged into the lower chamber.

Some of the benefits of this modified embodiment over the first are: (1) The base area occupied by the heater is less.

(2) The construction cost is slightly lower, as there is less metal.

(3) The combustion chamber rear wall 24 is the hottest and most vulnerable area of the combustion chamber.

In this embodiment the whole air flow is directed over this area, thus providing substantially better cooling than in the aforesaid known unit wherein the area is cooled by only a small proportion of the total air flow. This modification reduces the surface temperature of the combustion chamber rear wall, and therefore reduces the thermal stress and increases its life-span.

In a second embodiment of the heater unit shown in FIGURES 3 and 4, the combustion chamber 16A is vertically instead of horizontally orientated. The burner 19 is fitted at the top of the combustion chamber and fires vertically downwards. The hot flue gases then pass out of the combustion chamber 16A through a bottom duct 40, into a heat exchanger 17A, up into a tee-piece flue pipe 20A to which the flue is connected, and then out of the building through the terminal 22. The heat exchanger in this embodiment is a twin parallel plate heat exchanger, of which the left hand heat exchanger plate is shown in an external view, whereas the right hand heat exchanger plate is shown in cross section. It will be seen that the hot flue gases travel up within the plates and then into the flue pipe.Each heat exchanger plate contains baffles (not shown) to cause turbulence to the hot flue gases for improving heat transfer. Alternatively or additionally, the surfaces of the heat exchanger plates may be dimpled or shaped to improve the rate of heat transfer from the plates to the air stream. The number and configuration of the heat exchanger plates may be varied, depending on the size of and output required from each model of the heater.

Cleaning access door(s) (not shown) are provided to enable each plate to be cleaned and internally inspected.

The return air, as previously described with reference to FIGURES 1 and 2, is drawn down the return air duct 28, collecting heat from the flue 21, but then in this embodiment it passes through the heat exchanger 17A, and then past the bottom duct 40 and into the lower chamber 11 of the housing 25. In FIGURE 3, an optional air filter 41, shown in broken lines, is located within this chamber 11, (which filter may be included in all embodiments), to filter the return air.

Also in this embodiment an optional fresh air supply duct 42 is provided to admit fresh air from outside the building to the recirculating air descending within the housing to improve the quality of the air in the building.

The fresh air supply duct 42 is fitted with a damper 43, which is used to regulate the amount of fresh air drawn in through a fresh air inlet grille 44. The fresh air damper 43 may be regulated manually or by an electric damper actuator from a remote control position. An equivalent air supply duct 42 and damper 43 may be provided in the other embodiments.

The return air is then blown by the centrifugal impeller 12 to a tapered base 45 of the combustion chamber 16A, past the sides of the combustion chamber, and out through the top air outlets 23. The descending air flow passing the heat exchanger 17A and the ascending air flow past combustion chamber 16A are separated by a divider partition 35A. The combustion chamber may be of any suitable elongate form, e.g. cylindrical or rectangular along its length, so that the ascending air flow is heated in a uniform manner to avoid development of excessively hot streams within the flow and localised overheating of the wall of the combustion chamber. The ascending air flow enters a mixing space 46 above a top plate 47 of the combustion chamber 16A, prior to being blown out through the air outlets 23.

FIGURE 4 also shows additional features omitted from FIGURE 3, including air outlets 23 at the sides of the housing. Each of the outlets 23 may be fitted with a manually adjustable air grille 48 by which the direction of the air stream flowing from the outlets may be directed as required within the building.

Combustion air may be drawn from outside the building through a combustion air inlet duct 49 and into an enclosure 50 containing the burner 19 so that neither the combustion air nor the flue gases have any connection to the air within the building compartment. Alternatively, the combustion air inlet duct may draw air from outside the building via an external inlet located at low level, e.g.

on an external wall of the building.

Also illustrated in broken lines in FIGURE 4 is an alternative orientation of the cooling air duct 38A which leads from a top inlet down to the electric motor. This duct may be connected to the fresh air supply duct 42 instead of a top inlet to the housing 25.

FIGURE 5 shows a modified form of the second embodiment of the heater shown in FIGURES 3 and 4. In this modification, the rear part ascending air flow path is divided into a first sub-path 51 proximal to the combustion chamber and a second sub-path 52 separated from the first sub-path by partitioning 53. Air flowing up this sub-path 52 is not in direct contact with either the combustion chamber 16A or the heat exchanger 17A, so that it is cooler than the air within the first sub-path 51. The sub-flows are kept separate by the partitioning above the combustion chamber, so that the cooler air stream is discharged over the hotter air stream from the front air outlet or outlets 23 whereby, after being discharged, to form a "layer" of cooler warm air over the hotter air. Heating of the upper parts of the burner unit is also reduced.

The remainder of the ascending air flow path may be divided by front and side partitioning 53A so that a cooler air stream prevents the front and side panels 54 becoming excessively hot, and thus these panels may be without the insulation shown in FIGURE 3. The partitioning 53A adjacent these panels 54 may be steel sheeting fixed to these panels, leaving a small air gap into which the air is forced, and this air mixes with the air heated by the combustion chamber prior to being discharged.

In both embodiments, at least some of the panels and walls of the housing are removable or are provided with hatches (not shown) for cleaning and maintenance purposes.

Similarly, the combustion chamber and heat exchanger may have access panels for cleaning and inspection purposes.

The flue pipe 13 can be inspected internally and cleaned by removal of the condensate drain cap 22 or a hatch in the flue pipe. Inspection doors may be fitted to the return air duct for external inspection of the flue if required.

The flue 21 is made of a suitable corrosion resistant material such as stainless steel. It may be a rigid pipe which is continuous in its length, or made of sections which have to be joined using jointing clips which form a gas-tight seal between the flue gases and the return air stream. In the case of rigid pipes, turbolators may be inserted into the pipe to cause turbulence of the flue gases, and thereby improve the heat transfer rate from the flue gas to the return air stream. Alternatively a flexible continuous spirally wound stainless steel flue pipe or specially manufactured flue pipes may be used, which improve the rate of heat transfer between the hot flue gases and the air stream, by means of fins or other devices.

The flue may be double-skinned with a vented gap between the inner pipe and outer skin for safety purposes, so that heat transfer from the inner pipe to the return air flow is indirect; and the reduction in heat transfer rate caused by the gap may be reduced by bridging the gap at intervals with thermally conductive material and by minimising the gap thickness.

The duct 28 is made of a similar or less expensive rigid continuous pipe, jointed pipework, spirally wound pipe or specially formed pipe to those pipe and pipework previously mentioned for construction of the flue, and again vanes or tubolators may be fitted within the duct 28 to increase the rate of heat transfer across the flue.

In the apparatus, the air inlet duct serves as a physical shield around the hot flue, as well as enabling the impeller to be employed for destratifcation. The housing serves to support the weight, or at least most of the weight, of the flue and inlet duct so that simple stays 58 may be employed to brace the duct and upper part of the flue in position in the building. The flue pipe is held in position inside the return air duct by stays, such as radial stays (not shown).

In addition to heat recovery, reduction of stratification and noise reduction offered by the first embodiment, the second embodiment offers highly efficient counter flow heating within the heater, reduced width and required floor area, direct impeller propelled cooling of the lower (hottest) end of the combustion chamber, efficient heat exchange under suction around the heat exchanger, positioning of the vulnerable projecting external parts of the burner assembly in a physically safe located, improved combustion and ease of access, cleaning and repair.

Referring to FIGURE 6 (and FIGURES 1 to 5), the heater unit is controlled by a control system which comprises a fan control thermostat 63, a burner thermostat 64 (which may be associated with the thermostat 53), and a safety thermostat 65 mounted in the housing, a low level thermostat 61 (which senses the low level ambient temperature and operates at a first set (or required) temperature, a timer 62, as well as the high level thermostat 60 (which senses the temperature of the air available for recirculation and closes at a second set temperature).

The system operates so that when the thermostat 60 is above the temperature at which it is set (itself greater than the required temperature) it energises the impeller via the fan thermostat 63. If the thermostat 60 is below its set temperature, it switches on the burner circuit via thermostat 64. The burner then is switched on, and when the housing air temperature has risen so that the thermostat 63 is satisfied, the air impeller motor is switched on. The heater continues to run on this basis, in response to the various thermostats, until the timer 62 switches off. When the burner is turned off, irrespective of the status of the components 60, 61 and 62, the fan thermostat 63 will keep the impeller motor energised until the fan thermostat has cooled to a predetermined temperature e.g. about 40"C.

Referring to FIGURE 7, in a modified control system, the high level thermostat 60 is not employed, but instead a dual low level thermostat arrangement 61, 61A is employed in which the thermostat 61 keeps its role and thermostat 61A responds at a second set temperature which is set a few degrees below the required temperature. This means that there is no necessity to install and wire up a thermostat or sensor at high level within the building. Only one sensor box is required at low level, thus reducing the cost of installation, and retaining all controls at a height where they are easy to adjust and maintain.

In this system, if the air temperature at low level is below the setting of 61, but above that of thermostat 61A, then the impeller only runs. If this produces enough heat at low level to raise the air temperature above the setting of thermostat 61, then the impeller switches off, and the burner does not operate at all. If, however, the impeller alone fails to raise the air temperature, and it falls below the 61A, then the burner circuit is switched on. Again the burner operation is governed by the thermostats 63, 64 and 65. If the air temperature is below both 61 and 61A, then the burner is switched on and the impeller is energised via thermostat 63 until the air temperature reaches the setting temperature of the thermostat 61A.The burner will then turn off, and the impeller will continue to run unless the air temperature rises above the setting of the thermostat 61, when it will turn off.

A time delay may be added to the thermostat 61A.

When the condition arises where the low level temperature is below both 61 and 61A, the time delay will allow the impeller to run for a predetermined period of time before 61A switches on the burner circuit. This provides the opportunity for the warm air from the upper zone to heat the lower zone on start up, thus reducing the amount of time the burner fires, and consequently reducing the fuel consumption.

The control system may be further modified to employ a purpose designed "intelligent" main electronic controller incorporating all of the above functions, and such others as may be necessary, to control the burner and impeller operation so as to provide the highest possible operational efficiency from the heater, within the building in which it is contained.

The electronic controller may utilise sensors rather than electric thermostats, so that the temperatures are set on the controller rather than at each thermostat, or may use thermostats and sensors as is convenient. This method of control will enable settings to be changed at a single location instead of multiple locations, thus providing an easier control system.

Further, the timer 62 may be replaced by an optimiser-timer control of known form which adjusts the start and stop times according to the prevailing air temperatures, to avoid unnecessary pre-heating and to curtail the period of heating.

In all the foregoing embodiments a two speed impeller motor 13 may be employed (and the thermostats need not be mechanically coupled e.g. to change the energisation path for the fan when the thermostat 64 shuts down the burner, or even to shut off the burner when the fan thermostat reaches its preset limit), the fan thermostat 63 being wired to energise the impeller motor to drive the impeller at high speed when said limit (appropriately reset) is reached in response to heat from the burner. In such a case, the impeller will run at low speed when the burner is not switched on, for recirculation purposes only. At low speeds the air stream from the heater unit will be slower, and therefore less likely to cause discomfort to people in the immediate vicinity of the heater.The impeller will be switched to high speed once the burner has been switched on, and the air temperature has risen so that the fan low limit thermostat 63 switches on the high energisation line to the motor 13.

In a further modification a two stage burner is used.

Such burners are commonly available, and have a high output level and a low output level. An additional thermostat is located within the recirculated air stream, e.g. adjacent to 63 and 64, to switch the burner to low level. This additional thermostat is set to a changeover limit, for example, at 604C so that in use the burner will be switched on to operate at high output (via the thermostat 64) when the air flow is below the changeover limit, before being switched to low output when this limit is exceeded. The system will turn off the burner when the air temperature rises to switch off the high limit thermostat 64. This modification may be used in conjunction with a two speed impeller motor.

Typical settings can be: Thermostat 60-24"C Thermostat 61-18"C (Fig. 6) or 20 (Fig. 7) Thermostat 61A-18"C Thermostat 63-40"C Thermostat 64-90 C Thermostat 65-120"C.

The thermostat 65 is preferably of non-self-resetting form and needs to be manually reset after it has tripped out.

Furthermore, the impeller motor and burner can be of kinds which permit the outputs thereof to be varied in other ways, e.g. fully modulated by means of a variant of the control system in which sensors are used to provide inputs to a programmable control unit in which such settings are stored as data for use in modulating said outputs to give an impeller speed proportional to burner output (under steady conditions) so that the air outlet temperature is maintained within comfortable limits.

Clearly, the control system may be simplified to have the impeller running all the time the system is switched on, and to employ a single thermostat to turn on the burner in response to temperature in said space.

For cooling or ventilation the control system may include an over-temperature thermostat to open the damper 43 for mixing fresh air with recirculated air when a relatively high temperature, e.g. 18"C is sensed in said space.

The invention is not confined to details of the foregoing examples and embodiments, and many variations are possible within the scope of the appended claims, and the invention further provides a heater unit or apparatus incorporating any novel part, functional feature or combination of parts or features disclosed herein or in the FIGURES. The invention further includes functional equivalents of said parts and features, for example, the air streams from the cooler sub-paths bounded partially by the partitioning 53A may be directed to form a thin layer of cooler air below the main heated layer, upon discharge.

However, in the housing above the combustion chamber there may be a mixing box which will allow the various sub-flows to be mixed and directed to selected outlets or combinations thereof, by means of, for example, manually adjustable vanes.

For example, the present invention further provides a heater unit comprising a down firing burner in an upstanding combustion chamber alongside an upstanding heat exchanger, and a duct connecting the lower ends thereof, in a housing partitioned and provided with an impeller to draw air to be heated past the heat exchanger prior to it being driven past the combustion chamber.

Claims

1. An air heating unit comprising a forced draught fuelburning heater having a flue connected thereto, a housing in which a combustion chamber of the heater is disposed, and an impeller within the housing to draw air from an inlet of the housing and propel it past the combustion chamber and a heat exchanger to an outlet of the housing; wherein a riser portion of the flue extends in thermal contact with air in an air inlet duct connecting an elevated air inlet opening with the inlet so that said air can be drawn down and pre-heated before entering the housing for selective recirculation or recirculation and further heating by the heater.

2. An air heating unit comprising a housing, a fuel burning heater, an impeller to propel air to be heated by the heater from an inlet of the housing to an outlet of the enclosure, and a flue pipe to convey the products of combustion from the heater; wherein the housing is internally partitioned to define a flow path for the air to be heated so that said air descends a descending limb of said path from the inlet and past said flue pipe to a lower chamber from which chamber the air is driven to ascend an ascending limb of said path past a combustion chamber of the heater to the outlet; wherein the heater is a forced draught heater having a heat exchanger, which heat exchanger is disposed in said flow path and conveys the products of combustion from the combustion chamber to said flue pipe; and wherein the unit is adapted for fitting of an air inlet duct to the inlet and a flue riser to the flue pipe so that part of the riser extends within the inlet duct.

3. An air heating apparatus comprising a unit as claimed in Claim 2 wherein a riser is connected to said flue pipe and an air inlet duct is connected to said air inlet to convey said air downwards from an elevated air inlet opening so that the air absorbs heat from the riser when the heater is operating, and comprising thermally sensitive control means to selectively actuate the impeller or both the impeller and the heater.

4. An air heating unit or apparatus as claimed in Claim 1, 2 or 3 wherein the combustion chamber extends horizontally from a burner at one end, and a wall at the opposite end of the combustion chamber is exposed to said air.

5. An air heating unit or apparatus as claimed in Claim 1, 2 or 3 wherein the combustion chamber extends downwards from a down firing burner.

6. An air heating unit or apparatus as claimed in Claim 5 as appended to Claim 2 wherein the ascending limb is partitioned to provide a hot sub-path proximal to the combustion chamber and a cooler sub-path spaced apart from the combustion chamber.

7. An air heating unit or apparatus as claimed in any preceding claim wherein the heat exchanger is disposed so that said air impinges on the heat exchanger prior to passing the impeller.

8. An air heating unit or apparatus as claimed in any preceding claim wherein the impeller is driven by a motor provided with a cooling air duct for intake of air from outside the housing.

9. An air heating apparatus or unit as claimed in any preceding claim including a filter to filter air passing through the housing.

10. An air heating apparatus or unit as claimed in Claim 9 wherein the filter is disposed upstream of the impeller.

11. An air heating apparatus or unit as claimed in any preceding claim wherein the heater is connectable or connected to a combustion air inlet separate from said air inlet.

12. An air heating apparatus or unit as claimed in any preceding claim wherein the housing has removable or openable panels or hatches providing access to the interior of the housing for inspection of the combustion chamber, impeller flue pipe and the air flow path through the housing.

13. An air heating apparatus or unit as claimed in any preceding claim wherein a condensate collector is disposed in said flue or said flue pipe.

14. An air heating apparatus or unit as claimed in any preceding claim having a fresh air inlet to admit an additional fresh air flow to mix with said recirculated air.

15. Apparatus as claimed in Claim 3 or any preceding claim as appended to Claim 3 wherein the control system including thermally responsive control means responsive to a temperature to determine when heating is required to maintain the sensed temperature within a predetermined range by selectively causing either the impeller only to run or the impeller to run and the heater to operate for heating the air.

16. Apparatus as claimed in Claim 15 wherein control means is constructed to sense a plurality of temperatures for controlling the operation of the impeller and the heater to provide either operation of the impeller only when a low heating demand is deemed to exist or operation of the impeller and burner when a higher heating demand is deemed to exist.

17. Apparatus as claimed in Claim 16 wherein said plurality of temperatures comprise a first or threshold temperature sensed externally of the apparatus below which a heating demand exists, and a second temperature at which said higher demand is deemed to exist; said second temperature maybe sensed: (a) at physically greater height than the height at which the threshold temperature is sensed, e.g. to respond to the temperature of the air available at about the level of the elevated air inlet opening; (b) within the air inlet duct or enclosure or adjacent the outlet to respond to the temperature of the recirculated air, or (c) at about the same height as that at which the first temperature is sensed, and in this case the second temperature is set lower instead of higher than that at which the first temperature is set.

18. An air heating unit in which a forced draught burner fires vertically or substantially vertically downwards in an upstanding combustion chamber from which the products of combustion are led away from the bottom of the combustion chamber through a bottom duct connected to the bottom of an upwardly extending heat exchanger exhausting into a flue pipe, and in which air to be heated is impelled to flow from an inlet past the heat exchanger and then the combustion chamber to an outlet.

19. A unit as claimed in Claim 18 wherein an impeller to propel said air flow is disposed in the air flow path between the heat exchanger and the combustion chamber.

20. A unit as claimed in Claim 19 wherein the impeller has a drive motor cooled by a cooling air flow separate from said air flow.

21. A unit as claimed in Claim 18, 19 or 20 wherein the combustion chamber has longitudinal corrugations or finning to increase the effective heat transfer area of the upstanding wall of the combustion chamber.

22. A unit as claimed in Claim 18, 19, 20 or 21 wherein the space around the combustion chamber is partitioned to provide a hot sub-path proximal to the combustion chamber and a cooler sub-path spaced apart from the combustion chamber.

23. A unit as claimed in Claim 18, 19, 20, 21 or 22 wherein a fresh air inlet is provided to admit for mixing with said air flow.

24. An air heating unit substantially as hereinbefore described with reference to any one of FIGURES 1 to 5 of the accompanying drawings.

25. Apparatus substantially as hereinbefore described with reference to any one of FIGURES 1 to 5, or any one of FIGURES 1 to 5 as modified by FIGURE 6 or 7 of the accompanying drawings.

26. An installation of an air heating unit or apparatus as claimed in any preceding claim in a space within a building providing a floor and a ceiling or roof bounding said space so as to be able to recirculate air from an upper zone of said space through the apparatus or unit to a lower zone of said space

27. An air heating unit, apparatus or installation comprising any novel part, functional feature or arrangement of parts or features disclosed herein or in the accompanying drawings and mechanical or functional equivalents thereof.

28. A flue and air inlet duct assembly adapted to be secured upon an upper part of a housing or cabinet of an air heating unit to convert said heating unit into apparatus for selective recirculation or recirculation and heating of air drawn down the air inlet duct by said unit.