GB2220475A

GB2220475A - Ventilating systems

Info

Publication number: GB2220475A
Application number: GB8912568A
Authority: GB
Inventors: Ian Thomas Roberts; Kenneth John Davies
Original assignee: Johnson and Starley Ltd
Current assignee: Johnson and Starley Ltd
Priority date: 1988-06-09
Filing date: 1989-06-01
Publication date: 1990-01-10
Anticipated expiration: 2009-06-01
Also published as: GB8912568D0; GB2220475B; GB8813644D0

Abstract

A ventilating system which uses the heat obtained from combustion flue gases from a fuel-fired heating appliance (10), and spent air exhausted from the premises, to warm incoming fresh air via a heat exchanger (20). Spent air may be collected via a cooker hood (26) or a bathroom or shower room extractor (28). As shown, heating appliance (10) heats the air which exits via a warm air supply duct (18). Alternatively the appliance (10) may be a central heating boiler; or the appliance (10) may heat water which is circulated by a pump through a heat store. <IMAGE>

Description

Ventilating Systems This invention relates to ventilating systems, and more particularly to ventilating systems incorporating heat recovery systems for improved thermal efficiency.

In new housing, changes in house construction techniques and customer attitudes have resulted in the more widespread adoption of mechanical ventilation combined with heat recovery as a method of providing a pleasant and comfortable indoor environment as well as reducing running costs.

Fresh air requires to be introduced into homes for a variety of reasons, including respiration, dilution and/or displacement of odours and smoke, control of internal humidity, and provision of air for fuel-burning appliances.

Natural ventilation by a combination of leaks in the building fabric and opening of windows can lead to uncontrolled ventilation, excessive loss of heat, and personal discomfort. Improved standards of house construction which reduce air leaks (draughts) and heat losses (by increased insulation) increase the demand for controlled ventilation that gives an adequate supply of fresh air without being thermally uneconomic or excessively variable in temperature. Ventilation by induced draught through the provision of extractor fans can control the withdrawal of stale air and fumes, but the withdrawn air is replaced by already-heated air from elsewhere in the room or the house, which in turn draws in unheated external air in a more or less uncontrolled manner. Thus simple extractor fan systems do not provide a full control of ventilation, and are thermally wasteful.

Control of both air supply to, and of air withdrawal from a leak-tight house can provide stability and predictability of ventilation patterns, and hence reduce gross demand for fresh air without cutting local ventilation below an acceptable minimum rate. A reduced gross intake of fresh air correspondingly reduces the heat required to bring the incoming air up to an acceptable temperature, and this required heat can be further reduced by exchange of heat between outgoing warm stale air and incoming cold fresh air.

In a ventilation system where the primary domestic heat source is fuel-burning, as for example a gas-fired heating appliance, the combustion gases must not be allowed into the atmosphere of the house. The consequent necessity of total venting of combustion gases has inevitably resulted in heat being lost with these gases.

It is therefore an object of the invention to provide a thermally efficient ventilating system for premises wherein a major heat source is a fuel-burning appliance producing combustion gases. The invention is particularly intended for domestic use, but the invention may also be used in nondomestic premises with similar or analogous ventilation and heating requirements, and references to domestic premises are to be construed accordingly. Where references are made to domestic-premises, this is intended to refer to premises in a range from the minimum size and number of rooms that are habitable (sometimes call starter homes" or "studio flats") through conventional family homes, to larger premises.

According to a first aspect of the invention, a method of ventilating premises wherein a major heat source is a fuelburning appliance producing combustion gases comprises supplying fresh air in to the premises, withdrawing spent air from the premises together with the combustion gases, and exchanging heat from the spent air and from the combustion gases to the incoming fresh air such as to recover heat from the spent air and from the combustion gases to produce heated fresh air.

Preferably, the spent air and the combustion gases are mixed before heat is recovered from them to provide heated fresh air. By this mixing, the temperature of the flue gases may be kept under 120 degrees C, This allows the use of ordinary materials rather than specially developed materials in the heat exchanger, and thus provides a very costeffective system.

The supply of fresh air and the withdrawal of spent air preferably take place substantially within a predetermined pattern of air flow paths between regions of said premises, and said predetermined pattern preferably includes at least a predominance of air flow from a region or regions of relatively lower humidity towards a region or regions of relatively higher humidity, for example from bedroom and lounge towards bathroom and kitchen.

The heat exchange preferably takes place by conductive transfer without mass exchange, but heat may alternatively -or additionally be exchanged by pumping of heat.

The heated fresh air may have additional heat supplied thereto either directly by said fuel-burning appliance, or indirectly by heat transferred from a heat store which is supplied with heat from said fuel-burning appliance. Where the heated fresh air has additional heat supplied thereto, the additional heat may either be supplied solely to said fresh air which is subsequently withdrawn as spent air without being further heated by said appliance (other than by subsequent exchange of heat with the combustion gases), or the additional heat may be supplied to a mixture of the incoming heated fresh air with previously admitted air which is being recirculated within the premises.Whether or not recirculation of air is carried out within the premises, the supply and withdrawal of air preferably take place at a rate which provides at least a predetermined minimum number of volume changes per unit time.

The method preferably includes adjusting the rate of supply and withdrawal of air.

The method preferably includes sensing the rate of withdrawal of combustion gases, and terminating combustion and/or the supply of fuel for combustion in the event of the sensed rate of withdrawal falling below a predetermined minimum appropriate to circumstances of operation.

According to a second aspect of the invention, a ventilating system for premises wherein a major heat source is a fuelburning appliance producing combustion gases comprises an air supply duct for supplying fresh air into the premises, an air exhaust duct for exhausting spent air from the premises, air propelling means for propelling air through said ducts, a vent for said combustion gases-, and heat exchange means pneumatically linked to said ducts and to said vent for exchanging heat from the spent air and from the combustion gases to the fresh air.

The air supply duct preferably terminates in one or more air inlets, and the air exhaust duct preferably commences at one or more air extraction points, the inlet or inlets and the extraction point or points being so disposed within the premises that the supply of fresh air and the withdrawal of spent air take place substantially within a predetermined pattern of air flow paths between regions of the premises.

The predetermined pattern preferably includes at least a predominance of air flow from a region or regions of relatively lower humidity towards a region or regions of relatively higher humidity. To achieve this preferred pattern, the air inlets may be disposed in the bedroom or bedrooms and/or in the lounge of a house, and the air extraction points may be disposed in the bathroom or shower room and kitchen. One of the extraction points may be in the form of an extractor hood fitted over a cooker, or in the form of a tumble drier exhaust. Preferably, filters are provided on these extraction points. Such filters may be washable and/or removable for easy cleaning.

The heat exchange means preferably takes the form of an airto-air heat exchanger but may alternatively or additionally comprise a heat pipe and/or a heat pump. Means may be provided to mix the extracted air and the combustion products before they enter the heat exchanger, and a multipass heat exchanger may be used.

The air propelling means preferably comprises an air circulator, for example a fan, mounted in each of the ducts.

Adjustable fan speeds may be available for allowing particular proportions of inlet and extract air to enter the system. By adjusting the flow rates of inlet and extract air, in relation to each other, the efficiency of the system may be improved.

The air circulator in the air exhaust duct is preferably mounted downstream of the heat exchange means to reduce the risk of combustion gases mixing with the fresh air supply in the event of a leak in the heat exchange means. The air circulator in the supply duct may be mounted upstream of the heat exchange means.

The fuel-burning appliance may be an air-heating appliance which may either be supplied solely with the incoming fresh air or with a mixture of the incoming air and air recirculating within the premises. Alternatively, the fuel-burning appliance may-be a water-heating appliance, and the premises may be heated by the circulation of heated water through one or more radiators or the heated water may be circulated through a heat store with heat being withdrawn from the heat store through the same or a separate water circuit and supplied to the incoming fresh air by a waterto-air heat exchanger to heat the premises.

The fuel-burning appliance may be an appliance which is constructed or adapted to burn a natural gas or a synthetic gas The fuel-burning appliance may be an integral part of the ventilating system, or the ventilating system may be a kit of parts which is retro-fitted to premises having a preexisting fuel-burning appliance so as to recover heat from the combustion gases thereof.

The system preferably includes means to sense the rate of exhaustion of spent air and of combustion gases from the premises, and-means to terminate combustion and/or to terminate the supply of fuel for combustion in the event of the sensed rate of exhaustion falling below a predetermined minimum appropriate to the circumstances of operation of the system. Fuel to appliances may also be terminated if the system becomes blocked.

The rate of this supply and withdrawal of air may be affected by differentials in pressure between the interior of the house and outside: the house could be pressurised so making ventilation more efficient - or depressurised, or a neutral pressure could be created. Means of effecting the desired differential may be incorporated into the method or the system.

Other controlling means may also be provided to control and monitor the temperature of gases entering the system and provide a shut-off if the temperature exceeds certain predetermined levels.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings wherein: - Fig. 1 is a schematic diagram of a ventilating system wherein the premises are heated solely by warm air; Fig. 2 is a schematic diagram of a ventilating system wherein the premises are heated princi pally by heat emitted from water-filled radiators; and Fig. 3 is a schematic diagram of a ventilating system incorporating a heat store.

Referring first to Fig. 1, this schematically illustrates the ventilating system applied as a retro-fitted modification of a pre-existing ducted warm air gas-fired domestic ventilating and heating system. However it could equally well be constructed as a totally new system. The pre-existing system consists of a gas-fired air heater 10 exhausting its combustion gases through a flue 12. Air to be heated is drawn in to the heater 10 partly as fresh air through a fresh air inlet 14, and partly as air already in the house through a recirculation inlet 16. The heated mixture of fresh and recirculated air is distributed throughout the house by a warm air supply duct 18 with suitably located outlets. In the pre-existing system, the flue 12 and the fresh air inlet 14 respectively terminated and started outside the house.

In accordance with the invention, the pre-existing system has an air-to-air heat exchanger 20 added, and coupled by suitable ducting to have the demanded fresh air passed entirely through one side of the heat exchanger 20 by an upstream air circulator 22. At the same time, the flue 12 is coupled by suitable ducting to have the combustion gases from the heater 10 drawn through the other side of the heat exchanger 20 by a downstream air circulator 24. Thereby the heat previously wasted by direct venting of the hot combustion gases from the heater 10 is recovered to a significant extent by the heat exchanger 20.

Even more heat can be recovered, with a corresponding increase in thermal efficiency, if the heat exchanger 20 or other form of heat recovery unit is operated to condense at least part of the water vapour comprised in the combustion gases, since condensation releases the latent heat of the vapour to make it available for recovery. This may be achieved using a condensate drainage system, for example, where a tray is provided with a connected outlet drain to entrain condensed water running down from the heat exchanger 20.

The efficiency of the domestic heat supply can be further improved by collecting spent air that would previously have been extracted and vented directly, or dissipated through the house, and mixing such spent air with the combustion gases before passing through the heat exchanger 20 to make available even more recoverable heat. This lowers the concentration and temperature of combustion gases and allows the use of non-special materials in the heat exchanger. By way of example, such additional efficiency measures are schematically illustrated in Fig. 1 as a cooker hood 26, and a bathroom or shower room extractor 28 coupled by respective ducts to the same side of the heat exchanger 20 as the flue 12. Additionally or alternatively, a duct can be provided from a tumble drier exhaust.Washable filters are provided on these extractors 26, 28 to remove undesirable matter from the spent air before it reaches the heat exchanger. The preferred position is at the entry to the extract ducts so that they are accessible for cleaning.

A potential disadvantage of the system illustrated in Fig. 1 is that a separate water-heating appliance is required.

However, a gas-fired water heater could be fitted in the same cabinet or housing as the air heater 10, with the water heater sharing the air heater flue 12 and hence contributing combustion gas heat to be transferred to the incoming air via the heat exchanger 20. The air heater 10 recovers at least 75% of the heat from the combustion products. The remaining 25% is directed through the heat exchanger 20, where at least 60% of it is recovered to warm the incoming fresh air.

Fig. 2,schematically illustrates a system in which a gasfired boiler 30 provides both domestic hot water and domestic central heating (by utilising a pump 32 to force circulation of hot water through radiators 34 with individual thermostatic control valves 36), Such a system can be a conventional pre-existing system to which the invention is retro-fitted, or a new system integral with the invention.

As in the Fig. 1 system, the Fig. 2 system has an air-to-air heat exchanger 20 fitted and coupled to carry the combustion gases from the boiler 30. The heat exchanger 20 recovers otherwise waste heat from the combustion gases and other sources such as the cooker hood 26, or a tumble drier extract duct, and transfers the recovered heat into the incoming fresh air supplied through the air inlet 14 for distributed ventilation. Since heat is primarily supplied by the radiators 34, the ventilation air is not heated in a fuelburning appliance (in contrast to the Fig. 1 system).

However, the heat recovered from the combustion gases and the spent air, and added to the ventilation air in the heat exchanger 20, reduces the heat demanded from the wet central heating system and increases overall thermal efficiency. In Fig. 2, as in Fig. 1 additional benefit is provided by the recovery of latent heat from a condensate drainage system.

In the Fig. 3 system, a gas-fired boiler 40 heats water which is circulated by a pump 42 through a heat store 44 to store the thermal energy obtained from the combustion. The combustion gases are vented through the boiler flue 46 and the heat exchanger 20 for waste heat recovery, as in the systems of Fig. 1 and 2.

Owing to the low heat demand resulting from the high standard of insulation, all the required heat (for space heating and excluding water heating) can be met in the Fig.

3 system by heating the fresh air distributed for ventilation, and without recirculation of air. Heat that has to be added to the incoming fresh air in excess of the heat added by the heat exchanger 20 is supplied downstream thereof by a water-to-air heat exchanger 48. The second heat exchanger 48 is fed by a pump 50 with hot water heated by thermal energy withdrawn from the heat store 44.

Supplementary heat added to the fresh air is thus drawn indirectly from the boiler 40 which consequently acts as the major heat source in the house. Further heat can be recovered from the cooker hood 26, or a tumble drier exhaust, as before.

The heat store 44 also provides heat for the domestic hot water supply 52, again drawing heat indirectly from the boiler 40.

The ventilation and heating system of Fig. 3 could (apart from the ducting, piping and cabling) be integrated into a single unit or package for compactness, ease of installation, and economic advantages.

Fig. 3 also may have the additional benefit of increased thermal efficiency by providing a condensate drainage system in the heat exchanger.

While the systems of Figs. 1 and 2 as described above are suited for retro-fitting to domestic premises of conventional construction, they may also be fitted in to newly-constructed premises, and are particularly suited to domestic premises that are insulated and draught-proofed to an exceptionally high standard, as is the case for the Fig.

3 system.

In each of the foregoing embodiments, the mixture of combustion gases and spent air entering the heat exchanger has a temperature in the region of 105-120 degrees C. This temperature may be reduced by the introduction of air at room temperature (20 degrees C) through a valve placed upstream of the heat exchanger. A monitor may also be positioned immediately before the heat exchanger to ensure that the inlet temperature does not exceed 120 degrees C.

This monitor could control a switch which would shut off the heat source and so reduce the temperature if it became too high.

The heat exchangers may have more than one pass for either stream of air to increase efficiency; as in a double-pass heat exchanger.

Thus, significant increases in thermal efficiency can be expected from each of the systems described above.

In each of the systems described above, it is desirable (and may be a legal requirement) for a safety cut-out system to be incorporated, which shuts off the supply of fuel and/or sounds an alarm in the event of a failure of the fume extraction arrangement. In normal circumstances, the secondary flue gas would pass through the orifice plate to cause a pressure drop from the upstream side of the orifice plate to the downstream side of the orifice plate. A differential pressure switch connected to both sides of the orifice plate would sense this normal pressure drop and give a permissive signal allowing combustion to proceed as required. However, in the event of an exhaust duct blockage and/or a failure of the exhaust fan 24, the reduced or halted gas flow through the orifice plate would result in reduction or disappearance of the normal pressure drop and a consequential tripping of the differential pressure switch.

-Where the fuel is a gas or a liquid, the fuel supply could be automatically shut off by a solenoid valve or other suitable arrangement controlled directly or indirectly by the differential pressure switch. Appropriate alarm and/or shut-down and/or emergency arrangements can be made to suit systems employing solid fuels.

While certain modifications and variations have been described above, the invention is not restricted thereto, and other modifications and variations can be adopted without departing from the scope of the invention.

Claims

1. A method of ventilating premises wherein a major heat source is a fuel-burning appliance producing combustion gases, comprising supplying fresh air into the premises, withdrawing spent air from the premises together with the combustion gases, and exchanging heat from the spent air and from the combustion gases to the incoming fresh air such as to recover heat from the spent air and from the combustion gases to provide heated fresh air.

2. A method of ventilating premises as claimed in Claim 1 wherein the spent air and the combustion gases are mixed before heat is recovered from them to provide heated fresh air.

3. A method of ventilating premises as claimed in Claims 1 and 2 wherein the supply of fresh air and the withdrawal of spent air take place substantially within a predetermined pattern of air flow paths between regions of said premises.

4. A method of ventilating premises as claimed in Claim 3, wherein said predetermined pattern includes at least a predominance of air flow from a region or regions of relatively lower humidity towards a region or regions of relatively higher humidity.

5. A method of ventilating premises as claimed in any of Claims 1 to 4, wherein the heat exchange takes place by conductive transfer without mass exchange.

6. A method of ventilating premises as claimed in any of Claims 1 to 5, wherein the heat exchange takes place by pumping of heat.

7. A method of ventilating premises as claimed in any of the preceding Claims, wherein the heated fresh air has additional heat supplied thereto.

8. A method of ventilating premises as claimed in Claim 7, wherein a fuel-burning appliance is used to supply heat directly or a heat store supplied with heat from said fuelburning appliance is used to supply heat indirectly.

9. A method of ventilating premises as claimed in any of the preceding Claims, in which the supply and withdrawal of air take place at a rate which is adjustable.

10. A method of ventilating premises as claimed in any of the preceding Claims, in which the rate of withdrawal of combustion gases is sensed by monitoring means.

11. A method of ventilating premises as claimed in Claim 10, in which the monitoring means includes means for terminating the fuel combustion under specified conditions.

12. A method of ventilating premises as claimed in any of the preceding Claims, in which the temperature of gases entering the system is monitored.

13. A method of ventilating premises as claimed in Claim 12, in which the system is shut down if the temperature falls outwith defined limits.

14. A ventilating system wherein a major heat source is a fuel-burning appliance producing combustion gases, comprising an air supply duct for supplying fresh air into the premises, an air exhaust duct for exhausting spent air from the premises, air propelling means for propelling air through said ducts, a vent for said combustion gases, and heat exchange means pneumatically linked to said ducts and to said vent for exchanging heat from the spent air and from the combustion gases to the fresh air.

15. A ventilating system as claimed in Claim 14, wherein the spent air and the combustion gases are mixed before heat is exchanged from them to the fresh air.

16. A ventilating system according to any of Claims 14 or 15 wherein the air supply duct terminates in one or more air inlets within the premises.

17. A ventilating system according to any of Claims 14 to 16 wherein the air exhaust duct commences at one or more air extraction points within the premises.

18. A ventilating system according to Claim 17 dependent on Claim 16 wherein the inlets and extraction points are disposed within the premises such that the supply of fresh air and the withdrawal of spent air take place substantially within -a predetermined pattern of air flow paths between regions of the premises.

19. A ventilating system according to Claim 18 in which the inlets and extraction points are located such that the predetermined pattern of air flow includes at least a predominance of air flow from a region-or regions of relatively lower humidity towards a region or regions of relatively higher humidity.

20. A ventilating system according to any of Claims 17 to 19, wherein filters are provided on the ducts at the extraction points.

21. A ventilating system according to any of Claims 14 to 20, wherein the heat exchange means is in the form of an air-to-air heat exchanger.

22. A ventilating system according to any of Claims 13 to 19, wherein the heat exchange means is in the form of a heat pipe.

23. A ventilating system according to any of Claims 13 to 20, wherein the heat exchange means is in the form of a heat pump.

24. A ventilating system according to any of Claims 14 to 23, wherein the heat exchange means provides a plurality of passes.

25. A ventilating system as claimed in any of Claims 14 to 24, wherein the heat exchange means includes a condensate drainage means.

26. A ventilating system as claimed in any of Claims 14 to 25, wherein the air propelling means comprises a respective air circulator in each of the ducts.

27. A ventilating system as claimed in Claim 26, wherein the air circulaters are adjustable in speed

28. A ventilating system as claimed in Claim 26 or 27, wherein the air circulator in the air exhaust duct is positioned downstream of the heat exchange means.

29. A ventilating system as claimed in Claim 26, 27 or 28, wherein the air circulator in the air supply duct is positioned upstream of the heat exchange means.

30. A ventilating system as claimed in any of Claims 14 to 29, in which the ventilating system includes a fuel-burning appliance as an integral part of the system.

31. A ventilating systems claimed in Claim 30, in which the fuel-burning appliance is an air-heating appliance.

32. A ventilating system as claimed in Claim 30 or Claim 31, in which the fuel-burning appliance is a water-heating appliance.

33. A ventilating system as claimed in any of Claims 14 to 32, in which the system is provided as a kit of parts which is retro-fitted to premises having a pre-existing fuelburning appliance.

34. A ventilating system as claimed in any of Claims 14 to 33, including a monitor for sensing the rate of exhaustion of combustion gases.

35. A ventilating system as claimed in Claim 34, including means responsive to the monitor to terminate the combustion of fuel under specified conditions.

36. A ventilating system as claimed in any of Claims 14 to 35, including a temperature sensor arranged to monitor the -temperature of gases entering the heat exchanger.

37. A ventilating system as claimed in Claim 36, including control means responsive to the temperature sensor to shut down the system if the temperature falls outwith defined limits.

38. A ventilating system substantially as hereinbefore described with reference to the accompanying drawings.

39. A method of ventilating premises, substantially as hereinbefore described with reference to the accompanying drawings.