This invention relates to a combustion devi and flue and particularly to an improvement by which t possibility of back draft, that is the reversal of t normal flow of gases in the flue, is prevented or least reduced.

Various combustion devices are availab mounted within buildings in which the heated gases gene ated in the combustion are extracted from the building a flue which extends from the combustion device to outlet in the exterior of the building. In some cas the flue gases are assisted or driven in their moveme by a fan arrangement or a flow induction arrangemen This technique is often used in cases where increas heat of combustion is required, in cases where the tem erature of the flue gases is reduced by the extraction heat to a level where they cannot themselves sustain t necessary convection current, or in cases where it desired that the combustion device operate without conventional flue stack which is of course necessary generate the updraft. Examples of these devices a shown in U.S. patents 4,262,608 (Jackson), 4,512,2

(Crawford), 2,979,322 (Dailey) , 3,134,345 (King), 2,951,457. (Kneass) , 4,149,453 (Reed), 3,570,423 (Hemmingson) , 3,527,177 (LaRue) , 1,689,241 (Haber) , and 4,424,792 (Shimek) . In addition similar examples are shown in Canadian patents 487,573 (Campbell), 514,680 (Edwards), and 317,336 (Delamere) .

Further patents use a device for assisting the flow through a flue by generating an air stream in the forward direction which induces increased draft. Examples of these devices are shown in U.S. patent 1,604,271 (Friedman) and 3,175,552 (Sutton) . Friedman uses the device to increase combustion when required by a fan which injects air into the combustion chamber and part of that air is bypassed into the flue to increase the draft within the flue. Sutton provides a device which increases the air flow through the flue by an in¬ duction system in view of the fact that the normal com¬ bustion gas flow is inhibited by heat exchanger posi¬ tioned in the fireplace.

Other patents relating to steam locomotives show various induction arrangements for improving the combustion. Examples are shown in Canadian patents 239,344, 384,749, 229,808. 216,732, 229,807.

The present invention is concerned with conven

tional furnaces or woodstoves which generally will when the combustion is operating at a reasonable level gener¬ ate enough heat in the combustion gases that the flue including a flue stack will cause the updraft to draw the flue gases away from the combustion device.

Conventional chimneys are intended to conduct combustion gases and smoke particles out of the building. However, they can just as well provide a path for move¬ ment of air into the building if the air pressure inside is less than the pressure outside. Interior air pressure can become reduced relative to ambient pressure if air is exhausted by bathroom, kitchen and clothes dryer fans and building heating equipment faster than it can be replaced by infiltration of outside air. In such cases, "combus¬ tion-venting failure" occurs and smoke is drawn into the building rather than moving along the flue. This can affect health and even cause death due to poisoning by gases such as carbon monoxide or due to fire started when flames or embers are drawn into a room. The problem occurs most often when a fire is first lit or when it is dying because the small fire at that stage does not heat the flue enough or produce sufficient hot gases to over¬ come pressure differences.

When the flue and the flue gases cool suffi-

ciently, particularly in cold outside temperatures, the convection effect on the vertical column of air in the flue can reverse, causing the movement of air in the flue to reverse in direction due to the fact that it is colder than the interior air of the building. This can cause an inflow of the cold air which may sweep the combustion materials from the combustion device into the building.

This problem is widely experienced particularly where the flue is exterior to the building and hence more susceptible to cooling due to the surrounding cold temp¬ eratures.

Despite the wide problem, little effective sol¬ ution is available at the present time. Attention has been given to the problem of pressure equalization in that in many cases the household now provides an inlet for outside air to ensure that the pressure inside the building does not significantly fall below that of the outside ambient air pressure. However this by itself does not solve the problem since the cooling effect on the flue gases itself can generate the reverse flow or back draft.

The above prior art patents show various de¬ vices for inducing air flow in the flue but these gener¬ ally operate continuously or are operated manually to

improve or increase combustion temperatures. SUMMARY OF THE INVENTION

It is one object of the present invention, therefore, to provide an improved combustion arrangement which prevents or inhibits the development of back draft in the combustion flue.

According to the present invention, therefore, there is provided a combustion and flue device comprising a combustion device mounted within the interior of a building, a flue leading from the combustion device and including at least a part thereof which rises to an out¬ let communicating with the exterior of the building at a height above the combustion device and arranged such that heat in the combustion gases generated by the combustion device is, at least part of the time, sufficient to gen¬ erate a forward flow of the gases in the flue from the combustion device to the outlet, a nozzle in the flue arranged to inject gases into the flue in a forward direction to tend to induce flow of the gases in the flue in the forward direction, a duct communicating from an inlet means to the nozzle, fan means in the duct for gen¬ erating a flow of gases from the inlet means to the nozzle, heating means in a duct for heating the gas as it travels from the inlet means to the nozzle, and means

responsive to a back draft condition in the flue to actu¬ ate said fan means and said heating means.

The back draft hazard is therefore prevented by injecting heated air into the flue once the back draft condition is detected. This hot air is directed toward the outlet of the flue to achieve two effects:

1. Air is drawn through the stove or other heating unit and up the chimney as of a result of the induction effect of the nozzle to restore combustion.

2. The air in the vertical column of the flue is quickly reheated thus reversing the down flow carried by the convection currents and reverting to the proper upward convection flow in the flue.

With the foregoing in view, and other advan¬ tages as will become apparent to those skilled in the art to which this invention relates as this specification proceeds, the invention is herein described by reference to the accompanying drawings forming a part hereof, which includes a description of the best mode known to the applicant and of the preferred typical embodiment of the principles of the present invention, in which: DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration partly in elevation and partly in cross-section of an attachment

unit according to the invention for attachment to a con¬ ventional combustion system.

Figure 2 is an isometric view on an enlarged scale of the nozzle by which the air is injected into the flue.

Figure 3 is a cross-sectional view of the nozzle of Figure 2.

Figure 4 is schematic illustration partly in cross-section and partly in elevation of the whole com¬ bustion apparatus including the attachment of Figure 1.

Figure 5 is an isometric view of the inlet con¬ trol device shown schematically in Figures 1 and 4.

In the drawings like characters of reference indicate corresponding parts in the different figures. DETAILED DESCRIPTION

Turning firstly to Figures 1 and 4, a conven¬ tional combustion device preferably a woodstove indicated at 15 and is connected to a flue duct 10 which communi¬ cates combustion gases from the combustion device 15 through an exterior wall of the building to a vertical flue stack 13 on the exterior of the building. This con¬ struction is strictly conventional and generally combus¬ tion in. the combustion device 15 is sufficient to gener¬ ate an updraft through the flue so that the combustion

gases are drawn away from the combustion device to sustain combustion and to ensure that no combustion products escape into the building.

However in situations where the interior pres¬ sure of the building drops or the combustion is insuffi¬ cient to maintain heat in the flue gases at a sufficient level to maintain the upwardly moving column of the gases in the stack 13, the flow can reverse. As soon as the column of air in the stack 13 becomes a column of cold air this will immediately overcome any remaining upward pressure and will cause the air to flow downwardly into the building ejecting the combustion products from the combustion device through the normal air intake open¬ ings.

The device according to the invention therefore includes a sensor 9 which comprises a conventional thermo couple for detecting the temperature of the air within the duct 10. The thermo couple can be adjusted to set the temperature at a position which is determined by simple experimentation. The sensor 9 is therefore responsive to the back draft actually commencing or to a preliminary stage at which is determined by experimenta¬ tion that the back draft will shortly commence. The sen¬ sor 9 communicates with a control unit 14 which is

supplied with power from a conventional 110 vac service Indicated at 16.

In order to prevent the back draft commencing or to restore a forward draft condition shortly after the back draft has commenced, there is provided an air injec¬ tion system. The air injection system comprises a duct 11 which connects to a nozzle or venturi device 1 mounted within the duct 10 of the flue. The duct 11 connects to an elbow 12 and to a horizontal portion of pipe which is generally located in the basement of the building in which the combustion device 15 is located. The horizon¬ tal pipe includes a heater 2 which in one example can be a 1,000 w electric heater of a type which can be simply mounted in the duct or can include a separate housing communicating with a part of the duct. Upstream of the heater 2 is provided a fan schematically indicated at 3 which acts to drive air through the duct into the nozzle 1. Upstream of the fan 3 is provided an inlet air regu¬ lator generally indicated at 17. This allows inlet air into the duct to be driven by the fan through the nozzle 1.

The nozzle is shown in more detail in Figures 2 and 3. From these figures it will be seen that the duct portion 10 is circular in cross-section with only the

lower wall portion being illustrated.

The nozzle 1 comprises an arched wall extending over a bottommost portion of duct 10 and defining there¬ with a duct portion longitudinal of the duct 10 and directed toward the stack 13. The nozzle or venturi air director 1 is connected to the duct part 11 which extends through an opening in the wall of the duct 10.

The flow control unit 17 is shown in more de¬ tail in Figure 5. The device comprises a housing which is rectangular in shape and is divided by a central wall 18 into two separate housing parts 4 and 5. One end wall of the part 4 includes a collar 19 for attachment to the duct upstream of the fan 3. The opposed end of the hous¬ ing includes a collar 20 for attachment to an outside air connector 8 which projects through the outside wall of the building and includes a hood on the outside to allow the ingress of air from the exterior of the building.

A front wall of the housing is divided by the separator 18 into two parts. Each of the parts has an opening provided therein of a size approximately equal to that of the collars 20 to allow air flow which is sub¬ stantially unrestricted. A conventional flap valve 6, 7 is provided in the opening to control flow of air through the opening. The flap valve 6 is arranged to allo

freely air to flow from the exterior of the building into the interior of the building but to prevent air flow in the reverse direction. The flap valve 7 is arranged to allpw flow of air from the interior of the building into the duct to feed the fan 3.

In operation, on detection of a back draft con¬ dition by the sensor 9, the control unit 14 actuates both fan and the heater to inject air through the nozzle into the flue. The air is injected in the direction longitu¬ dinal and forwardly of the flue so as to tend to generate a flow in the forward direction. In addition the air is heated so that the forwardly moving air also generates convection currents when that forwardly moving hot air reaches the stack 13.

The operation of the fan and the heater can be terminated either by detecting a rise in the temperature in the flue to a predetermined higher temperature, or by a timer. Thus, when the back draft condition is removed, the injection system is halted to allow the normal com¬ bustion process to continue.

The air feeding the fan 3 is pulled into the flow control unit 17 through the opening in the front wall as controlled by the flap valve 7. If the air

_• pressure in the interior of the building is sufficient,

the air is drawn fro the interior of the building and directed to the flue. If the air pressure in the build¬ ing has fallen, air is drawn into the interior through the outside duct 8 and via the flap valve 6. As the openings in the front face are arranged closely adjacent, when exterior air is drawn into the building for supply to the flue, that exterior air passes substantially directly from the opening in the front face connectin with the outside duct 8 to the opening in the front face connecting to the fan. Thus little cold draft is gener¬ ated in the area of the flow control device 17.

However it will be appreciated that the valves 6 and 7 can operate independently. If there is no nee for the valve 6 to open, air is drawn into the unit fro the interior of the building and this can act to reduc concentrations of noxious or dangerous gases in the base ment area, for example, radon. When the fan 3 is no operating, the flap valve 6 can control ventilation int the basement to maintain the pressure within the buildin substantially at atmospheric pressure and this by itsel will to some extent reduce the likelihood of the bac draft condition developing.

Since various modifications can be made in m invention as hereinabove described, and many apparentl

widely different embodiments of same made within the spirit and scope of the claims without departing from such spirit and scope, it is intended that all matter contained in the accompanying specification shall be in¬ terpreted as illustrative only and not in a limiting sense.