A flue restrictor to control air flow in a combustion apparatus having an exhaust system that includes a flue. The restrictor has a housing to be attached in the flue. There is a first valve member located in the housing and a second valve member located in the housing. Each valve member is composed of open and closed areas, and cooperates with the other valve member to act as a valve. Movement of one valve member relative to the other controls air flow in the restrictor, and thus in the combustion apparatus. The restrictor provides sensitive control and ease of adjustment in position.

FIELD OF THE INVENTION 
This invention relates to a flue restrictor for use on a combustion 
apparatus, usually a furnace, boiler or water heater. 
DESCRIPTION OF THE PRIOR ART 
The gas or oil burning furnace that finds wide application in central 
heating systems in the United States and Canada is a commendably efficient 
mechanical device. In the Northern United States and Canada such a furnace 
will typically be in daily use for six months of the year yet for the 
first ten or fifteen years of its life the only maintenance that is 
necessary is usually the occasional application of a small amount of 
lubricating oil and the annual changing of the filter. 
From a combustion point of view the furnace is less commendable. When 
tested under laboratory conditions the typical efficiency is about 80% but 
field tests of a typical furnace boiler or water heater show that the 
actual working deficiency is closer to 50%. However even here the fault 
lies not with the furnace but with the exhaust or venting system through 
which the combusted gases pass to atmosphere. 
It has been recognized for some time that this efficiency loss stems from 
the venting system allowing too much air to flow through the applicance. 
When a combustion device is running the gases flow from a combustion 
chamber, through a heat exchanger, into the flue and then into the 
atmosphere. Air is fed to the combustion chamber for the combustion but 
the air fed is not in any way controlled in the usual combustion apparatus 
and this is the basis of the problem. Excess air lowers the temperature of 
the flue gases flowing through the heat exchanger. Furthermore the greater 
the draft of the flue then the more air is present to dilute that used by 
the burner and less heat reaches the heat exchanger. 
The excess gas introduced is also undesirable in the actual combustion 
step. The excess air upsets the desirable ratios for combustion so that 
unburned fuel escapes into the flue. 
Even when the combustion apparatus is not in operation air can still flow 
rapidly through the furnace causing a further loss of heat by taking heat 
from the apparatus as well as from the surroundings and exhausting that 
heat up the flue. 
It is therefore clear that excess air flow through the combustion apparatus 
is undesirable. What is desirable is an optimum flow, able to support 
combustion at the maximum possible level and not so great that inefficient 
combustion and heat loss are induced. 
Attempts at solving the above problems include those set out in Canadian 
Pat. Nos. 1,119,497 and 1,134,229. 
Unfortunately these devices, although obviously helping to solve the 
problem, do so in a relatively unsatisfactory way. In particular although 
they are adjustable the adjustment means is relatively imprecise and the 
actual adjusting step laborious. Although they act to restrict the air 
flow through the combustion apparatus they do so in a way that cannot be 
fine tuned for an individual apparatus. 
SUMMARY OF THE INVENTION 
The present invention seeks to provide an apparatus in which fine tuning of 
the air flow through the combustion apparatus can be achieved. 
Accordingly the present invention provides a flue restrictor to control air 
flow in a combustion apparatus having an exhaust system that includes a 
flue, the restrictor comprising a housing to be attached in the flue; a 
first valve member located in the housing and a second valve member 
located in the housing, each valve member being composed of open and 
closed areas, and cooperating with the other valve member to act as a 
valve whereby by movement of one valve member relative to the other, air 
flow in the restrictor, and thus in the combustion apparatus, is 
controlled.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the drawings FIG. 1 shows a conventional furnace 2 having an air inlet 
4. No details of the interior of the furnace are shown. They are entirely 
conventional. The furnace is fitted with a vent 6, typically a five, six 
or seven inch pipe. Only the first part of the vent, generally known as 
the flue, is shown in FIG. 1. Further although a furnace is shown the 
restrictor of the invention is useful with related devices such as water 
heaters and boilers. 
In the flue 6 a restrictor 8 according to the present invention is 
attached. 
As shown in more detail in FIG. 2 the restrictor 8 comprises a housing 10 
attached in the flue 6. The housing 10 comprises end pieces 12 having 
projections 14 to engage flue 6. In accordance with conventional practice 
the upstream projection fits outside flue 6 and the downstream projection 
14 fits within flue 6 to facilitate gas flow. The lower projection 14 is 
downstream of the upper projection 14, that is flow is upward. 
There is a valve 16 associated with the housing 10. 
In the illustrated embodiment of FIG. 2 the valve comprises a first valve 
member 18 that is fixed within housing 10 by flaps 20 that are attached to 
housing 10 by spot welding. The valve is shown in FIG. 3. A second or 
lower valve member 22 is pivotable within the housing 10 and, as shown 
most clearly in FIG. 3, lever 24 extends outwardly from the valve member 
22 to extend out from the housing 10. The lever 24 extends through slot 26 
in the housing 10. 
As shown particularly in FIG. 3 each valve member 18 and 22 comprises a 
plate. In the embodiment of FIG. 3 the plate has a central opening 28 
formed within an inner ring 30. Discrete fins 32 extend outwardly from the 
inner ring 30 and there are spaces 34 between each fin 32. Generally 
speaking the area of a fin 32 is the same as the area of space 34. Second 
valve member 22 is generally the same as the first valve member 18 and 
differs only by having lever 24 and being without tabs 20 although a 
single tab 36 is desirable to assist in keeping the valve member 22 
aligned in housing 10. 
The arrangement of fins 32 and spaces 34 ensures that as the valve member 
22 is rotated the valve moves from a fully open position, the solid line 
position shown in FIG. 3, to a fully closed position, the broken line 
position for the lower valve member 22 in FIG. 3. 
The embodiment of FIG. 4 differs from that of FIG. 3 by the provision of a 
central hub 38 for each valve member. Discrete fins 40 extend outwardly 
from the central hub 38 and there are spaces 42 between each fin 40. Again 
the fully open position is shown in solid lines in FIG. 4, the fully 
closed position is shown by the use of broken lines for the lower valve 
member in FIG. 4. The tabs 20 and 36 are as in the FIG. 3 valve member. 
It should be noted from both FIGS. 2 and 5 that the restrictor 8 is larger 
in cross section than the flue 8 that receive the restrictor. This is done 
to ensure that when the valve members are in the fully opened position the 
flow through the flue restrictor is the same as though the flue restrictor 
were not present. 
The restrictor of the present invention is useful either in existing 
heating systems, or, may, of course, be installed when the heating system 
is installed. Installation is simple. The existing system is cut and part 
of the exhaust system removed, sufficient to enable the device according 
to the present invention to be inserted. The device may be screwed or 
riveted in position, using conventional techniques. Typically screws or 
rivets will be inserted at the overlapping parts of the system at the top 
and bottom of the restrictor. 
Once the device is installed and valve member 22 is moved so that the flow 
through the restrictor is at its maximum, that is the valve is fully open. 
A draught reading is taken, using a conventional flow meter, below the 
restrictor 8. If the draught is excessive, as would be the case in a 
conventional system, then the valve member 22 is moved by pushing lever 24 
to ensure at least partial overlap of the fins 32. This decreases the 
draught and thus the air supply. When optimum efficiency is reached the 
member 22 is locked in position by bending down the lever 24 and locating 
it, for example by riveting, to the exterior of the housing 10. 
The optimum flow for highest efficiency for any one furnace can, of course, 
be determined from known figures. Such information is available in tables 
produced by various authorities. 
It has been found desirable to arrange the area of the spaces 34 of the 
valve members 18 and 22 to ensure that flow through the restrictor can 
vary from 30% to 100% of the flow through an unrestricted exhaust system. 
Of course figures above 100% flow through the restrictor can be reached 
but there is no point in exceeding that figure. Figures below 30% are 
usually prohibited by local authorities as below that figure exhaust fumes 
can easily be forced back into the building. 
The embodiment of FIG. 5 functions precisely as the embodiment of FIGS. 2 
to 4. The embodiment of FIG. 5 differs from that of FIG. 2 in comprising 
an upper housing 41 and a lower housing 43 that are rotatable relative to 
each other. Each housing has located within it a valve member, for example 
as shown in FIGS. 3 and 4. However unlike the FIG. 2 embodiment, each 
valve member is located, within its respective housing, for example by the 
provision of flaps 44 that are riveted to the housing. There is a bead 46 
provided on the lower housing 43 to control the depth of telescoping of 
the housings 41 and 43 and, in particular, to ensure the proper location 
of the valve members. 
To use the device of FIG. 5 the restrictor is installed in a system as for 
the embodiment of FIG. 2 but the housings 41 and 43 are not located within 
the system. The housings are then rotated relative to each other to ensure 
that the most efficient flow is achieved, again by taking simple flow 
measurements using conventional, prior art equipment. Once the position is 
achieved the housings are each located within the exhaust system, for 
example by riveting. 
The present invention reduces excess air flow through a furnace. Only the 
optimum amount of air for combustion is allowed to flow. Furthermore when 
the burners are switched off the flow is restricted and losses of heat due 
to draught are thus reduced. When the furnace is not combusting the heat 
loss is not as rapid because the air flow is not as rapid. 
The illustrated devices may be made of the usual galvanized sheet metal 
common in gas fittings.