Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to furnace controls and in
particular to the control of the flow of air through the furnace
by closing a flue damper or air valve in the primary air duct
to the burner thereby to reduce heat losses up the stack when the
furnace burner is not operating.
Present oil burning furnaces are provided with a
barometric stack control in the form of a draft regulator or
damper on the flue pipe to ensure that vacuum or draft at the
burner does not exceed a specified amount. This control is
automatic and it admits ambient air from the furnace room into
the stack when the draft is considered too great and reduces the
amount of raw air into the stack as the draft or vacuum on the
burner decreases.
Present gas furnaces are provided with secondary air
inlets to the combustion chamber and an unrestricted opening in
the flue pipe. These presently practiced draft controls are
wasteful and unnecessary, since a large percentage of the oxygen `~
in the air that is admitted to the furnace system is not required
for the combustion of the fuel. The amount of air to the furnace ¦
burner is preset to give proper combustion.
All heating installations are designed to provide ample
heating capacity for maximum requirements to be encountered and
the burner, therefore, is only in operation for a small
percentage of time for a given period. With the amount of air
to the furnace burner being manually preset, it will be
appreciated that there is a large loss of heat through the furnace
and up the stack when the burner is inoperative.
In order to reduce heat losses in the stack under the
above conditions, the position of the flue damper or the primary
air inlet valve must be controllable and in accordance with the
present invention the opening and closing of the damper or the
primary air inlet valve is operated by motor means which is
actuated in response to room temperature through a thermostat.
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The position of the motor output shaft, that is directly
related to the damper angle, controls the switch which actuates
the furnace burner. Moreover, a delay in closing the damper -
or the primary air intake valve after the burner shuts off is
included in the control so that purging time is provided to rid
the combustion chamber of the furnace and the flue pipe of
combustion gases. This purging time can be varied in duration
by means of adjustments to suit whatever conditions that are
encountered in specific burner installations.
The general concept of furnace damper control is known,
U.S. Patent 2,085,912 of July 6, 1936 to J.K. Lencke being one
example. However, the Lencke patent discloses the use of a
solenoid type of motor to activate the damper, a very abrupt
and instantaneous method of operation. While such a method may
be acceptable for starting a burner, it does not provide any
purging time in closing.
In the present invention I provide an important
improvement over the prior art by including means in the control
to give a variable "purging" time between the shutdown of the
burner and the stopping of the flow of air. The draft control
consists basically of a thermostat to actuate an electrical
mechanism to open and close the flue damper or primary air inlet
valve and a switch responsive to damper positioning to activate
the fuel burner. In one embodiment a three wire type
thermostat energizes two circuits, the first being closed, with
the second open, when calling for heat and the second circuit
being closed, with the first open, when heat is not required.
The electrical mechanism such as a small motor to operate the
stack damper, or primary air inlet valve is actuated by the
thermostat. When the thermostat calls for heat, the motor is
energized to open the stack damper or inlet valve and when the
second circuit is energized at the time when heat is no longer ;
required, the mechanism closes the stack damper or inlet valve.
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The limit switch on the motor output axis closes the circuit to
the burner when the damper or air inlet valve is moving to an
open position and, conversely, opens the circuit to the burner
when the damper or inlet valve is moving to a closed position.
It is important that the flue pipe have a minimum of
openings, and no secondary air inlets to the combustion chamber.
The only openings that would be acceptable in the flue pipe are
those resulting from the fit of standard galvanized smoke pipe.
In accordance with a broad aspect, the present invention
relates to an automatic draft control for furnaces having a
burner and a damper in the exhaust stack between the furnace and
the chimney or a primary air duct valve to the burner. The draft
control effectively interrupts the flow of air through the
exhaust stack when the burner is not in operation and it
comprises motor means operatively connected to the damper or ~ -
primary air duct valve to the combustion chamber for opening and
closing the same; a temperature responsive switch to actuate the
motor means; switch means responsive to the position of the -
damper or the primary air duct valve for actuating the burner; and ~
20 means for providing a delay to a) the actuation of the burner -
from the beginning of the opening of the damper or air duct
valve and b) the closing of the damper or air duct valve from
the stopping of the burner.
The invention is illustrated by way of example in the
accompanying drawings wherein:
Figure 1 is a diagrammatic sketch of one embodiment of
the mechanism for operating the burner switch responsive to
damper or air duct valve position;
Figures 2a and 2b illustrate another embodiment of the
burner switch actuating means;
Figures 3a and 3b are schematic views of another means
of actuating the burner operating switch;
Figure 4 is an elevation view of a preferred form of
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motor used for controlling the position of the damper or the
primary air duct valve;
Figures 5a through 6b are side and end elevations
respectively of two forms of mounting the burner actuating
switch on the motor of Figure 4;
Figure 7 is a schematic wiring diagram of the control
system;
Figure 8 is a schematic illustration of the present
invention applied to a gas fired, intermittently operated .
furnace, and
- Figures 9 and 10 are end and plan views respectively
of Figure 9.
As shown in the schematic diagram of Figure 7, a
typical home type furnace 1 is provided with a burner 2 and a -
draft damper 3. The movement of the damper 3 is operated
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through interconnection with an electric motor 4 of suitable -~
size and power. I have, for example, successfully used a
Minneapolis Honeywell Electric Janitor motor, type M26A, lOJ4,
20 volts, 8 watts, but any equivalent motor will do. Motor 4 is
connected to a three wire type thermostat 5 which, as usual, is
positioned in the "living" area of the house. Switch means,
shown here in the form of a mercury switch 6, is operatively
connected to the damper 3 and is electrically connected to the
burner 2.
Functionally, the thermostat 5 can energize the two
illustrated circuits. The first circuit C-l is closed (with the
second circuit C-2 open) when the thermostat 5 calls for heat; ;
and the circuit C-2 is closed lwith C-l open3 when heat is not
required. Motor 4 which operates the damper 3 is operated by the
thermostat 5 so that a) when the thermostat 5 calls for heat,
motor 4 is energized to open the stack damper; b~ when the
damper 3 is moving to its open position, the switch means 6
- actuates the burner 2; c) when the thermostat 5 energizes the
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second circuit, when the desired room temperature is reached, the
motor burner 2 is shut off when motor 4 moves the stack damper
3 towards its closed position. It is important to appreciate
- that, following the shut-off of the burner 2, the action of the
motor 4 provides a delay in closing the damper 3 so as to give
ample time for purging of gases up the stack from the
combustion chamber of the furnace 1.
Referring now to Figure 1 there is schematically shown
one form of mounting and operating the burner switch 6 by means
Of a single cam 7.
The mercury switch 6 is securely positioned in a frame
1~ which is pivotally mounted at 12. Frame 10 is oscillated
by means of a pin 14 on its lower end being engaged by a yoke
16 on one end of a bell crank 18. The latter is pivotally
mounted at 20 and, at its other end, is provided with a cam -~
follower 22. As shown, the follower end of the bell crank 18
may be held in contact with the cam 7 by means of a spring 24 1
connected at one end to the crank 18 and at the other end to a I
stationary point 26.
Cam 7 is attached, for example, to a flange 28 on the
; output shaft 30 of motor 4, not shown. By using a single cam 7
as illustrated, only one setting could be selected for purging
purposes. By example, the line X-Y in Figure 1 represents the
position of the output shaft 30 when the damper 3 is in a
fully closed position. Cam 7 would permit an angular movement
of the damper shaft equal to angle X 0 B before the follower 22
dropped into the cam profile to cause ignition of the burner.
Likewise, after the burner 2 is shut off, the cam 7 would
permit an angular movement equal to C 0 Y of the damper for
purging purposes before closing of the damper. The "shaded" --
areas on the cam 7 represent portions which could be removed to
alter the timing of the unit, i.e. to give angle X 0 A as the
dwell before ignition of the burner from the closed position of ;~
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the damper 3 and the dwell for purging would be reduced to
angle Y 0 D.
In Figures 2a and 2b a pair of symmetrical cam plates
32, 34 provide a variable type of profile. In Figure 2a the
discs or plates are shown in a position to give the maximum
amount of time that the burner 2 can be operated during the
period when the damper is moving from a completely closed to a
completely open position. Accordingly this would be equal to
150 of the angular movement of the damper 3 thereby leaving
15 for partial opening of the damper before the burner starts
and 15 for the damper 3 to be partially open when the burner 2
shuts off, for purging purposes. In Figure 2b, the two plates
32, 34 have been moved relative to one another to give only
90 of the angular movement of the damper shaft for the burner
2 to be operating, leaving 45 on either end of the cycle for
opening the damper for accepting products of combustion or for
closing the damper for purging time. It will be appreciated
that any position between the above extremes would be available
simply by altering the relative positions of the discs 32, 34
which can be releasably secured together by various known means.
Figures 3a and 3b illustrate another embodiment of
actuating the mercury switch 6. As shown in Figure 3b the motor
4 has extensions to either end of its output shaft 30. On one
end, the mercury switch 6 is mounted in a T-shaped frame 36 which
is pivotally mounted at 38, the free end of the frame being
pivotally connected at 40 to one end of an adjustable length
connecting rod 42 which has its other end pivotally connected
at 43 to a crank 44 mounted on the output shaft 30. As seen in
Figure 3a, the other end of shaft 30 is provided with a slotted
crank 46 interconnected with the damper 3 through a rod 48 and
slotted damper arm 50. It will be observed that the length of
time that the burner 2 is in operation during the period that
the damper 3 takes to move from a closed to an open position,
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can be altered to suit any conditions that may be encountered ¦-
in practice merely by changing the length of the connecting -
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rod 42.
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Another, preferred embodiment of the invention is ¦
shown in Figures 4 through 6b, where a reversing type motor is ¦
used which will rotate in opposite directions when energized ¦~
by the two separate circuits C-l or C-2 from the thermostat 5.
In Figure 4, the motor 52 has an output shaft which is connected
at one of its ends 54 to the damper 3 by means of a crank arm ¦ ~ -~
10 and connecting rod similar to 46, 48 of Figure 3a. The mercury
switch 6 is mounted on the other, auxiliary end 56 of the output ¦
shaft by way of a spring clip 58 which is attached to a collar -;~
60 that, in turn, is secured on the end 56 of the shaft by set
screws 62 or the like. Another form of securing the switch
6 to the motor shaft is shown in Figures 6a and 6b wherein a
pair of half collars 64, 66 are secured onto the shaft 56 by
bolts 68 which also retain a pair of clips 72 securely holding
the switch 6 therebetween.
In this embodiment, the stroke or angular movement
20 of the output shaft 54 would be about 90 and the time to move
through this angle should be about 15 seconds. This will
provide ample time for purging the combustion chamber and to place
the damper 3 in a sufficiently open position when the burner is
ignited. It will be noted from a comparison of Figure 3a and I `
Figure 5b that, when a reversing type motor is used, the ,
activating mechanism for the mercury switch controlling the
operation of the burner 2 is substantially simplified.
The present invention is applicable also to gas fired
intermittently operated furnaces wherein the control of the -~
30 invention is applied to the air supplied to the burner. By
virtue of the fact that most gas fired furnaces require the use of
a pilot flame instead of a high tension spark as in the case of
an oil fired furnace, combustion gases are being created all of
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I the time that the furnace is in operation and therefore one
cannot restrict the combustion gases through the flue pipe.
In a gas fired furnace utilizing my invention, the flue pipe
from the furnace to the chimney should be as airtight as
possible and all secondary inlets to the combustion chamber
should be eliminated. An opening to the combustion chamber
should be provided to permit visual inspection of the flame and
manual purging when required. This opening should be closed -
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by means of a door seated against the outside frame of the
furnace and hinged from the top.
In Figure 8, a gas fired, intermittently operated
furnace 74 has a service door 76 and a primary air duct 78 -
with a movable valve 80 therein, associated with a main gas
pipe 82 feeding a nozzle 84.
It will be appreciàted that the air requirements for
the pilot flame are extremely small and, as shown in Figure 8, ¦~
this air can be supplied via a duct 87 of small diameter
through which the tube 86 supplying the fuel to the pilot is
` located and that discharges beneath and uniformly around the
pilot light. This duct 87 is open at all times when the furnace
is operating and, to insure this, the valve 88 controlling the
duct 86 should be interlocked with the main electrical supply to
the furnace. The primary duct 78 which supplies air to the
main burner of the furnace is equipped with a valve 80 to be
operated in the same manner as the flue damper control described
with regard to Figures 4-6b.
Accordingly, a motor 90 is connected to the valve 80
by means of a slotted crank 92 mounted on one end 94 of the motor
output shaft and an adjustable connecting rod 96. A mercury
switch 98 is mounted on the other end of the motor output shaft
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in the same manner as figures 4 through 6b for operating the
burner.
It will be appreciated that whether the invention is
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used on an oil fired or gas fired furnace, the principle of
the invention is to control the amount of air going through
` the furnace when the burner is not operating. Therefore, the f
terms and expressions that have been used in the abstract and .
disclosure have been used as descriptive terms and not terms
of limitation and there is no intention in the use of such ~-~
terms or expressions to exclude any equivalents of the features
shown and described or portions thereof but it is recognized :
that various modifications are possible within the scope of the
invention claimed. :
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