CA1110499A - Positive automatic draft control - Google Patents

Abstract

POSITIVE AUTOMATIC DRAFT CONTROL
Abstract of the Disclosure A furnace control means is provided for reducing heat losses in the chimney stack to a minimum. The flow of air through the furnace and the flue pipe is controlled by a means to interrupt the flow of air during the period when the burner is not in operation and the usual leakage of combustion gases or the combination of such gases and air from the flue pipe to the furnace room, through the use of such means, is prevented by the use of an airtight radiator and flue pipe up to the means location.

Description

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This invention relates to furnace controls ana 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 ~urnace 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 ;
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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 draf$ or vacuum on the burner decreases.
Present gas furnaces are prov~ded with secondary air inlets ko the combustion chamber and an unrestricted opening in the ~lue pipe. These presently practiced dra~t controls are wastefuL and unnecessary, since a large pércentage 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 t~me ~or a given period. With the amount of air to the furnace burner being manually preset, it will be appreciated that there is a large los5 of heat through the furnace and up the stack when the burner is inoperative.
In order to reduce heat losse~ in the ~tack under the`
above condition~, the position of the flue damper or the primary air inlet valve must be controllable and in accordance with the present in~ention tha opening and closing of the damper or the , ', .
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primary air inlet valve is operated ~y motor means which is actuated in response to room temperature through a thermostat.
The position of the motor output shaft, that is directly related to the damper angle, controls the sw-itch which actuates the furnace burner. Moreover, a delay in closing ~he 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 o~
combustion gases. This purging time can be varied in auration by mèans 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.X. Lencke being one example. However, the Lencke patent discloses the use o~ a solenoid type o~ motor to activate the damper, a very abrupt and instantaneous method of operation. While such a method may be acceptable Eor starting a burner, it does not provide any purging time in closing.
In the present invention I provide an ~mportant 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 o the Elow o air. The dra~t 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 responsi~e 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 circui~
being closed, with the first open, when heat is not required.
The electrical mechanism such a$ a small motor to operate the stack damper, or prim~ry air inlet valve is actuated by the thermostat. When the thermostat calls for heat, the motor is

-2-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.
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 cirruit to the burner when the damper or inlet valve is moving to a closed position.
- When the flow of gases is stopped completely, in accordance with the present invention, it will be appreciated that no openings in the flue pipe up to the damper location will exist.
Any openings in the flue pipe between the damper and the chimney although they may not create a leakage problem would tend to reduce the saving~ available and should be sealed with suitable sealer such as urnace cement or heat resistant tape. These openinss would be those such as joints of galvanized flue pipe from manufacturer's tolerances. A~y other openings to duct work, down- -spout to a cistern,or passageways into partitions to upper floors etc. that tend to permit the circulation of air should be eliminated. It is therefore important to ha~e an air-tight ~lue pip~ from the radiator to the damper. In the prior art, standard galvanized smoke pipe is intended to be used and the customary openings at the joints and seams are accep~ed. Under such conditions however, the leakage of gases from the f~ue pipe to the furnace room could not be prevented when a severe restriction such as a closed damper in accordance with the present invention or even ~ closed damper without a damper seat, were installed in the flue pipe.
Any air in the furnace room that is permitted to ri~e , .: . . ................ . . . . ..... .
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in a more or less unrestricted manner to the outside such as through partitions, duct work connected to the outside or to a chimney etc. should be prevented within prac~ical limits. For ~his reason, it is preferred to have all joints and seams in the standard galvanized smoke pipe from the damper to the chimney to be either sealed with urnace cement or taped with a fire resistant,tape or the like. Tape would be pre~erable at points-where the flue pipe would be required to be dismantled for cleaning purposes. In gas furnace design, no intentional openings in the flue pipe could be permitted between the radiator and the chimney and the standard galvanized flue pipe would be sealed similarly to the pipe in an oil furnace installation between the damper and the chimney. A dual sa~ety control gas 3uppl~ device would be provided in ~eries and this would provide double protection that the gas would,be shut off in the event -that the pilot light was extLnguished.
, $wo other features o,f the present' invention, while not actually necessary, should be provided for maximum savings. The first feature is the iocation of the damper. As the height,of the column of comhustion gases is increased in the flue pipe to the damper location, a differential in static pressure between that in the flue pipe compared to that in the furnace room, increases. Thi~ increase in diferential would eventually become such a serious problem that seals would have to be added , to the clearance openlngs between the damper shaft and the flue - pipe. Accordingly, the damper location preferably ~hould be selected as close to the ~urnace as possi~le thereby el~min~ting flue pipe openiny~ and where the differential of the sta~ic pressure between ~the flue pipe and the furnace room would b~
at a minimum. A Eurther feature is ~he sealing o~ ~11 seams in the standard flue pipe used between the damper and t:he l-himn2y as mentioned abov~.
In accord~nc~ with a broad aspect, the present The invention is illustrated by way of example in the accompanying drawings wherein:
Figure 1 is a diagrammatic sketch of one embodiment of the mechanism for operating the burner switch responsiYe to damper or air duct valve position;
F$gures 2a and 2b illu~trate another embodiment of the burner ~witch actuatiny me~n~;
Figures 3a and 3b are schemat:ic views o another means of actuat~ng the burner operat~ng swit:ch;
Figure ~ is an elevati~n ~iew of a preferred ~orm of motor used for controlling the position of the dampe~ or the primary air duct valves Figures 5a throuyh 6~ are side and end elevation~
respect~vely of two forms o~ mounting the burner actu~ting switch on the motor o~ Figure 4~
Figure 7 is ~ ~chematic wiring diagram o~ ~he ¢ontrol I
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invention relates to an automatic draft contxol for furnaces having a burner and a damper in the exhaust stack between the furnace and the chimney or a primary air duct val~e to the burner. ~he draft control effectively interrup~s the flow of air through the exhaust stack when the burner is not in operation and when used in conjunction with an airtight radiator and flue pipe up to the damper location, prevents any leakage of gases from the flue pipe to the furnace room and it comprises motor means operatively connected to the damper or 10 primary air duct valve to the combustion chamber for opening and closing the same; a temperature responsive switch to ac~uatP
the motor means; switch means responsive to the position of the damper or the primary air duct valve for actuating the burner; and means for providing a delay to a) the aatuation of the burn~r from the beginning o~ the opening o* the damper or ~ir duct valve and b) th~ closing of the damper or air duat valve from the stopping of the burner.
In accordance with a further aspect, the invention relates to the combination of a furnace having mtermitten discharge of products of combus~ion and a ~lue pipe for accepting and channeling the same up a stack, a d~r operably mLunted in said flue pipe for completely inte~n~ting the flow of air ~ugh said flue pipe when said burner is not in operatlonl stop mean~ mounted in s~id flue pipe and positioned to operably engage ~aid damper on closure thereof in a manner to prevent the leak~ge of air through said flue pipe from about the periphery of said damper when said damper i8 closed, motor means operatively connected t~ said da~per for opening and closing the ~ame, a temperature respo~sive switch for actuation of said motor means, switch means ~or actuating said furnace, said switch means being responsiv~ to the position of saild damper, and means for providing a delay to the actu tion of said furnace from the beginning of the opening of said damper and to the ~lo~ing o~ saia ~mper from the de~ctivation of ~Ld furnace.

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, system;
Figure 8 is a schematic illustration o the present nvention applied to a gas fired, intermittently operated furnace, Figures 9 and 10 are end and plarL ~iews respectively of Figure 9;
Figure 11 is a plan view of the clraft control with motor mounted axially on a damper sha~t;
Figure 12 is a sectional view taken along the line 12-12 of Figure 11;
- Figure 13 is an end elevation view of the appaxatus in Figure 11;
Figure 14 is a section view from line 14-14 of Figure 13; and Flgure 15 is a graph illustrating the performance o~
the pre~ent invention.
As shown in the sch~matic diagram o Figure 7 t a typical ~ ~ furnace not shown is provided with a burner 2 and a draft damper 3 operably mounted within flue pipe 13. The movement of the damper 3 is operated through interconnection with an electric motor 4 o suitable slze and powerO I have, ~or example, successfully used a Minneapolis ~oneywell Electric Janitor motor, type M26A, lOJ4, 20 vol~sl 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 "livingn area of the house. Switch means, shown here in the form of a mercury switch 6; is operatively connected to the aamper 3 and is electrically connected to the burner 2.
Functionally, the thenmostat 5 can energize the two illustr~ted circuits. The f~rst circuit C-l is closecl (with the second circui~ C-2 open) when ~he thermostat 5 calls for heat;
and the circuit C-2 is closed (with C-l open~ when heat is not .

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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 second circuit, when ~he desired room temperature is reached, the furnace 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 oE 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 f~rm of mounting and operating the burner switah fi by means o~ a single cam 7.
~ he mercury switch 6 is s~curely positioned in a Erame 10 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 connected at one end to the crank 18 and at the other end to a stationary point 26.
Cam 7 i5 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 purposeq. By example, the line X-Y in Figure 1 represents the position of the output ~haft 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 O B before the ~ollower 22 dropped into the cam profile to cause ignition oiE the burner.
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Likewlse, after the burner 2 is shut off, the cam 7 would permit an angular movement equal to ~ O 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 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 ~a tha disc~ or plates are shown in a position to-give the maximum amount of time that the burner 2 can be operated ~uring the period when the damper is moving from a completely closed to a completely open position. Accordingly this would be equal to 150 o~ the ang~lar movement o~ the damper 3 thereby leaving 15 for partial opening of the damper before the burner stsrts and 15 for the damper 3 to be partially o~en when the burner 2 shuts off, for purging purposes. In Flgure 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 cyc:Le for opening the damper or accepting products of combustion or for closing the damper for purging time. I~ will be appreciAted that any position between the above extremes would be av~ilable simply by altering the relative positions of the discs 32, 34 which can be releasably secured together by ~arious Xnown means.
Figures 3a and 3b illustrate another embodiment o~
actuating the mercury -switch 6. A~ shown in Figure 3b motor 4 is preferably mounted on furnace 1 and is provided with an extended output shaft 30 on which may be supported the swltch and link means ~or operation of d~mper 3. Also, as shown in Figures 3a and 3b, damper 3 is preferably positioned within 4~9 ~-flue pipe 13 at a location closely adjacent furnace 1, at a juxtaposed location whera the differential in the static pressure between the column of air in flue pipe 13 and the ambient air in the furnace room, is the least when aamper 3 is closed. On one end, the mercury switch 6 is mounted in a T-shaped frame 36 which is pivotally mounted at 38, the ~ree end of the frame being pivotally connected at 40 to one end of an adjustahle 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 moves ~om a closed to an open position, can be altered to ~uit any conditions thak may be encountered in practice merely by adjusting the length o~ the connecting ro~ 42.
As shown in Figure 3a, stop means 51 may be provided within flue pipe 13 at juxtaposition to operably engage damper

3 on closure thereof in a manner to prevent the leakage of air through the flue pipe 13 from about the periphery of damper 3.
Another, preferred embodiment of the invention is shown in Figures 4 through 6~, wher~ a reversing type motor ~s used which will rotate in opposite directions when energized by the two separate circuits C-1 or C-2 from the thermostat 5r .
In Figure 4, the motor 52 has an output shaft which is connected at one of its ends 54 to the aamper 3 by means of a crank arm and connecting rod similar to 46, 48 of Figure 3a. The mercury -switch 6 is mounted on the otherl auxiliary end 56 of the output sha~t 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. ~nother form of securing the switch 6 to the motor shaft is shown in Figures 6a and 6b wherein a pair of .
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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 -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 Figure 5b tha~, when a reversing type motor is used, the activating meGhani~sm for the mercury switch controlling the operation'Qf the burner 2 is substantially simplified. ' .
The present invention'is applicable also to gas ired intermittently operated furnaces wherein the contro} o~ the inVention i9 applied to the air supplied to the burner. ~y ' virtue of the'fact that ~ost gas ~ired furnaces reguire the use of a pilot flame instead of a high tension spark as in the case of an oil fired furnac'e, combustion gases are being created ' all of 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 air~ight 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 purg~ ng when required. This Openlng should be ~losed -by means of a door sèated against the outside frame of ~he furnace and hinged from the top.
In F~gure 8, 2 gas fired, intermittently operated furnace 7~ has a service door 76 and a primary air duct 78 with a movable valve 80 therein, associated with a main gas --10-- , pipe 82 feeding a no~zle 84.
It will be appreciated that the air requirements for the pilot flame are extremely small andr 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 ~he furnace is operating and, to ensure this, the valve 88 controll~ng 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 e~uipped with a valve 80 to be operated in the same manner as the flue damper control described with regard to Figures 4-6b. Accordingly, it is important that valve 80 controlling the main air duct 78 should be at leas~
9S~ airtight or pre~erably ~ur~ace ground to give 100~ control.
Accord~ngly, a motor 90 is connected to the valve 80 by means o 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 in the same manner as Figures 4 through 6b-for operating the burner.
Referring now to Figures 11~15, a preferred embodiment of the in~ention i~ illustrated and the flue pipe 113 i8 combined with a short exten~ion 110 of the 8ame flue pipe material used in th~ furnace design. One end of the extension 110 could be extruded to a depth o approximately an inch and a quart~r and to a ~ize that would fit over the existing flue pipe to permit sealing with furnace cement at the joint 114. The naxrower portion 116 of the extension pro~ides space for the dampar shaft 118 and the circular or sem~circular damper stops 120 and to attach stand~rd galvani~ed smoke pipe thereto maXing up the flue pipe from the d~mper to the chimney.

4g9( Figures 13 and 14 illustrate the preferred arrangement of a reversing motor means and damper shaft and stops location.
The motor 121 is a reversing type motor controllable by a single pole double throw switch such as a three wire ~oneywell thermostat with the angular movement controlled by two snap acting switches 122, 124 shown in Figure 14. It will be appreciated however that other electrical circuit motors which are reversible could be used and which could be controlled by a similar switching appaxatus.
The damper shaft 118 is provided adjacent the motor 121 with a contact arm 126 to actuate switches 122 ana 124 in response ko the control means. ~he output shaft 128 of the motor means is received ln one end of,the d~mper sha~t 118 and ig secured therein by a cotter key 130 or the like.
The present dovlce ha~ been tested in an installation in a home with a burner nozzle having a capacity ~ 1 ~.S. gallon per hour and the furnace stack was approximately 30 feet high and the air opening to the burner was set at two and five e~ghths square i~ches to give complete combustion. It wlll be noted from the;graph of Figure 15 which illustrates savings percent against chimney openings in square inches, that to leave a clearanc~ o~ one slxteenth of an inch be~tween the damper and six inch 1ue pipe to prevent binding in the closed position, resulted in a reduction of available savings in excess of ten percent and this is produced with a burner operating approximately twenty-five percent o~ the timeO Ac~ordin~ly, it will be appreciated that the use of semi-circular damper stops such as those shown ~n Figures 11 through 13, are important and their attachment to the flue pipe in such a position as to permit a seat for the damper in the closed position and thereby completely seal the annular opening from a .

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practical point of view.
It will be appreciated that whether the invention is 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 operati.ng. Therefore, the terms and expressions that have been used in the Abstract and disclosure have been used as descriptive t:erms and not terms of limitation and there is no intention in the use o~ such terms or expressions to exclude any equivalents of the features shown and described or portions thereof but it is recoynized that various modifications are possible within the scope of the invention claimed.
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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A positive automatic draft control for intermittently operated furnaces having an air valve, comprising apparatus for completely interrupting the flow of air through the valve to the exhaust stack when the burner is not in operation and an airtight radiator and flue pipe up to the air valve to prevent any leakage of gases from the flue pipe to the furnace room, said apparatus comprising motor means operatively connected to the air valve of the furnace for opening and positively closing the same; a temperature responsive switch to actuate said motor means; switch means, responsive to the position of the air valve, for actuating said burner; and means for providing a delay to the actuation of the burner from the beginning of the opening of the valve and to the closing of the valve from the stopping of the burner.

2. An automatic draft control according to claim 1 wherein said furnace is oil fired and the air valve comprises a damper in the exhaust stack juxtaposed to the furnace.

3. An automatic draft control according to claim 1 wherein the furnace is gas fired and the air valve comprises a damper in the primary air duct to the combustion chamber of the furnace.

4. A draft control accordingt to claims 1, 2 or 3 wherein the temperature responsive switch is a thermostat and the switch means for actuating said burner is a mercury switch operatively connected to the air valve motor means whereby the burner is ignited after the valve has partially opened and is shut off prior to closure of the valve.

5. A draft control according to claims 1, 2 or 3 wherein the means for providing a delay to the burner operation from the beginning of the valve movement includes a profiled cam on said motor means and a cam follower operatively connecting the cam with said burner actuating switch.

6. A draft control according to claims 1, 2 or 3 wherein the means for providing a delay to burner operation from the beginning of the valve movement comprises crank means mounted on the output shaft of said motor, and an adjustable length rod connecting said crank with the valve; the burner actuating switch being operable in response to predetermined movement of the motor output shaft.

7. In combination with a furnace having intermittent discharge of products of combustion and a flue pipe for accepting and channeling the same up a stack, a damper operably mounted in said flue pipe for completely interrupting the flow of air through said flue pipe when said burner is not in operation, stop means mounted in said flue pipe and positioned to operably engage said damper on closure thereof in a manner to prevent the leakage of air through said flue pipe from about the periphery of said damper when said damper is closed, motor means operatively connected to said damper for opening and closing the same, a temperature responsive switch for actuation of said motor means, switch means for actuating said furnace, said switch means being responsive to the position of said damper, and means for providing a delay to the actuation of said furnace from the beginning of the opening of said damper and to the closing of said damper from the deactivation of said furnace.

8. A combination as specified in claim 7 wherein said damper is mounted within said flue pipe at a juxtaposed location adjacent said furnace where the differential of the static pressure between the column of air in said flue pipe and the ambient air in the furnace room is the least when said damper is in a closed position.