Abstract:
The combustion gases of a burner in a heating furnace are discharged to a vent through a draft hood having three ports. The first and second ports communicate with the furnace and the vent respectively and define a first flow path for combustion gases through the chamber of the hood. The third port is open to the ambient atmosphere and defines a second flow path for dilution air through the chamber. Separate temperature responsive, bimetallic elements sense the temperatures in the two paths and cause a vent damper to be moved toward and away from a position in which it closes the second port in response to the sensed temperatures to minimize the flow of dilution air. In a modified embodiment the draft hood is vertically elongate with the bimetallic elements positioned in a zone filling with hot products of combustion for minimizing the flow of dilution air and minimizing spillage of products of combustion during start-up and sudden changes in draft.

Description:
BACKGROUND OF THE INVENTION 
     Gas-fired natural draft appliances, such as the typical water heaters and furnaces in American homes, are equipped with a draft hood which protects the combustion process from excess draft and from backdraft. Excess draft in the vent draws ambient air through the draft hood. In case of backdraft or no draft conditions the flue gases flow from the heat exchanger through the draft hood out of the appliance. Without a draft hood a backdraft would cause incomplete combustion (excessive CO 2  and CO) and might blow out the flame. 
     If the appliance is installed in a home and draws conditioned air for combustion the vent acts like an open window during the standby periods, allowing warm room air to escape, or cold outside air to enter. Automatic vent dampers reduce this loss by closing the vent during the standby periods of the heating cycle. 
     Thermally controlled vent dampers have been used for this purpose for many years. They operate by sensing the temperature prevailing in the damper. In addition to their primary job of closing the vent during standby they adjust the vent opening during the operation of the appliance by flexing to a partially closed position if the bimetallic elements are cooled by a large amount of dilution air aspirated through the draft hood. This modulating function of thermally controlled vent dampers reduces heat loss during the operating period of the heating cycle. 
     While known designs of thermally controlled vent dampers reduce excessive dilution air to a varying degree they cannot optimize this function since they sense the temperature in the vent connector. At that point the vent gases constitute a mixture of the flue gases coming from the heat exchanger and the dilution air coming through the draft hood. The temperature of the flue gases varies widely from one appliance to the other and even in the same appliance during one heating cycle. The automatic adjustment of the amount of excess dilution air (and thereby the reduction of heat loss) to a practical minimum by sensing only the temperature of the flue gas/air mixture is therefore an approximation. 
     OBJECT AND SUMMARY OF THE INVENTION 
     It is a primary object of this invention to provide a new and improved draft control arrangement for combustion apparatus. 
     Another object is to provide a draft control arrangement incorporating a draft hood which operates in a manner to hold to a minimum the inherently wasteful flow of conditioned dilution air through the draft hood into the vent. 
     The invention in summary provides a draft control arrangement for a combustion apparatus which includes a draft hood bounding a chamber and formed with first, second, and third ports spaced from each other. A first flow path through the chamber is defined by the first and second ports, and a second flow path by the second and third ports. A first connector communicates with the first port for admitting combustion gases to the chamber. A second connector communicates with the second port for venting the chamber to a vent. The third port is open to the ambient atmosphere for admitting air to the chamber. A vent damper is mounted in the chamber for movement toward and away from a position in which the damper assembly essentially closes the second port. First and second temperature sensing devices are respectively associated with the first and second flow paths for sensing the temperatures in the associated flow paths. A damper actuating mechanism connects the vent damper assembly to the sensing devices and responds to the sensing of rising or falling temperatures by either one of the sensing devices for moving the vent damper away from or toward that position thereof. 
     Other features, additional objects, and many of the attendant advantages of this invention will readily be appreciated as the same becomes better understood by reference to the following detailed description of the preferred embodiments when considered in connection with the appended drawing in which: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a vertical section view of a gas-fired combustion apparatus incorporating a draft control arrangement of the invention; 
     FIG. 2 is a vertical section view of another embodiment of the invention; 
     FIG. 3 is a vertical section view of another embodiment. 
     FIG. 4 is a partial vertical front section view of another embodiment; 
     FIG. 5 is a bottom plan view of the embodiment of FIG. 4; 
     FIG. 6 is an end view, partially broken away, of another embodiment; 
     FIG. 7 is a vertical section of the embodiment of FIG. 6; 
     FIG. 8 is a vertical section of another embodiment; 
     FIG. 9 is a vertical section of another embodiment; 
     FIG. 10 is a vertical section of another embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring initially to FIG. 1, there is shown a combustion apparatus 10 comprising a domestic heating furnace which incorporates the invention. The walls of the apparatus enclose a combustion space 12 and a heat exchanger 14. A conventional gas burner 15 is arranged under the heat exchanger 14 so that the combustion products of the burning gas heat the air flowing through heat exchanger 14 which functions as a heat transfer medium for room heating. 
     A draft hood 16 is provided and is separated from the heat exchanger by a sheet metal wall 18. A first port 20 defines a flow path from the combustion space into the upper portion of the draft hood. A second port 22 in the top wall of the hood 16 leads to a vent 24. The draft hood encloses a chamber 26 which communicates with the ambient atmosphere through a continuously open third port 28. 
     In the illustrated, non-operative mode of the furnace, a vent damper 30 blocks the port 22. The vent damper 30 comprises a flat sheet-metal plate 32 from which narrow leaf springs 34, 36 depend. The ends of the leaf springs remote from the plate are S-shaped for greater flexibility and are fastened to respective first ends of bimetal strips 38, 40. The other ends of the bimetal strips are fixedly fastened to a bracket 42 in the chamber 30. 
     The bimetal strips 38 are located in a flow path 43 of combustion gases from the combustion zone through the port 20 into the chamber 26. Heating of the strips 38 by the hot gases causes the strips to flex so as to swing counterclockwise (as viewed in FIG. 1) about the bracket 42 into the position indicated by broken line at 38&#39;. The damper plate is moved thereby into a correspondingly indicated position 32&#39; to permit escape of the combustion gases through the port 22 into vent 24. The thermal up-draft in the vent draws a small amount of ambient air through the third port 28 into another flow path 44 to second port 22. The bimetal strips 40 are located in the flow path 44 and as long as the strips 40 are adequately cooled by the air stream they do not change their curvature, nor do they affect the position of the plate 32. 
     If the draft in the vent 24 is insufficient for drawing all of the combustion gases through the partly closed port 22, then hot gases accumulate along a flow path leading from port 20 to the port 28 and heat the bimetal strips 40. The strips 40 then flex and swing clockwise (as viewed in FIG. 1) about bracket 42 to position 40&#39;. This causes the damper plate to fully retract from port 22 to position 32&#34; and thereby increase the gas flow into vent 24. When the hot gases near the strips 40 are replaced by cooler diluting air drawn into port 28 by the draft in the vent these strips flatten and move plate 32 back toward position 32&#39;. 
     The dimensions and thermal responses of the strips 38, 40 are readily selected to minimize the flow of dilution air in each combustion unit. Unnecessary loss of heated or otherwise tempered inside air through the vent 24 is avoided. The temperature of the combustion gases and their amount may be varied by adjusting the gas supply and the air supply to the burner in a conventional manner, but varying combustion conditions in the space 12 do not affect the control of the diluting air by the bimetal strips. 
     The bimetal strips in the specific embodiment of the apparatus shown in FIG. 1 are selected to move the plate 32 toward closure of the port 22 when their temperatures fall below about 150° F., and to move the plate away from the port at higher temperatures. 
     The leaf springs 34, 36 provide yieldably resilient connections between the bimetal elements and the plate 32. They avoid the damage to the bimetal strips and the damper plate that might otherwise result from excessive pressure exerted by the bimetal strips at extremely low temperatures. They also permit the draft control arrangement to be built without hinges, bearings, shafts, or other elements subjected to friction during use, and which would otherwise require periodic lubricating or other maintenance. No external power supply is needed. All connections between the bimetal strips, the leaf springs, the damper plate, and the bracket 42 are fixed, and preferably riveted. 
     FIG. 2 illustrates another embodiment providing a draft control arrangement for a furnace having a heat exchanger 44 and a draft hood 46 and vent 48 similar to the structure described for FIG. 1. A damper plate 50 is carried by leaf springs 52, 54 which in turn are mounted on respective bimetal strips 56, 58. 
     The damper assembly shown in FIG. 2 may be assembled in a suitably shaped frame to facilitate installation in existing draft hoods as a unit. 
     Another embodiment of the invention shown in FIG. 3 is similar to the structure of FIGS. 1 and 2 and includes bimetal strips 60, 62 which extend in approximately right angle directions from the associated leaf springs 64, 66. The bimetal strips are mounted so that the strips 60 extend across a flow path 68 from the furnace to the port 70 which is closed by damper plate 72. The strips 62 are located well below the port 70, and the mode of operation is similar to that described for the embodiment of FIG. 1. 
     FIGS. 4 and 5 illustrate another embodiment of the invention with only the top portion of a typical draft hood being shown in frontal elevation with the front plate of the draft hood removed for clarity. A damper comprising two half-plates 73, 74 is mounted below a vent 75 within the draft hood 76. Each half-plate is carried by respective pairs of leaf springs 77, 78 and 79, 80. The leaf springs of half-plate 73 are connected to upper and lower bimetal strips 81, 82 while the leaf springs of the other half-plate 74 are connected to another pair of upper and lower bimetal strips 83, 84. In operation, the upper bimetal strips 81 and 83 respond to the flow of combustion gases while the lower bimetal strips 82 and 84 respond to either the relatively cold dilution air by moving dampers 73 and 74 toward the vent port or to the heat of the combustion gases if they accumulate in the draft hood by moving damper half-blades 73 and 74 away from the vent port, thereby minimizing the flow of dilution air. The operation is similar to that described for the embodiment of FIG. 1. 
     FIGS. 6 and 7 illustrate another embodiment adapted for use with a typical water heater. A draft hood 86 is mounted above a flue pipe 87 leading from the water heater, and the port 88 in a vent 89 above the hood is opened and closed by a damper plate 90. The damper plate is controlled by three or more bimetal strips, with one or more strips 91 mounted to extend in the flow path of flue gases from the heater, and two or more strips 92, projecting in the flow path leading from the open lower end 94 of the hood toward vent port 88. Leaf springs 95, 96 connect the respective bimetal strips with the damper plate. The operation of the embodiment of FIGS. 6 and 7 is similar to that described for the embodiment for FIG. 1. 
     FIG. 8 illustrates another embodiment providing a draft control arrangement incorporating a vertically elongate draft hood 98. A damper 100 closes vent port 102 and is carried on leaf springs 104, 106 which in turn are carried on one or more pairs of bimetal strips 108, 110 mounted on brackets 112, 114 within the hood. 
     In heating appliances equipped with natural draft burners some spillage of products of combustion may occur at the draft hood relief opening during start-up and when the draft changes suddenly, especially in installations with short vent systems. If such an appliance is equipped with a conventional thermally controlled vent damper then spillage may occur during start up even under normal draft conditions. The embodiment of FIG. 8 minimizes both spillage and flow of dilution air by a combination of: (a) the vertically elongated draft hood, which allows more accumulation of the flue gases before spillage and thereby more time for the bimetal elements to react and to open the damper further, and (b) the thermally controlled vent damper having its heat sensing element located in a portion of the draft hood which becomes filled with hot products of combustion if the draft decreases or becomes filled with relatively cool ambient air if the draft increases. 
     Another embodiment of the invention is illustrated in FIG. 9 and includes four stacks 113, 115, 116, (only three of which are illustrated) with each stack comprising a plurality, shown as twelve, of generally U-shaped bimetal strips 118, 120. The stacks extend upward from a bracket 122 which is fixedly mounted in the chamber 124 of an elongate draft hood 126 similar to that described for the embodiment of FIG. 8. The bimetal strips are riveted or otherwise fixedly fastened together in each stack in such a manner as to extend from the bracket 122 to a damper plate 128 which is riveted to the four topmost bimetal strips. The use of bimetal stacks is a known method for achieving large linear thermal expansion and contraction of the temperature-sensitive unit. 
     The two or more longitudinal layers of metals having different coefficients of thermal expansion are combined in each strip 118, 120 in such a manner that the legs of the U-shape move toward each other as the temperature rises. The bimetal strips above the port 130 thus contract primarily in response to the flow of hot combustion gases along path 132 while the bimetal strips below the level of port 130 provide the desired independent control of the diluting air. 
     The U-shaped strips 118, 120 are resilient enough to assume the functions of the leaf springs in the draft control arrangement shown in FIG. 2. Another embodiment of the invention is illustrated in FIG. 10. A draft hood 120 is mounted above a flue pipe 122 leading from a heating appliance, and ports 124 and 126 in vent 128 are opened and closed by damper sections 130 and 132. Damper section 130 is actuated by one or more bimetal elements 134 responding to the hot combustion gases flowing through flue pipe 122 and draft hood 120 into vent 128. Damper section 132 is actuated by one or more bimetal elements 136 responding to either the relatively cold dilution air by moving the damper section toward the vent port or to the heat of the combustion gases if they accumulate in the draft hood by moving the damper section away from the vent port, thereby minimizing the flow of dilution air. 
     Many variations are possible in the draft control arrangement of the invention without affecting its desired operation. The draft hoods shown in FIGS. 1 through 5 are of a type commonly used in domestic heating furnaces. Draft hoods on gas-fired water heaters generally may have the form shown in FIGS. 6 and 7. A port leading to a vent need not be installed in the top wall of a draft hood, as is well known. Yet, the invention is applicable to the various modified draft hoods as long as the temperatures of the combustion gases (or of a gas mixture predominantly consisting of combustion gases) and the temperature of the diluting air (or of a gas mixture predominantly consisting of diluting air) is sensed separately, and with a damper plate moved accordingly toward and away from the position in which it occludes the port to the vent, or where the heat sensing elements are installed in an area within the draft hood which becomes filled with hot products of combustion if the draft decreases and becomes filled with relatively cool ambient air if the draft increases. 
     The leaf springs and bimetal strips described above are generally quite narrow so as not to interfere with gas flow through the hood chambers. For adequate stability of the damper plates, it is preferred to provide at least three leaf springs and three bimetal strips. If at least one relatively wide spring and a corresponding strip are acceptable, a total of two strips and two springs may be adequate. More than four springs and strips, of course, are permissible. 
     Bimetal strips are the least expensive temperature sensing elements available at this time, but other temperature sensing elements may be employed in conjunction with actuating elements such as the described leaf springs, or the sensing elements themselves may function as direct actuating elements in the manner shown in FIG. 9. 
     Other variations of the illustrative embodiments of the invention described above will readily suggest themselves to those skilled in the art. It should be understood, therefore, that the foregoing disclosure relates only to presently preferred combustion apparatus and to draft control arrangements for such apparatus, and that it is intended to cover all changes and modifications of the examples of the invention chosen herein for the purpose of the disclosure which do not depart from the spirit and scope of the invention set forth in the appended claims.