Patent Publication Number: US-5293860-A

Title: Standing pilot furnace with vented vestibule

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to furnaces, and, more particularly, to induced draft, fuel-fired furnaces having a standing pilot. 
     2. Description of the Related Art 
     High efficiency fuel-fired furnaces of the type herein concerned include a combustion chamber wherein fuel such as natural or liquified petroleum (LP) gas is burned creating hot gaseous products of combustion, i.e., exhaust gas. A burner having one or more exhaust ports formed therein is connected to a fuel source and burns the fuel which is ignited. The hot exhaust gas is circulated through a heat exchanger which is heated thereby. Air circulates past the heat exchanger and is heated thereby via convection heat transfer. The heated air is circulated through a living space by a circulating blower. In an induced draft furnace, an exhaust blower is located between the heat exchanger and the flue outlet of the furnace and draws the products of combustion through the heat exchanger and discharges them into an exhaust vent. 
     To ignite the fuel exhausted from the burner, it is known to use either an electronic ignition apparatus or a standing pilot. An electronic ignition apparatus is operably controlled when fuel is exhausted from the burner to ignite the fuel. An electronic ignition system is considerably more costly and complex than a standing pilot. A standing pilot maintains a continuously burning open flame regardless of whether fuel is exhausted from the burner. During periods of inactivity of the burner, therefore, it is necessary to vent the relatively small amount of combustion gas produced by the standing pilot to the flue and ambient environment. A furnace using a standing pilot is less expensive to manufacture than a furnace using an electronic ignition system and may therefore be preferred. 
     It is known to vent the combustion products produced by a standing pilot through the heat exchanger and into the flue. However, some high efficiency furnaces provide increased heat exchanger efficiency by increasing the pressure drop of the exhaust gas between the inlet and outlet of the heat exchanger. This may be accomplished by providing a longer flow path and/or appropriate sized conduits at various locations within the flow path. This higher pressure drop increases the efficiency of the heat exchanger when exhaust gas is circulated therethrough, thus requiring less heat transfer area for a given output; however, it also prohibits the venting of the standing pilot combustion products through the heat exchanger during periods of inactivity of the burner, i.e., idle flow conditions. 
     The higher pressure drop therefore further increases internal heat exchanger resistance prohibiting the safe venting of standing pilot combustion products. Thus, a standing pilot may be preferred to reduce manufacturing costs, but may not be safely vented into the heat exchanger and ultimately to the vent. 
     One method of obtaining the advantages of using a standing pilot while at the same time avoiding the problems of unsafe pilot gas venting mentioned above is to provide a vent tube disposed immediately above the standing pilot. The vent tube is connected to and in fluid communication with the outlet section of a draft inducing fan. Such an apparatus, however, requires relatively precise placement of the vent tube above the standing pilot flame and a specially manufactured or modified blower. Moreover, to prevent back flow of exhaust gas during operation of the blower, a special venturi section forming member must be rigidly fixed at a precise location within the blower adjacent the outlet of the vent tube, thereby further increasing manufacturing costs. 
     What is needed in the art is an apparatus and method which allows a standing pilot to be used with a high efficiency furnace by easily and inexpensively bypassing the combustion products of the standing pilot directly to the flue of the fuel-fired furnace. 
     SUMMARY OF THE INVENTION 
     The present invention provides an opening at the upper end of a vestibule section of a furnace using conventionally available components to allow venting of combustion products from a source of combustion products to the flue. Moreover, a drafthood is provided having openings formed therein for allowing the standing pilot combustion products to flow therethrough to the flue and may include a temperature sensor to detect a blocked flue condition. 
     The invention comprises, in one form thereof, an induced draft, fuel-fired furnace having a heat exchanger with an inlet and an outlet. A flue inlet defines a first cross-sectional area. A vented vestibule in fluid communication with the flue has disposed therein a burner in fluid communication with the heat exchanger inlet, a standing pilot adapted to ignite fuel operably discharged from the burner, a blower having an inlet in fluid communication with the heat exchanger outlet, and an outlet disposed at the inlet of said flue. The blower outlet defines a second cross-sectional area less than the first cross-sectional area. The portion of the first cross-sectional area not having the second cross-sectional area disposed immediately adjacent thereto defines a vent inlet opening allowing standing pilot combustion products to flow into the flue and ambient environment. 
     In another form of the invention, a drafthood is disposed in the vestibule and forms a plenum in fluid communication with the vent inlet opening. The drafthood includes an undersurface sealingly engaged with the blower outlet which extends immediately adjacent to a panel of the furnace. The undersurface is formed with an inlet adapted to receive combustion gases from the standing pilot, and a blocked flue detecting means adapted to detect a blocked exhaust gas flow condition in said flue. 
     An advantage of the present invention is that the combustion gas from the standing pilot is not circulated through the heat exchanger, thereby inhibiting potential associated corrosion of the heat exchanger and providing safe venting of the pilot. 
     Another advantage is that less expensive materials can be used to form the heat exchanger. 
     Yet another advantage is that the present invention can provide dual functionality of preventing heat exchanger corrosion and detecting a blocked vent. 
     Still another advantage is that the components within the vestibule need not be extensively and physically modified to vent the standing pilot to the vent. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of the induced draft, fuel-fired furnace of the present invention with the louvered front panel in place; 
     FIG. 2 shows the embodiment of FIG. 1 with the louvered front panel removed; 
     FIG. 3 is a partial side sectional view of the vestibule section shown in FIG. 2 with the upper end of the blower and flue shown in perspective; 
     FIG. 4 is a perspective view of the drafthood illustrated in FIGS. 2 and 3; 
     FIG. 5, shows another embodiment of the present invention utilizing an electronic ignition. 
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings and particularly to FIGS. 1-3, there is shown an induced draft, fuel-fired furnace of the present invention. Furnace 10 generally includes an air inlet plenum 12, supply air plenum 14 and vestibule 16 separated by a horizontal wall 18 and vertical wall 20. 
     Air inlet plenum 12 is defined by side walls 22, bottom 24, front panel 26 (FIG. 1), back panel 27 and horizontal wall 18. Air inlet plenum 12 is connected to suitable duct work (not shown) and receives return air therethrough from a space to be heated (not shown). Horizontal wall 18 is formed with an opening therein (not shown) disposed below a heat exchanger generally designated as 28 disposed in supply air plenum 14. A supply blower 30 is suitably fixed within air inlet plenum 12 such that the outlet of supply blower 12 is disposed below the opening formed in the horizontal wall 18. In the embodiment shown, supply blower 30 is a centrifugal blower fixed to an appropriate framework 32 which in turn is fixed to horizontal wall 18. The outlet of centrifugal supply blower 30 exhausts return air through the opening formed in horizontal wall 18 and through heat exchanger 28. 
     Heat exchanger 28 is disposed within supply air plenum 14 and fixed therein at a desired location with a plurality of mounting brackets, one of which is shown and designated as bracket 34. Heat exchanger 28 is a relatively high pressure drop, high efficiency heat exchanger including three clam shell type heat exchangers 36 formed with a relatively long fluid flow path therethrough. Clam shell type heat exchangers 36 each include an inlet in fluid communication with one of three burners 38 (FIGS. 2 and 3), and an outlet in fluid communication with the inlet of an induced draft blower 40. 
     In the embodiment shown, heat exchanger 28 is shown as including clam shell type heat exchangers 36. An example of a clam shell type heat exchanger which may be used with the present invention is disclosed in U.S. Pat. No. 4,739,746, which is assigned to the assignee of the present invention and incorporated herein by reference. Moreover, it is also possible and within the scope of this invention to use other type heat exchangers. For example, U.S. Pat. No. 5,094,224, also assigned to the assignee of the present invention and incorporated herein by reference, discloses an enhanced tubular type heat exchanger which may be used with the present invention. 
     Vestibule 16 (FIGS. 2 and 3) is defined by side walls 22, horizontal wall 18, vertical wall 20, front louvered panel 26 and top member 73. Front panel 26 is provided with a plurality of combustion and ventilation air openings B6 and vents 88 allowing air flow from the ambient environment into vestibule 16. In the embodiment shown, front panel 26 is formed from two pieces of sheet metal providing respective removable covers for air inlet plenum 12 and vestibule 16. However, front panel 26 may be formed from a single piece of sheet metal providing a removable vertical panel adjacent air inlet plenum 12 and vestibule 16. 
     Vestibule 16 generally includes therein a burner assembly 42, induced draft blower 40 and drafthood 46. Burner assembly 42 includes a gas valve 48, burners 38, standing pilot 50 and housing 52. Gas valve 48 is of conventional design and operably supplies fuel, e.g., natural or LP gas, to the burners 38 and standing pilot 50. Burners 38, shown schematically in FIG. 3, are in-shot burners fixed within housing 52 having flame producing ends 54, e.g., such as an orificed jet, which respectively extend toward the inlet ends of the clam shell type heat exchangers 36. Burners 38, however, can be formed with other known constructions producing products of combustion which are circulated through heat exchanger 28. For example, U.S. Pat. No. 4,616,994 to Tomlinson, which is assigned to the assignee of the present invention, discloses a gas burner including means for reducing nitrous oxide emissions which may be used with the present invention. 
     In-shot burners 38 are affixed within housing 52 which in turn is affixed to the vertical wall 20. Housing 52 includes two upper horizontal surfaces 56 and 57 (FIG. 2) which are vertically offset and define an opening 58 allowing combustion gas produced by standing pilot 50 to flow therethrough via natural draft during idle conditions. 
     Induced draft blower 40 is fixed to vertical wall 20 and includes an inlet 60 in fluid communication with the outlets of clam shell heat exchangers 36. Induced draft blower 40 also includes a housing 62 with an outlet end 64 disposed at the upper end thereof. Disposed within housing 62 is an impeller 66 which is rotatably driven by a motor 68. Outlet end 64 is disposed immediately adjacent to the inlet end 70 of a flue or vent 72 fixed to top member 73. In the embodiment shown, inlet end 70 of flue 72 has a circular cross section with a diameter from 3 to 5 inches, and outlet end 64 of induced draft blower 44 has a circular cross section with a diameter of about 3 inches. 
     Because the cross-sectional area of the outlet end 64 of induced draft blower 40 is less than the cross-sectional area of the inlet end 70 of flue 72, an area of lower pressure 74 is formed in the space between the periphery of the outlet end 64 and inlet end 70. In the embodiment shown, lower pressure area 74 is defined as a vent inlet opening or venturi section having a generally annular shape. Lower pressure area 74 allows a natural draft within vestibule 16 when induced draft blower 40 is not operating. This fluid flow path is illustrated by the directional arrows indicated generally as 75. Thus, by providing an outlet 64 having a cross-sectional area less than the inlet end 70 of flue 72, the combustion products produced by the standing pilot 50 may be exhausted into the flue 72 and out into the ambient environment. 
     Although outlet 64 of induced draft blower 44, and inlet end 70 of flue 72 are shown with a generally circular cross-section, it is possible and considered within the scope of this invention to use an outlet 64 and inlet end 20 of different cross-sectional shapes. 
     A drafthood 46 is disposed within vestibule 16 at the upper end thereof (FIGS. 2, 3 and 4). Drafthood 46 includes an under surface 78 with a first opening 79 disposed around and closely adjacent outlet end 64 of induced draft blower 44, and two side members 80 forming a plenum when front panel 26 is in place (FIG. 1). A flange 83 having holes 85 formed therein allows attachment of drafthood 46 to the underside of upper surface 73. 
     Front panel 26 is formed with an aperture 86 disposed adjacent drafthood 46. Aperture 86 allows ambient air to enter the drafthood 46 and flue 72 and mix with and dilute the exhaust gases therein. Aperture 86 may be a single opening (as shown) or multiple openings (not shown), and the aperture(s) may be louvered. 
     Formed in the under surface 78 of drafthood 46 is at least one opening 82 (shown schematically in the drawings) allowing combustion gas produced by standing pilot 50 to flow therethrough and into flue 72. In the embodiment shown in FIGS. 2 and 3, two openings 82 are provided in undersurface 78 for allowing natural or induced draft through the vestibule 16. In an alternative embodiment shown in FIG. 4, five openings 82 are provided in undersurface 78 for allowing draft through the vestibule 16. Thus, it is apparent that the exact number of openings 82 formed in undersurface 82 is not critical so long as the combustion products from the standing pilot 50 are allowed to adequately flow therethrough into the drafthood 46. 
     One of the side members 80 is formed with a second opening 81 adapted to receive a blocked flue sensor means 84 (FIGS. 2 and 3) for detecting a blocked exhaust gas flow condition within flue 72. Sensor means 84 is connected to and operably controls gas valve 48 to prevent fuel flow through gas valve 48 when a blocked vent condition exists, thereby effectively stopping the operation of furnace 10. In the embodiment shown, blocked flue sensor means 84 is a temperature sensor for detecting when hot exhaust gas backflows from the vent into drafthood 46 from vent 72 because of a blocked vent condition. In an alternative embodiment (not shown), it may be possible to limit the size and configuration of openings 82 and aperture 86 and use a pressure sensing means disposed within one of the side members 80 to detect a blocked flue condition. 
     FIG. 5 shows another embodiment of the present invention utilizing an electronic ignition 51 discharged from burners. While electronic ignition 51 does not produce products of combustion during idle conditions of burners 38, it may nonetheless be desirable to use the drafthood of the present invention to detect, e.g., a blocked vent condition which might occur. The drafthood is in fluid communication with the vent inlet opening and dilution air opening, and includes an inlet trace or residual combustion products within vented vestibule 16 which may be produced by burners 38. 
     During an idle condition, a natural draft condition exists in vestibule 16 allowing products of combustion from standing pilot 50 to draft upwards through vestibule 16 and drafthood 46 into flue 72 and the ambient environment. Vents 88 formed in front panel 26 provide oxygen for combustion of fuel exhausted by burners 38 and standing pilot 50, and allow a draft to occur through vestibule 16 such as indicated by directional arrows 75. As combustion products flow upwardly though vestibule 16 and drafthood 46, dilution air flows inwardly from the ambient environment through aperture 86 into drafthood 46 and mixes with and thereby dilutes the standing pilot combustion products. Because induced draft blower 40 is not operating, an area of lower pressure forming a venturi 74 does not exist between the peripheries of outlet 64 and inlet end 70. That is, the area of the induced draft inlet opening between outlet 64 and inlet 70 has a pressure about the same as that existing within vestibule 16, drafthood 46 and flue 72. The products of combustion produced by standing pilot 50 are therefore free to flow upwards via a natural draft through vestibule 16 and into flue 72. The products of combustion of standing pilot 50 are not circulated through heat exchanger 28 during periods of inactivity of burners 38, thereby inhibiting the propensity for causing internal corrosion of heat exchanger 28 and unsafe pilot venting as described above. 
     During operation, an externally located thermostat (not shown), sends a signal initiating the production of heat from fuel-fired furnace 10. Fuel exhausted from in-shot burners 38 is ignited and drawn through heat exchanger 28 by induced draft blower 40 and exhausted upwardly through flue 72. The exhaust gas flows from outlet 64 of induced draft blower 40 and through inlet end 70 of flue 72, and an area of low pressure forming a venturi section 74 occurs between outlet 64 and inlet 70. Because the pressure existing within venturi section 74 is less than the pressure existing within vestibule 16, vestibule 16 is vented to flue 72 while induced draft blower 40 is operating. 
     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.