Patent Publication Number: US-RE37128-E

Title: Standing pilot furnace with vented vestibule

Description:
This is a continuation of application Ser. No.  08 / 389 , 268  filed Feb.  17 ,  1995 , abandoned.  
    
    
     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 by 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  75 . Front panel  26  is provided with a plurality of combustion and ventilation air openings B 6   86 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 alternate 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.