Patent Publication Number: US-8985999-B2

Title: Fuel/air furnace mixer

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     None. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not applicable. 
     BACKGROUND 
     Heating, ventilation, and/or air conditioning (HVAC) furnaces are widely used in commercial and residential environments for heating and otherwise conditioning interior spaces. To reduce emissions, HVAC furnaces may premix fuel/air completely prior to combustion. To help achieve this, HVAC furnaces sometimes comprise a premixer, such as a venturi premixer used to mix air and fuel prior to combustion. Some premixers may be designed for efficiently mixing fuel/air while also minimizing both pressure losses and the size of the premixer. In some furnaces, the air-fuel mixture outputted by the premixer may not be mixed to an effective level to provide for efficient burning of the air-fuel mixture. 
     SUMMARY 
     In some embodiments of the disclosure, a heating, ventilation, and/or air conditioning (HVAC) furnace is disclosed as comprising a venturi premixer and a disturber disposed downstream relative to the premixer and in an undivided output of the venturi premixer. 
     In other embodiments of the disclosure, a receiving tube for a furnace is disclosed as comprising a flowspace disposed within the receiving tube and a disturber at least partially disposed within the flowspace wherein the flowspace is configured to be at least one of (1) directly connected to an output of a venturi premixer and (2) substantially enveloping an output of a venturi premixer, and wherein the receiving tube is configured to allow passage of fluid therethrough in an undivided flowpath. 
     In yet other embodiments of the disclosure, a method of operating a heating, ventilation, and/or air conditioning (HVAC) furnace is disclosed as comprising mixing air and fuel using a venturi to generate an air-fuel mixture, outputting the air-fuel mixture from the venturi into a receiving tube along an undivided flowpath, disturbing the air-fuel mixture using a disturber disposed within the interior of the receiving tube, and outputting the disturbed air-fuel mixture from the venturi along the undivided flowpath. 
     These and other features will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present disclosure, reference is now made to the following brief description, taken in connection with the accompanying drawings and detailed description, wherein like reference numerals represent like parts. 
         FIG. 1  is an oblique exploded view of a furnace according to an embodiment of the disclosure; 
         FIG. 2  is an orthogonal side view of the furnace of  FIG. 1 ; 
         FIG. 3  is an oblique exploded view of a venturi premixer of the furnace of  FIG. 1 ; 
         FIG. 4A  is an orthogonal front view of the premixer of  FIG. 3 ; 
         FIG. 4B  is an orthogonal cut-away top view along lines A-A of the premixer of  FIG. 4A ; 
         FIG. 4C  is an orthogonal cut-away side view along lines B-B of the premixer of  FIG. 4A ; 
         FIG. 4D  is an orthogonal bottom view of the premixer of  FIG. 3 ; 
         FIG. 5A  is an orthogonal bottom view of a venturi premixer according to another embodiment of the disclosure; 
         FIG. 5B  is an oblique cut-away view of a venturi premixer according to another embodiment of the disclosure; 
         FIG. 5C  is an oblique cut-away side view of a venturi premixer according to another embodiment of the disclosure; 
         FIG. 6  is an oblique view of a mixture distributing box of the furnace of  FIG. 1 ; 
         FIG. 7  is an oblique view of a post-combustion chamber of the furnace of  FIG. 1 ; 
         FIG. 8A  is an orthogonal cut-away view of an intake assembly of the furnace of  FIG. 1 ; 
         FIG. 8B  is a schematic top view of the intake assembly of  FIG. 8A ; 
         FIG. 9  is a flowchart of a method of operating a furnace according to an embodiment of the disclosure; and 
         FIG. 10  is a schematic view of a furnace according to another embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Effectively mixing air and fuel in a furnace prior to combustion may be accomplished by disposing a disturber downstream of the furnace&#39;s premixer. It may be desirable to further mix the air-fuel mixture downstream of the premixer in order to allow for a more complete combustion of the mixture that may result in relatively lower emissions. Accordingly, a furnace with a disturber located downstream of the premixer for more effectively mixing the air-fuel mixture is provided. The disturber may be configured to shape a velocity distribution of the air-fuel mixture in order to more evenly distribute the air-fuel mixture downstream. The furnace may comprise one or more premixers, each having one or more disturbers located downstream therefrom. A post-combustion chamber may be disposed downstream of the premixer and disturber for distributing the air-fuel mixture to a plurality of outputs. A heat exchanger tube may be located downstream of each outlet for receiving the air-fuel mixture after combustion. 
     Referring to  FIGS. 1 and 2 , an oblique exploded view and an orthogonal side view of a furnace  100  are shown, respectively. The furnace  100  may comprise a partition panel  110 , a mixture distributing box  122 , a burner  125 , a post-combustion chamber  126 , at least one first or upstream heat exchanger  130 , a manifold  132 , a second or downstream heat exchanger  134 , and a heat exchanger exhaust chamber  140 . An intake assembly  195  may comprise a premixer  160 , mixture distributing box  122 , burner  125  and post-combustion chamber  126 . 
     The mixture distributing box  122  may be mounted to the partition panel  110  so that an inlet  123  of distributing box  122  may direct an air-fuel mixture received from the premixer  160  to the burner  125 . The mixture distributing box  122  may promote even distribution of the air-fuel mixture across a cross-sectional area of an air-fuel mixture flowpath and/or may promote even distribution of the air-fuel mixture across an upstream side of the burner  125 . The mixing of the air and fuel prior to entering the distributing box  122  may be aided by a mixing device such as the premixer  160  (see  FIG. 2 ) to promote homogenous mixing of the air and fuel prior to entering the mixture distributing box  122 , as will be discussed further herein. 
     In some embodiments, the burner  125  may extend across substantially an entire cross-sectional area of the air-fuel mixture flowpath. The air-fuel mixture may flow from the mixture distributing box  122  through the burner  125  and into the post-combustion chamber  126 . The burner  125  may be permeable, such as to allow the air-fuel mixture to travel through the burner  125  without a substantial pressure drop across the burner  125 . For example, the burner  125  may comprise a great number of small perforations over a substantial portion of the upstream and downstream sides of the burner  125 . Alternatively, a substantial portion of the upstream and downstream sides of the burner  125  may comprise one or more layers of woven material configured to allow the air-fuel mixture to flow therethrough. Still further, in other alternative embodiments, the burner  125  may comprise a combination of both perforations and woven material. 
     The burner  125  may be received within a cavity formed by the coupling of the mixture distributing box  122  and the post-combustion chamber  126 . In some embodiments, a flange  129  of the burner  125  may be sandwiched between the mixture distributing box  122  and the post-combustion chamber  126  so that substantially all of the air-fuel mixture may pass through the burner  125  prior to exiting the above-described cavity. When the burner  125  is received within the above-described cavity the upstream side of the burner  125  may face the mixture distributing box  122  and an opposing downstream side of the burner  125  may face the post-combustion chamber  126 . Post-combustion chamber  126  may be configured to output the combusted air-fuel mixture into multiple parallel flowpaths, as will be discussed further herein. 
     The one or more upstream heat exchangers  130  may be configured to receive an at least partially combusted air-fuel mixture downstream of the burner  125  and each upstream heat exchanger  130  may form a separate flowpath downstream relative to the burner  125 . The downstream heat exchanger  134  may be configured to receive the at least partially combusted air-fuel mixture from the upstream heat exchangers  130 . Heat exchanger  134  may comprise a fin-tube type heat exchanger and/or plate-fin type heat exchanger, either of which may comprise one or more tubes  136 . In other embodiments, the heat exchanger may comprise a so-called clamshell heat exchanger. 
     In some embodiments, the at least partially combusted air-fuel mixture may be transferred from the one or more upstream heat exchangers  130  to downstream heat exchanger  134  through the manifold  132 . While furnace  100  is described above as comprising one burner  125 , alternative furnace embodiments may comprise more than one burner  125 . In some cases, additional burners  125  may be utilized to increase an overall heating capacity. In some embodiments, several burners  125  may be aligned in parallel, so that multiple parallel air-fuel mixture flowpaths may be formed. Further, while furnace  100  is disclosed as comprising at least one upstream heat exchanger  130  and a downstream heat exchanger  134 , alternative furnace embodiments may comprise only one upstream heat exchanger no downstream heat exchanger  134 , and/or multiple downstream heat exchangers  134 . 
     An igniter  154  (see  FIG. 2 ) may be mounted partially within the post-combustion chamber  126  proximal to the downstream side of the burner  125  to ignite the air-fuel mixture a short distance downstream from the downstream side of the burner  125 . The air-fuel mixture may be moved in an induced draft manner by pulling the air-fuel mixture through the furnace  100  and/or in a forced draft manner by pushing the air-fuel mixture through the furnace  100 . The induced draft may be produced by attaching a blower and/or fan downstream, such as inducer blower  150  (see  FIG. 2 ) relative to the heat exchanger exhaust chamber  140  and pulling the air-fuel mixture out of the system by creating a lower pressure at the exhaust of the heat exchanger exhaust chamber  140  as compared to the pressure upstream of the burner  125 . Inducing flow in the above-described manner may protect against leaking the at least partially combusted air-fuel mixture and related products of combustion to the surrounding environment by ensuring the at least partially combusted air-fuel mixture is maintained at a pressure lower than the air pressure surrounding the furnace  100 . With such an induced flow, any leak along the flowpath of the air-fuel mixture may result in pulling environmental air into the flowpath rather than expelling the at least partially combusted air-fuel mixture and related products of combustion to the environment. In alternative embodiments, the air-fuel mixture may be forced along the air-fuel mixture flowpath by placing a blower or fan upstream relative to the burner  125  and creating higher pressure upstream of the burner  125  relative to a lower pressure at the exhaust of the heat exchanger exhaust chamber  140 . In some embodiments, a control system may control the inducer blower  150  to an appropriate speed to achieve desired fluid flow rates for a desired firing rate through the burner  125 . Increasing the speed of the inducer blower  150  may introduce more air to the air-fuel mixture, thereby changing the characteristics of the combustion achieved by the burner  125 . In some embodiments, a so-called zero governor regulator and/or zero governor gas valve may be additionally utilized to provide a desired fuel to air ratio in spite of the varying effects of an induced draft and/or other pressure variations that may fluctuate and/or otherwise tend to cause dispensing or more or less fuel in response to the pressure variations and/or negative pressures relative to atmospheric pressure. 
     Substantially enclosing the burner  125  within a cavity formed by the connecting of the mixture distributing box  122  and the post-combustion chamber  126  and substantially combusting the air-fuel mixture near the burner  125  may reduce the surface temperatures of the post-combustion chamber  126  and upstream heat exchangers  130  as compared to embodiments utilizing other types of burners. While the downstream side of the burner  125  is disclosed as facing the post-combustion chamber  126  while the upstream side of the burner  125  faces the mixture distributing box  122 , in alternative embodiments, the burner  125  may be positioned differently and/or the flow of the air-fuel mixture may be passed through the burner  125  in a different manner. The post-combustion chamber  126  may be connected to the upstream heat exchangers  130  so that the at least partially combusted air-fuel mixture enters directly into the upstream heat exchangers  130 , as will be discussed further herein. The post-combustion chamber  126  may seal the air-fuel mixture flowpath from secondary dilution air as well as position the burner  125  in a manner conducive for transferring the at least partially combusted air-fuel mixture to the upstream heat exchangers  130 . While the upstream heat exchangers  130  are disclosed as comprising a plurality of tubes, in alternative embodiments, the upstream heat exchangers may comprise clamshell heat exchangers, drum heat exchangers, shell and tube type heat exchangers, and/or any other suitable type of heat exchanger. 
     Referring now to  FIG. 2 , the furnace  100  is shown as comprising the inducer blower  150 , the air-fuel premixer  160 , the igniter  154 , and the flame sensor  156 . Premixer  160  may comprise a Venturi style air-fuel mixer or any other suitable style of air-fuel mixers. The igniter  154  may comprise a pilot light, a spark igniter, a piezoelectric device, and/or a hot surface igniter. The igniter  154  may be controlled by a control system and/or may be manually ignited. The flame sensor  156  may comprise a thermocouple, a flame rectification device, and/or any other suitable safety device. 
     Referring now to  FIG. 3 , an oblique exploded view of a venturi mixer or premixer  160  is shown. The premixer  160  may comprise a central or longitudinal axis  161  and comprise an outer tube  162  and a venturi  185 , which may comprise a diffuser or lower portion  180  and an upper portion  190 . Upper portion  190  may comprise a first end  190   a , second end  190   b  and a central or longitudinal axis concentric with axis  161 . Upper portion  190  may also comprise an inner surface  192  that defines a bore  193 . The inner surface  192  may be configured such that the radius of bore  193  decreases logarithmically moving from first end  190   a  towards second end  190   b . Lower portion  180  may comprise a first end  180   a , a second end  180   b  and a central or longitudinal axis concentric with axis  161 . Lower portion  180  may also comprise an inner surface  182  that defines a bore  183  having a diameter  183   a  at second end  180   b . The inner surface  182  may be configured that the radius of bore  182  increases linearly moving from first end  180   a  towards second end  180   b.    
     The outer tube  162  may comprise a first end  162   a , second end  162   b , a fuel injector  163  and a port  164  providing fluid communication between injector  163  and the interior of the tube  162 . Premixer  160  may be configured to provide for mixing between fuel from injector  163  and air, which may enter premixer  160  via a bore  193  that may be formed within upper portion  190  of venturi  185 . The effective and thorough mixing of air and fuel within premixer  160  may allow for more efficient burning of the air-fuel mixture by a burner (e.g., burner  125  of  FIG. 1 ), in turn providing for relatively fewer emissions as the mixture is burned. 
     Referring now to  FIGS. 4A-4D , an orthogonal front view, two orthogonal cut-away views (the first along lines A-A and the second along lines B-B), and an orthogonal bottom view of the premixer  160  are shown, respectively. The tube  162  may comprise an inner surface  165  that defines a bore  167  having a diameter  167   a . A flange  166  may extend radially inward from inner surface  165  and into bore  167 . The lower portion  180  of venturi  185  may be disposed within bore  167  and comprise a flange  184  that extends radially outward from an outer surface  181  of lower portion  180 . A lower radial surface  184   a  of flange  184  may physically engage an upper radial surface  166   a  of flange  166 . The physical engagement between flange  184  of lower portion  180  and flange  166  of tube  162  may seal or at least substantially restrict fluid flow between surfaces  184   a  and  166   a . Thus, engagement between flanges  184  and  166  may divide bore  167  into an upper section  167   b  and a lower section  167   c . Fluid communication between sections  167   b  and  167   c  may be provided for via a bore  183  of lower portion  180 . 
     Tube  162  may further comprise an upper flange  168  at upper end  162   b . Upper portion  190  of venturi  185  may comprise a flange  194  at first end  190   a  that extends radially outward from an outer surface  193  of upper portion  190 . Upper portion  190  of venturi  185  may be disposed within bore  167  of tube  162  such that a lower radial surface  168   a  of the flange  168  of tube  162  may engage an upper radial surface  194   a  of the flange  194  of upper portion  190 . The physical engagement between flange  194  of upper portion  190  and flange  168  of tube  162  may seal or restrict fluid flow between surfaces  194   a  and  168   a . Fluid communication between the area exterior of premixer  160  and bore  167  (e.g., section  167   c ) may be provided for via bore  193  of upper portion  190 . 
     Lower portion  180  and upper portion  190  of venturi  185  may be axially positioned relative to each other within bore  167  such that second end  190   b  of upper portion  190  may extend partially into bore  183  of lower portion  180 , providing a gap  186  between outer surface  193  of upper portion  190  and inner surface  182  of lower portion  180 . Thus, a path of fluid communication may be provided between section  167   b  of the bore  167  of tube  162  and bore  183  of lower portion  180  via gap  186 . Also, flange  184  of lower portion  180  may be positioned on the outer surface  181  such that second end  180   b  extends partially into lower section  167   c  of the bore  167  of tube  162 . 
     Tube  162  may further comprise a disturber  169  in the form of a longitudinal member disposed axially between second end  180   b  of the lower portion  180  of venturi  185  and second end  162   b . Disturber  169  may be configured to disturb or obstruct a fluid that is flowing axially from first end  162   a  towards second end  162   b  of tube  162 . Disturber  169  may also be configured to shape a velocity profile of the air-fuel mixture by creating additional mixing zones towards the center of the fluid flow downstream of the disturber  169 . Disturber  169  may comprise an outer surface  169   a  having a width  169   b  (as shown in  FIG. 4B ) and may extend radially across bore  167  to contact inner surface  165  (as shown in  FIG. 4B ) of tube  162  at diametrically opposed locations (as shown in  FIG. 4C ). Further, disturber  169  may be positioned within bore  167  such that it intersects axis  161  of premixer  160 . The outer surface  169   a  of disturber  169  may be positioned at an axial distance  169   c  from second end  180   b  of lower portion  180 . The width  169   b  of disturber  169  and the axial distance  169   c  may be a function of the diameter  167   a  of bore  167  and the diameter  183   a  of bore  183  at second end  180   b  of lower portion  180 . For instance, the length scale for a mixing zone height (H mix ) may be defined as being equal to half the difference between diameter  167   a  (D tube ) and diameter  183   a  (D diff ):
 
 H   mix =( D   tube   −D   diff )/2
 
     In the embodiment of  FIGS. 3-4D : the ratio of D tube  over D diff  (i.e., D tube /D diff ) may have a value in the range of about 1.2 to 4; the ratio of axial distance  169   c  (L) over H mix  (i.e., L/H mix ) may have a value in the range of about 1 to 15; and the ratio of width  169   b  (W) over D diff  (i.e., W/D diff ) may have a value in the range of about 0.1 to about 1.1. Disturber  169  and distance  169   c  may be configured to enhance mixing of an at least partially mixed air-fuel mixture flowing toward disturber  169  from bore  183  of lower portion  180 . However, distance  169   c  may be different depending upon the application (e.g., degree of pressure differential, type of fuel used, size of premixer, etc.). For example, in some embodiments, D tube /D diff  may equal 1.6, L/H mix  may equal 3, and W/D diff  may equal 0.3. 
     An air-fuel mixture flowpath  170  may be created by inducing a relatively lower pressure at end  162   b  of tube  160  (e.g., via a blower disposed downstream of mixer  160 ) such that air from an area exterior of premixer  160  may enter bore  193  of the upper portion  190  of venturi  185  and fuel from injector  163  may enter bore  167   c  of tube  162  via port  164 . Thus, air-fuel mixture flowpath  170  may be formed from the mixing of air from an air flowpath  170   a  and fuel from a fuel flowpath  170   b  within bore  167  and venturi  185 . Flowpath  170  may extend across disturber  169  and may be disturbed such that additional mixing of air and fuel may take place as a result of extending across disturber  169 . 
     Flowpath  170  may also include upstream air-fuel mixing zones  170   c  disposed in section  167   c  upstream from disturber  169  and proximal to inner surface  165 , which may allow for additional mixing of the air-fuel mixture within zones  170   c . Mixing zones  170   c  may arise from the expansion in diameter between diameter  183   a  of lower portion  180  and diameter  167   a  of tube  160 . Flowpath  170  may also include a downstream mixing zone  170   d  disposed within section  167   c  but downstream of disturber  169  and proximal to longitudinal axis  161 , which may allow for additional mixing of the air-fuel mixture. 
     Referring now to  FIGS. 5A-5C , several different embodiments of disturbers are shown.  FIG. 5A  is a bottom view illustrating an embodiment of a premixer  260  that may comprise a disturber  269  in the form of a cross-member. Disturber  269  may include two longitudinal members  269   a  and  269   b , which may span across a bore  267  defined by an inner surface  265  to intersect at a central location  269   c .  FIG. 5B  illustrates another embodiment of a premixer  360  that includes a disturber  369  in the form of a tab member disposed below a venturi  385 . Disturber  369  may extend partially into a bore  367  from an inner surface  365  of premixer  360 . Thus, disturber  369  may extend radially through an axis  361  of premixer  360  and terminate at a terminal end  369   a .  FIG. 5C  illustrates another embodiment of a premixer  460  that may comprise a disturber  369  and a disturber  469  disposed axially below disturber  369 . Disturber  469  may be a helical member and may extend both axially within premixer  460  and radially into a bore  467  from an inner surface  465  of premixer  460 . Also, disturber  469  may protrude helically from inner surface  465  between a first end  469   a  and a second end  469   b.    
     Referring now to  FIG. 6 , an oblique view of mixture distributing box  122  is shown. The mixture distributing box  122  may comprise an inlet  123  and a deflector  124 . Deflector  124  may be connected to and received within mixture distributing box  122 . The shape and positioning of deflector  124  within mixture distributing box  122  with respect to inlet  123  may be configured to promote even distribution of the air-fuel mixture entering mixture distributing box  122  over a cross-sectional area of the flowpath of the air-fuel mixture and/or to promote even distribution of the air-fuel mixture over an upstream side of the burner  125  disposed downstream of the deflector  124 . The above-described increased even distributions of the air-fuel mixture may promote a more homogenous temperature distribution within the post-combustion chamber  126  and/or the upstream heat exchangers  130 . While deflector  124  is shown as comprising a rectangular plate with an upstream side facing inlet  123 , in alternative embodiments, a deflector may comprise any another shape and/or device configured to disturb fluid flow entering mixture distributing box  122 . 
     Referring now to  FIG. 7 , an oblique view of post-combustion chamber  126  is shown. In this embodiment, igniter  154  and flame sensor  156  are disposed within an inlet  127  of post-combustion chamber  126 . Post-combustion chamber  126  may further comprise a plurality of outlets  128  that may be configured to directly couple to the upstream heat exchangers  130 . Burner  125  may be disposed upstream of post-combustion chamber  126 , an inputted air-fuel mixture may be ignited by igniter  154 , and the at least partially combusted air-fuel mixture may pass through a substantially undivided space of the post-combustion chamber  126  prior to passing into a plurality of separate flowpaths via outlets  128 . 
     Referring now to  FIGS. 8A and 8B , an orthogonal cut-away side view and an orthogonal cut-away top view, respectively, are shown of an intake assembly  195 . An intake flowpath  197  (as shown in  FIG. 8B ) may generally illustrate a flow of an air-fuel mixture within intake assembly  195  which may be caused by providing a pressure difference between outlets  128  of post-combustion chamber  126  and first end  162   a  of premixer  160 . Intake flowpath  197  may extend through premixer  160  and may be disturbed via disturber  169 , which may result in additional mixing of air and fuel within the intake assembly  195 . As the flowpath  197  enters mixture distributing box  122  via inlet  123  it may be deflected by deflector  124 , which may aid in distributing fluid within intake assembly  195  across a width  126   a  (see  FIG. 8B ) of the mixture distributing box  122  before entering the plurality of outlets  128 . Following the exit of post-combustion chamber  126  via the plurality of outlets  128 , intake flowpath  197  may extend into a plurality of heat exchanger tubes of heat exchangers  130 . 
     Referring now to  FIG. 9 , a block diagram depicting a method  400  of operating a furnace is shown. The method may begin at block  410  by mixing a fuel and air together. An air-fuel mixer and/or so-called premixer, such as premixer  160 , may be utilized to accomplish the mixing of the fuel and the air. The fuel may comprise natural gas available from a gas valve attached to a mixture distributing box, such as mixture distributing box  122 , or to an air-fuel premixer upstream of the mixture distributing box. Alternatively, the fuel may comprise propane and/or any other suitable fuel. The air may be introduced to the mixture distributing box or to the air-fuel mixer by a so-called forced draft or a so-called induced draft. 
     The method  400  may continue at block  420  where the air-fuel mixture may be disturbed via a disturber, such as disturber  169 , positioned at least partially within the air-fuel mixture flowpath, such as intake flowpath  197 . The disturber may be positioned within a tube downstream of the air-fuel mixer and/or so-called premixer and may additionally mix the air and fuel within a flowpath or flowspace disposed downstream of the mixer. The disturber may take the form of a longitudinal member extending radially across at least a portion of the flowpath or flowspace, however, in other embodiments, the disturber may take other forms (e.g., a tab, a helical member, etc.). 
     The method  400  may continue at block  430  where the air-fuel mixture may pass through a mixture distributing box to be more evenly distributed across an upstream side of a burner, such as burner  125 . The mixing process may be aided by a deflector located within the mixture distributing box that may comprise the effect of deflecting or disturbing the flow of the air-fuel mixture. For example, the deflector may be placed in front of the outlet of the air-fuel mixing box, altering the flow of the air and fuel within the air-fuel mixing box and thereby causing the air-fuel mixture to be more evenly distributed across a cross-sectional area of the air-fuel mixture flowpath. 
     The method  400  may continue at block  440  where the air-fuel mixture may be moved through a burner. The burner may comprise a thin and elongate body with an upstream side and a downstream side. The upstream side and downstream side of the burner may be permeable to allow the air-fuel mixture to pass through the burner. For example, the burner may comprise a great number of small perforations and/or a woven material over a substantial portion of the upstream and downstream sides of the burner. Further, the burner may be contained within a cavity comprising internal space of a mixture distributing box and internal space of a post-combustion chamber so that the air-fuel mixture leaving the air-fuel mixture distribution box passes through the upstream and downstream sides of the burner. 
     The method  400  may continue at block  450 , where the air-fuel mixture may be ignited. The downstream side of the burner may face the post-combustion chamber. An igniter may be mounted in the post-combustion chamber near the downstream side of the burner. The igniter may comprise a pilot light, a piezoelectric spark, or a hot surface igniter. As the air-fuel mixture may pass through the burner, the igniter may ignite and cause at least partial combustion of the air-fuel mixture to begin near the downstream side of the burner. 
     The method  400  may continue at block  460  by directing the at least partially combusted air-fuel mixture into a heat exchanger, such as heat exchanger  130 . Combustion may at least partially occur near the downstream side of the burner so that heat is generated and forced downstream of the burner and into the post-combustion chamber. In this embodiment, the combustion may occur generally at or near the downstream side of the burner. In alternative embodiments, combustion may occur both at the upstream and downstream sides of the burner as well as within an interior of the burner. The post-combustion chamber may be configured to divide a single flowpath associated with the burner into multiple parallel flowpaths. One or more of the multiple parallel flowpaths may extend through a heat exchanger. The heat exchangers may be tubular in design with an upstream end connected to the post-combustion chamber and a downstream end connected to either a heat exchanger exhaust chamber or to a manifold. An upstream end of a downstream heat exchanger may be connected to the manifold and a downstream end of the downstream heat exchanger may be connected to a heat exchanger exhaust chamber. A heat exchanger exhaust chamber may be disposed downstream from the heat exchanger(s) and may be configured to recombine the plurality of parallel flowpaths associated with the heat exchanger(s) into a single and/or fewer flowpaths. The at least partially combusted air-fuel mixture may comprise NO x . The level of NO x  in the at least partially combusted air-fuel mixture may be lowered by varying the combustion temperature of the air-fuel mixture and/or the ratio of air to fuel within the mixture. 
     The method  400  may continue at block  470  by conditioning air outside of the heat exchanger. As the at least partially combusted air-fuel mixture moves through the heat exchanger(s) toward the heat exchanger exhaust chamber, the heat exchanger(s) may be heated. Air that is exterior to the heat exchanger(s) may be moved into contact with the heat exchanger(s). As the air moves across the heat exchanger(s), heat may be transferred from the heat exchanger(s) to the air contacting the heat exchanger(s). 
     The method  400  may continue at block  480  by venting the conditioned air into an air conditioned space, for example, an office space or living area of a home. The heated air may be used to warm the space in order to increase comfort levels for occupants and/or to maintain the contents of the space at a pre-determined temperature. 
     Referring now to  FIG. 10 , a furnace  500  is shown. Furnace  500  may comprise a circulation air blower  502  that receives incoming airflow  504  and passes incoming airflow  504  into contact with downstream heat exchanger  134  and upstream heat exchanger  130  to transfer heat from the heat exchangers  134 ,  130  to the air. Exiting airflow  506  may be distributed to an area that is to be conditioned with the heated air. A partition panel  110  may isolate the air-fuel mixture that may be at least partially combusted from the incoming and exiting airflows  504 ,  506 . Due to a thin and elongate burner that may be disposed between the mixture distributing box  122  and post-combustion chamber  126 , a size of the furnace  500  may be reduced relative to other furnaces that do not comprise a premix burner configured for use with an inducer draft. 
     At least one embodiment is disclosed and variations, combinations, and/or modifications of the embodiment(s) and/or features of the embodiment(s) made by a person having ordinary skill in the art are within the scope of the disclosure. Alternative embodiments that result from combining, integrating, and/or omitting features of the embodiment(s) are also within the scope of the disclosure. Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example, whenever a numerical range with a lower limit, R l , and an upper limit, R u , is disclosed, any number falling within the range is specifically disclosed. In particular, the following numbers within the range are specifically disclosed: R=R l +k*(R u -R l ), wherein k is a variable ranging from 1 percent to 100 percent with a 1 percent increment, i.e., k is 1 percent, 2 percent, 3 percent, 4 percent, 5 percent, . . . 50 percent, 51 percent, 52 percent, . . . , 95 percent, 96 percent, 97 percent, 98 percent, 99 percent, or 100 percent. Moreover, any numerical range defined by two R numbers as defined in the above is also specifically disclosed. Use of the term “optionally” with respect to any element of a claim means that the element is required, or alternatively, the element is not required, both alternatives being within the scope of the claim. Use of broader terms such as comprises, includes, and having should be understood to provide support for narrower terms such as consisting of, consisting essentially of, and comprised substantially of. Accordingly, the scope of protection is not limited by the description set out above but is defined by the claims that follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated as further disclosure into the specification and the claims are embodiment(s) of the present invention.