Patent Publication Number: US-2010122806-A1

Title: Compact and Efficient Heat Exchanger, Furnace, HVAC Unit, Building, and Method of Making

Description:
FIELD OF THE INVENTION 
     This Invention relates to apparatuses for transferring heat, heat exchangers, furnaces, heating, ventilation, and air conditioning (HVAC) units, HVAC systems, buildings having such devices, and methods of manufacturing such devices. 
     BACKGROUND OF THE INVENTION 
     Apparatuses for transferring heat, such as heat exchangers, have been used in the past to transfer heat from one fluid (e.g., liquid or gas) to another. In furnaces, for example, one or more fuels, such as natural gas, propane, liquefied petroleum gas (LPG, LP, or GLP), butane, methane, heating oil, gasoline, alcohol, coal, wood, or biomass, has been burned and one or more heat exchangers having one or more stages has been used to transfer heat from the products of combustion to indoor air (e.g., air inside or delivered to a building or other occupied space), for instance. Further, furnaces have been incorporated into or formed heating, ventilation, and air conditioning units, various embodiments of which have included one or more fans, and some of which have included direct expansion air conditioning systems, for example. Furnaces and HVAC units have been used, for example, to change or control the temperature within buildings to provide a comfortable and safe environment for people to live or work, for example. 
     Counter-flow heat exchanges are known in the art, and furnaces have been made with multiple stages, some of which have had multiple passes. Typical furnaces have had a first stage having three passes, joined to a second stage that had a single pass and that had fins to enhance heat transfer. The efficiency of such furnaces was limited to about 90% (AFUE), but notions of adding further stages were rejected in order to keep heat exchangers and furnaces compact and inexpensive. 
     Thus, needs or potential for benefit exist for apparatuses for transferring heat, heat exchangers, furnaces, and HVAC units, that are more efficient, and yet are more compact than prior art alternatives having comparable efficiency or cost. Needs or potential for benefit also exist for such apparatuses for transferring heat, heat exchangers, furnaces, and HVAC units, that have connections between stages that provide for compactness and yet are reliable and cost competitive. Needs or potential for benefit also exist for methods of manufacturing or making such apparatuses for transferring heat, heat exchangers, furnaces, and HVAC units, that are conducive to mass production, cost effective, and reliable. Room for improvement exists over prior art in these and other areas that may be apparent to a person of ordinary skill in the art having studied this document. 
     SUMMARY OF PARTICULAR EMBODIMENTS OF THE INVENTION 
     This invention provides, among other things, various apparatuses for transferring heat, heat exchangers, furnaces, HVAC units, and methods of manufacturing or making such devices. Various embodiments provide, as objects or benefits, for example, that they are more efficient, more compact, less expensive, or a combination thereof, in comparison with various alternatives. Further, some embodiments are reliable, have short manufacturing times, produce high quality units, or a combination thereof. Other benefits of certain embodiments may be apparent to a person of ordinary skill in the art. Other embodiments of the invention, include various HVAC systems and buildings having such apparatuses for transferring heat, heat exchangers, furnaces, or HVAC units, as further examples. 
     In specific embodiments, this invention provides various apparatuses for transferring heat or heat exchangers, for example. In various embodiments, the apparatus may be, or may be part of, a furnace, an HVAC unit, an HVAC system, or a building that has an HVAC system, as examples. In a number of embodiments, such an apparatus may include, for example, a first heat-exchanger stage and a second heat-exchanger stage. In certain embodiments, the first heat-exchanger stage may include, for example, multiple parallel first-stage tubes. And in some embodiments, each first-stage tube may have a first 180 degree bend and a second 180 degree bend, for example. Further, in particular embodiments, the second heat-exchanger stage may include, for example, multiple parallel second-stage tubes. In some embodiments, each second-stage tube may have a third 180 degree bend, for instance. 
     In some such embodiments, the first-stage tubes and the second-stage tubes may be configured to contain within a flowing first fluid, for instance, and may be configured to transfer heat between the first fluid and a second fluid external to the first-stage tubes and to the second-stage tubes, for example. Further, in some such embodiments, when the apparatus, unit, or system is in operation, for instance, the second fluid may flow in a predominant flow direction past the first-stage tubes and past the second-stage tubes, for example. In some embodiments, this predominant flow direction may be up, for example. 
     Further, in a number of embodiments, the first 180 degree bend may be oriented at a first angle from the predominant flow direction, and in some such embodiments the absolute value of the first angle may be between 15 degrees and 75 degrees, for example. Moreover, in particular embodiments, the second 180 degree bend may be oriented at a second angle from the predominant flow direction, and in some such embodiments the absolute value of the second angle may be between 15 degrees and 75 degrees, for instance. Furthermore, in certain embodiments, the third 180 degree bend may be oriented at a third angle from the predominant flow direction, and in some such embodiments, the absolute value of the third angle may be less than 15 degrees, for example. In some such embodiments, the first angle and the second angle have opposite signs, for instance. 
     In other embodiments, the absolute value of the first angle may be between 30 degrees and 60 degrees, the absolute value of the second angle may be between 30 degrees and 60 degrees, the absolute value of the third angle may be less than 10 degrees, or a combination thereof. Further, in some embodiments, the absolute value of the first angle may be between 40 degrees and 50 degrees, the absolute value of the second angle may be between 40 degrees and 50 degrees, the absolute value of the third angle may be less than 5 degrees, or a combination thereof, as other examples. 
     In some embodiments, the first heat-exchanger stage and the second heat-exchanger stage may be arranged and connected in an order such that the first fluid passes first through the first heat-exchanger stage and then through the second heat-exchanger stage, for example. And in some embodiments, the first heat-exchanger stage and the second heat-exchanger stage may be arranged in an order such that the second fluid passes first through one pass of the second heat-exchanger stage, then through at least one pass of the first heat-exchanger stage, then through one pass of the second heat-exchanger stage, and then through at least one pass of the first heat-exchanger stage, for instance. Further, in various embodiments, the first-stage tubes have a first diameter and the second-stage tubes have a second diameter. In some embodiments, the first diameter may be substantially larger than the second diameter, for example. 
     The apparatus or unit may further include, in various embodiments, a junction plate, and in some such embodiments, one end of each of the first-stage tubes may terminate at the junction plate, at least one end of each of the second-stage tubes may terminate at the junction plate, or a combination thereof, for example. Further, in some embodiments, the apparatus or unit may further include, for instance, a first collector that seals against the junction plate and forms a first enclosed passageway that connects the first-stage tubes to the second-stage tubes. The first enclosed passageway may transfer the first fluid (e.g., combustion gasses) from the first-stage tubes to the second-stage tubes, for example. 
     Certain embodiments may include, for example, at least one, or even multiple burners. In some such embodiments, for instance, each of the first-stage tubes has an entrance end where air enters the first-stage tube, and a burner may be located at the entrance end to one or more (e.g., each) of the first-stage tubes. In a number of embodiments, each burner may be configured to burn a fuel (e.g., in one of the first-stage tubes), for example, forming combustion gasses. In some embodiments, for instance, combustion gasses from the air and the fuel specifically form the first fluid. 
     Some such apparatuses or units may further include, for example, a third heat-exchanger stage which may include, for instance, multiple parallel third-stage tubes. In some of these embodiments, the third heat-exchanger stage may include, for instance, multiple fins, which may be external to the third-stage tubes, for example. In specific embodiments, the first heat-exchanger stage has three passes through the second fluid, the second heat-exchanger stage has two passes through the second fluid, the third heat-exchanger stage has one pass through the second fluid, or a combination thereof, for example. 
     Moreover, in some embodiments, the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage may be arranged in an order such that the second fluid passes first through the third heat-exchanger stage, and then through at least one pass of the second heat-exchanger stage, and such that the second fluid passes through at least one pass of the first heat-exchanger stage last of the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage. Furthermore, in some embodiments, the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage may be arranged in an order such that the second fluid passes first through the third heat-exchanger stage, then through at least one pass of the second heat-exchanger stage, then through at least one pass of the first heat-exchanger stage, then through at least one pass of the second heat-exchanger stage, and then through at least one pass of the first heat-exchanger stage. In various embodiments, the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage may be arranged and connected in an order such that the first fluid passes first through the first heat-exchanger stage, then through the second heat-exchanger stage, and then through the third heat-exchanger stage. 
     Further, in a number of embodiments, the first-stage tubes have a first diameter, the second-stage tubes have a second diameter, and the third-stage tubes have a third diameter. In addition, in various embodiments, the first diameter may be substantially larger than the second diameter, the second diameter may be substantially larger than the third diameter, or both, for instance. In addition, or instead, in some embodiments, the first heat-exchanger stage has a first number of tubes, the second heat-exchanger stage has a second number of tubes, and the third heat-exchanger stage has a third number of tubes. Furthermore, in some embodiments, the first number of tubes may be equal to or one less than the second number of tubes, the third number of tubes may be substantially larger than the second number of tubes, or both, for example. 
     Further still, in certain embodiments, one end of each of the first-stage tubes terminates at the junction plate, two ends of each of the second-stage tubes terminate at the junction plate, one end of each of the third-stage tubes terminates at the junction plate, or a combination thereof, for example. In some embodiments, the apparatus may further include, for instance, the first collector (e.g., that seals against the junction plate and forms a first enclosed passageway that connects the first-stage tubes to the second-stage tubes), a second collector that seals against the junction plate and forms a second enclosed passageway that connects the second-stage tubes to the third-stage tubes, or both the first collector and the second collector, for example. The second enclosed passageway may transfer the first fluid (e.g., combustion gasses) from the second-stage tubes to the third-stage tubes, for example. 
     This invention also provides various methods, including methods of manufacturing or making certain apparatuses for transferring heat, heat exchangers, furnaces, HVAC units, HVAC systems and buildings, as examples. Particular embodiments include certain methods of making one or more compact and efficient furnaces, for example, to heat indoor air by burning a fuel. Such methods may include, for example, (e.g., in any order, unless a particular order is required or indicated) at least certain acts. In some embodiments, such acts may include, for example, an act of forming or obtaining first-stage tubes. In particular embodiments, each first-stage tube may have a first diameter, a first 180 degree bend, a second 180 degree bend, or a combination thereof, for example. Another act found in various embodiments is an act of forming or obtaining second-stage tubes. In certain embodiments, each second-stage tube may have a second diameter, a third 180 degree bend, or both, for instance. 
     Some methods also include an act of assembling a first heat-exchanger stage, for instance, using multiple of the first-stage tubes. In some such embodiments, the first heat-exchanger stage may include, for instance, multiple passes, such as a first pass, a second pass, and a third pass. Another act in various methods may be an act of assembling a second heat-exchanger stage, for instance, using multiple of the second-stage tubes. In some such embodiments the second heat-exchanger stage may include, for instance, one or more passes, such as a fourth pass and a fifth pass, for example. Further, some methods include an act of installing a burner, for example, at an entrance end of (e.g., each) first-stage tube. And some embodiments include an act of connecting the second heat-exchanger stage to the first heat-exchanger stage, for instance, so that, when the furnace is in operation, products of combustion from the burning of the fuel (e.g., at the burner) pass first through the first heat-exchanger stage and then through the second heat-exchanger stage, for example, passing first through the first-stage tubes and then through the second-stage tubes. 
     In some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, assembling the first heat-exchanger stage, and assembling the second heat-exchanger stage, for example, (or other acts) may include, for instance, forming or obtaining the first-stage tubes and the second-stage tubes, and assembling and arranging the first heat-exchanger stage and the second heat-exchanger stage, for example, so that indoor air passing through the furnace (e.g., when the furnace is in operation), passes first through the fifth pass, then through the third pass, then through the fourth pass, then through the second pass, and then through the first pass, for example. 
     In various embodiments, various acts, such as the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, assembling the first heat-exchanger stage, and assembling the second heat-exchanger stage, for example, may include, for instance, forming or obtaining the first-stage tubes and the second-stage tubes, and assembling and arranging the first heat-exchanger stage and the second heat-exchanger stage so that (e.g., when the furnace or unit is in operation), the indoor air flows in a predominant flow direction, for example, past the first-stage tubes, past the second-stage tubes, or both. In some such embodiments, the first 180 degree bend may be oriented at a first angle (e.g., from the predominant flow direction), and the absolute value of the first angle may be between 15 degrees and 75 degrees, for example. Similarly, in some embodiments, the second 180 degree bend may be oriented at a second angle (e.g., from the predominant flow direction), and the absolute value of the second angle may be between 15 degrees and 75 degrees, for instance. Further, in some embodiments, the third 180 degree bend may be oriented at a third angle (e.g., from the predominant flow direction), and the absolute value of the third angle may be less than 15 degrees, for example. In other embodiments, some or all of these angles may fall within other ranges described herein, as examples. Moreover, in a number of embodiments, various acts, such as the acts of forming or obtaining first-stage tubes and assembling the first heat-exchanger stage, for example, may include, for instance, forming or obtaining the first-stage tubes and assembling and arranging the first heat-exchanger stage so that the first angle and the second angle have opposite signs. 
     Certain embodiments may further include, for example, an act of forming or obtaining a third heat-exchanger stage, which may have multiple third-stage tubes, for example. In some such embodiments the third heat-exchanger stage may include, for instances a sixth pass. Further, in some embodiments, the act of forming or obtaining a third heat-exchanger stage may further include, or another act in a method may be or include, for instance, forming or obtaining a third heat-exchanger stage which may have multiple fins external to the third-stage tubes. 
     Various methods may further include, for example, an act of connecting the third heat-exchanger stage to the second heat-exchanger stage, for instance, so that (e.g., when the furnace or unit is in operation), products of combustion from burning of the fuel (e.g., at the burner) pass first through the first heat-exchanger stage, then through the second heat-exchanger stage, and then through the third heat-exchanger stage, for instance, passing first through the first-stage tubes, then through the second-stage tubes, and then through the third-stage tubes. Further, in some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, forming or obtaining the third heat-exchanger stage, assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and connecting the third heat-exchanger stage to the second heat-exchanger stage, for example, may further include, (or one or more other acts may include) for instance, assembling and arranging the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage so that indoor air passing through the furnace, for example, (e.g., when the furnace or unit is in operation) specifically passes first through the sixth pass, then through the fifth pass, then through the third pass, then through the fourth pass, then through the second pass, and then through the first pass, for example. 
     Additionally, various embodiments of methods may include, for example, (e.g., in any order) the acts of forming or obtaining a junction plate, forming or obtaining a first collector, forming or obtaining second collector, or a combination thereof, for instance. Further, in some embodiments, various acts, such as the acts of assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and connecting the third heat-exchanger stage to the second heat-exchanger stage, for example, may further include, for instance, (e.g., in any order) the acts of terminating one end of each of the first-stage tubes at the junction plate, terminating two ends of each of the second-stage tubes at the junction plate, terminating one end of each of the third-stage tubes at the junction plate, or a combination thereof, for instance. Some such embodiments may further include one or both acts of installing the first collector against the junction plate forming a first enclosed passageway, for example, that connects the first-stage tubes to the second-stage tubes, or installing the second collector against the junction plate forming a second enclosed passageway, for example, that connects the second-stage tubes to the third-stage tubes. 
     Furthermore, in some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes, forming or obtaining second-stage tubes, and forming or obtaining the third heat-exchanger stage, for example, may include, for instance, forming or obtaining the first-stage tubes and the second-stage tubes and forming or obtaining the third heat-exchanger stage so that the third-stage tubes have a third diameter. In some embodiments, the first diameter may be substantially larger than the second diameter, the second diameter may be substantially larger than the third diameter, or both, as examples. 
     Moreover, in some embodiments, particular acts, such as the acts of assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and forming or obtaining the third heat-exchanger stage include, for instance, assembling the first heat-exchanger stage, assembling the second heat-exchanger stage, and forming or obtaining the third heat-exchanger stage so that the first heat-exchanger stage has a first number of tubes, the second heat-exchanger stage has a second number of tubes, the third heat-exchanger stage has a third number of tubes. In certain embodiments, the first number of tubes may be equal to or one less than the second number of tubes, the third number of tubes may be substantially larger than the second number of tubes, or both, as examples. In addition, various other embodiments of the invention are also described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view illustrating, among other things, an example of an apparatus for transferring heat, which is an example of a heat exchanger, and which may be used in a furnace or an HVAC unit, as examples; 
         FIG. 2  is a side view illustrating a building having an HVAC system and an HVAC unit, which may include the apparatus of claim  1 , for example; 
         FIG. 3  is an isometric view of the apparatus for transferring heat of  FIG. 1 , from a different angle, showing, among other things, the fan that moves air and products of combustion through the apparatus; 
         FIG. 4  is a side view of the apparatus for transferring heat of  FIGS. 1 and 3 , showing, among other things, the junction plate; 
         FIG. 5  is the same side view as  FIG. 4 , of the apparatus for transferring heat of  FIGS. 1 and 3 , except with the junction plate removed, showing, among other things, the orientation angles of various 180 degree bends in the tubing, in the embodiment depicted, and the order of different passes of the heat exchanger; 
         FIG. 6  is a partial side view, taken from the same angle as  FIG. 5 , also with the junction plate removed, but showing the 180 degree bends of the second heat-exchanger stage oriented at a slightly different angle; and 
         FIG. 7  is a flow chart illustrating an example of a method of manufacturing a heat exchanger or a furnace for heating indoor air by burning a fuel, for example. 
     
    
    
     The drawings illustrate, among other things, various particular examples of embodiments of the invention, and certain examples of characteristics thereof. Different embodiments of the invention include various combinations of elements or acts shown in the drawings, described herein, known in the art, or a combination thereof, for example. 
     DETAILED DESCRIPTION OF EXAMPLES OF EMBODIMENTS 
     Among other things, various embodiments of the invention are, or include, apparatuses for transferring heat, or heat exchangers, for example. In various embodiments, the apparatus may be, or may be part of, a furnace, an HVAC unit, an HVAC system, or a building that has an HVAC system, as examples. Further, a number of embodiments are, or include, methods, including methods of manufacturing or making certain apparatuses for transferring heat, heat exchangers, furnaces, HVAC units, HVAC systems and buildings, as examples. Particular embodiments include certain methods of making one or more compact and efficient furnaces, for example, to heat indoor air by burning a fuel. 
     In various embodiments, examples of which are described in detail below, a second heat-exchanger stage may be added to an apparatus for transferring heat or a heat exchanger (e.g., in comparison with a typical prior art apparatus or heat exchanger). In different embodiments, this second heat-exchanger stage may be added in an unconventional way or configuration, examples of which are described herein, that improves efficiency without unduly increasing the size of the apparatus, while maintaining required radiuses of bends in heat exchanger tubing. In some embodiments, the second heat-exchanger stage is connected to the other stages in a novel way, for another example. 
     As described herein, various embodiments orient bends (e.g., 180 degree bends in heat-exchanger tubing) at particular angles to allow heat-exchanger stages to fit together more closely. Some embodiments provide improved structure for connecting stages, for example, using two collectors that seal against a common junction plate. Further, some embodiments provide for indoor air passing through the heat exchanger or apparatus to pass through different passes of the heat exchanger, apparatus, or stages, in a particular unconventional order in order to allow for surprisingly greater compactness. Some embodiments further restrict or combine such features, as described herein. 
     Specifically,  FIGS. 1 and 3  illustrate an example of an apparatus for transferring heat or a heat exchanger, apparatus  100 . In various embodiments, apparatus  100  may be part of a furnace, a heating, ventilation, and air conditioning unit (an HVAC unit), an HVAC system, or a building that has an HVAC system, as examples. For example,  FIG. 2  illustrates building  200  with HVAC unit  250  mounted on roof  202  of building  200 . HVAC unit  250 , along with supply air ductwork  275 , supply air registers  271 ,  272 , and  273 , thermostat or controller  274 , return air grille  278 , air filter  279 , and return air ductwork  277 , among other components, form HVAC system  270 . 
     Building  200 , in the embodiment illustrated, includes, besides HVAC system  270 , roof  202 , ceiling  203 , floor  201 , and walls  204 , among other components. In the embodiment shown, walls  204 , ceiling  203 , and floor  201  form enclosure  215  enclosing space  220 , which contains indoor air  210 . In the embodiment illustrated, indoor air  210  is drawn through return air grille  278 , through filter  279  and return air ductwork  277  to HVAC unit  250  by fan  255  (which is a component of HVAC unit  250 ). Indoor air  210  then passes through apparatus or heat exchanger  100  where indoor air  210  is heated, for example, and then indoor air  210  is blown (e.g., by fan  255 ) through supply air ductwork  275  and registers  271 ,  272 , and  273  back to space  220  within enclosure  215 . Fan  255  is shown in  FIG. 2  in one example of a location within HVAC unit  250 . In other embodiments, fan  255  may be located below heat exchanger  100  and may blow indoor air  210  upwards (e.g., in direction  110  shown in  FIGS. 1 and 3 ) through heat exchanger  100 , as another example. 
     In the embodiment illustrated, apparatus or heat exchanger  100  may be used to heat indoor air  210  when conditions warrant. In some embodiments, HVAC unit  250  may also include an air conditioning system, such as direct expansion air conditioning, for cooling indoor air  210  when conditions require cooling of space  220 . Thus, in various embodiments, air conditioning unit  250  may include a compressor, evaporator coil, condenser coil, expansion valve, condenser fan, etc. In the embodiment illustrated, HVAC unit  250  is a packaged unit, but in other embodiments, split system air conditioning systems may be used, as another example. Various fans may be driven by electric motors for example. In other embodiments, HVAC unit  250  may be (just) a furnace, and may lack an air conditioning system, for example, for climates or applications where cooling is not required. As used herein, the phrases “HVAC unit” and “HVAC system” are general terms and include units and systems that do not have air conditioning. As used herein, a stand-alone furnace, having a ventilation fan, is considered to be an HVAC unit. A furnace may also be part of an HVAC unit that further includes air conditioning, as such terms are used herein. 
     Returning to  FIGS. 1 and 3 , and also referring to  FIGS. 4 and 5 , in the embodiment illustrated, apparatus  100  includes, for example, first heat-exchanger stage  121 , second heat-exchanger stage  122 , and third heat-exchanger stage  123 . Some embodiments require, or involve improvements to, just first heat-exchanger stage  121  and second heat-exchanger stage  122 , for example. In the embodiment shown, first heat-exchanger stage  121  includes, for example, five parallel first-stage tubes  131 . Other embodiments may have a different number of first-stage tubes  131 , such as 1, 2, 3, 4, 6, 7, 8, 9, 10, 12, 15, or 20 first-stage tubes  131 , as examples, which (e.g., if multiple first-stage tubes are used) may be positioned in parallel as shown or in one or more rows. 
     In different embodiments, tubes (e.g.,  131 ) of a heat-exchanger stage (e.g.,  121 ) may be geometrically parallel (e.g., to within a manufacturing tolerance), may be parallel with respect to the flow of a fluid [e.g., the first fluid or combustion products contained within the tubes (e.g.,  131 ), as described below], or both. As used herein, unless apparent otherwise, “parallel” means either geometrically parallel (e.g., to within a manufacturing tolerance), parallel with respect to the flow of a fluid (e.g., the first fluid or combustion products, as described below), or both. Further, as used herein, “fluids” may be liquids, gasses, or a mixture thereof, in various embodiments. 
     In the embodiment illustrated, each first-stage tube  131  has a first 180 degree bend  141  and a second 180 degree bend  142 . Further, in the embodiment shown, the second heat-exchanger stage  122  includes five parallel second-stage tubes  132 . Other embodiments may have a different number of second-stage tubes  132 , such as 1, 2, 3, 4, 6, 7, 8, 9, 10, 12, 13, 15, 16, 20, or 21 second-stage tubes  132 , as examples, which may be positioned in parallel as shown (e.g., in one or more rows). In this embodiment, each second-stage tube  132  has a third 180 degree bend  143 . As used herein, 180 degree bends may vary slightly from an exact 180 degrees, but may need to meet required tolerances. In some embodiments, for example, tolerances for angle of bends, parallelness of tubing, orientation of angles, diameters, lengths, etc., may be consistent with other tolerances employed in similar prior art equipment, for instance. 
     In the embodiment illustrated, first-stage tubes  131  and second-stage tubes  132  are configured to contain within a flowing first fluid, and are configured to transfer heat between the first fluid and a second fluid (e.g., indoor air  210 ) external to first-stage tubes  131  and to second-stage tubes  132 . Further, in this embodiment, when apparatus  100 , HVAC unit  250 , or HVAC system  270 , for example, is in operation, the second fluid (e.g., indoor air  210 ) flows in a predominant flow direction  110  past first-stage tubes  131  and past second-stage tubes  122 , for example. In this embodiment, the predominant flow direction  110  is upwards (up). In other embodiments, however, the predominant flow direction (e.g.,  110 ) may be in another direction, such as downwards, horizontal, or at an angle, as examples. 
     Referring to  FIG. 5 , in a number of embodiments, first 180 degree bend  141  (e.g., of each first-stage tube  131 ) may be oriented at a first angle  501  from the predominant flow direction  110 . As used herein, the orientation of a 180 degree bend is measured from the perspective of 180 degree bends  141 ,  142 , or  143  shown in  FIG. 5 , for example. Further, as used herein, the orientation of a 180 degree angle, relative to the predominant flow direction  110 , for example, is, considered to be a positive angle (i.e., have a positive sign) if the orientation angle (less than 90 degrees) is to the right, and the orientation of a 180 degree angle, relative to the predominant flow direction  110 , for example, is, considered to be a negative angle (i.e., have a negative sign) if the orientation angle (less than 90 degrees) is to the left. Thus, first angle  501  of first 180 degree bend  141  (e.g., of each first-stage tube  131 ) has a positive angle (e.g., a positive sign). 
     In the embodiment illustrated, first angle  501  is about 45 degrees, specifically, and is positive. In various other embodiments, first angle  501  may have another angle, may be negative, or both, for instance. For example, in a number of embodiments, the absolute value of first angle  501  may be between 15 degrees and 75 degrees. In fact, in some embodiments, the absolute value of first angle  501  may be between 30 degrees and 60 degrees, and in particular embodiments, the absolute value of first angle  501  may be between 40 degrees and 50 degrees, as examples. 
     Additionally, in the embodiment illustrated, second 180 degree bend  142  (e.g., of each first-stage tube  131 ) is oriented at second angle  502  from the predominant flow direction. In the embodiment illustrated, second angle  502  is about 45 degrees, specifically, and is negative. In various other embodiments, second angle  502  may have another angle, may be positive, or both, for instance. For example, similar to first angle  501 , in a number of embodiments, the absolute value of second angle  502  may be between 15 degrees and 75 degrees. In fact, in some embodiments, the absolute value of second angle  502  may be between 30 degrees and 60 degrees, and in particular embodiments, the absolute value of second angle  502  may be between 40 degrees and 50 degrees, as examples. 
       FIG. 6  illustrates an alternate embodiment to second heat-exchanger stage  122 , namely, second heat-exchanger stage  622 . In this embodiment, third 180 degree bend  143  (e.g., of each second-stage tube  132 ) is oriented at third angle  603  from the predominant flow direction  110 . In the embodiment illustrated, third angle  603  is about 30 degrees, specifically, and is negative. In various other embodiments, third angle  603  may have another angle, may be positive, or both, for instance. For example, in a number of embodiments, the absolute value of third angle  603  may be less than 15 degrees. In fact, in some embodiments, the absolute value of third angle  603  may be less than 10 degrees, or even less than 5 degrees, as examples. In the embodiment shown in FIGS.  1  and  3 - 5 , for instance, the third angle (e.g., corresponding to angle  603 ) is about zero degrees, as another example. 
     Certain embodiments may include, for example, at least one, or even multiple burners. In the embodiment shown in FIGS.  1  and  3 - 5 , for instance, each of the first-stage tubes  131  has an entrance end  151  (e.g., shown in  FIG. 1 ) where air enters the first-stage tube  131 , and a burner  155  (e.g., shown in  FIGS. 1 and 3 ) is located at the entrance end  151  to each of the first-stage tubes  131 . In a number of embodiments, each burner (e.g.,  155 ) may be configured to burn a fuel, for example, in one of the first-stage tubes  131 . Such a fuel may be natural gas, propane, LPG, butane, or heating oil, as examples. In various embodiments, the burning of the fuel may form flue products or combustion gasses. In many embodiments, for instance, flue products or combustion gasses from the air and the fuel specifically form the first fluid referred to herein. 
     As shown in  FIGS. 1 ,  3 , and  4 , in the embodiment shown, apparatus  100  further includes junction plate  160 . In this embodiment, one end  351  (e.g., shown in  FIG. 3 ) of each of the first-stage tubes  131  terminates at junction plate  160 . In addition, in this embodiment, both (two) ends  352  of each of the second-stage tubes  132  terminate at junction plate  160 . Further, as shown in  FIG. 1 , in this particular embodiment, apparatus  100  further includes first collector  161 . First collector  161 , in this embodiment, seals against junction plate  160  forming a first enclosed passageway that connects first-stage tubes  131  to second-stage tubes  132 . The first enclosed passageway (e.g., formed between junction plate  160  and first collector  161 ) transfers the first fluid (e.g., combustion gasses) from the first-stage tubes  131  to the second-stage tubes  132 , in this embodiment. Thus, in the embodiment shown in FIGS.  1  and  3 - 5 , first heat-exchanger stage  121  and second heat-exchanger stage  122  are arranged and connected in an order such that the first fluid or combustion gasses pass first through first heat-exchanger stage  121  and then through second heat-exchanger stage  122 . 
     Further still, in the embodiment shown, one end of each of the third-stage tubes  133  terminates at junction plate  160 . In this embodiment, apparatus  100  further includes second collector  162  that seals against junction plate  160  and forms a second enclosed passageway that connects second-stage tubes  132  to third-stage tubes  133 . The second enclosed passageway, in this embodiment, transfers the first fluid (e.g., combustion gasses) from second heat-exchanger stage  122  to third heat-exchanger stage  123 . The first collector  161  and the second collector  162  may be located in or attached to the same (e.g., vertical) plane of junction plate  160 , for example, as shown. The first collector  161  and the second collector  162  may be attached to junction plate  160  with fasteners, in some embodiments, such as sheet metal screws, for instance. 
     Moreover, in the embodiment illustrated, first heat-exchanger stage  121 , second heat-exchanger stage  122 , and third heat-exchanger stage  123  are arranged and connected in an order such that the first fluid (e.g., combustion gasses) passes first through first heat-exchanger stage  121 , then through second heat-exchanger stage  122 , and then through third heat-exchanger stage  123 . Specifically, in the embodiment illustrated, fan  355  (shown in  FIG. 3 ) draws air or the first fluid (e.g., combustion gasses) from third heat-exchanger stage  123 , which draws (through connector  162 ) from second heat-exchanger stage  122 , which draws (through connector  161 ) from first heat-exchanger stage  121 , which draws air or the first fluid (e.g., combustion gasses) through burners  155 . In a number of embodiments, fan  355  may exhaust the first fluid or products of combustion outside of HVAC unit  250  and outside of building  200 , for example. 
     In a number of embodiments, since fan  355  draws the combustion gasses through apparatus  100  (e.g., rather than blowing air into burners  155 ), the pressure of the combustion gasses is lower than atmospheric pressure. In addition, in many embodiments, the pressure of the second fluid (e.g., indoor air  210 ) within apparatus  100  may be kept above atmospheric pressure (e.g., by blowing indoor air  210  into apparatus  100  with fan  255 ). Thus, if a leak develops, for example, within first heat-exchanger stage  121 , second heat-exchanger stage  122 , third heat-exchanger stage  123 , or connections (e.g., collectors  161  or  162 ) therebetween, the products of combustion do not leak into space  220  threatening occupants thereof, for example, with asphyxiation. 
     Referring to  FIG. 4 , in the embodiment illustrated, apparatus  100  includes third heat-exchanger stage  123 , which includes multiple parallel third-stage tubes  133 , which are arranged in two rows  471  and  472 . In the embodiment depicted, the third heat-exchanger stage  123  includes multiple fins  333  (e.g., shown in  FIG. 3 ), which are external to third-stage tubes  133 . In other embodiments, a third heat-exchanger stage may have third-stage tubes formed into 1, 3, 4, 5, 6, 7, 8, or 10 rows, as other examples. Further, in some embodiments, the third heat-exchanger stage may omit fins. 
     In some embodiments, some or all of the stages (e.g., first heat-exchanger stage  121 , second heat-exchanger stage  122 , and third heat-exchanger stage  123 ) may have multiple passes (e.g., through the second fluid or indoor air  210 ). Referring to  FIGS. 1 and 5 , in the embodiment illustrated, for example, first heat-exchanger stage  212  has three passes  571 ,  572 , and  573  (e.g., the first, second, and third passes) through the second fluid, the second heat-exchanger stage  122  has two passes  574  and  575  (e.g., the fourth and fifth passes) through the second fluid, and the third heat-exchanger stage  123  has one pass  576  (e.g., the sixth pass) through the second fluid. In different embodiments, some or all of the stages (e.g., first heat-exchanger stage  121 , second heat-exchanger stage  122 , and third heat-exchanger stage  123 ) may have 1, 2, 3, 4, 5, 6, or 7 passes (e.g., through the second fluid or indoor air  210 ), for example, in various combinations. 
     Furthermore, in the embodiment illustrated, the first heat-exchanger stage  121  and the second heat-exchanger stage  122  are arranged in an order such that the second fluid (e.g., indoor air  210 ) passes (e.g., flowing in the predominant airflow direction  110 , for example, up) first through one pass (e.g., pass  575 ) of second heat-exchanger stage  122 , then through one pass (e.g., pass  573 ) of first heat-exchanger stage  121 , then through one pass (e.g., pass  574 ) of second heat-exchanger stage  122 , and then through two passes (e.g.,  572  and then 571) of first heat-exchanger stage  121 . In various other embodiments, the first heat-exchanger stage and the second heat-exchanger stage may be arranged in an order such that the second fluid passes first through one or more passes of the second heat-exchanger stage, then through one or more passes of the first heat-exchanger stage, then through one or more passes of the second heat-exchanger stage, and then through one or more passes of the first heat-exchanger stage, for instance. 
     Such unconventional configurations or departures from typical multi-pass counter-flow heat exchangers, for example, may allow the first and second heat-exchanger stages (e.g.,  121  and  122  respectively) to be packed more closely together, for example, with a given radius of bend of the different 180 degree bends (e.g.,  141 ,  142 , and  143 ). This may allow the apparatus or heat exchanger (e.g.,  100 ) to be more compact, for example. The radius of the 180 degree bends (e.g.,  141 ,  142 , and  143 ) may be limited (e.g., to a minimum radius) based on the diameter of the tubing, wall thickness, material used, internal flow resistance that is acceptable, structural considerations, etc. In some cases, tighter bends may require the use of fittings, which may increase cost, provide more potential for leakage, increase internal flow resistance (and therefore fan power, for example, for fan  355 ), require more structural components, or the like, as examples. 
     In addition, in some embodiments, this unconventional configuration may surprisingly allow more stages (e.g., three stages instead of two) to be packed within the available space for the apparatus or heat exchanger (e.g.,  100 ), thus, allowing for a greater heat transfer efficiency. In the embodiment of apparatus  100 , for example, heat exchanger efficiency increased from 90% to 95% Annual Fuel Utilization Efficiency (AFUE) by adding second heat-exchanger stage  122 , and reached a steady state efficiency of 99%. Other embodiments may provide other levels of performance. Improvements in heat transfer efficiency reduce the amount of fuel that must be burned, thereby reducing fuel bills, reducing the emission of greenhouse gasses, and reducing dependency on sources of fossil fuels, among other potential benefits. 
     Further, in the embodiment illustrated, first-stage tubes  131  have a first diameter and second-stage tubes  132  have a second diameter. In the embodiment illustrated, the first diameter is substantially larger than the second diameter, for example. As used herein, substantially larger means larger by more than ten percent (10%). Specifically, in some embodiments, the first diameter (e.g., of first-stage tubes  131 ) is 1.75 inches, and the second diameter (e.g., of second-stage tubes  132 ) is 1.0 inches, for example. In other embodiments, the first diameter (e.g., of first-stage tubes  131 ) may be 1.25, 1.5, 1.625, 1.875, 2.0, 2.25, or 2.5 inches, or 3.5, 4.0, 4.5, 5.0 5.5, or 6.0 cm, as examples, and the second diameter (e.g., of second-stage tubes  132 ) may be 0.75, 0.875, 1.125, 1.25, or 1.5 inches, or 2.0, 2.25, 2.5, 2.75, or 3.0 cm, for example. 
     In some embodiments, first-stage tubes  131 , second-stage tubes  132 , plate  160 , covers  161  and  162 , or a combination thereof, may be made of aluminized steel, or stainless steel, as examples. In some embodiments, the third heat-exchanger stage  123  may be made of stainless steel, for example, 29-4C stainless steel. Further, in a number of embodiments, third-stage tubes  133  have a third diameter, and, in various embodiments, the second diameter (e.g., of second stage tubes  132 ) may be substantially larger than the third diameter, for instance. In the embodiment illustrated, the second diameter of second-stage tubes  132  is shown to be substantially larger than the third diameter of the third-stage tubes  133 . 
     Moreover, in some embodiments, the first heat-exchanger stage (e.g.,  121 ), the second heat-exchanger stage (e.g.,  122 ), and the third heat-exchanger stage (e.g.,  123 ) may be arranged in an order such that the second fluid (e.g., indoor air  210 ) passes first through the third heat-exchanger stage (e.g.,  123 ), and then through at least one pass of the second heat-exchanger stage, (e.g.,  122 ) and such that the second fluid passes through at least one pass of the first heat-exchanger stage (e.g.,  121 ) last of the first heat-exchanger stage, the second heat-exchanger stage, and the third heat-exchanger stage. In the specific embodiment illustrated, for example, first heat-exchanger stage  121 , second heat-exchanger stage  122 , and third heat-exchanger stage  123  are arranged in apparatus  100  in an order such that the second fluid (e.g., indoor air  210 ) passes first through pass  576  of third heat-exchanger stage  123 , and then through pass  575  of second heat-exchanger stage  122 , and such that the second fluid passes through pass  571  of first heat-exchanger stage  121  last of first heat-exchanger stage  121 , second heat-exchanger stage  122 , and third heat-exchanger stage  123 . 
     Furthermore, in some embodiments, the first heat-exchanger stage (e.g.,  121 ), the second heat-exchanger stage (e.g.,  122 ), and the third heat-exchanger stage (e.g.,  123 ) may be specifically arranged in an order such that the second fluid passes first through the third heat-exchanger stage (e.g.,  123 ), then through at least one pass of the second heat-exchanger stage (e.g.,  122 ), then through at least one pass of the first heat-exchanger stage (e.g.,  121 ), then through at least one pass of the second heat-exchanger stage (e.g.,  122 ), and then through at least one pass of the first heat-exchanger stage (e.g.,  121 ). In the specific embodiment illustrated, for example, first heat-exchanger stage  121 , second heat-exchanger stage  122 , and third heat-exchanger stage  123  are arranged in apparatus  100  in an order such that the second fluid (e.g., indoor air  210 ) passes first through pass  576  of third heat-exchanger stage  123 , then through pass  575  of second heat-exchanger stage  122 , then through pass  573  of first heat-exchanger stage  121 , then through pass  574  of second heat-exchanger stage  122 , and then through pass  572  and pass  571  of first heat-exchanger stage  121 . 
     In various embodiments, the first heat-exchanger stage (e.g.,  121 ) has a first number of tubes, the second heat-exchanger stage (e.g.,  122 ) has a second number of tubes, and the third heat-exchanger stage (e.g.,  123 ) has a third number of tubes. In some embodiments, the first number of tubes may be equal to or less than (e.g., one less than) the second number of tubes, the third number of tubes may be substantially larger than the second number of tubes, or both, for example. In the embodiment illustrated, for example, the first number of tubes and the second number of tubes are both five, and the third number of tubes is 36, which is substantially larger than the second number of tubes. As mentioned, other embodiments may have different numbers of tubes. For example, the third number of tubes (e.g., of third heat-exchanger stage  123 ) may be 10, 12, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 38, 40, 42, 44, 46, 48, or 50, as examples. 
     Various embodiments of the invention are, or include, methods, such as methods of manufacturing or making certain apparatuses for transferring heat, heat exchangers, furnaces, HVAC units, HVAC systems and buildings, such as those described herein, as examples. Particular embodiments include certain methods of making one or more compact and efficient furnaces, for example, to heat indoor air by burning a fuel. Such methods may include, for example, (i.e., in any order, unless a particular order is required or indicated) at least certain acts. 
       FIG. 7  illustrates an example of such a method, method  700 . In the embodiment shown, method  700  includes act  731  of forming or obtaining first-stage tubes (e.g., tubes  131  described herein). In particular embodiments, each first-stage tube (e.g.,  131 ) may have a first diameter (e.g., 1.75 inches), a first 180 degree bend (e.g.,  141 ), a second 180 degree bend (e.g.,  142 ), or a combination thereof, for example. Another act found in various embodiments, and illustrated in  FIG. 7 , is act  732  of forming or obtaining second-stage tubes (e.g.,  132 ). In certain embodiments, each second-stage tube (e.g.,  132 ) may have a second diameter (e.g., 1.0 inches), a third 180 degree bend (e.g.,  143 ), or both, for instance. In some embodiments, acts  731 ,  732 , or both, may include obtaining tubing of the desired diameter and material, cutting the tubing to the desired lengths, and bending the tubing into the desired shapes (e.g., forming bend  141 ,  142 ,  143 , or a combination thereof), for instance, using a tubing bender. In other embodiments, such tubing may be ordered and obtained already cut to length, already bent, or both, as further examples. 
     In the embodiment illustrated, method  700  also includes act  733  of forming or obtaining third-stage tubes (e.g., tubes  133  described herein). In some embodiments, act  733  may include obtaining tubing of the desired diameter and cutting the tubing to the desired lengths, for example. In other embodiments, the third-stage tubes (e.g., tubes  133  described herein) may be obtained already cut to length, may have fins (e.g.,  333 ) already attached, or both, as other examples. Additionally, method  700  further includes, act  760  of forming or obtaining a junction plate (e.g., junction plate  160  described herein), act  761  of forming or obtaining a first collector (e.g., first collector  161  described herein), and act  762  of forming or obtaining second collector (e.g., second collector  162  described herein). These components may be made of sheet metal, such as steel, aluminized steel, or stainless steel, as examples, and acts  760 ,  761 , and  762  may include cutting the sheet metal, stamping, bending, etc. In other embodiments, these components may be obtained precut, pre-bent, or both, as examples. 
     In the embodiment depicted, method  700  also includes act  721  of assembling a first heat-exchanger stage (e.g., stage  121  described herein). Act  721  may be accomplished using multiple (e.g., five) of the first-stage tubes (e.g.,  131 ) formed in act  731 , for example. In some such embodiments, the first heat-exchanger stage (e.g.,  721 , for instance, assembled in act  721 ) may include, for example, multiple passes, such as a first pass (e.g., pass  571  shown in  FIG. 5 ), a second pass (e.g., pass  572 ), and a third pass (e.g., pass  573 ). Another act in method  700  is act  722  of assembling a second heat-exchanger stage (e.g., stage  122  described herein), for instance, using multiple (e.g., five) of the second-stage tubes (e.g.,  132 ) formed in act  732 , for example. In some such embodiments the second heat-exchanger stage (e.g.,  121 , for example, assembled in act  722 ) may include, for instance, one or more passes, such as a fourth pass (e.g.,  574 ) and a fifth pass (e.g.,  575 ), for example. In other embodiments, the first or second heat-exchanger stages (or both) may be obtained already formed, as other examples. 
     Yet another act in method  700 , in the embodiment shown, is act  723  of forming or obtaining a third heat-exchanger stage (e.g., stage  123  described herein), for instance, using multiple (e.g.,  36 ) of the third-stage tubes (e.g.,  133 ) formed in act  733 , for example. In some such embodiments the third heat-exchanger stage (e.g.,  123 , for example, formed or obtained in act  723 ) may include, for instance, one pass, such as a sixth pass (e.g.,  576 ). In some embodiments, act  723  may include installing multiple fins (e.g.,  333  shown in  FIG. 3 ), which may be external to the third-stage tubes (e.g.,  133 ), and may be bonded to the third-stage tubes (e.g.,  133 ), for example, with solder or an interference fit, as examples. In some embodiments, for example, the third-stage tubes (e.g.,  133 ) may be expanded to create an interference fit with holes in the fins (e.g.,  333 ) by passing a bullet through the third-stage tubes (e.g.,  133 ). In some embodiments, the third heat-exchanger stage (e.g., stage  123  described herein) may be obtained already assembled, as another example. 
     Further, method  700  includes act  755  of installing one or more burners (e.g.,  155  shown in  FIGS. 1 and 3 ). In some embodiments, for example, a burner (e.g.,  155 ) may be installed (e.g., in act  755 ), at an entrance end (e.g.,  151 ) of (e.g., each) first-stage tube (e.g.,  131 ). Further still, method  700  also includes act  781  of connecting the second heat-exchanger stage (e.g.,  122 ) to the first heat-exchanger stage (e.g.,  121 ) (or vice versa). In some embodiments, the second heat-exchanger stage (e.g.,  122 ) may be connected (e.g., in act  781 ) to the first heat-exchanger stage (e.g.,  121 ), for instance, so that, when the furnace or unit (e.g., HVAC unit  250  shown in  FIG. 2 ) is in operation, products of combustion from the burning of the fuel (e.g., at burner or burners  155 ) pass first through the first heat-exchanger stage (e.g.,  121 ) and then through the second heat-exchanger stage (e.g.,  122 ), for example, passing first through the first-stage tubes (e.g.,  131 ) and then through the second-stage tubes (e.g.,  132 ). 
     In some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes (e.g., act  731 ), forming or obtaining second-stage tubes (e.g., act  732 ), assembling the first heat-exchanger stage (e.g., act  721 ), and assembling the second heat-exchanger stage (e.g., act  722 ), for example, may include, for instance, forming or obtaining the first-stage tubes (e.g.,  131 ) and the second-stage tubes (e.g.,  132 ), and assembling and arranging the first heat-exchanger stage (e.g.,  121 ) and the second heat-exchanger stage (e.g.,  122 ), for example, so that indoor air (e.g.,  210  shown in  FIG. 2 ) passing through the furnace (e.g., HVAC unit  250 , for example, when the furnace or unit is in operation, for instance, when fan  255  is operating), passes (e.g., in predominant air flow direction  110 ) first through the fifth pass (e.g., pass  575  shown in  FIG. 5 ), then through the third pass (e.g., pass  573 ), then through the fourth pass (e.g., pass  574 ), then through the second pass (e.g., pass  572 ), and then through the first pass (e.g., pass  571 ), for example. 
     Further, in various embodiments, various acts, such as the acts of forming or obtaining first-stage tubes (e.g., act  731 ), forming or obtaining second-stage tubes (e.g., act  732 ), assembling the first heat-exchanger stage (e.g., act  721 ), and assembling the second heat-exchanger stage (e.g., act  722 ), for example, may include, for instance, forming or obtaining the first-stage tubes (e.g.,  131 ) and the second-stage tubes (e.g.,  132 ), and assembling and arranging the first heat-exchanger stage (e.g.,  121 ) and the second heat-exchanger stage (e.g.,  122 ) so that (e.g., when the furnace or unit, for example, 250, is in operation), the indoor air (e.g.,  210 ) flows in a predominant flow direction (e.g.,  110 ), for example, past the first-stage tubes (e.g.,  131 ), past the second-stage tubes (e.g.,  132 ), or both. 
     In some such embodiments, the first 180 degree bend (e.g.,  141 , which may have been formed or obtained in act  731 , for example) may be oriented at a first angle (e.g.,  501  shown in  FIG. 5 , for instance, from the predominant flow direction  110 ), and the absolute value of the first angle (e.g.,  501 ) may be between 15 degrees and 75 degrees, for example. Similarly, in some embodiments, the second 180 degree bend (e.g.,  142 , which may have been formed or obtained in act  732 , for example) may be oriented at a second angle (e.g.,  502  from the predominant flow direction  110 ), and the absolute value of the second angle (e.g.,  502 ) may (e.g., also) be between 15 degrees and 75 degrees, for instance. Further, in some embodiments, the third 180 degree bend (e.g.,  143 ) may be oriented at a third angle (e.g., shown in  FIG. 6 , for instance, from the predominant flow direction  110 ), and the absolute value of the third angle (e.g.,  603 ) may be less than 15 degrees, for example. In other embodiments, some or all of these angles (e.g.,  501 ,  502 , and  603 ) may fall within other ranges described herein, or may have values identified herein, as examples. 
     Moreover, in a number of embodiments, various acts, such as the acts of forming or obtaining first-stage tubes (e.g., act  731 ) and assembling the first heat-exchanger stage (e.g., act  721 ), for example, may include, for instance, forming or obtaining the first-stage tubes (e.g.,  131 ) and assembling and arranging the first heat-exchanger stage (e.g.,  121 ) so that the first angle (e.g.,  501 ) and the second angle (e.g.,  502 ) have opposite signs (e.g., are in opposite directions from predominant flow direction  110 ). 
     Method  700  also includes, in the embodiment illustrated, act  782  of connecting the third heat-exchanger stage (e.g.,  123 ) to the second heat-exchanger stage (e.g.,  122 ), for instance, so that (e.g., when the furnace or unit, for example, HVAC unit  250 , is in operation), products of combustion from burning of the fuel (e.g., at burners  155 ) pass first through the first heat-exchanger stage (e.g.,  121 ), then through the second heat-exchanger stage (e.g.,  122 ), and then through the third heat-exchanger stage (e.g.,  123 ), for instance, passing first through the first-stage tubes (e.g.,  131 ), then through the second-stage tubes (e.g.,  132 ), and then through the third-stage tubes (e.g.,  133 ). 
     Further, in some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes (e.g., act  731 ), forming or obtaining second-stage tubes (e.g., act  732 ), forming or obtaining the third heat-exchanger stage (e.g., act  723 ), assembling the first heat-exchanger stage (e.g., act  721 ), assembling the second heat-exchanger stage (e.g., act  722 ), and connecting the third heat-exchanger stage to the second heat-exchanger stage (e.g., act  782 ), for example, may further include, (or one or more other acts may include) for instance, assembling and arranging the first heat-exchanger stage (e.g.,  121 ), the second heat-exchanger stage (e.g.,  122 ), and the third heat-exchanger stage (e.g.,  123 ) so that indoor air (e.g.,  210 ) passing through the furnace (e.g., HVAC unit  250  shown in  FIG. 2 ), for example, (e.g., when the furnace or unit  250 , or fan  255 , for instance, is in operation) passes (e.g., flowing upwards or in predominant airflow direction  110 ) first through the sixth pass (e.g.,  576  shown in  FIG. 5 ), then through the fifth pass (e.g.,  575 ), then through the third pass (e.g.,  573 ), then through the fourth pass (e.g.,  574 ), then through the second pass (e.g.,  572 ), and then through the first pass (e.g.,  571 ), for example. 
     Further, in some embodiments, various acts, such as the acts of assembling the first heat-exchanger stage (e.g., act  721 ), assembling the second heat-exchanger stage (e.g., act  722 ), and connecting the third heat-exchanger stage to the second heat-exchanger stage (e.g., act  782 ), for example, may further include, for instance, (e.g., in various orders) terminating one end (e.g., end  351  shown in  FIG. 3 ) of each of the first-stage tubes (e.g.,  131 ) at the junction plate (e.g.,  160 ), terminating two ends (e.g.,  352 ) of each of the second-stage tubes (e.g.,  132 ) at the junction plate (e.g.,  160 ), terminating one end (e.g., end  453  shown in  FIG. 4 ) of each of the third-stage tubes (e.g.,  133 ) at the junction plate (e.g.,  160 ), or a combination thereof, for instance. Some such embodiments may further include (e.g., within act  781 ), installing a first collector (e.g.,  161  shown in  FIG. 1 ) against the junction plate (e.g.,  160 ) forming a first enclosed passageway, for example, that connects the first-stage tubes (e.g.,  131 ) to the second-stage tubes (e.g.,  132 ). Further, some embodiments may further include (e.g., within act  782 ), installing a second collector (e.g.,  162  shown in  FIG. 1 ) against the junction plate (e.g.,  160 ) forming a second enclosed passageway, for example, that connects the third-stage tubes (e.g.,  133 ) to the second-stage tubes (e.g.,  132 ). 
     Furthermore, in some embodiments, various acts, such as the acts of forming or obtaining first-stage tubes (e.g., act  731 ), forming or obtaining second-stage tubes (e.g., act  732 ), forming or obtaining the third-stage tubes (e.g., act  733 ) and forming or obtaining the third heat-exchanger stage (e.g., act  723 ), for example, may include, for instance, forming or obtaining the first-stage tubes (e.g.,  131 ) and the second-stage tubes (e.g.,  132 ) and forming or obtaining the third heat-exchanger stage (e.g.,  123 ) so that the third-stage tubes (e.g.,  133 ) have a third diameter. In some embodiments, the first diameter (e.g., of first-stage tubes  131 ) may be substantially larger than the second diameter (e.g., of second-stage tubes  132 ), the second diameter (e.g., of second-stage tubes  132 ) may be substantially larger than the third diameter (e.g., of third-stage tubes  133 ), or both, as examples. 
     Moreover, in some embodiments, particular acts, such as the acts of assembling the first heat-exchanger stage (e.g., act  721 ), assembling the second heat-exchanger stage (e.g., act  722 ), and forming or obtaining the third heat-exchanger stage (e.g., act  723 ), for example, may include, for instance, assembling the first heat-exchanger stage (e.g.,  121 ) and assembling the second heat-exchanger stage (e.g.,  122 ) and forming or obtaining the third heat-exchanger stage (e.g.,  123 ) so that the first heat-exchanger stage (e.g.,  121 ) has a first number of tubes, the second heat-exchanger stage (e.g.,  122 ) has a second number of tubes, and the third heat-exchanger stage (e.g.,  123 ) has a third number of tubes. In certain embodiments, the first number of tubes (e.g.,  5 ) may be equal to or one less than (or a different number less than) the second number of tubes (e.g.,  5  or  6 ), the third number of tubes (e.g.,  36 ) may be substantially larger than the second number of tubes, or both, as examples. 
     Other embodiments may be apparent to a person of ordinary skill in the art having studied this document, and may include features or limitations described herein, shown in the drawings, or both. Various methods may include part or all of the acts shown in  FIG. 7 , described herein, or known in the art, as examples.