the embodiments of the invention describe below are adapted for use in either a compact residential hot air furnace or a commercial roof top furnace although it is to be understood the invention defines like use in other furnace units . the heat exchanger described below is adapted for use either by itself or in combination with a series of other heat exchanger elements . furthermore , the heat exchanger as hereinafter described will be of a particular configuration but it is to be understood that variations of that configuration are included within the scope of this invention . more specifically variations in the arcs formed by the curved portions , whether or not the straight portions are parallel , the particular cross section dimensions of the various portions of the heat exchanger , the number of straight sections or curved sections , or in general many of the specific details hereinafter described may be altered without adversely effecting the overall design and efficiency of the heat exchanger . referring now to the drawings . fig1 shows an orthographic view of a typical compact hot air furnace 10 . therein the furnace 10 has a sheet metal outer covering 28 which encases a series of five heat exchangers 20 , blower 14 , burners 18 , one for each heat exchanger and gas and pressure regulator 16 . burners 18 are so arranged that they receive natural gas from the gas and pressure regulator 16 . this gas is injected by burner 18 into the open end of heat exchanger 20 . as a part of the injection process air is drawn into the heat exchanger so that the gas and the air may be combusted within the heat exchanger . header 22 connected to the exhaust portion of each of the heat exchangers is also connected to induction draft unit 24 which creates a suction pressure through the heat exchangers to exhaust the discharge gases resulting from combustion through opening 26 to the discharge flue . blower 14 receives cold room air from the area which is to be heated , forces that air over the heat exchanger surfaces in the direction indicated by arrow 12 and said air is then collected and returned to the rooms to be heated ( the collection and return of the air not being shown ). since the induction draft blower 24 is only operated when the furnace is being fired by the burners , the blower serves to impede exhaust gases from exiting the furnace after shutdown performing a similar function as a flue damper . fig2 an elevational view of the heat exchanger element , discloses the various portions of the heat exchanger 20 when shown in the embodiment of a four pass unit . the gas and air mixture from the burners enters the heat exchanger through entry portion 101 . entry portion 101 is located in registration with the burner so that there is a smooth flow of both fuel and air into the heat exchanger . combustion portion 102 of the heat exchanger connected to portion 101 is the area in which the bulk of the fuel is combusted with the air to provide heat , both radiant and convectant . combustion portion 102 is generally elliptical in cross section as is entry portion 101 and increases in cross sectional thickness from the juncture of combustion portion 102 with entry portion 101 to cross section line iv -- iv which indicates the thickest point of the heat exchanger . thereafter combustion portion 102 gradually converges in cross sectional thickness . connected to combustion portion 102 is a first curved portion 103 elliptical in cross section and generally converging in cross sectional thickness . connected to first cuved portion 103 is first straight portion 104 elliptical in cross section and generally converging in cross sectional thickness along the flow path of the products of combustion from entry into the heat exchanger to discharge from the heat exchanger . connected to the first straight portion 104 is second curved portion 105 also generally elliptical in cross section and decreasing in cross sectional thickness . transition portion 106 is connected to second curved portion 105 and at their intersection is generally elliptical in cross section and at the other end of transition portion 106 , said portion is generally rectangular in cross section . second straight portion 107 is connected to transition portion 106 is generally rectangular in cross section and has a relatively constant cross sectional thickness . the third curved portion 108 is connected to second straight portion 107 and has a generally rectangular cross section of increasing thickness along the flow path . the third straight portion 109 is generally rectangular in cross section and relatively constant in thickness and is also connected to the third curved portion 108 . exit portion 110 is connected to the third straight portion 109 is generally rectangular in cross section and a relatively constant thickness . exit portion 110 is adapted to be connected to the induction draft header from which the combustion products are exhausted from the heat exchanger . fig3 , 5 , 6 , 7 , 8 , 9 and 10 all reflect elliptical cross sections having varying thicknesses and configurations depending upon the location within the heat exchanger . fig1 , 12 and 13 all reflect cross sections having generally rectangular configurations of varying thicknesses depending on their position within the heat exchanger . fig1 is a view of a portion of transition portion 106 showing the transition from an elliptical cross section to a rectangular cross section and the consequent narrowing in the thickness of the heat exchanger at that point . furthermore dimples 30 as shown on fig2 and as shown in cross section on fig1 are provided within the rectangular cross sectional area portions of the heat exchanger . a reducing section comprises the part of combustion portion 102 from point 115 to the junction of combustion portion 102 with first curved portion 103 , first curved portion 103 , first straight portion 104 and second curved portion 105 . this reducing section is designed so that the cross sectional thickness is constantly converging along the flow path so that the flow of the products of combustion through the heat exchanger must constantly increase in velocity . this increase in velocity together with the combination of the decrease in temperature of the combustion products by the transfer of heat through the heat exchanger surfaces is designed to result in a relatively constant surface temperature throughout the reducing section . the advantage of having a relatively constant surface temperature is that upon heating of the unit on startup and on cooling of the unit after the burner is turned off , the reducing section will expand and contract uniformly , thereby limiting thermal stresses within the heat exchanger . the level section of the heat exchanger , second straight portion 107 , third curved portion 108 , third straight portion 109 , and exit portion 110 , are all designed to have the narrowest practical thickness possible for the desired flow rate . this reduction in cross sectional area enables the combustion products to be accelerated so that the surface temperature of the heat exchanger is maintained as high as possible while the temperature of the combustion products is being reduced to that temperature just above the condensation point for the combustion products . in other words it is desirable to have the combustion products at the point of discharge at a temperature slightly higher than the temperature at which condensation will occur in the unit during the operation . should condensation occur in the unit , the unit will be subject to the corrosive and other damaging forces due to condensate collection and the overall heat exchanger life will be greatly reduced . the elliptical portions of the heat exchanger are designed with sufficient wall thickness that upon heating and cooling the convex half will expand outwardly thereby preventing any severe reduction in the cross sectional area of the heat exchanger at any specific point . the level section of the heat exchanger is provided with a plurality of dimples 30 , each dimple being a depression in the heat exchanger surface designed to contact a similar depression in the opposite surface so that the heat exchanger surfaces are held apart thereby providing the minimum flow area desired . the general direction of the flow of air over the heat exchanger surface is as shown by arrow 12 perpendicular to the direction of the flow path in the various straight portion , the second straight portion and the third straight portion . the various straight portions which intersect the direction of the air flowing over the heat exchanger are all connected by semicircular portions heretofore referred to as the first curved portion , the second curved portion and the third curved portion . it is to be understood that providing a series of heat exchanger portions perpendicular to the direction of flow of the air to be heated results in a maximum of heat transfer to the air to be heated for a given volume flow rate . furthermore , the particular cross sectional configurations as shown in fig3 - 14 are designed to prevent any hot spots from occurring across the cross section of the various heat exchanger portions . the dimples 30 have an additional function besides preventing collapse of the heat exchanger unit upon heating and cooling . these dimples further create obstacles within the air flow stream of the level section of the heat exchanger thereby creating mixing which further increase the velocity of the combustion products at that point . the heat exchanger herein described is furthermore capable of being mounted in any flow direction i . e . it may be mounted for horizontal or vertical burner input and upflow , downflow or across flow or any combination thereof of the air to be heated . another embodiment of this invention , a straight thru heat exchanger is shown in fig1 and 17 . this particular configuration is designed for a commercial furnace wherein the overall length of the heat exchanger is less critical than the size requirements for residential units . the straight thru heat exchanger is adapted to receive fuel and air from an in - shot type burner and is adapted to be connected to an induction draft blower to create a specific internal flow rate . air to be heated is passed over the surface of the heat exchanger while the fuel and combustion air pass thru the heat exchanger . referring now to fig1 and 17 it can be seen that entry portion 201 is flared to receive the fuel and air for combustion , said portion being mounted in registration with the burner ( not shown ). connected to entry portion 201 is combustion portion 202 having elliptical cross section of increasing or constant cross sectional thickness . connected thereto is reducing portion 203 having an elliptical cross section of gradually decreasing cross sectional thickness . connected to reducing portion 203 is first level portion 204 wherein the cross section is transformed from a generally elliptical shape to a generally rectangular shape . second level portion 205 having a constant thickness rectangular cross section is connected to the first level portion 204 . discharge portion 206 is flared and connected to second level portion 205 , the discharge portion being adapted to be connected to an induced draft blower for drawing the fuel and air through the heat exchanger . as can be seen in fig1 , 17 , and 22 - 28 the transitions from the reducing portion 203 to the first level portion 204 , and the first level portion 204 to the second level portion 205 are parabolic in shape . hence a single cross section such as shown in fig2 contains part of all three of these portions . since in this transition area no portion extends fully across a section of the heat exchanger the reference to a portion being rectangular or elliptical in cross section applies only to that part of the cross section within the portion . both the first level portion 204 and the second level portion 205 have dimples 30 therein . each half of the heat exchanger has the dimples so that upon heating and cooling the flow path of the heating fluid , fuel and air , will not be reduced in cross sectional area as a result of internal thermal stresses . this straight thru embodiment provides for a relatively constant surface temperature in the reducing portion since the velocity of the heating fluid is increased to compensate for the transfer of heat to the heat exchanger . the overall cross sectional area in the first level portion is less than the least area part of the reducing portion to provide for a further increase in heating fluid velocity . the second level portion is of a yet smaller cross sectional area so that the velocity is further increased to the point at which the heating fluid leaving the heat exchanger is only slightly above the temperature at which condensate will form on the heat exchanger surfaces during operation . many modifications to the embodiments described above can be used with this invention . for example , altering the length of any particular portion , the thickness of any particular portion , the cross sectional configuration of any particular portion , the heat input or any of the other numerous factors involved results in only slight overall efficiency changes . these types of variations are contemplated within the spirit and the scope of this invention . the invention has been described in detail the particular reference to two embodiments thereof , but it is to be understood that variations and modifications can be effected within the spirit and scope of the invention .