United States Patent 3813039

A heat exchanger connected to the smoke pipe of a central heating furnace below the horizontal plane of the smoke pipe and with an induced draft such that upon operation of the induced draft, the smoke pipe is blocked requiring the hot flue gasses to bypass through the heat exchanger and when the furnace operates without operation of the induced draft, the smoke pipe is open for direct flow of hot flue gasses.

Application Number:
Publication Date:
Filing Date:
Primary Class:
Other Classes:
110/304, 122/DIG.1, 126/110R, 165/122, 165/901
International Classes:
F28D21/00; (IPC1-7): F24B7/04
Field of Search:
237/55 126
View Patent Images:
US Patent References:
2468909Auxiliary air heater1949-05-03Yeager et al.
2362940Control for fuel economizers1944-11-14Skerritt
2174710Air heating and conditioning apparatus1939-10-03Wagner
1840836Heat exchanging apparatus1932-01-12Davis, Jr.
1785334Air heater1930-12-16Black
1720536Heat exchanger1929-07-09Young

Primary Examiner:
Wayner, William E.
Assistant Examiner:
Tapolcai Jr., W. E.
Attorney, Agent or Firm:
Tarrant, Thomas N.
I claim

1. A heat exchanger for insertion in the smoke pipe of a central heating unit between the heating unit and a chimney flue therefore comprising:

2. A heat exchanger according to claim 1 wherein said damper is normally open.

3. A heat exchanger according to claim 1 wherein operation of said induced draft means produces a pressure differential in said smoke pipe across said damper causing it to close.

4. A heat exchanger according to claim 1 wherein said direct smoke pipe is in a substantially horizontal position entirely above said plurality of smoke passages.

5. A heat exchanger according to claim 1 wherein said induced draft means is a blower.

6. A heat exchanger according to claim 1 wherein said means to provide a flow of heat distributing medium is a blower to provide a flow of air.

7. An auxiliary heat exchanger for a furnace in combination with and bypassing a direct section of smoke pipe for connection between a furnace and a chimney flue comprising:

8. An auxiliary heat exchanger according to claim 7 wherein said induced draft means is a blower located in said outlet passage.

9. An auxiliary heat exchanger according to claim 7 wherein said induced draft means coacts with said damper by placing a pressure differential across said damper effecting closure thereof.

10. An auxiliary heat exchanger according to claim 7 wherein said spaced flues are positioned vertically within an enclosure and air is circulated through said enclosure around said spaced flues and out as a space heating medium.


1. Field of the Invention

The present invention relates to heat exchangers and in particular to auxiliary heat exchangers for central heating units improving the efficiency of such units.

2. Relation to the Prior Art

In the everlasting battle to decrease the cost and size of central heating installations, it has become a common practice to minimize the heat exchanger so that frequently a substantial portion of the generated heat passes wastefully up the chimney. In many instances, where space is available, it is economical to provide an auxiliary heat exchanger to absorb heat from the flue gasses of the smoke pipe. Many such heat exchangers are known which surround and take heat from the smoke pipe itself. U.S. Pat. Nos. 3,124,197; 2,468,909 and 2,362,940 are exemplary. More effective compact heat exchangers require a tortuous path for the flue gasses impeding and interfering with the draft. To overcome the draft problem, an induced draft fan is known as disclosed in U.S. Pat. No. 2,174,710.

When a tortuous path external to the furnace unit is provided for flue gasses, a number of safety hazards occur. Not only is the draft for proper operation of the burner likely to fail, but there is a much greater likelihood of dangerous leaks where the flue gasses may end up in living space.


In accordance with the present invention, a heat exchanger has been provided bypassing a section of smoke pipe from a central heating furnace in which the heat exchanger lies below the horizontal plane of the bypassed smoke pipe and the bypassed section of smoke pipe contains a damper. This damper is forced to the closed position by operation of an induced draft device in the heat exchanger unit and opens during operation of the furnace when the induced draft device is not operating whereby normal flue draft is effective and the heat exchanger is effectively isolated from the flow of flue gasses in the absence of induced draft. Thus, it is an object of the invention to provide a novel auxiliary heat exchanger for central heating systems.

It is a further object of the invention to provide an induced draft system for an auxiliary heat exchanger in which the induced draft device coacts with a smoke pipe damper so as to place the heat exchanger in the path of flue gasses only during operation of the induced draft.

Further objects and features of the invention will be better understood after reading the following description together with the drawing.


FIG. 1 is a side elevation partially in section of a heat exchanger according to the invention.

FIG. 2 is a plan view in section along line 2--2 of FIG. 1.

FIG. 3 is a sectional view of the smoke pipe of FIG. 1 during heating unit operation in the absence of induced draft.


The invention provides an auxiliary heat exchanger for a central heating system which bypasses a section of the furnace smoke pipe only during operation of the auxiliary unit and substantially extracts the heat from the hot flue gasses before they pass up the chimney. Thus, FIG. 1 depicts smoke pipe 10 connected to a furnace (not shown) at inlet end 11 and to a chimney flue (not shown) at output end 12.

Damper 14 is supported on pin 15 which in turn is mounted on bearings (not shown) in the walls of pipe 10. Pin 15 supports damper 14 off center being considerably closer to the upper edge of damper 14. In order that damper 14 may open fully even though suspended off center, damper 14 is made smaller in its transverse direction than the inside diameter of pipe 10 and flanges 16 and 17 are positioned around the inside circumference of pipe 10 to act as sealing flanges when damper 14 is closed. Flange 16 is secured in the bottom circumference of pipe 10 on the inlet side of damper 14 while flange 17 is secured to the upper portion of pipe 10 on the outlet side of damper 14. Flanges 16 and 17 are discontinuous in the vicinity of pin 15 permitting damper 14 to swing through an arc of substantially 90°. Adjustable balance weight 18 is centrally located in damper 14 above pin 15 for adjusting the balance of damper 14 to open under conditions of normal operating draft. Auxiliary heat exchanger 20 is connected to smoke pipe 10 through downpipe 21 on the inlet side of damper 14 and through induced draft pipe 23 is connected to smoke pipe 10 on the outlet side of damper 14. Downpipe 21 makes a tee connection with pipe 10 at its upper end and connects to inlet 22 of heat exchanger 20 at its lower end. Inlet 22 is a passage to the bottom of heat exchanger 20 connecting to open ends of a plurality of vertically positioned cylindrical tubes 24. First plurality of cylindrical tubes 24 is depicted as 11 tubes shown in FIG. 2. The bottom ends of tubes 24 are supported in spaced relationship and spaces between them are blocked by sealing plate 25. A supporting partition 26 terminates inlet passage 22 so that any incoming hot flue gasses must pass vertically upward through tubes 24. The upper ends of tubes 24 are spaced below the upper surface of heat exchanger housing 27 and supported in spaced relation by sealing plate 28. Tubes 24 open at their upper ends into space 30 between housing 27 and sealing plate 28. Space 30 is open to the upper ends of tubes 24 and also the upper ends of second plurality of tubes 31. Space 30 is terminated by a supporting seal 32 so that flue gasses passing upward through tubes 24 must in turn pass downward through tubes 31.

Referring to FIG. 2 it will be seen that tubes 31 are a second plurality of vertically positioned cylindrical tubes depicted as 11 tubes. The bottom ends of tubes 31 open into a further space 34 which is continuous across the open bottom ends of third plurality of vertically positioned cylindrical tubes 35. The bottom ends of tubes 31 and tubes 35 are supported in spaced relationship by sealing plate 36. Space 34 is an enclosed space such that hot gasses passing downwardly through tubes 31 must pass upwardly through tubes 35. Referring again to FIG. 2 it will be seen that tubes 35 are also depicted as 11 tubes. Space 37 communicating with the upper ends of tubes 35 is connected to induced draft blower 38 by passage 40. Tubes 35 are supported in spaced relation at their upper ends by a sealing plate 41.

Baffle 42 supported adjacent opening 44 between passage 40 and space 37 serves to improve dynamic flow of gasses into passage 40.

Blower 38 suitably of the squirrel cage electrically driven type is connected to pull gasses through passage 40 driving them through induced draft pipe 23 into smoke pipe 10. Induced draft pipe 23 is depicted as angled toward outlet 12 of pipe 10 at its connection point with pipe 10. The angle of this pipe will be discussed further in connection with the operation of the heat exchanger.

Second blower 45 has an intake opening 46 into ambient space around the heat exchanger. An air filter (not shown) is desirably positioned across this opening to filter out dust and opening 46 may be connected to outdoors or some other location as desired. Blower 45 is depicted in FIG. 2 as connected to a side wall 47 of heat exchanger 20 and blows into passages around tubes 24, 31 and 35 to carry heat from the tubes. Looking at the sectional plan view in FIG. 2 it will be seen that blower 45 blows air in past and around the third plurality of tubes 35. A baffle 50 separates tubes 35 from tubes 31 extending from wall 47 to a position short of opposite wall 51 so that the air from blower 45 must pass by and around tubes 35 before encountering tubes 31 after passing around the end of baffle 50. A second baffle 52 separates tubes 24 from tubes 31 extending the confined circuitous travel of the air from blower 45 so that it is forced by and around tubes 31 and then tubes 24 before reaching outlet 54 in wall 51. Outlet 54 is suitably connected to a register for providing heated air into the immediately adjacent space or may be connected to ducting so as to provide heated air at a distant location.

The number and diameter of tubes 24, 31 and 35 is determined so as to provide efficient flow of flue gasses. However, the efficiency of heat transfer is determined by the area of exposed metal surface of the tubes from which the heat is picked up by air passing from blower 45 to the outlet 54. For a more compact design than that depicted, fewer tubes of a larger diameter may be utilized or merely a shorter length of tubing having the same diameter with fins extending from the tubing walls normal to the axis of the tubing so as to increase thermal transfer.

In any kind of a combustion heating system a draft which provides a constant flow of air for combustion purposes and carries the waste products of combustion away is of significant importance. Commonly the draft is based upon the fact that hot gasses are lighter in weight than cold gasses and therefore rise. Accordingly, if a gas such as air is heated at the bottom of a vertical duct (i.e., chimney flue) and a much colder air exists at the top of the duct, the heated air will rise through the duct causing a "draft."

Thus, when a furnace is ignited at the bottom of a vertical chimney flue, a definite pressure differential occurs between the combustion gasses in the furnace and the relatively cold air at the top of the chimney causing a dynamic flow. This is well known and in FIG. 1 will cause damper 14 in smoke pipe 10 to open under the pressure differential tending to allow the hot gas from the furnace entering at inlet 11 to pass through on to outlet 12 and up the chimney. Damper 14 can be adjusted in balance by weight 18 either so that it will be normally open under residual draft that exists even when the furnace is not operating or so that it remains just barely closed to open on the smallest increase in heat on the furnace side. This latter is considered to be the preferred condition since it reduces loss of warm air up the chimney when the furnace is quiescent. However, on operation of blower 38, air or flue gas is pulled through the heat exchanger through inlet passage 22 and connecting pipe 21 producing a low pressure zone on the inlet side of pipe 10. At the same time, air and/or flue gasses are forced through draft pipe 23 into the outlet side of pipe 10 increasing the pressure in the outlet side of the pipe. Thus a pressure differential is produced across damper 14 forcing the damper closed. With damper 14 closed, all flue gasses must pass through heat exchanger 20. Thus as depicted in FIG. 1, low and high pressure zones produced by the induced draft means 38 operate damper 14. Some angling of pipe 23 is desirable for efficient utilization of induced draft means 38. Since the arrangement of draft means 38 tends to affect pressure on both sides of damper 14 simultaneously, any venturi effect is unlikely. Nevertheless, pipe size relationships, angles and opening characteristics must be designed to avoid venturi action.

It will be recognized that other electrical or mechanical interlock means may be used to provide coaction between damper 14 and induced draft means 38. For example, the switch operating the electric motive power for induced draft means 38 may operate a solenoid actuating damper 14 from its closed to its open position. It will be recognized that this same switch during the heating season of the year would normally be interlocked with a switch operating the heating furnace or else even more desirably with a thermal switch reacting to the temperature of the smoke pipe. Additional thermostatic means could also be interconnected to avoid heat being provided in excess of a desired room temperature.

In the event that induced draft means 38 does not operate, heat exchanger 20 would be effectively isolated from the smoke pipe since damper 14 would open providing a more direct flow of flue gasses during operation of the furnace.

While the invention has been described with relation to a specific embodiment, it will be seen that a number of variations are available and are contemplated as within the scope of the invention to the extent covered by the appended claims.