Auxiliary air heater for fireplaces

There is disclosed an air-to-air heat exchanger which is positioned in a fireplace. The heat exchanger comprises a grate structure on which logs are burned. Ambient air is circulated through the heat exchanger and is heated. Hot air exiting from the heat exchanger is delivered into a metal conduit extending vertically into the chimney. An opening is formed in the chimney wall in the attic of the building and hot air is delivered into ductwork in the building attic. A damper is provided in the fireplace which seals about the conduit extending vertically into the chimney in the closed position of the damper. Air flow through the conduit and heat exchanger may be reversed.

This invention relates generally to heating devices and more particularly 
to a heating system which extracts heat from a fireplace and distributes 
heated air into the ductwork of a building. 
The heat generated in a fireplace is typically not utilized in an effective 
manner for room heating largely because much of the heat is discharged 
through the chimney in the form of hot combustion products and heated air. 
The heating that is actually accomplished by a fireplace occurs primarily 
because of the radiant heating of the area immediately to the front of the 
fireplace. Accordingly, fireplaces are normally provided merely for 
appearance and decorative effect while other heating systems are normally 
provided as the primary heating system of a building, such as a home. 
A number of fireplace grate heat exchanger type structures have been 
devised for supporting burning fuel and providing air conveying tubes 
formed by the grate for receiving air from the room to be served by the 
fireplace. Air is passed through the tubes in adequate heat exchange 
relation with the burning fuel and is discharged into the room containing 
the fireplace to promote heating efficiency. Typical disclosures of the 
prior art of this type are found in U.S. Pat. Nos. 3,901,212; 3,939,496; 
3,942,509; 3,955,553; 4,018,208 and 4,018,210. It is apparent that while 
fireplace heat exchangers of this type promote heating efficiency, they 
have the disadvantage of delivering heated air into the area that least 
requires it, which is immediately to the front of the fireplace. This 
area, of course, is normally much too warm and the delivery of hot air 
into this region largely aggravates this problem of conventional 
fireplaces even though heating efficiency is improved. 
The concept of ducting heated air from a fireplace heat exchanger to an 
area other than immediately to the front of the fireplace is disclosed in 
U.S. Pat. No. 4,008,703. Although the broad suggestion is disclosed, the 
device and technique of this invention introduces a number of features 
which makes this concept practical particularly for incorporation in an 
existing residence where modifications are necessary to conduct heated air 
to other rooms. 
In summary, this invention comprises a fireplace grate heat exchanger in 
which relatively cool air is forceably circulated through air passages in 
the heat exchanger. The heat exchanger is designed to provide a support 
for burning fuel. Air exiting from the heat exchanger passes into a 
flexible metal conduit which extends vertically into the chimney. Heated 
air passing into the flexible metal conduit is additionally heated by 
contact with hot air and combustion products moving upwardly through the 
chimney by convection. At the least, the temperature of the air in the 
flexible metal conduit does not begin to cool off until it moves out of 
the chimney. 
Above the ceiling of the room in which the chimney is located, typically in 
an attic or crawl space, an opening is made into the interior of the 
chimney and a pipe connection is made to the flexible metal conduit for 
connecting it to suitable ductwork disposed in the attic for delivering 
hot air from a conventional air handling system to the various rooms of 
the building. 
Reverse circulation of air through the heat exchanger may be accomplished 
merely by providing a fan capable of withstanding high temperatures and 
reversing the direction of fan operation. 
It is accordingly an object of this invention to provide a heating system 
which more efficiently utilizes the heat produced within a fireplace in 
order to provide heat to rooms remote from the fireplace. 
Other objects and a fuller understanding of the invention may be had by 
referring to the following description and claims taken in conjunction 
with the accompanying drawings.

Referring to FIGS. 1 and 3, there is illustrated a building 10 such as a 
single family dwelling, apartment or other such structure comprising a 
room 12 having a fireplace 14 therein. The room 12 includes a ceiling 16 
secured to joists 18 and separating the room 12 from an attic or crawl 
space 20 which is closed or covered by a roof 22. 
The fireplace 14 is of more-or-less conventional design having a brick or 
firebrick floor 24, a pair of vertically extending sidewalls 26, a back 
wall 28 and a wall 30 covered by a suitable facade or decorative mantle 
32. The fireplace 14 thus provides an opening 34 facing into the room 12 
and a smoke exhaust opening 36 communicating with a passage 38 provided by 
a chimney 40 which is typically of brick. 
Positioned in the fireplace 14 is an air-to-air heat exchanger 42 of this 
invention comprising, as major components, an air inlet manifold 44, a 
plurality of tubular sections 46 communicating with the manifold 44 and an 
air exhaust manifold 48. 
The air inlet manifold 44 is conveniently made of generally square tubing 
providing an offset leg 50 conncted to the discharge conduit 52 of a fan 
or blower 54 which is desirably located in an enclosure immediately 
adjacent the fireplace 14. There are many situations where the fireplace 
14 may be adjacent a wall so that the fan 54 may be located in an adjacent 
room or closet. If the location of the fireplace 14 is such that this is 
impractical, the fan 54 may be located forwardly of the front wall 56 of 
the fireplace 14 and the offset leg 50 repositioned. 
The tubular sections 46 are conveniently of square or rectilinear 
configuration providing a first portion 58 which is shaped or positioned 
to provide a support for wood, coal or other solid fuel. The section 58 
are accordingly desirably generally horizontal or downwardly inclined away 
from the manifold 44. To provide additional support for the solid fuel and 
to enhance heat exchange efficiency, a similar piece of square tubing 60 
is welded onto the bottom of the horizontal portion 58. Suitable 
supporting legs 62 may be welded or otherwise secured to the tubing pieces 
60 to support the heat exchanger 42. 
The tubular sections 46 also comprise a generally vertically extending 
portion 64 and a generally horizontally extending portion 66 which 
overlies the portion 58 and which resides in the same vertical plane as 
the portion 58. The horizontal portions 66 terminate in suitable openings 
in the air exhaust manifold 48 which is desirably a piece of square 
tubing. 
Connected to a single outlet opening in the exhaust manifold 48 is a 
conduit 68 which is generally parallel to the top wall 30 of the fireplace 
14 and passes through the exhaust opening 36 into the chimney passage 38 
to a location above the ceiling 16 and preferably above the joists 18. The 
conduit 68 is preferably made of a thermally conductive material. As used 
herein, a thermally conductive material is defined to mean a material 
which has a thermal conductivity value at least on the order of about that 
exhibited by common metals. The conduit 68 is desirably flexible to 
negotiate the inclined top wall 30 and the corner between the top wall 30 
and the chimney passage 38. Accordingly, the conduit 68 may comprise a 
conventional flexible metal conduit including a multiplicity of 
substantially identical sections 70 which are relatively pivotable or 
articulatable. In the mode of operation of the invention where hot air is 
moving upwardly through the conduit 68, the existance of leaks in the 
conduit 68 is undesirable but not troublesome because the conduit 68 is 
pressurized above the pressure in the chimney 40. In this circumstance, 
any leakage will be out of the conduit 68 into the chimney 40 so that 
smoke or carbon monoxide does not enter the conduit 68. 
As shown best in FIG. 3, the conduit 68 may be connected to a wall of the 
chimney 40 by suitable clamps 72. The upper end of the conduit 68 is 
connected to a duct 74 in the attic 20 of the building 10. The duct 74 is, 
in turn, connected to a duct 76 comprising part of an air handling system 
78 which may be a central heating or central air conditioning system. The 
duct 76 normally branches off and extends to a plurality of rooms in the 
building 10 and terminates in one or more registers 80. 
To connect the conduit 68 to the duct 74, an opening 82 is cut or broken 
through the chimney wall 84 to communicate between the attic 20 and the 
chimney passage 38. An elbow 86 is attached to the top of the conduit 68 
which is in turn connected to a sleeve 88 carrying a flange 90 sealing 
against the chimney wall 84 around the periphery of the opening 82. 
Typically located in the chimney passage 38 and normally adjacent the 
exhaust opening 36 is a damper structure 92 for controlling the rate of 
air flow vertically in the chimney 40. A typical damper construction 
includes a metal rim 94 affixed to the interior of the walls of the 
chimney 40 and a valve or damper plate 96 connected by a pivotal 
connection 98 to the rim 94. Suitable means (not shown) are provided for 
manipulating the damper plate 96 to move it between an open generally 
vertically extending position and a closed position engaging the rim 94. 
In order to clear the conduit 68, a generally U-shaped slot 100 is cut, as 
by the use of an acetylene torch or the like, into the side of the damper 
plate 96 opposite from the pivotal connection 98. It will be apparent that 
the slot 100 will allow movement of the damper plate 96 between its normal 
open and closed positions. 
In order to seal off the gap between the generally circular conduit 68 and 
the U-shaped slot 100, a fitting 102 is secured to the conduit 68. The 
fitting 102 comprises a collar 104 which may be affixed to the conduit 68 
in any suitable manner, as by the provision of one or more set screws 106 
or by welding, soldering, brazing or the like. Comprising part of the 
fitting 102 and integral with the collar 104 is a flange 108 which, in the 
embodiment illustrated, is of generally circular shape having a chord 
removed to leave a generally linear edge 110 which fits flush against the 
inner edge of the rim 94 to position the top of the flange 108 generally 
coplanar with the top of the rim 94. The damper plate 96 accordingly 
engages the rim 94 and flange 108 in the closed position to prevent upward 
air movement through the chimney 40. It will be apparent that the flange 
108 may be further trimmed to more closely correspond to the shape of the 
slot 100 to thereby minimize the restriction to air passing through the 
passage 38. 
It will be evident that the fireplace heat exchanger 42 may be used merely 
as a grate for burning solid fuel merely by leaving the fan 54 
unenergized. When it is desired to deliver heated air through the heat 
exchanger 42 to another room of the building 10, the fan 54 is energized 
to pass relatively cool air into the inlet manifold 44, through the 
tubular sections 46, through the exhaust manifold 48 and then through the 
conduit 68 for delivery into the ductwork 74, 76. As will become more 
fully apparent hereinafter, the temperature of the air entering the duct 
74 is quite high. It will accordingly be desirable to operate the fan (not 
shown) of the air handling system 78 in order to mix the hot air from the 
conduit 68 with ambient interior air. 
In addition to providing a very convenient means for ducting heated air 
away from the fireplace 14, the positioning of the metal conduit 68 in the 
chimney has an additional heat exchanging advantage. It will be evident 
that the temperature of the air and combustion products in the region 
where the conduit 68 is located is quite high. Thus, at the very minimum, 
there will be little or no cooling off of the heated air in the conduit 68 
during air passage from the exhaust manifold 48 to the sleeve 88. 
Indications are that it is more likely that the air traveling through the 
conduit 68 is heated still further during vertical movement into the duct 
74. In a test situation where a very intense fire was ignited on the heat 
exchanger 42, temperature measurements indicated that the air temperature 
in the conduit 68 immediately above the manifold 48 was 412.degree. F. 
while the temperature of air exhausting from the conduit 68 was 
414.degree. F. The exact hot air temperatures will depend, of course, on 
factors such as the rate of air movement through the heat exchanger 42, 
the type and quantity of fuel being burned, the efficiency of heat 
conductance through the tubular sections 46 and the like. 
As heretofore described, the direction of air flow is such that heated air 
passes upwardly through the conduit 68. It will be evident, however, that 
air flow may, in the alternative, be downwardly through the conduit 68 
with minor modifications. The fan 54 need only be capable of withstanding 
relatively high temperatures and be driven so that relatively high 
pressure air is discharged through a conduit 111 either into the room 12 
or into another room. In this circumstance, air is drawn through the 
registers 80 connected to the duct 76 and passes downwardly through the 
conduit 68 into the manifold 48, through the tubular sections 46 and into 
the manifold 44. In this event, it will be evident that the air to be 
heated in the heat exchanger 42 is pre-heated in the conduit 68. From a 
purely thermal efficiency standpoint, this mode of operation is preferable 
because the heat exchange relation of the air in the conduit 68 and the 
air passing upwardly in the chimney 40 is in a countercurrent 
relationship. It is, of course, well known that countercurrent heat 
exchangers are more efficient than heat exchangers in which the hot and 
cold fluids are passing in the same direction. 
Although the fan 54 may be capable of withstanding only low temperatures 
and positioned in the attic 20, it is preferred that the fan 54 be 
selected to withstand relatively high temperatures and be of a reversible 
nature so that it can be driven in either direction so that the heat 
exchanger 42 has two modes of operation. 
During installation, it is desirable to wrap the duct 74 with a suitable 
heat insulating material, such as asbestos, to prevent the exterior 
temperature of the duct 74 from getting too high and causing a fire 
hazard. In addition, it is desirable to place a flapper or check valve 
(not shown) in the duct 74 or sleeve 88 to prevent reverse air circulation 
through the heat exchanger 42 during the summer when the air handling 
equipment 78 may be used for cooling. 
Referring to FIG. 6, there is illustrated an electric circuit 112 for 
selectively energizing a motor 114 driving the fan 54 and for selectively 
energizing a motor 116 driving a fan 118 of the air handling equipment 78. 
The circuit 112 includes a pair of leads 120, 122 connecting the motor 114 
to a source of power 124. A controllable thermostat such as a bimetal 
switch 126 is disposed in the lead 120 for energizing the fan motor 114 in 
response to the occurrence of a preselected or changeable low temperature 
sensing in the room where the switch 126 is located. Accordingly, 
circulation of air through the heat exchanger 42 may be in response to the 
cooling off of a room remote from the fireplace 14. 
The circuit 112 also includes a pair of leads 128, 130 connecting the motor 
116 to the source of power 124. A thermostat such as a bimetal switch 132 
is disposed in the lead 128 for energizing the fan motor 116 in response 
to the occurrence of a preselected or changeable temperature sensing 
occurring in the room where the switch 132 is located. The leads 128, 130 
and switch 132 will be recognized as part of a conventional central 
heating and/or cooling system comprising the air handling equipment 78. 
In addition, the subcircuit energizing the fan motor 114 may be 
interconnected with the subcircuit energizing the fan motor 116 for 
automatically mixing heated air from the heat exchanger 42 and ambient 
interior air. To this end, the circuit 112 may comprise a lead 134 
connecting one leg of the switches 126, 132 together and a lead 136 having 
a switch 138 therein connecting the other leg of the switches 126, 132 
together. With the switch 138 closed, at any time either of the 
thermostatic switches 126, 132 senses a preselected low temperature, both 
of the fan motors 114, 116 will be energized to deliver a mixture of 
heated and ambient interior air through the registers 80. When the switch 
138 open, the fan motors 114, 116 operate independently to deliver heated 
air from the heat exchanger 42, to deliver heated air from the air 
handling system 78 or to deliver cool air from the air handling system 
depending on the selected mode of operation of the system 78. 
It will be apparent that the system of this invention is adapted for 
incorporation in new building construction and is particularly desirable 
for modifying an existing structure to increase heating efficiency. 
Although the invention has been described in its preferred form with a 
certain degree of particularity, it is understood that the present 
disclosure of the preferred form has been made only by way of example and 
numerous changes in the details of construction and combination and 
arrangement of parts may be resorted to without departing from the spirit 
and scope of the invention as hereinafter claimed. It is intended that the 
patent shall cover, by suitable expression in the appended claims, 
whatever features of patentable novelty exist in the invention disclosed.