Prefabricated fireplace exhaust plenum structure

A novel wood/gas burning fireplace is provided with an open box shaped outer housing. A combustion chamber is mounted inside said housing having a plurality panels that are juxtaposed the panels of the outer housing so as to provide an open wall at the bottom, back and top of the fireplace which serve as a heat exchanger. An insulating panel is mounted in the top wall of the heat exchanger dividing the top wall into two separate passageways. The air passageway below the insulating panel comprises a part of a heat exchanger for heating room air of the space being heated. The air passageway above the insulating panel forms a plenum chamber with apertures in the outer housing panel(s) for conducting room air into the plenum chamber. A stack opening aperture in the top panel of the outer housing conducts cooling air into a coaxial exhaust stack for cooling the top panel as well as the exhaust stack pipe.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to wood burning fireplaces. More 
particularly, the present invention relates to a novel stand off and 
improved top and exhaust structure for gas and wood burning fireplaces. 
2. Description of the Prior Art 
Heretofore exhaust stacks on stoves, fireplaces and gas water heaters have 
been known to overheat and create a hazardous condition that could result 
in igniting flammable walls and roof structures. 
A common cure for a hot stack on a gas water heater is a well known vent 
collar which mixes room air with the exhaust gas to lower the stack 
temperature. Domestic boilers and furnaces have adapted the same principal 
as a vent collar by directing hot exhaust gases through a hood or plenum 
which pulls room air into a box shaped chamber and mixes it with room air. 
The cooled mixture is then directed into a conventional exhaust stack. The 
problem with vent collars and vent hoods is that they often require a 
large number of individual pieces that add to the stack cost. 
Another remedy for a hot exhaust stack for fireplaces is to provide a 
coaxial or two piece exhaust stack which is mounted on and supported by 
the fireplace. As will be explained in greater detail hereinafter, the 
room air is pulled into the space between the larger diameter pipe and the 
smaller exhaust pipe to insulate or isolate the inner pipe which contains 
the hot exhaust gas. The outer pipe with an air space between it and the 
exhaust pipe forms an effective insulator. Prior art coaxial pipe stacks 
are known to be expensive and can be complex. 
It would be desirable to provide in a wood or gas fireplace, an improved 
lightweight and low cost cooled exhaust stack and top support structure 
which would permit it to be directly coupled to a conventional coaxial 
exhaust stack without expensive insulating structures or vent collars. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide a zero clearance 
wood burning fireplace capable of being installed in a walled enclosure or 
against an outside wall of a room or building to be heated. 
It is another primary object of the present invention to provide a 
fireplace exhaust structure which is constructed with fewer parts, yet has 
an improved efficiency of operation. 
It is another primary object of the present invention to provide a new top 
structure for a fireplace that eliminates the need for conventional 
standoff supporters, vents, collars and complex insulation. 
It is a general object of the present invention to provide a novel 
prefabricated base model wood burning fireplace structure which may be 
completed upon installation in the field in the form of a radiant heater 
or with a circulating air heat exchanger. 
It is a general object of the present invention to eliminate the need for 
radiation shields and top panel standoffs for a novel cooled fireplace. 
It is another general object of the present invention to provide in a heat 
exchanger wall an insulating panel that divides the heat exchanger wall 
into two air passageways which provide cooling of the outer housing panels 
sufficient to employ the fireplace in a wood burning or a gas burning 
fireplace without modification. 
According to these and other objects of the present invention, there is 
provided an outer housing and a combustion chamber mounted inside the 
outer housing having a plurality of similar juxtaposed panel which form 
walls having an airspace between similar panels. A heat exchanger is 
formed by the interconnection of the bottom back and top walls which warms 
the room air and cools the panels of the outer housing with the exception 
of the top panel of the outer housing which is separated from the heat 
exchanger by an insulation panel mounted in the top wall of the heat 
exchanger and forming a top air passageway above the insulating panel. 
Apertures are provided in the side panels of the outer housing for the 
entrance of room air into the top air passageway. The air entering the top 
air passageway is coupled into the outer pipe of a coaxial exhaust stack 
for cooling the top panel of the outer housing as well as insulating and 
cooling the hot exhaust stack in the center of the coaxial exhaust stack.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Refer now to FIG. 1 showing a state of the art prior art prefabricated wood 
burning fireplace 10 of the type which may be enclosed behind a wall 11. 
The wall 11 is shown having vertical studs 12 and horizontal studs 13 
which rest on or are attached to metal standoffs 14. Wallboard 9 is 
attached to the wood studs 12, 13 to complete the wall 11. The fireplace 
10 comprises an open box of outer panels of which the top 15, back 16 and 
bottom panels 17 are shown. A layer of insulation 18 is attached to top 
panel 15 yet still requires standoffs 14 to prevent organic studs 12, 13 
from being overheated. 
A combustion chamber 19 is mounted inside of the outer panels forming the 
open box 10 and is spaced apart from the top, back and bottom panels 15-17 
(and the side panels not numbered) forming an air space in the walls 
formed by the combustion chamber panels 21-23 juxtaposed panels 15-17. The 
bottom and back panels 23, 22 are lined with a refractory material 24, 25 
of the type known and used in the wood burning fireplace art that meets 
underwriter's specifications and is impact resistant. 
Walls 26, 27 and 28 are formed by juxtaposed panels 15 to 17 and 21 to 23 
and are interconnected. In the FIG. 1 embodiment, a radiation shield 29 is 
formed behind back panel 22 in the very hot upper portion of combustion 
chamber 19. Similar shields may be provided around the side panels above 
the refractory insulation 25. A second radiation shield 31 is provided in 
bottom wall 28 and is preferably mounted on outer bottom panel 17, 
however, it could be inverted and mounted on panel 23. Baffles 32 and 33 
are mounted at the front and back of top of the combustion chamber 19 to 
direct the hot exhaust gases of combustion into the exhaust stack 34. 
The fireplace 10 is provided with short vee shaped legs 35 which attach to 
outer panel 17. Formed metal shapes like 36 and 37 are provided in the 
wall 28, 27 to support the combustion chamber 19 within the open box 10. 
A header panel 38 and a foot panel 39 are extended across the top and 
bottom of the opening of the combustion chamber 19. Header panel provides 
support for a mesh screen 41 which is slidably opened to provide access to 
the wood burning combustion chamber 19. 
Upper and lower closure panels 42 and 43 block the opening in walls 26 and 
28 respectively, thus creating a dead air space in walls 26 to 28. 
Refer now to exhaust stack 34 shown as a coaxial pipe having an outer 
insulating pipe 44 and an inner hot exhaust pipe 45, the outer pipe 44 is 
connected to top outer panel 15 via outer collar 46 which is provided with 
apertures 47 that conducts cooling air into the space between pipes 45 and 
44. The inner pipe 45 is connected via inner collar 48 to an adapter 
collar 49 in turn connected to top panel 21 of combustion chamber 19. The 
pipes, collars and adapters are provided with male and female ends such as 
corrugations, intercepted screws or flanges as are well known in the 
fireplace art. 
Refer now to FIG. 2 showing a side elevation of a prior art wood burning 
fireplace 20 which is a modified version of fireplace 10 shown in FIG. 1. 
The numbered elements in FIG. 2 which are the same as those shown in FIG. 
1 are numbered the same and have the same mode of operation. 
In FIG. 2 the closure panels 42 and 43 are removed. Upper and lower grills 
51 and 52 replace the closure panels 42 and 43, thus permitting room air 
to circulate through the heat exchanger formed by connected wall 26 to 28. 
A blower motor 53 is preferably employed in the bottom wall 28 to increase 
the efficiency of heat transfer and to permit the removal of radiation 
shields 29 and 31. Arrows show the direction of the air flow through the 
heat exchanger. Damper blades may be employed in the grills 51, 52 to 
direct air entry and exhaustion to and from the heat exchanger. 
Refer now to FIG. 3 showing a schematic drawing in side elevation of a 
first preferred embodiment of the present invention. For purposes of 
explaining the novel improvement over the prior art FIGS. 1 and 2, a 
similar shaped open box housing 30 is shown even though the box can be 
made smaller so it will resemble a masonry fireplace as will be explained 
hereinafter. 
A set of wooden logs or artificial logs 50 are shown supported on a grate 
54. An opening 55 in the side of the combustion chamber provide an access 
for gas and/or combustion air which connects to a gas burner system 56 
when converted to gas, otherwise the opening 55 remains closed by a knock 
out plug. It is possible to bring in both air and gas into the wall space 
28A below the logs 50 and to provide holes in the refractory floor 24 as 
well as the bottom panel 23 of the combustion chamber 19A. 
The front opening of the fireplace 30 is provided with upper and lower 
closure panels 42A and 43A which are preferably positioned above and below 
header panel 38A and foot panel 39A. The panels 38A and 39A and adapted to 
provide support for a fireplace mesh screen 41A (partially shown), or to 
permit mounting of a glass doors (not shown) especially when a gas burner 
conversion 56 is employed. 
The top panel 15A of the outer housing is substantially flat and horizontal 
and is spaced apart from the top panel 21A of the combustion chamber 
forming a deep wall 26A. A thick semi rigid panel of insulating material 
57 separates the wall 26A into and upper air passageway 26B and a lower 
air passageway 26C. The lower air passageway forms a part of a convection 
heat exchanger with walls 27 and 28. In the preferred embodiment slots 
and/or apertures 58 are provided in the header and foot panels 38A and 
39A. It is also possible to modify the closure panels 42A and 43A to 
provide openings therein which enhance the flow of room air in the heat 
exchanger. 
An exhaust stack adapter 49A is shown connected to top panel 21A. The 
connection provides a hot seal and prevents combustion gases from leaking 
into air passageway 26C and may be made from a single component. The 
adapter 49A is further provided with apertures 59 of a size, number and 
location that permits aspiration of room air in passageway 26C to be 
sucked into the adapter 49A as a result of the flow of exhaust gases from 
the combustion chamber 19A. A small amount of room air may be used to 
lower the slack temperature about five percent. A coaxial exhaust stack 34 
comprising an outer insulating pipe 44 is connected to top panels 15A. The 
inner exhaust pipe 45 is connected to the adapter 49A. This arrangement 
can reduce the number of components in the exhaust stack from as many as 
six pieces down to two. 
A feature of the present invention is that the panel of insulating material 
57 is slightly compressible and when forced onto adapter 49A and the 
panels of the outer housing form gas and air tight joints. Bracket for 
positioning panel 57 may be attached to the housing panels and adhesive 
sealers may be employed but are not required, thus, reducing the cost of 
manufacturer. 
The upper air passageway 26B is preferably provided with apertures 61 in 
the outer vertical panels of the housing. The apertures are of a size and 
number that provide adequate air in the insulating space between pipes 44 
and 45 to meet underwriter's requirements for placement through or against 
organic structural members such as ceiling and roof structures. It will be 
understood that air passageway 26B acts as a cooling chamber for top panel 
15A which is cooled sufficiently to eliminate standoffs 14 of the type 
used in the prior art. 
The cooling effect provided by apertures 58 and the convection heat 
exchanger is sufficient to eliminate radiation panels 29 and 31 without 
any forced circulation of room air. 
Refer now to FIG. 4 showing a schematic drawing of the fireplace 40 shown 
in FIG. 3 modified to include a high efficiency heat exchanger and 
controllable blower motor 53A. The numbered elements in FIG. 4 which are 
the same as those shown and described in FIG. 3 are numbered the same and 
do not require additional explanation. 
In the modified preferred embodiment, motor 53A is provided with a source 
of power (not shown) which is controlled. The motor may be variable speed 
selectable by a controller 62 or switched on and off by a lower thermostat 
63 and/or an upper thermostat 64 designed to prevent motor overheating and 
to permit the heat exchanger to warm up before withdrawing heat. In this 
embodiment the upper and lower closure panels 42A and 43A are replaced 
with grills 51 and 52 which preferably have directional vanes. 
Having explained the present invention with reference to a wood burning 
fireplace, it will be understood that only one open box fireplace housing 
30 and 40 needs to be prefabricated at the factory. The grills 51, 52 and 
closure panels 42, 43 are removable and replaceable in the factory or in 
the field. When gas conversion is desired or required a gas burner 
assembly and decorative doors may be added at the factory or during field 
installation. When a high efficiency heat exchanger is desired, a blower 
assembly 53, 62 etc. may be added in wall 28A by removal of the lower 
closure panel 43A and/or grill 52. In the preferred embodiment, slots 58 
may be left open or covered by a dress plate (not shown). 
Having explained a preferred embodiment and modifications thereof, it will 
be appreciated that not every fireplace will be fitted with all of the 
features explained, but certain combinations of individual features may be 
selected for solving different problems. In the preferred embodiments, the 
structure in the novel wall 26A simplifies the exhaust adapter structure 
as well as cooling top panel 15A and exhaust stack 34 which does not 
require a vent collar. Radiation shields are eliminated without the need 
of a blower motor which permits the fireplace to be made smaller and more 
efficient. Since the top and exhaust stack of the fireplace is made 
cooler, typical top and stack temperature encountered in the prior art of 
up to 900-1000.degree. F. are easily reduced to where the fireplace is 
directly mountable against walls or enclosure structure and meet 
underwriter's specification for heat and safety without standoffs.