Furnace

A furnace for the burning of wood in a combustion chamber to generate heat. Primary air is brought in at a grate level of the furnace. Secondary air is brought in through a secondary air inlet, past a preheat chamber and through a secondary air duct to be expelled through holes in the secondary air duct at the top level of flames. The secondary air is preheated in the preheat chamber which is located between the combustion chamber of the furnace and the outside air inlet. A heat exchanger in the top of the oven transfers the heat from the combustion chamber of the furnace to ductwork for distribution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to improvements in a wood furnace, 
and more particularly, pertains to a new and improved furnace which 
preheats secondary air before expelling the secondary air into the top 
level of the flame of the fire. 
2. Description of the Prior Art 
Those concerned with the development of furnaces have long recognized the 
need for a furance which ignites a high percentage of the gases in the 
fire and operates with a high combustion efficiency. The present invention 
fills this need. 
Prior art furnaces in the past have failed to completely ignite all of the 
gases being expelled from the combustion chamber of the furnace into the 
smoke pipe. Such furnaces have been unsatisfactory in that more fuel is 
required to generate heat resulting in lower efficiency. If all of the 
gases in the combustion chamber were to be ignited, less fuel would be 
required to generate the same amount of heat increased efficiency. 
The problem in the art with introducing secondary air into the combustion 
chamber of the furnace to ignite the uncombusted gases is that the 
temperature of the secondary air being introduced into the combustion 
chamber is not sufficient to ignite the gases. On account of this factor, 
a large percentage of gases leave the combustion chamber unburned for the 
reason of the temperature in the combustion chamber not being high enough 
to ignite all the gases. 
Prior art patents issued to Brunner, U.S. Pat. No. 1,430,936 and to Swift, 
U.S. Pat. No. 1,760,183, disclose apparatus for introducing secondary air 
into the combustion chamber of a furnace. 
SUMMARY OF THE INVENTION 
The present invention obviates the foregoing disadvantages of prior art 
furnaces by providing a preheat chamber for secondary air in the upper 
portion of a furnace and adjacent to the combustion chamber and a 
secondary air duct to distribute the preheated secondary air increasing 
the efficiency of combustion of unignited gases. 
According to one embodiment of the present invention, there is provided a 
furnace with a combustion chamber, a grate at the level bottom of the 
chamber through which primary air is drawn to support a wood fire, and 
inlet through which secondary air is drawn, the secondary air passing 
through a preheat chamber and into a secondary air duct which surrounds 
the combustion chamber at the flame level of the fire whereby the 
preheated air is equally distributed in the combustion chamber by the 
secondary air duct so that there is an increased efficiency of combustion 
of unburned gases. 
Having briefly described the embodiment of the present invention, it is a 
principal object thereof to provide a new and improved furnace with a 
preheat chamber for secondary air for a furnace. 
An object of the present invention is to provide a furnace with a primary 
air inlet and a secondary air inlet. 
Another object of the invention is to provide a preheat chamber to heat the 
secondary air after its intake into the furnace but before it is 
discharged into the combustion area of the furnace. 
A further object of the invention is to provide a U-shaped secondary air 
duct structure to equally distribute the preheated secondary air in the 
combustion area at the top of the flame of the fire to ignite all of the 
unburned gases. 
A significant aspect and feature of the present invention is a furnace 
which preheats the secondary air in a preheat chamber and equally 
distributes the secondary air in the combustion chamber so that the 
secondary air has a high temperature necessary to ignite all the unburned 
gases thereby increasing the efficiency of combustion of the wood.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 illustrates a side view of the preferred embodiment of the furnace 
10, the invention. A fire door 12 on the front of the oven 10 permits wood 
logs 14, 16 and 18 to be inserted into the furnace 10. A grate 20 in the 
bottom of the furnace 10 permits primary air to be brought into the 
furnace 10 from an outside source which may be a duct if so desired, and 
further permits ashes to fall through the grate 20 for collection by any 
well known means in the art. 
Secondary air is brought into through a secondary air inlet 21, past a 
preheat chamber 22 adjacent a wall of a combustion chamber 30, and piped 
into a secondary air duct 24. The preheat chamber 22 is usually located in 
an upper wall of the furnace, but may be placed anywhere on the furnace 
for maximum heat transfer and may take any physical shape. The secondary 
air duct 24 takes the shape as shown in the figures and is provided with a 
plurality of holes 26 in the U-shaped portion 28 of the secondary air duct 
24. The surface area percentage opening of the holes all the way around 
the three legs of the U-shaped portion of the secondary air duct 24 is 20% 
that of the surface area of the primary air inlet at the grate level 20. 
The secondary air is discharged and equally distributed at the top of the 
flame in the combustion chamber 30. A heat exchanger 32 transfers the heat 
in the combustion chamber 30 to air which is ducted accordingly. A smoke 
pipe 34 removes the combusted and ignited gases from the combustion 
chamber 30 and exhausts the gases to the outside environment through 
ductwork accordingly. 
FIG. 2 illustrates a rear view of the furnace 10 taken on line 2--2 of FIG. 
1 showing wood logs 14, 16, 18; grate level 20; the introduction of 
primary air; the secondary air duct 24; and holes 26 in the U-shaped 
portion 28 of the secondary air duct 24. The secondary air is discharged 
through holes 26 in the U-shaped portion 28 of the secondary air duct 24 
at the level of the top of the flames from the logs 14 through 18. 
Vertical tubes 36 and 38 connect the secondary air duct 24 to the preheat 
chamber 22 and have a length such that the holes 26 discharge preheated 
secondary air at the top of the flame from the logs 14-18. By discharging 
the preheated secondary air at the top level of the flames, the total 
ignition of unburned gases occurs resulting in an increased efficiency of 
combustion internal to the furnace 10 in the combustion chamber 30. 
FIG. 3 illustrates a top view of the furnace taken on line 3--3 of FIG. 2 
showing the wood logs 14-18 in the furnace 10, the secondary air inlet 21, 
the preheat chamber 22, and the holes 26 in the U-shaped portion 28 of the 
secondary air inlet 24. 
PREFERRED MODE OF OPERATION 
Referring now to the drawings, particularly FIG. 1, primary air and 
secondary air is introduced into the furnace 10 at the grate 20 and the 
secondary air inlet 21, respectively. Primary air is introduced from the 
bottom of the grate 20 or ducted in from the front of the furnace 10 to 
the bottom of the grate 20 to be passed up through the grate 20 into the 
fire of the wood logs 14-18. Below the grate 20 is an ash pit and fire 
bricks surround the lower portion of the combustion chamber 30 which 
supports the combustion of the wood. Secondary air which is induced into 
the furnace 10 is brought in through the secondary air inlet 21 and is 
preheated to a temperature above ambient temperature of the air in the 
preheat chamber 22. The preheat chamber 22 takes its heat from the 
combustion chamber 26 through heat transfer and as air is passed from the 
secondary air inlet 20 through the preheat chamber 22, the secondary air 
rises from ambient temperature to a higher temperature necessary for the 
total ignition of uncombusted gases in the combustion chamber 26. From the 
preheat chamber 22, the secondary air is passed into the secondary air 
duct 24 which is a U-shaped tube 28 surrounding the three sides of the 
furnace 10 and having a plurality of holes 26 drilled into the secondary 
air duct 24 tube such that the surface area of the holes 26 is equal to 
20% of the primary air surface area of the grate 20. The preheated 
secondary air is discharged through the holes 26 into the combustion 
chamber 30 at the level of the fire of the wood logs 14-18 to provide for 
total ignition of unburned gases thereby increasing the efficiency of the 
combustion. 
It is an important feature of the invention that the secondary air be 
equally distributed within the combustion chamber 26 by the holes 26 in 
the U-shaped portion 28 of the secondary air duct 24 for maximum 
efficiency of combustion of unignited gases. 
Various modifications can be made to the furnace of the present invention 
without departing from the apparent scope thereof. While "20%" has been 
determined to be the ideal ratio of the surface area of the holes in the 
secondary air duct to be to the primary surface, it is recognized that a 
deviation of plus or minus from the percentage figure is permissable while 
achieving almost the same efficiency as disclosed.