Combined furnace and heat recovery system

The present invention relates to a combined furnace and air recovery system. The system is comprised of a furnace housing having an inlet opening for connection to a cold air return duct and an outlet opening connected to a heating duct. A negative pressure compartment is located within the housing and is in communication with the inlet opening. A positive pressure compartment is also located within the housing and is in communication with the outlet opening. A fan is provided, communicating between the negative pressure compartment and the positive pressure compartment. a heating unit is located within the positive pressure compartment. An air-to-air heat exchange unit is provided having first and second air flow paths, wherein the first air flow path connects an exterior inlet duct to the negative pressure chamber to provide a supply of fresh air to the negative pressure chamber and the second air flow path connects the positive pressure chamber to an exterior outlet duct to provide for exhausting stale air from said positive pressure chamber.

The present invention relates to a combination furnace and air-to-air heat 
exchange unit. Such a unit can be called a combined heat recovery/furnace. 
It should be understood that the term furnace is defined as any unit that 
adds heat energy to a forced air system. The term air handler can be used 
interchangeably with the term furnace. 
With the advent of totally sealed homes, it has become necessary to 
exchange a percentage of stale warm air within such a sealed home with 
cool fresh air from outside the home. In doing so the efficiency of the 
heating system within the sealed home is reduced. One method of recovering 
that efficiency is to transfer the heat energy of the exhausted stale air 
into the incoming cooler fresh air. This is accomplished by using an 
air-to-air heat exchange unit. In the past, such units have been separate 
from the furnace system itself. The air-to-air exchange unit has had to 
employ two fan systems, one to force the warm stale air through the 
exchange unit to the outdoors, and one to draw in the cool fresh air 
through the exchange unit. 
The present invention combines the two units so that the overall efficiency 
of the system is improved and the use of only one fan, the fan that was 
formerly only used to operate the furnace, can now be used to not only 
operate the furnace but drive both the exhaust air and the incoming air 
through the air-to-air heat exchange unit. 
In addition to increasing the efficiency of the overall system by using 
only one fan, the combined system enjoys a further efficiency improvement 
because of the temperatures available to the unit. This will be shown 
hereinbelow. 
In accordance with one aspect of the present invention, there is provided a 
combined furnace and air recovery system for use in a building comprising: 
a furnace housing having an inlet opening for connection to a cold air 
return duct and an outlet opening connected to a heating duct; a negative 
pressure compartment located within said housing and in communication with 
said inlet opening; a positive pressure compartment located within said 
housing and in communication with said outlet opening; a fan means 
communicating between said negative pressure compartment and said positive 
pressure compartment; a heating means located within said positive 
pressure compartment; and an air-to-air heat exchange unit having first 
and second air flow paths; wherein said first air flow path connects an 
exterior inlet duct to said negative pressure chamber to provide a supply 
of fresh air from outside the building to said negative pressure chamber 
and said second air flow path connects said positive pressure chamber to 
an exterior outlet duct to provide for exhausting stale air outside the 
building from said positive pressure chamber.

The present invention will be now described in detail with reference to 
FIGS. 1 and 2. FIG. 1 is a schematic diagram of a combination furnace 
system and an air-to-air exchange unit. FIG. 1 shows a furnace housing 10 
in which there is located a fan 12. Negative air pressure chamber 14 is 
provided within the furnace housing 10 and is defined by interior walls 16 
and 18. Air is drawn through a filter element 20 into negative air 
pressure chamber 14 by fan 12. The air comes from the cold air return duct 
22. The fan 12 forces the air into a positive pressure chamber 24 located 
within the furnace housing 10. Positive air pressure chamber 24 is defined 
by interior walls 18 and 26. A heating unit 28 is located within positive 
air pressure chamber 24 and heats the air exiting the chamber into hot air 
duct 30. Heating air unit 28 can be any conventional heating element, for 
example, a hot water to air heat exchanger. The hot water can be generated 
by a boiler not shown. The heating air unit 28 could also be an electrical 
heating unit or a heat pump unit. 
An air-to-air heat exchanger 32 is located within the furnace housing 10. 
One example of heat exchange unit 32 is described in detail in U.S. Pat. 
No. 4,554,719 which issued on Nov. 26, 1985. U.S. patent application Ser. 
No. 08/798,341, which was filed on Feb. 10, 1997, describes another such 
unit. Cool outside air is drawn from a duct 34, into a negative air 
pressure chamber 36 and through the air-to-air heat exchange unit 32 into 
negative air pressure chamber 14 by the action of fan 12. A portion of the 
warm air in positive air pressure chamber 24 is exhausted through the 
air-to-air exchange unit 32 into positive air pressure chamber 38 and out 
to the exterior of the building via exhaust duct 40. 
The warm air exhausting through air-to-air heat exchange unit 32 warms the 
cool air being drawn into the building via the duct 34. 
This combination of a heating furnace unit and an air-to-air heat exchange 
unit is more efficient than two separate units. For one reason, the 
combination unit makes use of a single fan unit to drive both the air 
through the furnace and also through the air-to-air exchange unit. 
Another reason why the efficiency of the combined unit is greater than that 
of a separate furnace and air-to-air heat exchange unit is the relative 
temperatures of operation. This improvement in efficiency can be shown as 
follows: 
Apparent Heat Recovery Effectiveness E={Ms(X.sub.2 -X.sub.1)}/{Mmin(X.sub.3 
-X.sub.1)} 
Where Ms=Mass flow of the supply air 
Mmin=Mass flow of the lower of the supply or exhaust air flows 
X.sub.1 =outdoor air temperature 
X.sub.2 =supply air temperature 
X.sub.3 =indoor air temperature 
X.sub.4 =from air-to-air heat exchange unit to outside temperature 
Supply air=air from the heat exchanger to indoors 
Exhaust air=air from indoors to the heat exchanger. 
X.sub.4 =X.sub.3 -X.sub.2 
Assume the following conditions for both the situations where there is a 
combined system and two separate systems. Outdoor air temperature is 
0.degree. C., indoor air temperature is 22.degree. C. and the temperature 
of the air after the air handler heating coil is 35.degree. C. The air 
flow rate through the air handler is 500 l/s and the air exchange rate is 
50 l/s. Also assume a heat exchange core with a fixed apparent heat 
recovery efficiency of 60%. 
Assume that in the heat exchanger the flow Ms=Mmin; 
then E=(X.sub.2 -X.sub.1)/(X.sub.3 -X.sub.1) 
therefore X.sub.2 =E.times.(X.sub.3 -X.sub.1)-X.sub.1 
also with a sensible energy balance, X.sub.4 =X.sub.3 -X.sub.2. 
For the present invention as shown in FIG. 2., 
X.sub.2 =0.6.times.(22-0)-0=13.2.degree. C. 
and X.sub.4 =22-13.2=8.8.degree. C. 
For a system that has a separate heat exchange unit and furnace as shown in 
FIG. 3, 
X.sub.2 =0.6.times.(35-0)-0=21.degree. C. 
and X.sub.4 =35-21=14.degree. C. 
Both systems have heat exchangers that are 60% effective and indoor 
temperatures of 22.degree. C. However, because the separate system uses 
the air heated by the air handler or furnace, which is 35.degree. C., 
warmer air is exhausted to the outside. 
The actual system effectiveness based on the 22.degree. C. indoor air can 
now be calculated for the separate system based on the temperature of air 
from the heat exchanger to the outside, X.sub.4 
Using an energy balance, X.sub.2 =X.sub.3 -X.sub.4 =22-14=8.degree. C. 
Then the Effectiveness E=(8-0)/(22-0)=0.364 or 36%. 
This compares to the effectiveness of the present invention which, as a 
system, maintains the 60% effectiveness of the heat exchanger. 
It is desirous that about 10% of the air within the heating system is 
exchanged at all times with fresh outside air that has been heated by the 
heat exchanger 32 with stale warm air that is being exhausted from the 
building. This ratio can be arranged by arranging the size of the openings 
connected to the cold air return duct and the hot air duct and the size of 
the openings of the air-to-air heat exchange unit 32. 
FIG. 2 shows an actual configuration of the present invention. In order to 
show the interior configuration of the combination furnace and air-to-air 
heat exchange unit, the exterior walls of the furnace housing 10 have been 
shown as if they are transparent. 
Fan 12 draws air into the furnace 10, into negative pressure chamber 14 
through inlet 50 that would be connected to the cold air return of the 
building. An air filter (not shown) could be located directly inside 
negative pressure chamber 14 across inlet 50 to filter air returning from 
the building to the furnace 10. Air-to-air heat exchanger 32 is located 
within the furnace housing 10. The negative air pressure in chamber 14, 
draws air from the exterior of the building, through duct 34 and through 
the air-to-air exchanger 32. Aperture 52 in the air-to-air exchanger 32, 
allows the air to enter chamber 14 from the exterior of the building. 
Fan 12 creates a positive air pressure in chamber 24. Chamber 24 is 
separated from chamber 14 by an internal wall 18. The positive air 
pressure in chamber 24 forces air through the heating element 28 and out 
of the furnace housing 10 via aperture 54. 
The positive air pressure in chamber 24 also forces air through aperture 56 
in the air-to-air exchanger 32 to exit via duct 40, to the exterior of the 
building. 
The warm stale air traveling through the air-to-air heat exchanger 32 from 
aperture 56 to duct 40 warms the fresh air being pulled into the system 
from duct 34 through the air-to-air exchanger 32 and out aperture 52. 
Since a single fan 12 is used to move all air flow through the combined 
system, the size of the apertures 50 and 54 of the furnace housing 10 and 
the size of the apertures 52 and 56 in the air-to-air heat exchanger 32 
control the ratio of the amount of air that travels through the furnace 
housing 10 and the amount of air exhausted from and fresh air introduced 
into the furnace housing 10 via the air-to-air heat exchanger. 
If the air exiting the system via the air-to-air exchanger has a very high 
moisture content, and if the outside temperature is very low, there can be 
so much energy transferred into the cold incoming air that the temperature 
of the outgoing air is lowered to the point where moisture freezes in the 
air-to-air exchanger 32. If this happens, a damper 60 can be opened and 
warm air from the positive air pressure chamber 24 will enter the 
air-to-air exchanger 32 via aperture 58, to warm it and melt the frozen 
moisture. This damper arrangement can be either automatic or manual. If 
automatic, air flow sensors (not shown) would determine that passages 
connecting aperture 56 and duct 40 were blocked with ice and open the 
damper 60. 
The heating element 28 can be of any conventional type. For example heating 
element 28 could be a hot water to air exchanger. In this case hot water 
would flow into input pipe 62 from a hot water source (not shown) and flow 
out of output pipe 64, back to the source of hot water.