View Full Version : Hot Air Furnace Mystery

Bob Winkler
08-22-2005, 8:09 AM
I encountered something yesterday that has me puzzled. My daughter is having her house resided with vinyl siding siding, and when her contractor removed the old wood shakes he uncovered two rectangular openings in the old pine sheathing. When I peered down these openings with a flashlight, I discovered that they we actually 4"x12" ducts that appeared to be cold air intakes to her 50+ year old gas furnace.

They have been covered for a long time, maybe since the house was built. Now, I'm trying to figure out if they should be covered again with the new siding, or should they be opened to the ouside world. I'm puzzled because I have always seen "cold air returns" return room air to the furnace, not outside air.

Any words of wisdom?:confused:

Thanks, Bob

David Wilson
08-22-2005, 8:48 AM
Those openings could have been for combustion air. The idea is that outside air goes into the combustion chamber rather than air which has been heated by the furnace.

Rob Russell
08-22-2005, 10:06 AM
It's also possible that those were some sort of "fresh air intake" for the system if you wanted to circulate outside air through the house. I'd guess that you'd need to go down and flip some dampers if you wanted to do that. Of course, you could also open the windows ...

Jerry Clark
08-22-2005, 10:12 AM
I think they used to require outside air for the cold air return-- I had a house with an electric furnace in California and it also had outside air for the return- not very efficent.:mad:

Frank Hagan
08-22-2005, 5:17 PM
Confusing two things here:

"Combustion air" is the air the furnace needs to create a fire, burning it with the fuel. Air + Fuel + Heat = FIRE (there used to be a nifty triangle they used in school). This air can be taken from the room the furnace is in, or in some cases, ducted in separately. You have to make sure the room has enough air coming into it to supply the combustion process, because the excess goes up the vent pipe (it had better, because otherwise you die).

"Return air" is the air you want to heat in your house, and this air never touches the flames. It passes through a heat exchanger and is separated from the flames so that you don't get carbon monoxide in your house. The heated air is then supplied to the house via the ducting.

Your furnace may draw both sources from the room, with a supply of air to the room from the house and, in some cases, a supply from the outside. In "very tight construction" you have to make sure you provide some fresh air or the fire will be starved for combustion air, burn dirty and the furnace will soot up and fail. The formula they use is that you need 1" per 4,000 BTU/hr of furnace size for this combustion air if the vent communicates directly with the outdoors (or 1" per 2,000 BTU/hr if you are using ductwork to bring the air in). Some appliances ask that this be supplied at both the top and the bottom of the room, so its not unusual to see a vent at the top of the room and ducting to the floor, providing the air at the floor even in a basement installation.

If you are seeing two ducts, this may be what you are seeing. You should have it checked by a pro to make sure you have enough combustion/ventilation air in the room to be safe. People die from doing this wrong.

Bob Winkler
08-22-2005, 9:29 PM
Thanks for all the comments guys. I'm sure that these intake ducts are NOT combustion air, because they are ducted to the main return air duct going right to the air plenum and heat exchanger.

In case I didn't make it clear, my main puzzlement is that these intakes were covered by the existing siding for many, many years. So, if they are the only source for return air, then the furnace must be getting air through duct leakage. Best guess, however, is that there are other return air ducts somewhere else INSIDE the house. I'll be looking for them later this week.

I'm still amazed that I have never heard of OUTSIDE air being used for return air (at least in a 60+ year old house).

Thanks again


Ernie Nyvall
08-22-2005, 11:04 PM
We had something similar set up for our fireplace. Two ducts from the outside and wrapped around and behind the fire brick and came into the house about six feet off the floor. With the fireplace going, it heated the air in the ducts and created a wind hard enough to blow your hair with no fan in the system. The colder it got, the faster it blew.