why does a furnace need outside air?

[bf]

I was doing some insulation in the basement, and I noticed that the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace. This ductwork lead to an outside vent, and was intended to bring in outside air to the furnace.

I assume that since the builder went through the trouble to put that in, that it's necessary.. but why is it necessary? Isn't there enough air in the house for the furnace to use? The furnace has been running for at least 4 years without it. I'm willing to repair this, but I'd like to know the reasoning behind it. I've found out that vent is a pretty major source of cold air coming into the basement.

Another bit of information: The house is very drafty, so there's no concern about it being so sealed up that all the oxygen is gone. LOL

[hallerb]

outside COLD air is more dense so it burns better, more energy efficent.:)

Indoor air sometimes has pollutants like bleach that can rot out high effceny furnace heat exchangers, this is a verified issue

burning indoor air means your pulling a vacuumn losing heated air and dragging even more outdoor cold air indoors making your gas bill go up.

put the original vent arrangement back if you want your gas bill to be lower

[Joseph Meehan]

Giving the furnace its own direct source of outside air will make you home less drafty. Really.

All furnaces (non-electric that is) use air to burn their fuel. That air needs to be replaced. It will come from those cracks and holes all around your home bringing cold air in and moving it around your home. If it has its own air source it will mean less drafts.

[mm]

-------------------- Are you talking about the cold air *return* via which the air heated by the furnace returns to the furnace to be heated again.

Or are you talking about a duct to supply combustion air to the fire, which then goes up the smoke stack or out the combustion air discharge?

I'm curious how big this duct is.

[RayV]

I'm with you the OP said "...the previous owner had disconnected some ductwork (4") which connected to the cold air return of the furnace...". That doesn't sound to me like it is being used for combustion.

[nicksanspam]

Air burns?

Nick

[dnoyeB]

Yes and no. I have such a connection in my home. There is a 4" or so duct that is piped into the return duct. That 4" pipe comes from outside. So anytime hot air leavs the house through the roof, or the bathroom fan, or furnace/hot water heater exhause, the cold air sucked in is through the furnace. (in theory)

Its not directly used for combustion, but its for pressure equalization. The air used for combustion which is exhausted is replaced by air coming in from this pipe and into the return.

[Tony Hwang]

Hi, They ducted fresh air like that in old days. Now thay don't do it any more. Combustion air intake is separate from cold air return duct. Just simply law of physics.

[Don Phillipson]

Consult your local building permits office. Many jurisdictions nowadays require an outside air duct to supply air to non-electric furnaces. The building code in Ontario, Canada, added this requiremennt about 20 years ago (and made iit retroactive.)

If your house is appropriately insulated against draughts, the outside air duct will be more efficient and cheaper rather than less.

[trader4]

Boy, I'd disconnect that crap real fast. This is an energy disaster. What makes you think outside air is only coming in to equalize pressure when air is leaving via say a bathroom fan? In reality, the powrful furnace blower is gonna draw outside air in all the time. And the inside air will make it's way out, via things like the bathroom fan, whether it's turned on or not, or any other ways to exit the house.

A normal house shouldn't need any air specifically brought in from the outside when the furnace is heating/cooling. Housed get enough from existing air leaks around doors, windows, opening/closing doors, etc. If you have a high energy efficiency house that is sealed very tight and you need more outside air, then an arrangement with a heat exchanger is used. Anything else is just wasting a lot of energy. IMO, the arrangement you have is similar to leaving a window partially open.

[trader4]

would make sense. He says the outside air connection is to the return duct.

[Randy R]

Outside air should be brought in through a fresh air exchanger. As Don said, this is a requirement in new homes because of how well sealed new houses in Ontario are. I don't think new houses in Ontario allow air in under the walls like US houses do. After the walls are insulated, the walls are covered with plastic, and caulking is applied around any boxes that protrude through the plastic. I don't know if there's any kind of sealant or insulation between the walls and the floor. I live in the US and I put a fresh air exchanger and high efficiency air filter to help deal with allergies. Our house smelled like paint for 6 months before we had this done. New carpet also puts out fumes. Another issue with new homes is black soot in the carpet at the base of walls where air from outside gets sucked into the house. I think the carpet acts as a filter.

Randy R

[Bud--]

And yet another opinion- The outside intake to the air return runs the house at a slightly elevated pressure which makes it less "drafty" (all the drafts are going out). Disconnected, your house would probably be draftier but cheaper to heat since you are not forcing air out through cracks. A new well sealed house might use an air-to-air heat exchanger but with your leaky house that wouldn't be effective. Having a source for combustion air that leaves up the flue is also an issue but not so much, as you said, in your drafty house. My house, drafty type, has a flexible insulated duct for combustion make-up air with the end bent in a U shape at the end - a block from cold air just flowing in.

bud--

[Pop]

Go back to sleep.

[trader4]

Of course that is only true for the short periods when the furnace blower is running. What about the rest of the time? And if the house is so drafty that it's noticeable, the drafts should be fixed.

Disconnected, your house would probably be draftier but cheaper to

Yeah, that's for sure. Plus, by drawing air into the house purposely to force air out cracks, you are doing exactly that. On a calm day with no breeze when there would not be much air flow through these cracks, and hence little energy loss, now you've got a blower creating air flow.

Having a raw outside air intake into the return plenum is simply one of the dumbest things imaginable

A new well sealed

[Doug Kanter]

:-)

[hallerb]

air intake should go to furnace burner area not plenum.

4 inch sounds right for air for combustion

[George E. Cawthon]

Pressure equalization? Don't think so. That cold air pipe is meant for a certain percentage of make up air from the outside. Required in many communities with newer homes that are built air tight and is likely to be 10 to 15 percent of the air. Seems like a bit much if in a cold place. But the purpose is to supply fresh air because much of the inside air is highly contaminated by the materials used in building, furniture, carpets, etc.

[Bubba]

Pay attention to trader. He got it right. Fresh air piped into the return duct from the outside is a large waste of energy dollars. Its typical on a rooftop commercial application where required amounts of air into the building are needed. If you have an atmospheric burner (open to your basement) many codes require piping in fresh for combustion. Most just leave the pipe hanging on the side of the furnace or down at the floor. I prefer to actually point or connect the pipe to an opening near the burners. Just make it easily disconnectable for easy service to your furnace. It does have to be sized properly or it will do no good. Bubba

[Mike Dobony]

You obviously never heard of Sick Building Syndrome! Indoor air is highly polluted and needs a replenishment of fresh air. In the winter time the house is usually shut up more than in the summer and contaminants just build up. This is especially bad if someone has allergies to indoor contaminants, a growing cause of asthma! Also the air exchanged with opening a door is contributing unheated air to the house. The outdoor air supply is being heated before entering the house.