why does a furnace need outside air?

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bf wrote:

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--
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Bud-- wrote:

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

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air intake should go to furnace burner area not plenum.
4 inch sounds right for air for combustion
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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
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