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This is my first winter in a house with an oil-heated, forced air system. I noticed my wall vents were pushing out warm air, but the floor cold air return vents were pushing out cool air. I followed all the ducts and found a booster fan pulling cold air from the outside, and pumping it into the ducts. I covered the outside hole with a piece of rigid insulation, and that has stopped the fan from turning on, and the cool air from coming up my vents. (they do suck cool air into the furnace) Did the previous owners make a mistake, or am I missing something?

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It would appear to be a poorly tuned fresh air vent (aka a "Make-up Air vent")

Blocking it completely is not a good choice.

Tuning it correctly is a good choice.

The intake should not be bringing in more air than the furnace blows out (far less, in general)

A page from 11 years ago...

http://www.nchh.org/Portals/0/Contents/Read_This_Before_You_Ventilate.pdf

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it sounds like somebody might have jury rigged something. older or malfunctioning oil furnaces have been known to "soot" the warm side if the supply air to the burner is too warm. the booster might have been installed to cool the supply side air. however, you should never have air coming out of your cold air returns. it is by design a suction port, not an exhaust port.

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