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This is my first winter in this rental home. Our furnace is in a utility closet that is accessible only from the outside of the home. The home is a rambler with a crawlspace and all of the ductwork to the vents is in the crawlspace (almost all the registers are on the floor). The "cold" air return is in the ceiling of the central hallway. I've never seen a return in a ceiling, but to each his own.

The mystery is that every time the furnace turns on, it pulls cold drafts from every imaginable crack or crevice in the house. For example, if you put your hand next to the door sill, then the moment the blower turns on, a strong draft of cold air starts coming in. This seems to be true in every room, including the utility closed where the furnace is located.

What is causing this low pressure vacuum and how can I fix it?

  • Is this draft a transient, or does it occur continually when the blower's running? – ThreePhaseEel Dec 3 '16 at 20:50
  • @ThreePhaseEel the draft is continuous. – reynoldsnlp Dec 4 '16 at 0:06
  • I take it your crawlspace is vented? – ThreePhaseEel Dec 4 '16 at 0:12
  • There's also combustion air. Does the draft persist with the burner off and the fan still running? – DJohnM Dec 4 '16 at 3:54
  • @ThreePhaseEel I don't know if the crawlspace is vented. Is there a way to tell without climbing down there. There is a trapdoor in a closet. I will see if there is a draft there. – reynoldsnlp Dec 4 '16 at 3:57
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Your furnace pulls air from your house (that return vent in the ceiling), heats it, and then blows it back into the house (the floor registers). It should pull out the same amount of air that it blows back in, but your experience shows otherwise; it's pulling more air out than it's blowing back in.

It looks like some of the (heated) air isn't making it back into the house. If, say, 1/4 of the air that's pulled from the house passes through the furnace is discharged to the great outdoors, then you'd have to have the same amount of air being pulled back in through "cracks and crevices" in the building envelope. If not, after a while the house would start deflating like a leaky balloon (and, since you didn't mention it, I'm guessing that isn't happening).

So, somehow hot air coming from the furnace is leaking outdoors. Check the furnace housing, check those ducts in the crawl space, check whether there's (warm) air blowing outdoors from the crawl space. Also try and compare the amount of air coming out of the registers to the amount going back in the return.

Good luck.

  • "Pulls in" & "blows out" are the proper terms to use when discussing what a furnace does; not "pulls out" & "blows in". – James Olson Dec 3 '16 at 21:03

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