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Bathroom exhaust fan discharging into the cold air return

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We live in a 1971 ranch house on slab. This winter we noticed a few drips of water coming out the cold air exhaust duct in the hallway. I went in the attic and noticed the bathroom exhaust fans ducts are attached to the cold air return. The ducts are insulated.

There is also a considerable amount of condensation on the underside of the roof by the soffits. During the summer we had an old bathroom exhaust fan replaced. I'm assuming the new exhaust fan is more powerful than the old one and therefore extracting more moisture. The moisture is caught in the cold air return and condensing because of the cold weather.

  • Is it best practice to hook the bathroom exhaust fans to the cold air return?
  • Does it sound like my assumptions are correct?
  • Should the fans be exhausted through the roof or soffit?
asked in House Ventilation by schaumburg (120 points)
edited by darekrudy
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1 Answer

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Hi Schaumburg,

I was wondering a few times about such installation myself. What a waste of moist air from an exhaust fan, at least during the cold season, this could almost work like a humidifier... and somebody did itsmiley.

Unfortunately it doesn’t work; it could if you’d be able to synchronize the furnace with the bathroom’s exhaust fan operation (both working at the same time). Even if you could, the fan exhaust pipes connection would need to be moved close to the heat exchanger, humidistat controller would be needed as well.

The best practice and the requirement is to have bathroom exhaust fans discharging to exterior. Although the bathroom exhaust fan discharge through the soffit is for some reason permitted in various jurisdictions I wouldn’t recommend it at all (never in a climate with temperatures occasionally dropping below freezing) because it usually results in condensation and mold in an area directly above the termination point.

  • I would disconnect both bathroom fan exhaust pipes from the cold air return ASAP and provide discharge through the roof.
  • I would also check the interior of the air return for any signs of mold, if there isn’t much room to look inside, slide a digital camera into the openings (after removing ventilation ducts) and snap a few pictures pointing in various directions.

If you have condensation in your attic there might be mold growing as well - check this link: Why is mold growing in my attic?

answered by darekrudy (21,730 points)