Shown is a basement of about 5,000 square feet with concrete floor and walls in a building originally built in 1935, acquired a year ago but with a 22" basement fan that's always been dead. Other than the two staircases and an elevator shaft, the only basement ventilation is the fan, which either blew into or sucked out of a 1,800 square foot dirt floor crawlspace that's about 3 feet high (the ceiling of the crawlspace is level to the ceiling of the basement). The crawlspace has a few outside wall vents and stays dry. The fan is installed about 6 feet above the basement floor with the motor end toward the basement and the blades toward the crawlspace, but it's unknown in what direction the fan rotated. The motor has no identifying information. This August, about half the basement floor nearest the fan appeared damp in spots and efflorescence formed.

When the fan is replaced, in what direction should the airflow go?

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  • 1
    What's the climate, and what problem is the fan supposed to be solving? Also, what is the heating plant like; blowing air out might suck exhaust gasses into the house.
    – keshlam
    Commented Sep 22, 2023 at 1:21
  • Central California coast, 3 blocks from the ocean. Don't know why the fan exists, and perhaps it's pointless. There are two Reznor gas duct furnaces suspended from the ceiling on the main floor above which are only used in winter; no heating or ducting in the basement.
    – joe
    Commented Sep 22, 2023 at 2:24
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    What does the rest of the envelope look like? I suspect it's an exhaust fan as they are far more common than wall mount intake fans.
    – KMJ
    Commented Sep 22, 2023 at 2:41
  • Does seem likely to be intended to blow air out of the basement into the crawl space. That said, I don't know that the fan alone will help much with the dampness on the basement floor. Couldn't hurt, though, so it's worth a try.
    – Daanii
    Commented Sep 22, 2023 at 5:15
  • I can't imagine wanting to blow exterior air into the home...is the basement HVACed?
    – Huesmann
    Commented Sep 22, 2023 at 13:26

1 Answer 1


The fan function would be to move the otherwise stagnant air to the outside.

However in doing so it will also draw in the air from outside.

In doing so the basement is vented.

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