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I have a classic brick chimney for a wood fireplace only (i.e. no hot water tank or other things sharing the stack).

In peak summer heat, the interior of the upstairs is well above the exterior temperature in the evenings for many hours.

I am wondering if a box fan pointing up the stack would be effective for pushing air out of the house.

It would be particularly convenient from a security point of view since the chimney is an existing hole in the structure, and "secure". We could even leave it running unattended 24x7.

Is this a bad idea? I'm OK if it's less efficient for some reason than a window or doorway placement. But if it would be completely ineffective, then, that would be a problem.

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    Warm air rises, so the warmest air will be near the ceiling. The opening for the fireplace is at/near the floor.
    – crip659
    Aug 1, 2021 at 11:43
  • Do you have any windows in that space that you could install a window-fan unit? They are purpose built for installing in an operable window, but depending upon the style of window they may or may not fit. Also, remember any air you expel from that space is going get replaced, perhaps drawing hot air into other parts of the home.
    – George Anderson
    Aug 1, 2021 at 15:33
  • You would be bushing air through a long, rough duct . It will have much more resistance than a window vent. There are also fans with shutters that go into a wall if you are concerned about security ( fan pushes shutter open , gravity closes it ),
    – blacksmith37
    Aug 1, 2021 at 15:51
  • it's not a bad or harmful idea, but it might not be very effective. The cost of trying is low so just give it a shot and see what you think.
    – dandavis
    Aug 1, 2021 at 22:21
  • It is a slick idea. I am a "whole house fan" groupie and this sounds like what you are trying to do, but without cutting a hole in the roof. I like it and you have got me thinking now.
    – Willk
    Aug 2, 2021 at 2:22

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A cheap box fan will not push air through any enclosed structure like a chimney unless it is sealed into the opening so air will not just recirculate around the blade. Also, a fan used to move air through a duct or space must have a venturi built into the fans construction so that the air is displaced and does not just recirculate. To understand the fan's construction, search for whole house exhaust fans or exhaust fan venturi. The hole the fan blade sits in does not allow for air to recirculate back to the fans inlet just to be recirculated. Hope this helps.

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  • +1 for acknowledging short circulating air in the chimney. And, the room will never cool down less than the existing outside air temperature that you’re bringing in as “make up air”. That’s why room fans are so popular. Having air blow across your skin (even though it’s warm air) cools the body.
    – Lee Sam
    Aug 1, 2021 at 17:57

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