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Hey guys,

So we have central air in our house on the main floor and top floor. The basement does not have AC. When the AC is off, the basement is crisp, very cool, and super refreshing, while the rest of the house is warm and usually humid (obviously). But when we turn on the central air upstairs and the house cools down, the basement gets very warm and incredibly humid (even though we have a dehumidifier running all day down there). Anyone have any idea why this happens? Or what can be done to fix it? It happens every time and goes back to normal when the AC is turned off.

Thanks.
 

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Check around the foundation & basement windows for air leaks, and seal up to not allow air flow. My basement stays around 67-69 in the summer, with just two vents & one return, while the upstairs stays around 72. House is air tight, but in my case, no insulation in the walls, due to older home built in the late 30's. Just go around and check to make sure storm windows are closed, no air leaks around doors & windows, and that bathroom fan vents have dampers on them to keep air from leaking out or outside air coming back in when the fans are off.

Also find out what the humidity & temp is down there, along with the main & attic, which you can use a remote temp/humidity unit to allow you to see this on the main floor, or in your office.
 

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Discussion Starter #3
Check around the foundation & basement windows for air leaks, and seal up to not allow air flow. My basement stays around 67-69 in the summer, with just two vents & one return, while the upstairs stays around 72. House is air tight, but in my case, no insulation in the walls, due to older home built in the late 30's. Just go around and check to make sure storm windows are closed, no air leaks around doors & windows, and that bathroom fan vents have dampers on them to keep air from leaking out or outside air coming back in when the fans are off.

Also find out what the humidity & temp is down there, along with the main & attic, which you can use a remote temp/humidity unit to allow you to see this on the main floor, or in your office.
Thanks for the response. Its a finished basement with two windows that are super sealed up. They actually have rubber weather stripping around them. No bathrooms down here, and none of the bathrooms in the house have fan vents. Also no air vents in the basement. Its almost as if the cold air is getting sucked out of the basement even though the door is closed. I usually eventually resort to opening the door to let some of the cold AC air from upstairs travel into the basement. But since the AC is off, its nice and cold down here again.
 

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If getting sucked downstairs, then that means that you have ductwork that needs to be sealed. If possible, you may want to check your cold air returns on both levels to make sure they are not closed, so the whole house is at the same air flow. I do not have a door going to my downstairs, so if you stand in the right spot, you can feel the air passing by. It also keeps the air fresh downstairs, not stale if you can keep air moving down there.
 
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