basement pressure changed by turning on HVAC fan
I have a one-zone HVAC system in my two story house. The furnace and air-conditioning unit are in the basement. When I turn on the fan at the thermostat control, with both the heating and air-conditioning off, I can feel a strong draft under the door at the top of basement stairway. The draft stops when the fan is off. I don't understand the negative pressure in the basement when the fan is running in a closed duct system. Does that mean the duct leak on the return side is much greater, so the basement air is sucked into the duct system?
You have a leak in your return system.
I had a similar situation and found it using a home made 'tell-tall' consisting of a long dowell with a 1" strip of facial tissue (long side) taped to the end. With that, run the strip over/around, near any seam, joint, panning, filter housing, etc. You will find it.
This should also be happening whenever the blower is running (heating, a/c)>
If you have a gas fired water heater (with vent) in the basement, the negative pressure could cause the exhaust to back draft (CO): Not Good.
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