Negative pressure/hvac nightmare
So I have a previously wood fireplace, when it’s hot and humid outside, and my ac is on, it is sucking air through my chimney and making the entire first floor smell like smoke!
I installed the best top damper, still smells! I left the flue damper in and closed that also, still smells.
I assume it’s a negative pressure issue, because when I turn on the gas starter, even a small flame, the smoke smell is gone in 5 minutes.
How do I fix this? An HVAC company said my ducts may be too small or leaking, my fan may be too high, and they need to balance the system.
Could I just install a fresh air vent in the basement to counter act the negative pressure?
My basement is freezing, main floor has the fireplace and normal, and top floor is 5 degrees warmer than main.
I also considered a ducted fireplace insert but not sure it would seal the fireplace enough to prevent smell leaking in.
HELP!!!