Re: Backdraft - Negative Air Pressure
Normally a 90% High Efficiency furnace should already be drawing in outside air on it's own PVC pipe. Depending on the furnace size, pipe length and number of elbows, it may only need to be 3".
I'm guessing this is a 2 story house? Possibly with high ceilings upstairs?
You problem is likely related to stack effect. You have air leaks upstairs to the attic or out upstairs windows that are making the downstairs and basement negative pressure. Made worse if you run a bath fan or kitchen hood exhaust fan vented outside. IF there are more leaks upstairs than downstairs, you might start backdrafting the hot water heater. The flue pipe might also be partially blocked or poorly designed... or even undersized.
IS the furnace exhaust run out of it's own PVC pipe? Or is it tied to the flue (I hope not... that's very ,very bad)
1925 Two-Story Stucco Beaux Arts Neoclassical