outside
That "extra" air resulting in the positive pressure comes from outside. You could have one source of incoming air and run that through filters, but it wasn't clear that's what you meant. But at that point you're talking saferoom with a separate air supply. If you're house is that tight, you'd probably have a stale air concern more than biohazards.
RThe local news reports a chemical spill or fire and says 'shelter in place' rules apply. One of those rules is to turn off your air conditioner.
But if I know that my air conditioner does NOT have an external source of air - by that, I mean that the air is just recirculating inside air, should I turn off my air conditioner?
My window unit and my car both have a control for introducing outside air - which I can close. My central unit does not. I wish it did but I can't afford an air exchanger.....
ps. I've used a water column to measure static pressure with the air on and it is neutral. Neither positive or negative pressure inside my home....
Unless you have had a true blower door test on your unit and home, you have no way of knowing what you have. EVERY unit leaks. It will leak at the returns, it will leak at the supply side....it will leak at the blower itself. Your home breathes too...if your home allows air to leave faster than the unit is putting additional air into the home from normal leakage from outside sources, then you will get a neutral pressure reading. If the units pulling in air faster than the home can let it out, then you get a positive...
Shut it off, unless you have a mini split ductless.
Actually, a positive pressure inside your home relative to the outside environment would be a good thing in case of a chemical/biological incident. Keeps the nasty stuff out.
But snice the average person doesn't habe that capability, I would think turning off all ventilation (blower too) would prevent a negative pressure situation - even in a single room - that could pull contaminated air in from the outside.
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