[Zeke]

The A/C guy is probably thinking of the balance of make up air, nothing too much more serious than that. A closed loop system keeps the intake and supply relatively balanced introducing a small amount of outside air, which is user adjustable on window units. Larger units are fairly well engineered with respect to exchange rates and make up air.

Wayne, your exhaust fan can't be more CFM than the A/C unit is pulling in or you will be pulling in dirty, unconditioned air. The A/C guy doesn't know what you have exactly, so he is playing it safe. You want a lightly positive system, certainly not negative.