neg pressure: air is sucked into the room because there is a slight vacuum in the room relative to the exterior.

pos: air is blown out of the room because of the difference in pressure.

Differences in pressure are created by fans blowing into or out of the room. A fan exhausting air causes neg. pressure. This is what you want, because pos. pressure blows the smelly air out of the room.

If all you had was an intake fan, the fan would create pos. pressure, so you always want a stronger exhaust fan than intake, or no intake fan at all. I usually don't use them.

For smell, replace the filter, if you need more smell control look into in-duct ozone generators. Carbon filters are rated for a certain amount of air movement through them, so make sure your filter matches your fan.
Dorje113 Reviewed by Dorje113 on . Q:positive vs neg. air pressure Good evening, all. Can anyone explain positive and negative air pressure? I've been reading and reading here and other places trying to figure out why a can fan, 5 months old, would still allow odors to be detectable outside. u We sed an automatic spray air freshener in the room, too. Could that be the reson? We have tried about everything, watch the humidity ( I read somewhere they aren't effective above 50%? We've taped , caulked, plugged any perceivable air leak, and am about to freak. Rating: 5