Your house has so much negative pressure because more air is being exhausted out of the house than is being brought back in, creating a vacuum effect that sucks outside air in through cracks. Common causes include running multiple exhaust devices at the same time, such as bathroom exhaust fans, kitchen range hoods that can exhaust 300 to 600 CFM, clothes dryers that exhaust 150 to 200 CFM, central vacuum systems, and fireplaces or wood stoves that consume large amounts of air for combustion. If your HVAC air handler is in an attic or crawlspace and has leaky return ducts, it can pull air from those unconditioned spaces and create negative pressure in the living area. Homes that are tightly sealed for energy efficiency without a dedicated fresh air makeup system are especially prone, because there is no easy path for replacement air. Other causes include wind pressure on one side of the house, stack effect in winter where warm air rises and escapes through upper leaks, pulling cold air in below, and unbalanced HVAC supply and return, where more return air is taken than supply air delivered. Signs of excessive negative pressure include doors that are hard to open, whistling sounds around windows, fireplace smoke coming back into the room, lingering odors, cold drafts, and furnace backdrafting that can bring carbon monoxide inside. This can increase energy bills and cause moisture problems.
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