Negative air pressure in a house is caused when more air is being exhausted out of the house than is being supplied back in, creating a vacuum that pulls outside air in through cracks and openings. Major causes include exhaust appliances that expel indoor air outside, such as kitchen range hoods that can exhaust 300 to 600 CFM, bathroom exhaust fans at 50 to 150 CFM each, clothes dryers at 150 to 200 CFM, central vacuum systems, and attic fans. Fireplaces and wood stoves also consume large amounts of combustion air, up to 300 CFM, contributing to negative pressure. HVAC system issues are another common cause, such as leaky return ducts located in attics, crawlspaces, or garages that pull air from those spaces into the system, unbalanced supply and return where more air is returned than supplied, or a furnace in a sealed closet without combustion air. Tight construction for energy efficiency without a dedicated fresh air makeup system makes houses more susceptible, because there is no designed path for replacement air, so air is sucked in through random leaks. Natural forces like stack effect in winter where warm air rises and escapes through upper leaks, and wind pressure on the building, can also create temporary negative pressure. Signs include doors that are hard to open, whistling around windows, cold drafts, and furnace backdrafting.
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