What is negative building pressure?
Negative building pressure results when an HVAC system inside a building draws in more air from the return ducts than it expels through the supply ducts. A building can be wholly under negative pressure, or exhibit zones of negative pressure in certain areas (e.g. a room contains a return air grille and no supply grille). Given the current configuration of most HVAC systems, most buildings operate under some degree of negative pressure. When buildings operate under negative pressure, some air must be introduced from outside to balance the pressure differential. Ideally, this air would enter the HVAC unit through a valve or duct and be dehumidified before being introduced to the indoor environment. Unfortunately, however, this is rarely the case. HVAC units often have no allowance for outside air, meaning the air must be brought in from different areas. This often results in air being sucked in around doors and windows, through cracks in the foundation, or through other penetrations in
