| Zone | Supply | Return | Exhaust | Net (SA−RA−EA) | Pressurization |
|---|
Air balance and building pressurization fundamentals
Air balancing confirms that supply, return, and exhaust airflows are distributed correctly and that each zone achieves its target pressure relationship with adjacent spaces. Getting this wrong causes real problems: negative pressure pulls in unconditioned outdoor air and pollutants through every crack and gap, while excessive positive pressure wastes conditioned air and can make doors difficult to open or close.
The core air balance equation
SA = RA + OA, where SA is supply air, RA is return air, and OA is outside air. For a mechanically ventilated space, OA further breaks down as OA = EA + RFA (exhaust air plus relief air) when the space is positively pressurized, or simply OA = EA when the space is negative and relief air drops to zero. The net airflow into a zone, supply minus return minus exhaust, determines whether that zone pressurizes positive, stays neutral, or goes negative relative to adjacent spaces.
Setting a target pressurization
Most general occupancy commercial buildings target 3-10% positive pressurization: outside air intake exceeds exhaust air by that percentage, which keeps unconditioned air from infiltrating through the envelope. Specific room types flip this: negative pressure isolation rooms, labs, and washrooms typically target 5-15% more exhaust than supply so contaminated or odorous air can't migrate into adjacent corridors. A simple field rule of thumb for negative pressure rooms is roughly 100 CFM more exhaust than supply per door, though a proper calculation accounts for the full room volume and envelope leakage. See the CFM calculator for individual room airflow requirements feeding into this balance.
Frequently Asked Questions
The core air balance equation is Supply Air = Return Air + Outside Air, and Outside Air = Exhaust Air + Relief Air for a positively pressurized space. Pressurization percentage is calculated as ((Outside Air minus Exhaust Air) divided by Exhaust Air) times 100. A positive result means the zone is pressurized (more air entering than leaving through mechanical exhaust), a negative result means the zone is under negative pressure, and zero means neutral.
Most general occupancy buildings target 3-10% positive pressurization, meaning outside air exceeds exhaust air by 3-10%. This prevents uncontrolled infiltration of unconditioned outdoor air while avoiding excessive energy loss from over-pressurizing. Specific spaces have different targets: isolation rooms and labs often require negative pressure (5-15% more exhaust than supply), while clean rooms may require much higher positive pressure. Always check the project's mechanical specification or applicable code for the exact target.