How to Calculate Airflow Through Door Undercuts and Transfer Grilles

nexoradesign.net
Jul 28
2 min read

In HVAC design, air supplied to a room must have somewhere to go.

Where a room has a dedicated supply-air diffuser but little or no return-air connection, the air normally leaves through a door undercut, transfer grille, transfer duct, corridor return path, or building leakage. Similarly, exhaust-driven rooms such as toilets, janitor rooms and utility spaces require replacement air to enter through one of these paths.

Although door undercuts are often treated as a minor architectural detail, they can significantly influence room pressure, airflow balance, door operation, noise and HVAC system performance. (How to Calculate Airflow Through Door Undercuts and Transfer Grilles)

ASHRAE defines transfer air as air moving from one room to another through openings in the room envelope, generally driven by a small pressure difference between the spaces.

This article explains how to calculate transfer airflow through door undercuts and transfer grilles, how pressure differential affects the result, and when a dedicated transfer grille should be used instead of relying on the door gap.