21/08/2026
HVAC Acronyms Explained | Airflow Mechanics & Static Pressure
SA (Supply Air): Conditioned air supplied into occupied spaces via ductwork, diffusers, and grilles.
RA (Return Air): Air pulled back from occupied spaces to the HVAC unit for reconditioning, mixing, or exhausting.
OA (Outside Air): Fresh outdoor air brought into the system to dilute indoor contaminants and satisfy ventilation standards.
EA (Exhaust Air): Contaminated or stale air expelled outdoors (e.g., restrooms, kitchens, laboratories).
TA (Transfer Air): Air passing directly from one room to another through grilles or door undercuts without returning to central equipment.
CFM (Cubic Feet per Minute): Volumetric rate measurement of airflow; critical for equipment sizing and air balancing.
FPM (Feet per Minute): Measurement of airflow velocity (speed) within ducts or across openings.
ESP (External Static Pressure): Resistance external to the air handling unit (ducts, dampers, diffusers) that the fan must overcome.
TSP (Total Static Pressure): Overall pressure drop across the entire fan assembly, reflecting total system load.
ACH / AC (Air Changes per Hour): Number of times the entire volume of air in a space is replaced within one hour.
