Airflow measurement quantifies the volume of air moving through an HVAC system, expressed in cubic feet per minute (CFM) at specific points like return vents, supply registers, and across the evaporator coil. Accurate airflow measurement ensures the system meets design specifications and delivers adequate conditioning to all occupied spaces. Measurement methods include anemometers, pitot tubes, and duct traverse techniques.
Measurement Techniques
Velocity measurements at supply and return vents multiplied by duct cross-sectional area yield CFM data. Technicians typically measure at multiple points across a duct cross-section using a pitot tube anemometer or hot-wire anemometer for accurate average velocity. Return air CFM should match supply air CFM within 5 percent, accounting for small exhaust or outdoor air portions.
Performance Verification
HVAC systems are designed for specific CFM ratings, typically 300 to 400 CFM per ton of cooling. Low airflow indicates filter restrictions, blower problems, or undersized ducts that reduce comfort and efficiency. High airflow may indicate oversized ducts or inadequate filtration. Proper airflow measurement identifies problems early and guides adjustments to optimize system performance and longevity.