Airflow Measurement
Residential airflow measurement methods including the TrueFlow grid, temperature rise calculation, flow hood measurement at registers, and verifying the 400 CFM per ton target for proper system operation.
- Measure total system airflow using the TrueFlow plate and digital manometer
- Calculate airflow using the temperature rise method with known BTU input and measured delta-T
- Measure individual register airflow using a powered flow hood and compare to room-by-room design requirements
- Apply the 400 CFM per ton standard and identify systems operating below the 350 CFM per ton minimum
Lesson 1
The 400 CFM Per Ton Standard and Why Airflow Matters
The Foundation of System Performance
Airflow is the lifeblood of a forced-air HVAC system. Every function the system performs - cooling, heating, dehumidification, filtration, and ventilation - depends on moving the correct volume of air. Insufficient airflow is the most common cause of comfort complaints, equipment problems, and energy waste in residential HVAC.
The industry standard for residential cooling airflow is 400 CFM per ton of cooling capacity. A 3-ton system should deliver 1,200 CFM, a 4-ton system should deliver 1,600 CFM, and so on. This 400 CFM/ton value is not arbitrary - it is the airflow rate that produces the optimal evaporator coil performance:
- The correct supply air temperature (approximately 55-58 degrees F)
- The correct temperature drop across the evaporator coil (approximately 18-22 degrees F delta-T)
- Proper dehumidification (the coil operates below the dew point long enough to remove moisture)
- Complete refrigerant evaporation (no liquid refrigerant returning to the compressor)
What Happens at Low Airflow
Below 400 CFM/ton: The evaporator coil gets colder than designed. The delta-T increases above 22 degrees F. Dehumidification may actually improve temporarily, but cooling capacity begins to decrease because the coil surface is too cold for optimal heat transfer.
Below 350 CFM/ton: The evaporator coil temperature drops below 32 degrees F and condensate begins to freeze on the coil surface. Ice insulates the coil and blocks airflow further, creating a self-reinforcing spiral. Liquid refrigerant may not fully evaporate and can return to the compressor (liquid slugging), causing compressor damage.
Below 300 CFM/ton: Severe coil icing, dramatically reduced capacity, and high risk of compressor damage. The system may run continuously without satisfying the thermostat.
What Happens at High Airflow
Above 450 CFM/ton: The evaporator coil warms up. The delta-T drops below 18 degrees F. The coil temperature rises above the dew point and the system loses dehumidification capability. In humid climates, occupants feel clammy and uncomfortable even though the temperature is at setpoint.
Above 500 CFM/ton: Very poor dehumidification. The supply air temperature may be 62-65 degrees F instead of the target 55-58 degrees F. The system must run longer to meet the cooling load, and indoor humidity may remain above 60% relative humidity.
Exam Tip - The Magic Numbers
Remember these three airflow values for the exam: 400 CFM/ton is the design target, 350 CFM/ton is the minimum before coil icing risk, and 450 CFM/ton is the maximum before dehumidification problems. Most residential systems operate well below 400 CFM/ton due to restrictive ductwork.
The target residential cooling airflow is 400 CFM per ton. Below 350 CFM/ton, the evaporator coil risks icing. Above 450 CFM/ton, the system loses dehumidification capability. Airflow is the most fundamental measurement for evaluating residential system performance, and most systems deliver significantly less than the 400 CFM/ton target.