Field guide
CFM per Ton: How Much Airflow a System Actually Needs
400 CFM per ton is where the conversation starts, not where it ends. The right target depends on climate, coil and what the system is being asked to do.
CFM per ton is the shorthand for how much air a cooling system moves for each 12,000 BTU/h of capacity. It decides how much of the equipment's capacity shows up as cooling in the house, and how much of it shows up as dehumidification.
Why the number matters
Move more air across the coil and the coil runs warmer: more sensible cooling, less moisture removal. Move less air and the coil runs colder: more dehumidification, less total capacity, and eventually a coil that freezes. The same equipment behaves like two different machines at 350 and 450 CFM per ton.
| Approximate target | Typical use | Trade-off |
|---|---|---|
| 350 CFM/ton | Humid climates, latent load priority | Lower sensible capacity, freeze risk if airflow drops further |
| 400 CFM/ton | General starting point | Balanced sensible and latent performance |
| 450 CFM/ton | Dry climates, sensible load priority | Poor dehumidification, possible blow-off |
Always confirm the manufacturer's own airflow range for the specific coil and equipment before choosing a target. The chart above is orientation, not a specification.
How to verify delivered airflow
- Measure total external static pressure, then read CFM from the equipment blower table at that pressure and speed tap.
- Cross-check with the temperature split across the coil against the expected split for the measured indoor conditions.
- Traverse the return or use a flow hood where the duct geometry allows a valid reading.
- Compare delivered CFM against the nominal tonnage — that ratio is your CFM per ton.
When the number cannot be reached
If the duct system cannot deliver the airflow at any available blower speed, raising the speed tap only raises noise and static pressure. At that point the fault is the duct system, and the fix is the duct system. Advanced HVAC Airflow Diagnostics Volume 1 works through how to prove that with measurements a homeowner can be shown.