400 CFM per Ton: Where the Rule Comes From and When It's Wrong

"400 CFM per ton" is the first airflow number every tech learns. It's a good default, but treat it as a starting point and check it against the manufacturer's spec and the actual airflow.

Where 400 comes from

A ton is 12,000 BTU/h. At about 400 CFM per ton, a typical evaporator coil runs cold enough to remove a reasonable amount of moisture while still delivering most of its capacity as sensible cooling. It's a middle-of-the-road balance between temperature and humidity control.

When to go lower or higher

Climate / goal	Typical airflow	Why
Humid (Gulf Coast, Southeast)	about 350 CFM/ton	A colder coil removes more moisture
Average	about 400 CFM/ton	Balanced
Dry (desert Southwest)	about 450 CFM/ton	Little moisture to remove, so more sensible capacity

Many variable-speed blowers and communicating systems have dehumidify or humid/dry climate settings that do this automatically. Too little airflow in any climate risks a frozen coil and floodback. Too much airflow cuts moisture removal and can blow condensate off the coil.

Always check the manufacturer's specs. Some equipment, especially inverter heat pumps, publishes its own airflow targets per mode and stage.

Measure, don't assume

Duct systems often restrict airflow well below the setting. Ways to measure actual airflow:

• Fan table + total external static pressure: read TESP, then look up CFM at that static and speed tap in the equipment's blower table. See total external static pressure.
• Temperature rise method (heating): on a gas furnace, CFM = output BTU/h ÷ (1.08 × temperature rise). Use the furnace airflow calculator.
• Flow grid or flow hood: direct measurement at the filter slot or at the registers.

Heat pumps and heating airflow

Heat pumps often run similar airflow in heating and cooling. Gas furnaces set heating airflow by temperature rise, not by tons. Check the rise against the rating plate range.

FAQ

How many CFM does a 3-ton system need?

At 400 CFM per ton, 1,200 CFM nominal. Somewhere between 1,050 (350/ton) and 1,350 (450/ton) depending on climate and the manufacturer's spec.

Does lower airflow improve dehumidification?

Yes, up to a point. A slower airflow makes a colder coil, so it removes more moisture, but too low risks freezing the coil.

How do I change blower airflow?

On PSC blowers, change the speed tap. On ECM and variable-speed blowers, change DIP switch, tap or control board settings per the installation manual.

Sources

• Workzen Academy: HVAC system optimization (CFM per ton guidance)
• measureQuick: total external static pressure