CFM per Ton Explained — The 400 CFM Rule & When to Break It
Every residential HVAC airflow conversation starts from one ratio: 400 CFM per ton. It's baked into ductulators, balancing charts and tech training — but the reason behind it, and the exceptions, decide whether your system dehumidifies or short-cycles.
Quick answer: Standard residential airflow is 400 CFM per ton of cooling: a 3-ton system moves about 1,200 CFM. Humid climates often run 350 CFM/ton (slower air = more moisture removal at the coil), dry climates can run up to 450. Wrong airflow shows up as a frozen coil (too little) or poor dehumidification (too much).
Where 400 comes from
Air carries heat at roughly 1.08 BTU per hour per CFM per °F. A 3-ton system delivers 36,000 BTU/h; with a typical 30°F across-coil temperature split, 36,000 ÷ (1.08 × 30) ≈ 1,111 CFM — call it 400 per ton with real-world margin. The ratio is physics, not folklore: change the split or the sensible fraction and the 'right' number moves, which is exactly when 350 or 450 enters.
When to run 350
- Humid climates (Gulf Coast, Southeast): slower coil air means colder coil surface and more moisture wrung out — latent wins over sensible.
- High-latency loads: a tight, shaded house with low sensible load dehumidifies better at reduced CFM.
- Equipment spec: many variable-speed air handlers list 350 as the nominal humid-climate setting.
When airflow is wrong, here's the symptom list
- Too little CFM: coil ice-over, low suction pressure, compressor stress, weak register output.
- Too much CFM: high suction pressure, cool-but-clammy rooms, poor moisture removal, higher sensible efficiency at comfort's expense.
- Chronically low return air (undersized returns) mimics low charge — and gets 'fixed' by adding refrigerant, which is the wrong fix.
From tons to rooms
System CFM (tons × 400) splits to rooms by floor-area share, then each branch's CFM sizes its duct via the equal-friction method. That chain — tonnage → CFM → room split → duct diameter — is the whole residential airflow design in four steps, and each step has its calculator on this site.