What it is

CFM per ton is the airflow rule that keeps a system balanced. A "ton" of cooling is 12,000 BTU/h of capacity, and that capacity is only realized if you move the right amount of air across the coil. Too little air and the coil runs too cold (low capacity, freezing, over-dehumidification); too much air and the coil runs too warm (poor dehumidification, less comfort). Setting blower speed to hit the right CFM per ton is one of the most basic and most-skipped steps in making a system actually perform.

How it works

The coil is sized to absorb a certain amount of heat from a certain amount of air. The standard cooling target is in the neighborhood of 350–400 CFM per ton. At that airflow, the coil hits its designed temperature drop, dehumidifies properly, and delivers rated capacity.

Move away from that and you trade off:

  • Toward 350 CFM/ton (and below): colder coil, more moisture removed (better dehumidification), but lower sensible capacity and rising freeze risk. Good for humid climates within reason; dangerous below ~300.
  • Toward 400+ CFM/ton: warmer coil, more sensible cooling, but weaker dehumidification. Good for dry climates; bad if humidity control matters.

In heating (gas furnace), airflow is set differently — by temperature rise, not CFM/ton. The furnace data plate gives a rise range (e.g., 35–65°F), and you set the blower so the supply-minus-return temperature lands inside that window. Too little airflow overheats the exchanger and trips the limit; too much airflow and the supply air feels cold and the rise falls below spec.

In the field

Pick the target: ~400 CFM/ton in dry climates or where sensible load dominates; ~350 CFM/ton where dehumidification matters. So a 3-ton system wants roughly 1,050–1,200 CFM in cooling.

PSC blower: change the speed by moving the blower motor lead to a different colored speed tap on the control board (per the wiring legend). Higher-speed taps = more CFM. It's coarse — you get the taps the motor offers.

ECM/variable-speed blower: set airflow by configuring CFM directly — dip switches, jumpers, or a control menu that lets you select CFM per ton and adjustments. Much finer control, and the ECM holds that CFM against varying static (up to its limit).

Verify, don't assume:

  1. In cooling, check the evaporator split — should land around 16–22°F at proper airflow and humidity. A very high split says low airflow.
  2. Measure total external static and compare to the blower table to estimate actual CFM.
  3. In heating, measure temperature rise and confirm it's inside the furnace's stamped range.
  4. The most accurate field method is an airflow measurement (flow hood, or a static-pressure-to-CFM lookup from the blower table).

Normal values & targets

  • Cooling airflow: ~350–400 CFM per ton. 400 for dry/high-sensible; 350 for humid/high-latent. Below ~300 CFM/ton invites freezing.
  • Per-ton math: 3 tons → ~1,050–1,200 CFM; 4 tons → ~1,400–1,600 CFM.
  • Evaporator split at proper airflow: ~16–22°F (humidity-dependent).
  • Heating temperature rise: set within the furnace data-plate range (commonly something like 35–65°F, but go by the plate).
  • Static limit: remember the blower only delivers rated CFM up to its rated static (~0.5" WC typical residential) — high static undercuts whatever speed you select.

Common faults & what they mean

  • High evaporator split, coil freezing: airflow too low — speed tap too low, dirty filter/coil/blower wheel, or restrictive ducts. Raise CFM and/or fix restriction.
  • Poor dehumidification, clammy house: airflow too high for a humid climate — drop toward 350 CFM/ton (or enable the ECM's dehumidify/lower-CFM profile).
  • Furnace limit tripping / short cycling on heat: airflow too low for the heat output — rise above the plate range. Increase blower speed or clear the restriction.
  • Cold-feeling supply air on heat: airflow too high — rise below the plate range. Lower the blower speed.
  • ECM set right but actual CFM low: static pressure is over the blower's limit — even an ECM can't make rated CFM through badly undersized ducts. Fix the ducts.

Tech tips & gotchas

  • Set airflow before you charge. Refrigerant readings are only meaningful at correct airflow. Charging on top of bad airflow gives a wrong charge. Airflow first, every time.
  • Heating is set by rise, cooling by CFM/ton — don't mix them up. A furnace blower speed that's perfect for the cooling coil may be wrong for the heat rise, which is why many systems use a different speed tap for heat vs cool.
  • Climate decides 350 vs 400. Humid Southeast: lean toward 350 for moisture removal. Dry Southwest: lean toward 400 for sensible. Don't blindly default to one number.
  • Verify with a measurement, not the dip-switch label. What you selected and what the duct system delivers can differ. Confirm with split, static-to-CFM, or a flow measurement.
  • The blower wheel and filter sabotage your setting. A caked wheel or restrictive filter eats CFM no matter what speed you pick. Check them when airflow comes up short.

Safety / code notes

For gas heat, setting airflow to keep temperature rise inside the data-plate range isn't just comfort — over-firing/under-airflow overheats the heat exchanger and can crack it, a CO hazard. Follow the furnace manufacturer's rise range and input rating. De-energize before changing speed taps inside the cabinet. Refrigerant work (charge verification after airflow is set) per EPA 608 (40 CFR Part 82, Subpart F).