What CFM Means for Your Heating and Cooling

CFM is cubic feet of air per minute. It is the blower's output after the filter, the coil, and the ducts have had their say. Tonnage says how much heat the refrigerant circuit can move. CFM says whether the house air actually passes across that coil and then arrives in the rooms. You need both.

The number people quote, and why houses miss it

A common cooling target is about 400 CFM for each ton of capacity. It is a setup target, not a law of every room. A restrictive filter, a dirty coil, a sagging plenum liner, and ducts that are too small all steal from it. So does closing a bunch of supply vents. The blower can be brand new and the bedroom at the end of a crushed flex duct can still be short of air.

Heating cares about airflow too. Too little air across a furnace and the heat exchanger runs hot and the limit switch shuts the burners down. Too little air across a heat pump coil and the house feels cool at the register even when the outdoor unit is doing its job.

What low airflow feels like

• A room that never catches up, with a weak grille.
• Whistling returns or a loud blower.
• A cooling coil that ices, or a furnace that short cycles on the limit.
• Dust, if return ducts are leaking and the blower is pulling attic air.

You cannot measure CFM by feeling a grille, but you can notice a grille that is nearly still while others are strong. That room's duct, damper, or connection is the question. Whole-house weakness with a clean filter points at the blower setting, the coil, or the plenums.

We measure and look, rather than sell a larger unit to shove more air through a duct that cannot take it. That is a diagnosis of the house, not only of the box.

Common questions