“What does CFM mean and why does my technician keep mentioning it?”
CFM in one line
CFM is cubic feet per minute — the volume of air the system actually moves. Everything a heating and cooling system does, it does by moving air. Capacity that never reaches a room is capacity you paid for and did not receive.
The rule of thumb, and its limits
Cooling equipment is generally designed around roughly 400 CFM per ton of capacity. A three-ton system is expecting somewhere near 1,200 CFM to perform as rated.
It is a starting point, not a verdict. The number that matters is what your system is actually delivering versus what this house needs, and the only way to know either is to measure — airflow on one side, a load calculation on the other.
What too little airflow does
In cooling, low airflow means the coil gets less warm air than it needs. The coil runs colder, removes less heat overall, and in bad cases freezes. In heating, low airflow means the heat exchanger cannot shed its heat into the air stream fast enough, so it runs hot and the safety limit shuts the burner down — then it restarts, and short-cycles.
In both cases the equipment looks faulty and is not.
How it is measured
There are several honest methods — a flow grid installed temporarily in the air stream, a balancing hood at each register, or airflow derived from temperature rise across the heat exchanger. They answer slightly different questions and a good technician will say which one they used and why.
What none of them is: a guess based on how the air feels at the register.