Airflow7 min
Airflow and static pressure
Most ‘low charge’ calls are filters, coils, and undersized returns. Prove airflow before you weigh in.
What to measure
- Total external static = supply plenum static + return static (absolute values), taken at the unit.
- Compare to the blower chart for that motor and speed tap / ECM profile. 0.5 in. w.c. is not a universal target.
- A dirty filter or coil shows up as high return static (filter) or high supply static (coil / duct).
Quick field tells
- Delta-T in cool of 16–22°F is a ballpark on a dry coil. High delta-T often means low airflow. Low delta-T can be low charge, bypass, or high latent.
- Ice on the suction or evaporator is airflow or undercharge — do not add gas on a frozen coil.
- ECM ‘constantly ramping’ with noisy returns is usually a duct or filter problem, not a bad board.
Fix order
- Filter, blower wheel, indoor coil, then duct. Charge last.
- Write static and blower setting on the job ticket so the next tech does not start over.