How is the airflow test performed?

The system is run at design conditions before any reading is taken. With the existing filter in place, a static-pressure probe is set through a 3/8-inch port in the supply plenum and the normal operating pressure is recorded. The filter is then removed and the Digital TrueFlow Grid is installed in its place, in an adapter plate sized to the slot, sealed so no air bypasses it and facing upstream into the airflow.

The grid carries its own pressure sensor and reports to the TrueFlow app by Bluetooth, averaging one hundred readings. The app takes the supply-plenum pressure a second time with the grid in place and applies the correction: the grid does not restrict air exactly as the filter did, so the measured flow is adjusted by the square root of the ratio of the two pressures. The result is total airflow through the air handler in cubic feet per minute, as the system runs with its own filter. At elevation the instrument's sea-level calibration is corrected for air density.

Static-pressure taps at the return, after the filter, before the coil, and at the supply separate a filter restriction from a coil restriction from a duct restriction, so the report says not only how much air the system moves but where it is being lost. Where the filter slot sits away from the air handler, at a return grille, the measurement is of that return, and each return is measured.

The airflow test applies to ducted systems only. It is a separate service from the Manual J load calculation, which applies to any system, ducted or ductless, mini-splits included. The client receives the PDF of the TrueFlow analysis exported from the app.

Last reviewed 2026-09-06. Accurate to the best of our knowledge as of that date. Rules change — that's why the date is there.