How to calculate total external static pressure
TESP = |supply static| + |return static|
- Run the blower at the speed you're testing (cooling speed is usual).
- Read return static between the filter and the blower.
- Read supply static on the supply side of the unit. If the coil is a separate box, the manufacturer's instructions say whether to read before or after it.
- Add the two readings, ignoring the signs.
Worked example
Supply reads +0.30 in. WC and return reads −0.25 in. WC. TESP is 0.30 + 0.25 = 0.55 in. WC, above a 0.5 in. WC rating.
What high static pressure means
High static means the blower is fighting the duct system. Airflow drops, coils freeze or heat exchangers run hot, and ECM motors work harder. Look for dirty filters or coils, undersized or restricted ducts, and closed dampers.
Questions
What is a normal total external static pressure?
Most residential furnaces and air handlers are rated for 0.5 in. WC. Check the data plate for the equipment you're testing.