Static Pressure Calculator (TESP)

How it works

TESP = supply static + |return static|. The supply side reads positive (the blower is pushing air into the ducts) and the return side reads negative (the blower is pulling), so the two readings are added as absolute values. The result is compared with the rated maximum external static on the data plate: at or under 80% of rated is comfortable, 80 to 100% is high, and over the rating means the blower is moving less air than the equipment was designed for.

Where you probe matters. Total external static is everything outside the equipment the rating applies to. On an air handler with an internal coil, probe the supply plenum after the coil and the return plenum before the filter. On a gas furnace with an add-on coil, the coil is external to the furnace, so the supply probe goes between the furnace and the coil (check the furnace manual). Drill probe holes in straight duct or plenum, away from turns and the blower outlet, and point the static tip into the airflow.

A common field budget for a system rated at 0.5 in. w.c. is about 20% for the filter (0.10) and 40% for the wet coil (0.20), with the rest split between the supply and return ducts. Enter the filter and coil drops to see how your system compares and how much is left for the ducts.

Worked example

Supply 0.25 in. w.c. and return −0.20 in. w.c. on a furnace rated 0.50: TESP = 0.25 + 0.20 = 0.45 in. w.c., 90% of rated, which is high. The filter measures 0.10 (20% of rated, right at the usual budget) and the wet coil 0.20 (40%), which leaves 0.15 in. w.c. for the ducts and grilles. Neither side clearly dominates, so the restriction is spread through the duct system rather than sitting in one part.