STATIC PRESSURE
TOTAL EXTERNAL STATIC PRESSURE
TESP = filter + coil + supply + return
in w.c.
in w.c.
in w.c.
in w.c.
RESULT
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FILL IN ABOVE
Most residential blowers are rated 0.5" w.c. TESP. >0.7" = airflow problems. Estimate only — verify with manometer measurement and a licensed HVAC contractor before equipment changes.
About this calculator
This static pressure calculator sums the four pressure drops a residential HVAC blower fights against: filter, evaporator coil, supply duct/registers, and return duct/grille. Total External Static Pressure (TESP) is what a Magnehelic gauge reads with probes drilled into the supply and return plenums. Most residential blowers are rated at 0.5" w.c. TESP — exceed that and airflow drops, the coil ices, the gas heat exchanger overheats, and the system dies young. Aim for 0.5" w.c. or lower; 0.7" is "in trouble." ESTIMATE ONLY — actual TESP must be measured with a Magnehelic gauge or manometer.
Common questions
What is "0.5 inches w.c." and why does it matter?
Inches of water column (in. w.c. or in. WC) is the unit the HVAC industry uses for low-pressure measurements. 0.5" w.c. is the static-pressure rating most residential PSC blowers are designed for. Run them at 0.7-0.8" and airflow drops 25-40%, the coil ices in summer, the heat exchanger overheats in winter, and the equipment dies in 5 years instead of 15.
Why is a 1-inch MERV 13 filter such a problem?
Surface area. A 1-inch pleated filter at MERV 13 has too little media to pass the airflow needed without a major pressure drop — typically 0.20-0.30" w.c. on a clean filter, and 0.40-0.50" once it loads up. A 4-inch or 5-inch media cabinet at MERV 13 has 4-5× the surface area and runs 0.10-0.15" — same filtration, half the cost in airflow.
How do I actually measure TESP?
Drill a 3/8" hole in the supply plenum after the coil and another in the return plenum before the filter. Insert a Magnehelic gauge probe (or a digital manometer) in each, set the gauge to read pressure differential, and read the result with the blower running on cooling speed. Plug the holes with grommets when finished. Total cost: $50 for a basic gauge, $300 for a digital manometer.