External static pressure · 2 min read
External static pressure, plainly
The number behind most ducted-unit complaints, and how to get it right.
External static pressure (ESP) is the pressure a fan unit has left, at a stated airflow, to push air through everything outside the unit: ductwork and fittings, dampers, grilles and diffusers, attenuators, and filters or coils in the duct. It is given in pascals.
Pressure loss rises roughly with the square of airflow, and fan power with its cube. Underestimate the ESP and the unit moves less air than designed: capacity drops and a DX coil can ice. Overestimate it and the unit moves more air, with more fan energy and noise, until its speed is trimmed at commissioning.
Ducted splits usually offer from about 25 Pa up to 200 Pa on high-static models; air handling units several hundred pascals.
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