Static Pressure

Metrics and Efficiency Ratings - HVAC Costs pillar

Also known as: external static pressure, ESP

The resistance to airflow inside a duct system, measured in inches of water column. High static pressure means restricted ducts and short blower life.

Static pressure is the pressure the blower fights against to push air through the duct system, expressed in inches of water column (in. w.c.). Modern variable-speed furnaces are designed for around 0.5 in. w.c. external static; oversized filters, undersized return ducts, and crushed flex duct push real-world systems to 0.8-1.2 in. w.c., which causes early blower failure, noise, and reduced cooling capacity. Measuring static pressure across the air handler is the single best diagnostic for an underperforming HVAC system.

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