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Airflow is the “other half” of HVAC performance. Static pressure is the resistance the blower must push against (measured in inches water column, in. w.c.). High static pressure reduces airflow, causing poor comfort and coil issues (freeze-ups or high heat exchanger temps). Key terms: • Total External Static Pressure (TESP) = supply static + return static (measured external to furnace/air handler) • Dirty filter, restrictive duct, closed registers → higher static → lower airflow • Low airflow in cooling → colder coil → possible icing; high superheat; low suction • Low airflow in heating → high temperature rise; possible limit switch trips
Worked example (concept): If return duct is undersized + filter clogged, blower sees high negative return static. Airflow drops → evaporator gets too cold → coil may freeze even with correct refrigerant charge.
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