CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A furnace blower has a rated total external static pressure (ESP) capability of 0.50 in. w.c. The installed evaporator coil has a pressure drop of 0.20 in. w.c., and the filter has a pressure drop of 0.08 in. w.c. How much static pressure is available for the supply and return ductwork?
- A0.30 in. w.c.
- B0.22 in. w.c.
- C0.14 in. w.c.
- D0.42 in. w.c.
Show answer & explanationAnswer & explanation
Correct answer: B. 0.22 in. w.c.
Available static pressure for ductwork equals rated ESP minus component pressure drops: 0.50 − 0.20 − 0.08 = 0.22 in. w.c. This remaining pressure budget is what the duct system design must not exceed.
Why the other options are wrong
- A. This only subtracts the coil drop and ignores the filter drop.
- C. This subtracts an extra, unlisted drop not given in the scenario.
- D. This only subtracts the filter drop and ignores the coil drop.
Available Static Pressure
The static pressure remaining for the duct system after subtracting all equipment component pressure drops from the blower's rated external static pressure.
- ESP = rated blower capability at design airflow
- Subtract coil, filter, and other component drops
- Remaining value budgets supply + return duct friction losses
Memory trick: Start with total ESP, then peel away every drop in the way.