CSLB C-20 HVAC ContractorPlanning and LayoutHard
A furnace blower has 0.50 in. wc of total external static pressure (ESP) available. Manufacturer data shows the cooling coil consumes 0.20 in. wc and the air filter consumes 0.08 in. wc of that available pressure. How much static pressure remains available for duct friction losses in the supply and return ductwork combined?
- A0.50 in. wc
- B0.30 in. wc
- C0.22 in. wc
- D0.12 in. wc
Show answer & explanationAnswer & explanation
Correct answer: C. 0.22 in. wc
Total available ESP is allocated among all system components. Subtracting the coil's 0.20 in. wc and the filter's 0.08 in. wc from the total 0.50 in. wc leaves 0.50 − 0.20 − 0.08 = 0.22 in. wc remaining for duct friction losses across the entire supply and return system.
Why the other options are wrong
- A. This ignores the pressure drop consumed by the coil and filter entirely.
- B. This only subtracts the coil's pressure drop and omits the filter's drop.
- D. This value results from subtracting too much pressure, exceeding actual component losses.
Static Pressure Budget
Total external static pressure (ESP) available from the blower must be divided among all system components (coil, filter, ductwork); remaining pressure after non-duct losses is the friction budget for the duct system.
- Available ESP = blower rated ESP
- Subtract fixed component losses (coil, filter) first
- Remaining ESP is allocated to supply + return duct friction per Manual D
Memory trick: Total pressure minus coil minus filter equals what the ducts get to spend.