CSLB C-20 HVAC ContractorPlanning and LayoutMedium
During Manual D duct design, a system has a total effective length (TEL) of 150 feet and an available static pressure of 0.08 in. w.c. What friction rate (in inches of water column per 100 feet) should be used to size the ductwork?
- A0.05 in. w.c./100 ft
- B0.08 in. w.c./100 ft
- C0.53 in. w.c./100 ft
- D0.15 in. w.c./100 ft
Show answer & explanationAnswer & explanation
Correct answer: A. 0.05 in. w.c./100 ft
Friction rate is calculated as (Available Static Pressure ÷ Total Effective Length) × 100. Here that is (0.08 ÷ 150) × 100 = 0.053, which rounds to approximately 0.05 in. w.c. per 100 feet.
Why the other options are wrong
- B. This simply repeats the available static pressure without dividing by TEL.
- C. This results from incorrectly multiplying instead of dividing by TEL.
- D. This value is roughly triple the correct friction rate.
Duct Friction Rate Calculation
Friction rate = (Available Static Pressure ÷ Total Effective Length) × 100, expressed in inches of water column per 100 feet of duct, and used to select duct sizes from Manual D tables.
- TEL includes straight duct plus fitting equivalent lengths
- Lower friction rate generally means larger required duct size
- Result is used with airflow (CFM) on a duct sizing chart
Memory trick: Divide the pressure by the path, then scale to a hundred.