CSLB C-20 HVAC ContractorPlanning and LayoutHard
A supply duct route requires 80 feet of straight duct, three 90-degree elbows each with an equivalent length of 15 feet, and one tee fitting with an equivalent length of 25 feet. What is the Total Effective Length (TEL) used for the friction rate calculation?
- A105 feet
- B120 feet
- C170 feet
- D150 feet
Show answer & explanationAnswer & explanation
Correct answer: D. 150 feet
TEL equals straight duct length plus the equivalent lengths of all fittings: 80 + (3 × 15) + 25 = 80 + 45 + 25 = 150 feet.
Why the other options are wrong
- A. This omits the tee fitting's equivalent length entirely.
- B. This only counts one elbow instead of all three.
- C. This overcounts by adding an extra elbow equivalent length.
Total Effective Length (TEL)
TEL is the sum of actual straight duct length plus the equivalent lengths of all fittings along the longest duct run, used to calculate friction rate in Manual D.
- Fittings add resistance equivalent to extra straight duct feet
- Longest run (critical path) determines system TEL
- Used with available static pressure to find friction rate
Memory trick: Straight duct plus every twist and turn adds up the length you earn.