CSLB C-20 HVAC ContractorPlanning and LayoutMedium
A furnace requires 100,000 Btu/hr gas input, and the gas piping run from the meter to the appliance is 40 feet including fittings. Using a simplified gas pipe sizing table, a 1/2-inch pipe at 40 feet carries 56,000 Btu/hr, a 3/4-inch pipe carries 117,000 Btu/hr, and a 1-inch pipe carries 220,000 Btu/hr. What is the minimum pipe size the technician must install?
- A1-inch pipe
- B1-1/4-inch pipe
- C1/2-inch pipe
- D3/4-inch pipe
Show answer & explanationAnswer & explanation
Correct answer: D. 3/4-inch pipe
The 1/2-inch pipe's 56,000 Btu/hr capacity is insufficient for the 100,000 Btu/hr requirement. The 3/4-inch pipe's rated capacity of 117,000 Btu/hr exceeds the requirement at this pipe length, making it the smallest acceptable size per code sizing tables.
Why the other options are wrong
- A. While adequate, a 1-inch pipe is larger than the minimum size needed.
- B. This size is unnecessarily oversized for the given load and length.
- C. 56,000 Btu/hr capacity is far below the 100,000 Btu/hr requirement.
Gas Pipe Sizing Table
Gas pipe capacity depends on both pipe diameter and total developed length; longer runs reduce capacity, requiring larger diameters to deliver the same Btu/hr rating.
- Capacity decreases as pipe length increases due to friction loss
- Always select the smallest pipe size meeting or exceeding the appliance Btu/hr input
- Code-based sizing tables (e.g., CMC) list capacity by diameter and length
Memory trick: Match the pipe to the pressure drop over distance, not just the Btu rating alone.