Journeyman Plumber ExamGas PipingMedium
A plumber is installing a gas-fired unit heater in a commercial garage. The heater has an input rating of 150,000 BTUs per hour. The gas meter is 75 feet away, and the gas pressure is 0.5 psig. The piping will be Schedule 40 steel. Using a 0.5 inches water column pressure drop, what is the minimum nominal pipe size required for this installation?
- A1 1/2 inch
- B1 inch
- C2 inches
- D1 1/4 inch
Show answer & explanationAnswer & explanation
Correct answer: D. 1 1/4 inch
Consulting a gas pipe sizing table (e.g., NFPA 54/IFGC Table 402.4(2) for 0.5 psig, 0.5 IWC pressure drop). For a 75-foot length and 150,000 BTU/hr, a 1-inch pipe can deliver approximately 136,000 BTU/hr, which is insufficient. A 1 1/4-inch pipe can deliver approximately 285,000 BTU/hr, which is sufficient.
Why the other options are wrong
- A. A 1 1/2-inch pipe is oversized for this application, leading to unnecessary material costs.
- B. A 1-inch pipe is insufficient, as it can only deliver up to 136,000 BTU/hr for this length and pressure drop.
- C. A 2-inch pipe is significantly oversized, indicating a miscalculation or misunderstanding of sizing principles.
Commercial Gas Pipe Sizing
The process of accurately determining the gas pipe diameter for commercial appliances based on higher BTU demands, longer runs, and specific pressure requirements.
- Uses the same principles as residential, but with larger demands.
- Sizing tables are critical.
- Accuracy prevents under-gassing or over-sizing.
Memory trick: Big BTUs, Long Run, Table's the Key, Get the Sizing Done!