CSLB C-20 HVAC ContractorHeating SystemsHard
A technician is sizing a gas supply pipe for a furnace with an input rating of 120,000 Btu/h using natural gas with a heating value of 1,000 Btu per cubic foot. What cubic feet per hour (cfh) demand value should be used when consulting the gas pipe sizing table?
- A200 cfh
- B150 cfh
- C120 cfh
- D100 cfh
Show answer & explanationAnswer & explanation
Correct answer: C. 120 cfh
Gas pipe sizing tables are based on volumetric flow (cfh), not Btu/h directly. Convert using the fuel's heating value: cfh = Btu/h ÷ Btu per cubic foot = 120,000 ÷ 1,000 = 120 cfh. This value, along with pipe length and allowable pressure drop, is used to select proper pipe diameter from the sizing table.
Why the other options are wrong
- A. 200 cfh significantly overestimates the load and would oversize the pipe unnecessarily.
- B. 150 cfh overestimates demand, possibly from using a lower heating value than specified.
- D. 100 cfh underestimates demand and would result from an incorrect heating value assumption.
Gas Pipe Sizing (cfh Conversion)
Gas pipe sizing tables require flow demand in cubic feet per hour (cfh), calculated by dividing the appliance's Btu/h input by the fuel's heating value (typically ~1,000 Btu/cf for natural gas).
- Natural gas heating value averages ~1,000-1,100 Btu per cubic foot
- Propane has a much higher heating value (~2,500 Btu/cf), requiring different cfh calculations
- Pipe size also depends on total developed length and allowable pressure drop (commonly 0.5 in w.c.)
Memory trick: Divide Btu load by heating value to get flow.