Gas Piping, Pressure, and Leak Testing
Domain 4, Gas Piping, Pressure, and Leak Testing, tests pipe sizing, delivered pressure, regulator settings, and safe leak-test procedure. The exam expects you to distinguish supply pressure, manifold pressure, and pressure drop.
Core rules
- Natural gas heating value is commonly about 1,000 Btu/ft³; LP is about 2,500 Btu/ft³. Pipe size depends on load, length, allowable drop, and specific gravity.
- Typical natural gas supply pressure is about 7 in. w.c. and manifold pressure about 3.5 in. w.c. LP supply is often 11–13 in. w.c. with a manifold setting near 10–11 in. w.c. Always verify the appliance rating plate.
- Allowable pressure drop is commonly 0.5 in. w.c. for low-pressure residential piping. Use the code table for the actual length and load.
- Leak testing commonly uses 1.5 times the maximum working pressure, with a minimum test pressure of 3 psi, held for the code-required duration. Follow the adopted fuel-gas code and local authority.
- Leak detection may use soap bubbles or an approved electronic detector. Never use a flame.
Worked example
A 150,000 Btu/h boiler is 50 ft from the meter. A 1/2 in. line may not carry that load within 0.5 in. w.c. drop; a 3/4 in. line is often required. Verify with the code table, then confirm actual inlet pressure under flow and set manifold pressure per the rating plate.
Traps
- Testing with the appliance gas valve open.
- Applying high pressure to a low-pressure gas valve.
- Confusing static supply pressure with flowing manifold pressure.
- Ignoring altitude derate or a long run with many fittings.