Gas Piping, Sizing, and Pressure Testing
What this domain tests
This domain tests pipe materials, sizing methods, pressure drop, CSST bonding, test media, test pressure, duration, and leak testing. You must size piping from the appliance load and the longest run, then prove it tight before commissioning.
Core rules and concepts
- Use approved pipe, fittings, and joining methods. Metallic pipe, copper where permitted, CSST, and polyethylene for underground use have specific restrictions and approval requirements.
- Size by the adopted code's tables using the longest equivalent length and the appliance input. Low-pressure systems commonly use a 0.5 in. w.c. pressure drop; higher-pressure systems use the applicable table and regulator arrangement.
- Add appliance loads correctly. Diversity may be allowed only where the code or design permits it; do not assume it for a residence.
- CSST must be bonded as required by the electrical code and the manufacturer's instructions.
- Pressure test with air or nitrogen, never oxygen or a flammable gas. Isolate regulators, relief valves, and appliances that could be damaged.
- A common test rule is 1.5 times the maximum working pressure with a code minimum and a timed hold; often 10 minutes for small systems and 30 minutes for larger ones. Confirm the adopted edition.
- After pressure testing, leak-test at operating pressure with an approved solution or detector, and purge safely.
Worked example
A natural-gas branch serves a 120,000 Btu/h appliance. The longest run is 60 ft (18 m), and the allowable drop is 0.5 in. w.c. Using the code's longest-length table for natural gas, 1/2 in. pipe at 60 ft may allow only about 75,000 Btu/h, while 3/4 in. pipe may allow about 150,000 Btu/h. Reasoning: select 3/4 in. because the load exceeds the 1/2 in. capacity; then verify the meter, regulator, and appliance connection can deliver the required pressure. Do not size from the branch length or from connected load without the longest run.
Traps that cost marks
- Using the branch length instead of the longest equivalent length.
- Sizing on connected load while ignoring actual appliance input or altitude derating.
- Testing with water or oxygen, or leaving gauges and regulators in the test section.
- Skipping CSST bonding.
- Shortening the test duration or failing to record gauge pressure.
- Purging into an occupied space instead of a safe location.
Exam tactic
Write down load, longest length, allowable drop, and table capacity before selecting pipe size.