The short answer
- A pressure test closes off the gas piping, pressurizes it with air and watches a gauge. If the pressure drops, there is a leak somewhere.
- The fuel gas code minimum is one and a half times the working pressure, never below 3 psi, for at least 10 minutes. Many inspectors want more.
- Buyers ask for one after an inspection finds a gas smell or questionable piping, and utilities and inspectors can require one before gas is restored.
- A failed test means the leak has to be found, fixed and the system retested before gas goes back on. Plan time for it before closing.
What a gas pressure test is
A gas pressure test checks whether the piping holds gas tight. The gas is shut off at the meter, the appliances are isolated or disconnected so they are not exposed to the test pressure, and the piping is filled with air or nitrogen to a set pressure. A gauge is read at the start and the end of the test period. If it holds, the piping is tight. If it drops, something leaks.
Never oxygen, and never the gas itself: the code calls for air or an inert gas as the test medium. The appliance controls and regulators have to be kept out of the test, because test pressure can damage them.
What the code requires
Section 406 of the International Fuel Gas Code sets the minimums. The test pressure must be at least one and a half times the working pressure, and never less than 3 psi. It has to hold for at least 10 minutes, with no drop that cannot be explained by temperature.
Local inspectors often ask for more. Spartanburg County's residential inspection list calls for a minimum 10 psi air test for the gas meter release inspection. Larger or more complex systems can be held for longer.
After the test passes and gas is turned back on, the piping and every appliance connection are checked for leaks with gas in the line, and each appliance is relit and checked.
When buyers, inspectors or utilities ask for one
A buyer's home inspector does not usually pressure-test the gas system. What they do is note a gas smell, a reading on a gas detector near a fitting, an old uncoated brass connector, rusted or unsupported pipe, unbonded CSST or a capped line with no valve. Any of those can lead the buyer to ask for a licensed contractor to test and repair.
Utilities and building departments can also ask. Gas that has been shut off for a leak or a safety problem will not be restored until the cause is fixed, and a permitted repair or alteration gets a pressure test as part of its inspection. A house that has sat vacant with the gas off for a long time is another common case.
What happens if it fails
A failed test is not a verdict on the whole system. Most leaks are at a single fitting, valve or connector. The contractor isolates sections of the piping, finds the leak with a soap solution or a detector, repairs it and tests again. The system does not go back in service until it holds.
Sometimes the answer is bigger: a corroded buried line to a detached garage, a run of old pipe in a crawlspace, or CSST that needs bonding. Those take longer and may need a permit. If the gas is off, the water heater and furnace are off too, which matters for a family moving in.
Advice for buyers and sellers
Sellers with an older gas system can have it tested before listing, so a leak becomes a repair on their timeline instead of a negotiation the week before closing. Keep the test results and any permits with the house papers.
Buyers who see gas findings on an inspection report should ask for a test by a licensed contractor, not a general handyman, and ask for the results in writing. If repairs are needed, ask whether they were permitted and inspected.
When to call a pro
Call a licensed gas contractor when an inspection report notes a gas smell, leaking fittings, old connectors or questionable piping, when the utility will not restore gas until the system is checked, or when you want the gas system tested before you list a house.