Pressure test (plumbing)
Also called: rough-in pressure test, hydrostatic test, air test plumbing, plumbing inspection test
The rough-in check on a new supply system: with every outlet capped, the pipes are pumped to a test pressure — commonly 100 psi, some jurisdictions 150 — and held for 15 minutes to an hour with a gauge attached. Any drop means a leak; inspectors witness it before walls close.
A repipe puts sixty or more joints inside walls that are about to be patched and painted. The pressure test is the one moment every joint is checked at once, under more pressure than it will ever see in service, while it can still be reached. Codes require it on permitted work at rough-in, and a competent plumber or DIYer does it before calling the inspector.
The method: cap every stub-out, connect a test gauge with a fill valve at a hose bibb or the water heater connection, and pump the system up with air (a compressor) or water (the house supply plus a hand pump). Hold at the required pressure for the required time and watch the needle. A steady gauge passes. A slow drop is a leak — on PEX almost always a crimp that wasn’t fully closed or a ring set too far from the tube end; on copper a joint that didn’t take solder all the way around. Soapy water on each joint finds an air leak; a paper towel finds a water leak.
Air is convenient and safe at these pressures on PEX; some jurisdictions require water for copper. Either way, test before the walls close, keep the result with the permit, and don’t let a contractor patch drywall before the inspector has seen the gauge.