What test does the inspector want on a new water supply rough-in, and can I use air?

The Test: The inspector wants a hydrostatic (water) pressure test per IPC 312.5 / UPC 712.1.

Procedure:

1. Fill the entire system with water.
2. Pressurize to working pressure (typically 40–80 psi) or 50 psi minimum, whichever is greater.
3. Hold for 15 minutes minimum (IPC) / 15 minutes (UPC).
4. Zero pressure drop and zero visible leaks at all joints.

Can you use air? Generally, NO.

• Code: IPC 312.5 / UPC 712.1 explicitly require water for the final inspection test on water distribution systems.
• Safety: Air stores massive compressive energy. A fitting failure at 50 psi air acts like a bomb; water at 50 psi just sprays. Many jurisdictions prohibit air testing on plastic (PEX/CPVC/PVC) entirely due to fracture risk.
• Manufacturers: Most PEX/CPVC manufacturers void warranties if air tested.

The "Air Exception" (Rare): Some jurisdictions may allow a low-pressure air test (≤ 5 psi) only for preliminary leak detection on metal piping (copper/steel) before the final water test, or if the system cannot hold water (freeze risk). You must get written AHJ approval first.

Pro Tip: Test with water. It finds weeping solder joints and push-fit seating errors that 5 psi air misses. Drain immediately after inspection if freeze risk exists.

For a new water supply rough-in, the inspector typically requires a hydrostatic pressure test using water, not air. This is standard under most plumbing codes, including the International Plumbing Code (IPC) Section 608.1 and the Uniform Plumbing Code (UPC) Section 609.4.

Why Water, Not Air?

• Safety: Compressed air stores significant energy. If a fitting fails during an air test, it can cause explosive rupture, posing serious injury or fatality risks. Water is nearly incompressible and releases energy slowly if a leak occurs.
• Accuracy: Water better simulates real operating conditions and reveals slow leaks or seepage that air might miss due to its lower viscosity and higher permeability through small gaps.
• Code Compliance: Both IPC and UPC explicitly require hydrostatic (water) pressure testing for water distribution systems. Air testing is generally prohibited or only allowed under very specific, controlled conditions (e.g., for DWV systems in some jurisdictions, but never for pressurized water supply lines).

Test Procedure (Typical):

1. Isolate the system (close valves, cap ends).
2. Fill with water and purge all air.
3. Pressurize to at least 150 psi (or 50 psi above the system’s working pressure, whichever is greater) — per IPC 608.1.
4. Hold pressure for 15 minutes (IPC) or 15–30 minutes (UPC, depending on jurisdiction).
5. Inspect for leaks, drops in pressure, or weeping at joints, valves, and connections.
6. Repair any leaks and retest.

Can You Use Air?

No — not for water supply rough-ins. Using air violates code and is unsafe. Some inspectors may allow a brief air test to check for gross blockages or disconnected lines before filling with water, but this is not a substitute for the required hydrostatic test. Never rely on air alone to pass inspection.

> ⚠️ Warning: If you use air and a fitting fails, you risk serious harm. Always follow code and use water for pressure testing potable water supply systems.

If your local jurisdiction has amendments, verify with the building department — but in 99% of cases, water hydrostatic test is mandatory. When in doubt, call the inspector — they’ll tell you exactly what they want to see.