Nitrogen tightness test
Pressure is the leak test. Vacuum is the moisture test. Do not skip nitrogen and hope the micron gauge will confess.
Before you start
- Nitrogen only. Never oxygen, never shop air — oil + air + pressure is a diesel event.
- Do not exceed the nameplate or OEM test pressure. A ‘300 psi because that is what the regulator will do’ is how you split a coil.
- Wear eye protection. A blown hose at 150 psi is a missile.
- A2L: check the area for ignition sources before you open anything that might still hold refrigerant.
On the truck
- Nitrogen cylinder and regulator
- Known-good gauges or a dedicated test port
- Bubble solution / electronic leak detector rated for the refrigerant family
- Core tools
Steps
1 Empty, isolated, cores under control
Recover first if the system had charge. Core tools on. Both indoor and outdoor in the test. Isolate the nitrogen bottle so a regulator failure cannot keep feeding the coil.
Expect: No refrigerant. Valves open to the lines you mean to test.
If it fails: A closed service valve tests six inches of pipe and lies about the coil.
2 Bring it up, write the number, wait
Raise nitrogen to the OEM standing-test pressure (field class is often ~150 psig on comfort splits — the I/O / nameplate is law). Write pressure, outdoor temp, and time. Wait. A 2–3 psig drop that matches a big temperature drop can be physics. A steady fall at a stable temp is a leak.
Expect: Pressure holds after the temperature correction.
If it fails: Soap every new joint, schrader, gasket, and the indoor coil. Electronic detector on A2L after you add the OEM-allowed tracer — or find it on nitrogen bubbles first.
3 Bleed to a whisper, then evacuate
Do not dump a 150 psi shot into a vacuum pump. Bleed most of the nitrogen, then pull the vacuum how-to. A2L new lines: this standing test plus the inert purge is the required discipline, not extra credit.
Expect: Safe residual pressure, then a deep vacuum and a decay.
If it fails: If it will not hold nitrogen it will not hold refrigerant. Repair before you weigh in.
Field notes
- Vacuum decay can look like a leak when it is moisture. Nitrogen standing test separates the two.
- Trace gas (a few ounces of the system refrigerant or an OEM-allowed hydrogen mix) plus nitrogen is a detector trick — recover that mix when you are done. Do not vent it.
Related
Sources
Practice distilled from public HVACR School and AC Service Tech material. OEM I/O and EPA 608 still win.