FieldBench
How To

Safety · 10 min

Nitrogen tightness test

Pressure is the leak test. Vacuum is the moisture test. Do not skip nitrogen and hope the micron gauge will confess.

Safety

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.

Tools: Nitrogen cylinder and regulator · Known-good gauges or a dedicated test port · Bubble solution / electronic leak detector rated for the refrigerant family · Core tools

  1. 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 not: A closed service valve tests six inches of pipe and lies about the coil.

  2. 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 not: 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. 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 not: If it will not hold nitrogen it will not hold refrigerant. Repair before you weigh in.

  • 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.