HVACopediaLeak test, vacuum & equipment limits
Test pressures, vacuum targets, gauge ratings and recovery rules.
Pressure and leak testing
| Test | Pressure / limit |
|---|---|
| Start looking for a leak with refrigerant | 5 to 30 psig through the system |
| Nitrogen strength / tightness test | 30 to 100 psig first, hold 1 hour. Then 170 psig, hold up to 24 hours with no drop |
| Nameplate test pressure unknown | Do not go above 170 psig with nitrogen or CO₂ |
| Regulator relief valve on the nitrogen bottle | Set to open at 180 psig |
| R-134a systems | Under 170 psig on both sides is enough |
| Trace gas mix | About 10% refrigerant and 90% nitrogen, up to the unit's condensing pressure |
| Refrigerant-only test | 15 to 30 psig, then up to the manufacturer's high-side pressure |
| Nitrogen with trace refrigerant after a compressor swap (domestic) | About 25 psig, then hold a vacuum for at least 1 hour: any rise is a leak |
| Epoxy repair | Test at 5 psig, then at 100 psig |
| Valve adapter or saddle valve | Test at 15 to 20 psig of refrigerant |
| Nitrogen flowing while brazing | 1 to 2 psi |
Vacuum targets
| Item | Value |
|---|---|
| Ready to charge | 500 microns or lower |
| Deep vacuum target | 250 microns, then isolate and watch for 1 to 3 minutes |
| Triple evacuation | About 1,500 microns three times, breaking each with dry nitrogen to 0 to 3 psig |
| A reading that stalls near | 5,000 microns means water or ice in the system |
| Pump alone | Should reach 500 microns or lower on a tee. Dirty or white foamy oil means water |
| Hose size | A 1/4 inch hose can take up to 8 times longer than a 1/2 inch hose |
Gauges and cylinders
- Do not run a gauge above 75% of its full scale for more than a few minutes.
- A compound gauge covers 30 in. Hg of vacuum up to roughly 240 to 350 psi. High-side gauges read 0 to 800 psi.
- Use only a high-side gauge for a nitrogen test. Never put pressure into a micron gauge.
- A recovery cylinder may be filled to 80% of its water capacity, never more.
- Recovery rest test: after pulling to a vacuum, close the low-side valve and wait 5 minutes. A rise of 10 psi or more means cold refrigerant is still in the system, so recover again.
- Nitrogen, CO₂ or argon only for pressure testing, through a regulator with a relief valve. Never oxygen or acetylene.
Take it with you
All of these tables, plus the calculators, are in the HVACopedia app, offline.
Calculation aids and reference figures for licensed technicians. Always follow the manufacturer’s data plate and manual for the exact model, and local codes. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Pressures are typical saturation values and change with load and airflow.