HVAC Evacuation: Pulling a Deep Vacuum
Before a refrigeration or air-conditioning system is charged, it is pulled into a deep vacuum. The goal is not just to remove air but to boil every trace of water out of the pipework, and that only happens at a few hundred microns.
Why evacuate?
Air and moisture left in the circuit cause real damage:
- Non-condensable gases (air, nitrogen) collect in the condenser and raise the head pressure and energy use.
- Moisture reacts with refrigerant and POE oil to form acids that attack windings and metal.
- Water can freeze at the expansion device and block the flow.
Why 500 microns?
Water boils when the pressure drops below its vapour pressure. At 500 microns (0.67 mbar) that happens at about −24 °C, so even moisture in a cold line or as ice will evaporate and be pumped away. At 5,000 microns water only boils at about +1 °C, which is why a "good enough" vacuum often leaves water behind.
Many equipment manufacturers specify 500 microns or lower; always follow the instructions for the specific system and refrigerant.
Decay (standing) test
After reaching the target, the pump is isolated and the micron gauge is watched. A reading that rises and then levels off indicates moisture still evaporating, so keep pumping. A reading that keeps rising steadily points to a leak. Our leak rate converter turns a pressure rise over time into a leak rate.
Good practice
Pump-down time depends far more on the hoses than on the pump size:
- Use short, large-diameter hoses and remove the Schrader cores; core removal tools can cut evacuation time several-fold.
- Measure with the micron gauge at the system, not at the pump.
- Change the vacuum pump oil often: wet oil limits the ultimate pressure.
- Open the gas ballast while large amounts of moisture are being removed.
Typical pressures
| Process stage | Pressure |
|---|---|
| Typical evacuation target | ≤ 500 microns (0.67 mbar, 66.7 Pa) |
| Decay test (common practice) | Stays below ≈ 1,000 microns for 10–15 min |
| Rough vacuum where water still boils above 0 °C | > 4,600 microns (6.1 mbar) |
| Two-stage rotary vane pump ultimate | ≈ 15–50 microns (with fresh oil) |
At what temperature does water boil off in the system?
| Microns | Torr | mbar | Pa | Water boils / ice sublimes at |
|---|---|---|---|---|
| 5,000 | 5 | 6.666 | 666.6 | 1.2 °C |
| 2,500 | 2.5 | 3.333 | 333.3 | -7.2 °C |
| 1,000 | 1 | 1.333 | 133.3 | -17.3 °C |
| 750 | 0.75 | 0.9999 | 99.99 | -20.3 °C |
| 500 | 0.5 | 0.6666 | 66.66 | -24.5 °C |
| 300 | 0.3 | 0.4 | 40 | -29.5 °C |
| 250 | 0.25 | 0.3333 | 33.33 | -31.3 °C |
| 100 | 0.1 | 0.1333 | 13.33 | -39.7 °C |
Suitable vacuum pumps
References & further reading
- IAPWS. SR1-86(1992): Revised Supplementary Release on Saturation Properties of Ordinary Water Substance. International Association for the Properties of Water and Steam. Saturation vapour pressure equation used for liquid water.
- D. M. Murphy & T. Koop. Review of the vapour pressures of ice and supercooled water for atmospheric applications. Q. J. R. Meteorol. Soc. 131, 1539 (2005). Sublimation pressure over ice below 0.01 °C.
- J. F. O’Hanlon & T. A. Gessert. A User’s Guide to Vacuum Technology, 4th ed.. Wiley (2023). Practical design of vacuum systems, pump-down and outgassing.
Frequently asked questions
How many microns should an AC system hold?
A common target is to reach 500 microns or lower and to stay below about 1,000 microns during a 10–15 minute decay test, but rules differ between manufacturers. A steady rise that does not level off indicates a leak.
Why does my vacuum stall around 1,000–2,000 microns?
Usually moisture is still evaporating, the pump oil is wet, or hoses and Schrader cores are restricting flow. Change the oil, use larger hoses, remove the cores and keep pumping.
Can I use inHg instead of microns?
No. 500 microns is only about 0.02 inHg from a perfect vacuum, far below the resolution of a manifold gauge. Use an electronic micron gauge.
Other applications
Vacuum Distillation
Vacuum distillation lowers boiling points to protect sensitive products: choosing the pressure, the delta-20 rule for rotavaps, industrial columns and pumps.
Freeze Drying (Lyophilization) Under Vacuum
How freeze drying works: sublimation below the triple point, chamber and condenser pressures, ice vapour pressure, drying phases and the right vacuum pumps.
Vacuum Furnaces and Heat Treatment
Heat treatment in vacuum furnaces: bright surfaces, brazing, sintering, typical pressures, partial pressure, gas quenching, pumps and leak-up rate tests.
Vacuum Pressure Impregnation (VPI)
Vacuum pressure impregnation for windings, porous castings and wood: evacuation, resin flooding, pressurisation, curing, typical pressures and pumps.
Vacuum Sewer Systems
How vacuum sewers move wastewater with air pressure instead of gravity: interface valves, the vacuum station, typical pressures, pumps and where they fit.