Evacuation and microns

Why evacuate

Evacuation removes two things that do damage in different ways. Rheem’s R-454B instructions put it plainly:

• Air or nitrogen left in the system raises condensing temperature and pressure, which means higher power draw, erratic operation and lower capacity.
• Moisture reacts with the refrigerant and oil to form acid, which attacks the compressor motor windings and internal parts and can cause compressor failure.

The filter drier does not replace a good vacuum. Rheem notes the drier only has adequate moisture-holding capacity if the system is properly evacuated, and that vacuum pumps will not remove moisture from POE oil. See refrigerant oil types for POE handling.

Evacuate only after the system has passed a pressure test with nitrogen. See the nitrogen pressure test.

Why microns, not inches of mercury

A micron is one thousandth of a millimeter of mercury, measured from a perfect vacuum. Because it is absolute, 500 microns means the same thing at any elevation. A compound gauge reads inches of mercury vacuum relative to local air: the same 500 microns reads 29.90 in Hg at sea level and 24.88 in Hg at 5,000 ft, and the whole range from 500 microns down to a perfect vacuum is about 0.02 in Hg. A compound gauge cannot show that. Use a micron gauge, and the pressure converter to translate units.

Target microns and decay tests by manufacturer

The numbers differ, and so does the pass rule. Follow the manual for the unit you are installing.

Rheem also requires the pump to be connected to both the high and low sides through the two pressure ports, and says to use the largest connections available because restrictive service connections can cause false readings from pressure drop through the fittings. The unbranded OEM manual says not to open the service valves until leak inspection and vacuuming of the lines and indoor coil are complete, and to do both through the service valves, warning that using a separate suction port leads to refrigerant loss.

Never use the compressor to pull a vacuum. Rheem warns that internal electrical arcing in near-vacuum conditions can damage the compressor, and that running a compressor while the system is in a vacuum will cause failure.

Reading the rise: leak or moisture

The Oxbox manual gives the standard interpretation. Close the valve to the vacuum pump so the system holds vacuum on its own, and watch the micron gauge for a few minutes:

• Steady, continuous rise: a leak. Leak test with dry nitrogen to find it, repair, and test again.
• Rise that then levels off and stays fairly constant: the system is tight but still holds moisture. Keep evacuating if the reading is above 350 microns.

Rheem’s 15-minute hold and the OEM’s one-minute rule are stricter or looser versions of the same test. A system that passes one may not pass the other, so read the manual first.

Triple evacuation

Fieldpiece’s vacuum pump manual calls triple evacuation “a great way to ensure a system is dry”, because the nitrogen flow helps carry moisture out:

1. Purge with nitrogen.
2. Evacuate down to 2,000 microns.
3. Purge with nitrogen.
4. Evacuate down to 1,000 microns.
5. Purge with nitrogen.
6. Evacuate down to below 500 microns.

The same manual suggests purging with a few psi of dry nitrogen before evacuation to pre-dry the system. None of the OEM manuals above requires triple evacuation; the final level and decay test still come from the equipment manual.

On a wet system, Fieldpiece says to open the pump’s gas ballast during the initial pull-down and close it at about 3,000 microns, when the pump quiets, so it can reach a deep vacuum.

Hoses, cores and gauge placement

Fieldpiece’s setup tips:

• Hoses as short as possible, widest diameter possible, vacuum rated, core depressors removed, ball-valve shut-offs instead of low-loss fittings, good seals, replaced when worn.
• Use a vacuum-rated Schrader core removal tool to temporarily remove valve cores from the service valves. Vacuum-rated hoses connected directly to core removal tools are best; manifold hoses and valves can limit or slow evacuation.
• If you use a manifold, a 4-port manifold typically has a larger bore and isolates the pump from both the system and the micron gauge without a shut-off valve on the hose. That keeps the micron gauge on the system side when you isolate the pump.
• During pull-down the vacuum is deeper near the pump, so place the micron gauge as far toward the back of the system, away from the pump, as possible.

Check the tools first. Fieldpiece’s vacuum isolation test: connect the micron gauge directly to a pump port, seal the other ports, close the gas ballast and run the pump. Below 200 microns within one minute means the pump and gauge work. If it does not reach 200 microns, the gauge, the pump or both have a problem. Fieldpiece also says pump oil left in the pump for more than a week may have absorbed enough moisture from the air to affect performance.

A2L systems

On R-454B, R-32 and other A2L systems, the refrigerant must be recovered before evacuation. A vacuum pump is not a recovery machine; Fieldpiece says so directly and says not to use its pump on pressurized systems. Use recovery equipment suitable for A2L refrigerants (see recovery machine compatibility).

Rheem’s A2L service section (10.8 Removal and Evacuation) and the unbranded OEM manual both say the outlet of the vacuum pump must not be close to any potential ignition source and that ventilation must be available. Fieldpiece’s VP55/VP85 is marketed as Class A2L refrigerant system compatible, but the manual still warns not to pump flammable media or hydrocarbons, and to keep the pump away from flames, sparks and flammable gases. Its remote exhaust port takes a garden hose to carry oil mist and vapor outside a building.

Fieldpiece also states that systems recovered of A2L refrigerants can be evacuated safely only by qualified personnel trained in handling those refrigerants. See the A2L refrigerants guide.

Not the same as recovery vacuum

The vacuum levels in EPA’s Section 608 rules (40 CFR 82.156), such as 0 in Hg or 10 in Hg for high-pressure appliances, are the minimum a recovery machine must reach before a system is opened. They are about recovering refrigerant, not dehydrating the system. Dehydration before charging follows the equipment manual’s micron target.