R-438A (MO99) PT Chart and Refrigerant Profile

Where you will find R-438A

R-438A, sold as Freon MO99 (formerly DuPont ISCEON MO99), is an R-22 retrofit blend for direct expansion (DX) residential and commercial AC and medium- and low-temperature DX refrigeration. The Chemours FAQ says it can also be used to retrofit R-22 heat pumps and to field-charge dry-shipped units.

Expect it in converted R-22 condensing units, package units and racks. The nameplate still says R-22, so the retrofit label (refrigerant and oil) is the main clue. If there is no label, do not assume. If R-22 and MO99 have been mixed, Chemours advises recovering the mixed gas at the next service and recharging with one refrigerant.

The 2.3% butane and isopentane is what carries mineral oil back to the compressor; the blend is still A1. Chemours warns that “BMP438” is not R-438A: it has no ASHRAE designation and no SNAP listing.

Charging R-438A

The Chemours retrofit guidelines give this sequence for R-22 DX systems:

1. Record baseline R-22 pressures, temperatures, superheat and subcooling.
2. Recover and weigh the R-22. Hold the system under vacuum to pull dissolved R-22 from the oil; break vacuum with dry nitrogen.
3. Replace the filter-drier with the same type, rated for HFCs.
4. Replace critical elastomeric seals (see below).
5. Evacuate to 500 microns and confirm it holds (evacuation targets).
6. Charge liquid from the cylinder, starting at about 85% of the R-22 charge weight. The final charge is usually about 95% of the original R-22 charge.
7. Adjust the charge and TXV in small steps until you reach the same superheat you had on R-22.
8. Watch the oil level, leak-check again and label refrigerant and oil.

MO99 is a zeotrope, so the cylinder vapor differs in composition from the liquid. Remove liquid only; flash it through the manifold or a throttling valve if feeding the suction side, and never let liquid reach the compressor.

A TXV sized correctly for R-22 normally stays; Chemours expects only a slight adjustment for glide and the small suction pressure difference. On pistons and cap tubes, weigh carefully and add toward the system’s target superheat; if head pressure climbs well above normal R-22 levels before the charge weight is in, stop and look for a restriction. On TXV systems, check subcooling against the equipment data.

Read the dew (vapor) column for superheat and the bubble (liquid) column for subcooling. On this chart, the computed glide is 9.6°F at 40°F dew and 7.5°F at 100°F bubble. Chemours quotes 7–8°F. Do not expect R-22 gauge readings: at 40°F, MO99 dew pressure is 60.8 psig versus 68.6 psig for R-22, and at 100°F its bubble pressure is 211.3 psig versus 195.9. Chemours says R-22 gauge sets can be used.

Chemours lists typical capacity 5–10% below R-22, with similar efficiency and lower discharge temperature. Some sight-glass bubbles are normal with this blend, so do not charge to a clear glass.

Oil and compatibility

MO99 works with mineral oil, alkylbenzene and POE, and Chemours says most retrofits need no oil change. Mineral oil and POE can be mixed in any proportion. Add POE (about 20% of the total oil charge, then small increments) in these cases:

• the oil level drops steadily or swings widely during a cycle
• the system has a liquid receiver and no oil separator
• the line set is long or complex, or the evaporator sits well below the condensing unit
• the heat exchangers use enhanced tubing and the system circulates a lot of oil

Keep the level below mid-glass right after adding, or drain an equal amount of mineral oil first. Trane 3-D and Danfoss SM scrolls get a single sump change to OEM-approved POE; screw compressors should go to POE while still on R-22. Copeland’s AE Bulletin 93-11 limits its MO99 approval to supermarket Discus racks (POE-32, MO or AB), so check the compressor maker’s list for anything else. More on lubricants in the refrigerant oil guide.

R-22 swells elastomers more than HFCs do, so seals that held on R-22 can shrink and leak after conversion. Replace Schrader cores and any seal you cannot reach without pulling the charge, and keep spares for solenoids, ball valves, receiver gaskets and flanges.

Safety, handling and recovery

R-438A is A1 (lower toxicity, no flame propagation). It is a Section 608 high-pressure refrigerant: Type II or Universal certification. Since Jan 1 2018, its sale has been restricted to certified technicians under 40 CFR 82.154, and venting it is prohibited.

Required recovery levels for high-pressure appliances made after Nov 15 1993:

• under 200 lb charge: 0 in Hg
• 200 lb or more: 10 in Hg

Chemours says most R-22 recovery machines handle MO99. Never leak-check with air and refrigerant mixtures; use an electronic HFC detector, since halide torches will not see it.

The legacy MO99 cylinder color is blue jay. Since Jan 1 2020, new cylinders are light green-gray (RAL 7044) under AHRI Guideline N, so read the label. See cylinder colors.

Status and replacements

EPA SNAP lists R-438A as acceptable for replacing R-22 in residential and light commercial AC (2007, 2010). Its AR4 GWP of about 2,265 is above the 700 limit that has applied to new residential and light commercial AC and heat pumps since Jan 1 2025 under 40 CFR 84.54. Its role is retrofit and service. AIM Act allowances are 60% of baseline in 2024–2028, 30% in 2029–2033, 20% in 2034–2035 and 15% from 2036.

See how MO99 stacks up against R-22 in R-22 vs MO99, and against R-407C, R-422B and others in R-22 replacement options. For the R-22 rules themselves, see the R-22 phase-out.

Common questions

Can you add MO99 to an R-22 system?

No. Chemours says MO99 should not be used to top off R-22. Recover the R-22 first. Topping off a system that is already on MO99 with more MO99 is acceptable, as long as the leak gets found and fixed.