R-22 vs MO99 (R-438A)

What changes in the field

MO99 (R-438A, sold by Chemours as Freon MO99, formerly ISCEON MO99) is an HFC blend with a small hydrocarbon content that Chemours says “closely matches the performance & operating conditions of R-22.” The PT data shows how close. At 40°F evaporating, MO99 dew is 60.8 psig against 68.6 psig for R-22, which is 7.7 psi lower. At 100°F condensing, MO99 bubble is 211.3 psig against 195.9 psig, which is 15.4 psi higher.

The bigger change is glide. R-22 is a single component. MO99 has about 9.6°F of glide at 40°F evaporating and 7.5°F at 100°F condensing. Use the dew column for superheat and the bubble column for subcooling. An R-22 chart gives the wrong saturation temperature on both sides. The glide calculator shows the spread at any pressure.

Oil is where MO99 differs from R-407C. Chemours lists it as compatible with mineral oil, AB and POE, and says no oil change is needed for most systems. Where oil return is a concern, such as complex piping or risers, Chemours says to add POE. Copeland’s lubricant bulletin lists R-438A for Copeland compressors only in supermarket Discus racks, so check the compressor maker’s approval before a retrofit.

Retrofit or replacement

You cannot mix the two. Chemours says not to top off an R-22 system with MO99: recover the R-22 first. The retrofit sequence, per Chemours:

1. Record operating data, then recover all of the R-22 into its own cylinder.
2. Leave the existing oil in most systems. Add POE where oil return is a concern.
3. Repair leaks and evacuate. OEM targets are 350–500 microns (evacuation in microns).
4. Remove MO99 from the cylinder as liquid. Start about 10% below the original R-22 charge weight, then adjust to superheat or subcooling using MO99’s dew and bubble columns.
5. Label the system with the new refrigerant.

Chemours also says MO99 is not for flooded evaporators. EPA SNAP lists R-438A as an acceptable R-22 substitute in residential and light commercial AC (listings dated Oct 4 2007 and June 16 2010). For other options, compare R-22 vs R-407C and the R-22 replacement overview.

Capacity, efficiency and cost

Beyond Chemours’ qualitative “closely matches” statement, the sources used here give no capacity or efficiency percentage for MO99 against R-22, so none is quoted.

Supply is the deciding factor. R-22 production and import ended Jan 1 2020. Servicing with recovered, recycled, reclaimed or pre-2020 stock remains legal (40 CFR 82.15). MO99 is an HFC blend with a GWP of 2,265 on the AR4 basis (EPA’s 40 CFR 84.64 calculation gives 2,264.5), so it falls under the AIM Act allowance phasedown and is far above the limits for new equipment in 40 CFR 84.54. Its role is retrofit and service of existing R-22 systems. More on dates is on the R-22 phase-out page.

Common questions

Can you add MO99 to R-22?

No. Chemours says not to top off R-22 with MO99. A mixed charge matches neither PT chart, and the only fix is to recover the whole charge.

Is MO99 a direct replacement for R-22?

It is a retrofit, not a top-off. In most systems it avoids the oil change, but it still requires full R-22 recovery, liquid charging, a reduced starting charge and glide-aware readings, and it is not for flooded evaporators.

How much MO99 do I charge compared to R-22?

Chemours says to start about 10% below the original R-22 charge weight, then trim to the manufacturer’s superheat or subcooling target using the correct MO99 column.