R22 vs MO99: The Mineral-Oil-Compatible R22 Retrofit
R-438A (Chemours Freon MO99, originally DuPont ISCEON MO99) is the R-22 retrofit blend whose defining property is compatibility with the existing mineral oil or alkylbenzene lubricant in most R-22 systems. Same A1 safety class, similar pressure envelope, similar cooling capacity, and lower discharge temperatures than R-22 across the operating range. The Chemours Freon MO99 Refrigerant Retrofit Guidelines (Publication C-10862, August 2016) documents an initial charge of approximately 85% of the R-22 nameplate weight, with the final charge typically settling near 95% of the R-22 charge after start-up adjustment.
PT curves, overlaid
Solid line = bubble, dashed = dew where the refrigerant has significant temperature glide.
Pressure comparison at service temperatures
Side-by-side pressure values at common service temperatures, computed from CoolProp 7.2.0. Useful for retrofit feasibility — pressure deltas within ±20% typically allow drop-in compatible service equipment; larger deltas require component pressure-rating review.
| Temperature | R-22 | R-438A | Δ vs R-22 |
|---|---|---|---|
| -20°F | 10 PSIG | 12 PSIG | +18.1% |
| 0°F | 24 PSIG | 27 PSIG | +11.8% |
| 40°F | 69 PSIG | 75 PSIG | +8.7% |
| 70°F | 121 PSIG | 131 PSIG | +7.8% |
| 95°F | 182 PSIG | 196 PSIG | +7.6% |
| 120°F | 260 PSIG | 279 PSIG | +7.3% |
Pressure delta visualization: positive = R-438A runs higher than R-22; negative = lower. Service equipment pressure rating matters when delta exceeds ±20% on the discharge side. For R-22 (zeotropic blend) bubble pressure is shown; for R-438A same rule applies.
Property differences side by side
- GWP impact: R-22 = 1,810, R-438A = 2,265 (+25% vs R-22). GWP delta is modest.
- Lubricant: R-22: MO/AB; R-438A: MO/AB/POE. Lubricant systems differ; check compatibility per manufacturer.
- Glide change: R-22 glide = 0.0°F; R-438A glide = 11.2°F. Service measurement (superheat / subcooling) needs dew/bubble curve awareness for the higher-glide blend.
- AIM Act status: R-438A is affected by AIM Act phase-down; the other is not. Drives new-equipment specification decisions in US market.
Properties side by side
| Property | R-22 | R-438A |
|---|---|---|
| Type | hcfc | hfc blend |
| ASHRAE class | A1 | A1 |
| Composition | Pure | 8.5% R-32 / 45.0% R-125 / 44.2% R-134a / 1.7% R-600 / 0.6% R-601a |
| GWP (AR5) | 1810 | 2265 |
| ODP | 0.055 | 0 |
| Lubricant | MO, AB | MO, AB, POE |
| Boiling point @ 1 atm | -40.8°C | -42.3°C |
| Critical point | 96.2°C / 709 PSIG | Blend (locus, not point) |
| Temp glide | 0.00°F | 11.20°F |
| AIM Act affected | No | Yes |
Choose R-22 if…
Servicing existing R-22 equipment through the remainder of its useful life. Reclaimed R-22 remains legal under EPA Section 608 (40 CFR Part 82) — virgin production was banned January 1, 2020, but the reclaim market continues to supply service demand. The economic trade-off is rising R-22 cost against retrofit labor + refrigerant cost, plus MO99's slightly higher GWP (2265 vs 1810 AR5) and ODP-zero position (vs R-22's 0.055). On equipment with 3+ years of remaining life, running through with reclaimed R-22 is often the cost-optimal path.
Choose R-438A if…
Retrofitting an existing R-22 system when reclaimed R-22 pricing or supply becomes a constraint, or when the ODP-zero position matters for the operator. MO99 is engineered specifically for the R-22 replacement path with (a) no lubricant flush required in most systems — the existing mineral oil or alkylbenzene stays in place per Chemours; (b) same A1 safety class, so no equipment recertification; (c) mass flow and pressure envelope close enough to R-22 that a properly-sized R-22 TXV usually doesn't need replacement; (d) lower discharge temperatures than R-22 (Chemours Table 1: -22°F at LT, -45°F at MT, -31°F at AC compared to R-22 baselines). The blend is not recommended for centrifugal compressor systems or chillers with flooded evaporators.
When neither is ideal
For new equipment installations rather than retrofits: R-32, R-454B, R-407C, R-410A, or one of the newer A2L blends depending on the application class (residential AC / commercial refrigeration / heat pump). Where GWP compliance is the primary driver, the sub-700 A2L blends are the AIM-Act-compliant target. Where preserving R-22 equipment economics is the driver, MO99 wins on the retrofit-friction axis.
Retrofit and transition
R-22 → R-438A (MO99) is the classic mineral-oil-compatible HFC retrofit path — the property that separates MO99 from R-407C, R-422D, and R-427A alternatives is that most systems retrofit without an oil change. The Chemours Retrofit Guidelines documents the procedure in detail; the compressed version:
Oil. MO99 is miscible with mineral oil, alkylbenzene, and POE lubricants. Field experience per Chemours shows most R-22 systems retrofit successfully with the existing lubricant in place. For complex systems — those with liquid receivers, screw compressors, or Trane 3-D / Danfoss SM scroll compressors that draw oil from the sump bottom — the guidelines recommend adding approximately 20% POE to the existing oil charge to maintain solubility and oil return. Multiple oil changes and full flushes are NOT required for the typical retrofit.
Charge. Initial charge target is approximately 85% of the R-22 nameplate weight per the Chemours Retrofit Guidelines. After start-up and stabilization, add refrigerant in small increments until the system reaches target superheat; the final charge amount typically settles at around 95% of the original R-22 charge. On critically-charged systems with fixed orifices or capillary tubes, monitor discharge pressure closely during charging — if head pressure rises significantly above expected R-22 levels before the target weight is reached, stop and check for restrictions.
Filter-drier and elastomeric seals. Replace the filter-drier during the retrofit (standard maintenance practice after opening any system). Replace critical elastomeric seals as a matter of course — Schrader valve cores, liquid receiver gaskets, solenoid valves, ball valves, flange seals, and open-drive compressor shaft seals — especially on older systems where R-22 has been in service long enough to swell the elastomers. Chemours notes that leaks appearing post-retrofit at these seals are common when switching R-22 to any HFC alternative, not specific to MO99.
Expansion device. Chemours states that mass flow rates and pressure-temperature curves for R-22 and MO99 are close enough that a TXV properly sized for R-22 typically does NOT need replacement — minor superheat set-point adjustment may be required, but the valve body and orifice usually stay. Fixed-orifice systems (capillary tubes, pistons) are similarly retrofit-compatible without device change in most cases.
Gauges and recovery equipment. Most recovery or recycle equipment used for R-22 can be used for MO99. Standard cross-contamination avoidance practices apply. R-22 manifold gauge sets work for MO99 service.
What NOT to do. Do not mix MO99 with other refrigerants or additives not specified by Chemours or the equipment OEM. Do not top off a CFC or HCFC charge with any Freon refrigerant. Do not use hand-held leak detectors as a substitute for oxygen monitors when working in enclosed spaces. Do not use halide torches (they detect chlorine, which MO99 does not contain).
For direct comparison of R-22 and MO99 at service temperatures, the Chemours PT chart publishes the two side by side — at 40 psig the R-22 saturation temperature is 17.1°F while MO99 bubble is 13.6°F / dew is 23.5°F. That side-by-side comparison from the manufacturer's own PT chart is the authoritative service reference for the pair.
Regulatory and transition context
Both refrigerants sit in an active regulatory transition driven by climate-impact rules. The transitions affect availability, pricing, and new-equipment specification.
- EPA AIM Act (40 CFR Part 84): US HFC production / import phase-down. Cap declines from 90% allocation (2022) to 15% by 2036. One or both refrigerants here are AIM Act-affected. New residential AC equipment over 700 GWP prohibited as of 2025.
- EU F-Gas Regulation (517/2014, updated 2024/573): European stationary refrigeration GWP cap typically 150 (much tighter than AIM Act). Drives earlier adoption of very-low-GWP options in European markets.
- Kigali Amendment to Montreal Protocol (2016): international HFC phase-down framework (198 countries). The AIM Act and EU F-Gas are regional implementations. Schedules differ by country group.
- ASHRAE 34-2022: safety classification (A1, A2L, A3, B1, B2L). For A2L refrigerants like R-32, R-454B, R-454C, R-455A: equipment must be A2L-certified, charge limits per IEC 60335-2-40 apply.
Standard transition procedure — R-22 → R-438A
Step-by-step service procedure for transitioning an existing R-22 system to R-438A, derived from the property differences above. Always cross-check equipment OEM service literature for the specific equipment being serviced. The steps below codify EPA Section 608 requirements (recovery, evacuation, documentation) plus refrigerant-specific accommodations for lubricant, safety class, pressure envelope, and glide differences. Skipping any of the regulatory steps (leak check, recovery, evacuation, documentation) creates compliance liability; skipping refrigerant-specific accommodations creates equipment-failure risk.
- EPA Section 608 leak-check first.Verify the existing system isn't leaking before any work. If it's leaking, find and repair the leak — adding refrigerant (existing or new) to a leaking system violates 40 CFR Part 82.
- Recover R-22. Use a recovery machine rated for A1refrigerants. Recover into properly-labeled cylinders; don't mix recovered R-22 with virgin or recovered R-438A (cross-contamination invalidates reclaim).
- Lubricant compatible — no oil change required. Both refrigerants run on MO lubricant family. Keep the existing oil charge; just replace the filter-drier and any compromised seals.
- Replace filter-drier. Install a new drier rated for R-438A (MOlubricant). Filter-driers are single-use after exposure to a refrigerant; the old drier may have absorbed contaminants you don't want carrying into the new charge.
- Pressure-test and evacuate to ≤500 microns. Pressure-test with dry nitrogen to verify no leaks. Pull deep vacuum and hold ≥30 minutes with vacuum pump isolated to confirm no leak-back. This step is non-negotiable — non-condensables (air, moisture) trapped in the system raise discharge pressure and damage the compressor.
- Charge R-438A by weight to nameplate. Use a calibrated recovery / charging scale. Charging by gauge feel produces frequent overcharge errors.
- Verify with SH and SC at steady state. R-438A is zeotropic with 11.2°F glide — use the dew curve at suction for SH, bubble curve at discharge for SC. Wrong-curve selection introduces error equal to the glide. Target SC = 8-12°F for TXV systems; target SH per OEM nameplate.
- Document and label. Update the equipment data plate to reflect R-438A. EPA Section 608 requires records of refrigerant added / recovered; OEM warranty may require documentation of approved-refrigerant substitution.
Lifecycle and operational context
Beyond the per-service-call decision, the R-22 ↔ R-438A choice sits inside a broader regulatory and lifecycle context. The transition direction (which is the predecessor, which is the successor) is driven by climate policy and the AIM Act phase-down, not technical preference alone.
- GWP profile: R-22 = 1,810 GWP (AR5); R-438A = 2,265 GWP. Switching from R-22 to R-438A increases direct refrigerant climate impact by 25%.
- AIM Act exposure: R-438A is AIM Act-affected; R-22 is not — the transition increases regulatory exposure (unusual direction). One or both refrigerants exceed the 700 GWP cap for new residential AC equipment (in effect since January 1, 2025).
- EU F-Gas Regulation: Both refrigerants exceed the EU F-Gas 150 GWP cap for new stationary refrigeration. Selection in European market favors very-low-GWP HFOs and natural refrigerants.
- Service supply outlook: Service supply of AIM Act-affected refrigerants persists during phase-down via reclaimed and allocated production, with prices rising as supply tightens. Plan for refrigerant cost escalation over equipment lifetime.
- TEWI / LCCP framing: Total Equivalent Warming Impact accounts for both direct refrigerant emissions (leakage, end-of-life) and indirect emissions from equipment energy consumption. For HVAC equipment with ≤5% annual leak rate, indirect emissions typically dominate TEWI by 80-90% — meaning equipment efficiency matters more than refrigerant GWP for total climate impact. For commercial refrigeration with higher leak rates, the balance can tip toward favoring low-GWP refrigerants.
Regulatory sources: EPA AIM Act (40 CFR Part 84), EU F-Gas Regulation 517/2014 and update 2024/573, Kigali Amendment to the Montreal Protocol (2016), Japan Fluorocarbon Emissions Control Law. GWP values per IPCC AR5 (2013) WG-I Table 8.A.1.
Service implications — R-22 → R-438A
What a service technician needs to know when transitioning from R-22to R-438A (or comparing them for new equipment specification). Two real-world scenarios show how the difference plays out in practice.
Pressure envelope check for R-22 → R-438A
Scenario · Field tech needs to know: do R-22 service tools handle R-438A, or does the pressure delta require new equipment? PT chart comparison at service temperatures gives the answer.
| Temp | R-22 | R-438A | Δ |
|---|---|---|---|
| 40°F | 69 PSIG | 75 PSIG | +8.7% |
| 70°F | 121 PSIG | 131 PSIG | +7.8% |
| 95°F | 182 PSIG | 196 PSIG | +7.6% |
Service-side implications: lubricant and safety
Scenario · Beyond pressure envelope, the switch from R-22 to R-438A affects lubricant, safety class, and operating procedure.
| Concern | R-22 | R-438A | Action |
|---|---|---|---|
| Lubricant | MO/AB | MO/AB/POE | No change |
| Safety class | A1 | A1 | No change |
| Glide | 0.0°F | 11.2°F | Curve awareness |
When to use which tool for this comparison
- R-22 full reference — properties, PT chart, lubricant, retrofit options for R-22.
- R-438A full reference — properties, PT chart, lubricant, retrofit options for R-438A.
- PT Comparison Tool — overlay any 2-4 refrigerants' PT curves interactively.
- Retrofit Compatibility Calculator — five-criterion compatibility analysis with verdict.
- Refrigerant Comparison Guide — long-form sourced reference for all common HVAC refrigerant comparisons.
Frequently asked
›Is MO99 a true drop-in replacement for R-22?
Close, but 'drop-in' is misleading. MO99 requires the standard retrofit procedure per the Chemours Retrofit Guidelines: recover R-22, replace filter-drier, replace critical elastomeric seals, evacuate, charge with MO99 at approximately 85% of the R-22 nameplate weight, adjust to target superheat, and label. The headline advantage over other R-22 retrofits (R-407C, R-422D, R-427A) is that most systems do NOT require an oil change — the existing mineral oil or alkylbenzene stays in place. Systems with liquid receivers or specific scroll compressor types (Trane 3-D, Danfoss SM) benefit from adding approximately 20% POE lubricant.
›What is the initial charge for R-22 to MO99 retrofit?
Approximately 85% of the standard R-22 charge for the system, per the Chemours Freon MO99 Retrofit Guidelines. After start-up and stabilization, additional refrigerant is added in small increments until the system reaches target superheat; the final charge typically settles at approximately 95% of the original R-22 charge weight. On critically-charged systems (fixed orifices, capillary tubes), extra care is warranted during charging — monitor discharge pressure closely.
›Do I need to change the compressor oil for MO99 retrofit?
In most systems, no. MO99 is miscible with mineral oil, alkylbenzene, and POE. The Chemours Retrofit Guidelines documents that field experience shows most R-22 systems retrofit successfully with the existing lubricant in place. Complex systems — particularly those with liquid receivers, screw compressors, or Trane 3-D / Danfoss SM scroll compressors that lubricate from the sump bottom — benefit from adding approximately 20% POE to the existing oil charge to maintain oil return. Multiple flushes and complete oil replacement are not the standard procedure.
›What is the GWP of MO99 compared to R-22?
MO99 (R-438A) has a 100-year GWP of 2265 (IPCC AR5, composition-weighted); R-22 has 1810 (AR5). The environmental case for MO99 is not GWP reduction but ozone protection: MO99 has ODP 0 while R-22 has ODP 0.055. The retrofit driver in most cases is either reclaimed R-22 supply/pricing or a mandate to phase out ozone-depleting refrigerants.
›Are R-22 and MO99 pressures the same?
Very close. The Chemours PT chart publishes both side by side — at 40 psig gauge, R-22 saturates at 17.1°F while MO99 bubble is 13.6°F and dew is 23.5°F. At 100 psig gauge, R-22 = 59.1°F, MO99 bubble = 54.9°F / dew = 63.9°F. MO99 is slightly cooler than R-22 at the same bubble pressure and has about 10°F of glide (the dew-minus-bubble spread) that R-22 doesn't have (R-22 is pure). The similarity is close enough that Chemours states the mass flow rates match well and R-22 TXVs typically retrofit without replacement.
›Can MO99 be used in centrifugal chillers or flooded-evaporator chillers?
No. The Chemours Retrofit Guidelines explicitly state that MO99 is not recommended for use in centrifugal compressor systems or chillers with flooded evaporators. For semi-flooded operations with low-pressure receivers, Chemours advises system-specific consultation with the equipment OEM.
R-22 full reference
PT chart, properties, retrofit guidance.
R-438A full reference
PT chart, properties, retrofit guidance.