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R-421A vs R-422D vs R-438A: Mineral-Oil Drop-Ins Compared
If you have decided against a full oil conversion, three blends dominate the shortlist: R-421A, R-422D and R-438A. All three work with the mineral oil already in an R-22 system. All three are non-ozone-depleting. All three turn a day-long conversion into a recover-and-recharge job.
They are close enough in performance that marketing material struggles to separate them. In the field, two questions usually decide it.
Question one: do you know what oil is in there?
On equipment you installed, you know. On equipment you inherited — a twenty-year-old rooftop unit with no service records and a compressor that may have been changed twice — you do not.
R-421A has the widest oil compatibility of the three: mineral, alkylbenzene or POE. If a previous technician converted the system partway and left POE in it, R-421A copes. That tolerance is its main practical advantage and the reason it gets specified for unknown-history equipment.
Question two: what temperature range?
R-422D was designed around existing R-22 air conditioning and refrigeration, and it is the most widely used of the three for straightforward AC retrofits. Expect slightly lower capacity than R-22 and somewhat lower discharge temperatures, which is generally kind to an older compressor.
R-438A targets direct expansion systems with positive displacement compressors, covering both air conditioning and refrigeration. It is a common choice on older commercial equipment where the system architecture is known.
If your work spans low, medium and high temperature and you would rather carry one product, R-427A covers the whole range — though it does require POE.
What all three share
- Some capacity loss against R-22 — the price of not touching the oil
- Real temperature glide, so read dew point for superheat (why that matters)
- Liquid charging only
- Recover the R-22 fully first — never mix
The honest summary
Any of the three will keep an R-22 system running. If the oil history is unknown, take R-421A. If it is a known AC system and you want the most field-proven option, take R-422D. If it is older commercial DX equipment, R-438A is a natural fit.
What you should not do is treat any of them as a permanent efficiency solution on a system you intend to run for another decade. For that, R-407C with a proper oil conversion earns back the labour. Full comparison in our R-22 replacement guide.
Products covered in this guide
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