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R22 Retrofit Tips
No, you shouldn't mix
There are NO refrigerants that you are supposed to use to top off an R22 system. If you do it, you are going against industry best practices. You are also contaminating the R22 for whoever might recover it later and, above all else, BEING VERY NAUGHTY!
Don't “top off” an R22 system with something else. It isn't supposed to be mixed, and you are going to have no clue what your saturation, superheat, or subcool values actually are.
Know the Consequences
A retrofit refrigerant will not perform as well either in capacity or efficiency. Before you retrofit, make sure you disclose this to the customer.
The Oil Will Be an Issue
Most R22 systems had mineral oil in them. All approved HFC refrigerants are incompatible with mineral oil and require POE oil. Can you get away without removing and dumping the compressor oil and adding POE? Maybe… but you are running the risk of oil logging. Sure, you could add an oil separator, but “ain't nobody got time for that.”
If you do retrofit, for heaven's sake, make sure you properly mark the unit.
So, here's my advice: don't retrofit unless it really makes sense. Most R22 systems are already older and may need to be replaced soon anyway. If you have an old system that you only want to limp through another year or two, then maybe you can retrofit without oil replacement and cross your fingers. However, I'm not a huge fan of the idea because of the risk and hassle.
If you are going to do it, I would suggest R-427a as my favorite alternative. It is listed as a good option by the EPA, it is readily available, and it has some of the best performance ratings in efficiency and capacity in testing.
R-438a (MO99) is also a decent option because it was specifically formulated to do better with mineral oil return than other options. However, it still isn't great at returning mineral oil.
My suggestions are based on typical A/C applications and R22 only. Different applications may require different retrofit options, but most of this will still hold true.
Just consider your options wisely… and don't call it a “drop-in,” please.
P.S. – here is a good write up by Forane on 438a and 427a