R22 is tough to retrofit. You would need to replace all the oil, blow it out and dilute with POE, and replace all of the elastomeric seals in the unit. Personally, I would use that effort and money (POE oil is not inexpensive) and just build a low temp stage for the unit instead. Probably cost 50% more, take 20% more time, and give you 30% less capacity at 50F lower evap temp for the effort. Good trade IMO
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