I have recently started researching some R-22 replacements, but have received mixed reviews. So far the Mfg says r421 is the most efficient substitute. R422 b and d are also a good substitute and can be dropped in or added to r22. Nu22 and r22a are Eco friendly and can also be dropped on or added to R22. I spoke with a Manufacturing Engineer who has tested the Nu22 and r22a and said that they had a very slight loss of efficiency. Does anyone know some facts they can contribute. I am getting ready to use the 422d ($230 for 25#), but I am hesitant to add on to R-22. Figured I would recover the R22 first and charge with R422d. Any thought?