Is flushing refrigerant lines always necessary?
Most technicians and business owners I speak to say yes, especially when replacing R-22 equipment with R-410A equipment and you're reusing the old refrigerant lines. My response is always the same; since when?
This leads to the person listing the many reasons a thorough flushing is not only good, but required, like-
a flush kit removes acid, sludge, moisture, residual mineral oil; basically all the "bad stuff"in the old refrigerant lines.
They then go on to tell me how the new refrigerants and POE oils have problems with acid and moisture, and how in the "good old days" you could flush lines with R-11.
Really? I was around in the "good old days" and I never once flushed a refrigerant line with anything.
I've had refrigerant systems go aquatic when a chiller cooler ruptured. I've had fifty ton split systems burn so badly the suction line looked like a diesel tail pipe. Not once did I flush a system with R-11.
Now, about the statement "POE oils and R-410A have problems with moisture". Are you kidding me? ALL refrigerant systems have problems with moisture. In fact, you wouldn't have moisture in the system in the first place if you followed basic service procedures.
What do I use to remove moisture, acid, and sludge? The same thing most people used in the "good old days", a vacuum pump and a filter drier, and if the contamination was bad enough, we'd change the oil in the compressor.
Some people will tell you that a vacuum pump cannot remove moisture from POE oil, and they're right, but it couldn't remove it from mineral oil all that well either, just for a different reason.
Everyone knows that POE oil bonds with water on a molecular level, and a filter drier is the only way to remove it. Well, a vacuum pump can remove moisture from mineral oil, but it takes a very long time because the moisture has a hard time "breaking through" the surface tension of the oil. In reality, the filter drier did most of the moisture clean up for R-22 systems. Besides, the majority of the oil is in the compressor, and you NEVER run flush through a compressor.
What about cleaning up the residual oil and other gunk in the lines?
I always blew the lines out with C02 and had someone hold a rag over the other end of the pipe to catch the small amount of crud and oil that would come out. Notice I said "small amount"; if the refrigerant lines are installed properly, there should not be any low areas to hold excess oil or crud. If there are dips, and low lying areas, fix them! It may be part of the reason the original system failed.
To summarize- in my opinion, the best way to combat system problems is to use standard refrigeration practice to keep systems clean, and use standard refrigeration practice to clean them.