I've been working on Janitrol 1.5 ton heat pump with a piston metering device. The initial call was a no cooling/frozen coil. I squeezed them in inbetween calls since I was in the neighborhood. I found a leaking shrader, so I replaced it added a pound got the super heat and sub cooling dialed in and told the homeowner there was a possibility there would be more but I didn't have time to look for other leaks. I explained this was temporary fix until I could get back and service the unit.
I got a call from the home owner a couple of days ago and said the coil was freezing up again. Went out there yesterday and pulled the charge, checked for debris in the piston, blew it out with nitrogen, put in a liquid line filter drier, pressurized to 200 psi (held for 30 min), pulled vacuum held for 30 min, started unit. Here are the conditions I left the unit in:
Indoor 68 wb 77 db
Suction line 40 F (68 psi) - 57 F = 15 F super heat
Liquid line 110 F(225 psi) -88 F = 32 F sub cooling
These were constant for 30 min.
Got a call from the home owner saying the coil was freezing up again! But he said it wasn't totally frozen over. I checked the coil it was clean, the filter was dirty but I've seen worse.