Quote Originally Posted by mad fridgie View Post
You can not use IR gun for measuring superheat.
You can not use rules of thumb to determine actual system performance.
The OP asked for an expanded explanation for evaporator coil icing, relative to "low refrigerant", with little information to do any analysis. If the coils are icing, and he's being told it's due to lost charge, we can only agree the diagnosis could be correct, since low charge generates low saturated temps that can dip far enough below 32˚F to freeze the condensed H2O.

Had he provided IDWB/OD temps, pressures, line temps and equipment SEER, we could offer a more specific analysis. 40˚- 50˚ evaporator temps are design values and 40˚ vapor lines are atypical occurrences. Though the IR measurement likely has an error factor, it still provides a glaring symptom of overcharge or low airflow with fixed metered systems, which was the only point I was attempting to make.

I suspect the only level of "performance" Doug's hoping for, is a system that will blow some cold air without freezing up.