First, a timeline:
April 30, 2009: Tech replaces evap. coil because pan has rusted through
June 14, 2009: A/C stops cooling altogether
June 15, 2009: Tech finds it frozen up and 6 lbs (out of ~8 lbs) low on refrigerant. Replaces refrigerant. Can't find a leak
June 16, 2009: Tech comes back because of inadequate cooling. Finds obstruction in filter drier and replaces it, and cleans condenser unit.
June 19, 2009: Tech comes back because of inadequate cooling. No problems found. Adds some refrigerant, but says it didn't really need it.
The A/C hasn't been working correctly since it initially froze up, though it is blowing cold air. It functions fine from about midnight to 10AM and has a return/supply differential of 20 deg F (78-58). Starting at around 10AM it struggles to maintain 80 deg F and the house slowly heats up to 85-87 deg F, peaking in the late afternoon. The return/supply differential shrinks to ~13-14 deg F. It takes until midnight to cool back down to 80. It runs constantly from 10ish until midnight.
It is an older system (16 years) and has been trouble free until the leaking pan issue. I would've had the whole thing replaced, but I am moving within a year and it had been performing wonderfully besides the leak.
The tech is coming back tomorrow to take a look again and I want to be able to ask intelligent questions when he does. Ignoring the potential of having a leak, what could cause this type of performance issue?
My guesses so far:
1) Compressor problems (he said he checked it and it showed signs of age, but it was functioning OK.)
2) Large cold air leak in attic (I looked and didn't find anything).
3) Blower problem
4) Expansion valve problem
5) Improper charging procedure
6) Improperly sized replacement evap. coil
Do all of these make sense for the issues I am having? Can you add any more?
I appreciate any help I get.