I recently serviced a unit that is about 25 years old ( I know , people think they should run for life ) and the problem was warm air from vents so I start trouble shooting, finding that the unit was severly low on freon. I evac.. and pump up with nitrogen to find a leak and did find a very small leak and fixed it.
So I Vacumm unit to 375 microns and let it stand and everything is good.
Weighed in the charge and started unit, checked superheat and it was great. Then I check Temp change acorss evep. coil and it was text book, So I left the owner with a great running unit.
Well today I get a call that the unit is blowing warm air so I go to check it out and when i got there it was severly low on freon ( Now i'm stratching my head for real) So I pump it up with nitrogen and bubble everything and I mean everything. Absolutely no leaks , No bubbles , Pressure remains Same for long period of time. I'm thinking If the unit is going to leak 3.5 pounds of freon in four days it's a substantial leak. but I can not find anything at all. My thought is maybe the compressor is leaking internally, But i'm not that familiar with compressors yet and i'm looking for some advice and the good news is that I finally got the homeowner to buy a new unit but i'm still wondering about the old one for general knowledge.