HVAC troubleshooting

[LeanMachine]

I have a 15 year old 3 ton Goodman unit that is not cooling well. It is not a heat pump. No TXV. It is a piston system. Filter is new. Fins on evaporator and condenser are clean. Initially I thought the r22 was low. The super heat numbers seemed a bit squirrelly. I added a bit to see if it improved. It did not. I thought maybe the liquid line filter drier was clogged and restricting flow so I replaced it. No change. My suction line yesterday was 52 degrees. Liquid lime was 86-89. Suction psi was around 65-70. Liquid psi was 200. Air out of vents is 66 with an inside temp of 75. I'm no sure what else it could be besides maybe the piston, but I'm just a joe blow homeowner. Thoughts? Really not wanting to replace the system right now. I replaced my ton system at the other end of the house last summer and it was zero fun. But I did learn a ton and the next one should be easier.

[Kelmat]

Have you checked the amp draw on the compressor? At 15 years old it may be getting weak and not working efficiently.

[Starvin]

Try pumping down the compressor by closing liquid line valve. If it won't pump down or leaks back pressure the compressor is probably bad

[bowhntrmatt]

Those numbers don't seem terrible. Depending on what the outdoor temp was at the time you did this, your superheat is pretty close. Your subcooling is around 10-15 by those numbers, so I doubt the charge is low. If you are checking the temp at the vents, and not right at the plenum, I would look at your ductwork for major leaks. Also, if it is an electric heat system, make sure your heat strips aren't coming on at the same time as your AC. You can do this easily by checking the amp draw of the indoor unit. The blower only pulls a few amps. if it's pulling 20 or more your heat is on.

[LeanMachine]

All good ideas. I will check them all tomorrow. I'm particularly curious if the heat strips are coming on because when the blower was running without the condenser running, the wife said it was blowing hot air and that internal temp was going up. I attributed this to just the attic duct work being warmer than room temp but I will investigate. Duct work is very short from the plenum. The vent I am measuring is within 10 feet. Temp outside at the time was 80-85. Wet bulb temp at return was 66 I think.