I live in AZ and one of my Goodman 3.5 ton SEER 12 units (only 2-year-old) is having trouble cooling the rooms below 80F in daytime after running for at least 3 hours straight (outside temp 100 to 110F). Using a laser thermometer, the temp coming out of the vents varies from 64F to 69F. My attic has R30 insulation. I've just changed the filter, and cleaned the condenser coils outside my house.
When the tech came, here are some measurements he took:
Sub cool: 11F / 10F
Superheat: 5F / 5F
Low side PSI: 75 / 77
High Side PSI: 280 / 260
Compressor: 22.1 - 18.3
Condenser: 1.6 -1.1
He told me those numbers look fine, and all he did is changing the piston from size 74 to 78 (according to him this will increase freon flow), and 'flush' the system.
Anyway, those didn't help at all, and now I have to call them back and do more diagnostics. What else can I ask them to look into (besides duct leaks)?