Rooftop AC
Check airflow first, then the capacitor, then think about the sealed system. In that order.
The short version
- Dirty filter or iced coil = airflow. Clean it, thaw it, retest.
- Compressor hums and won't start, or trips: check the run/start capacitor. Discharge it before you test it.
- Measure air in at the return and out at the vent after 15 minutes running.
- Low voltage at startup kills compressors. Check volts at the unit when it kicks on.
- Most roof units have no service ports. Sealed system work is a specialist job, and handling refrigerant takes EPA 608 certification.
On the job
- Clean filter, vents open, divider in place. Run it 15 minutes.
- Split: return air minus supply air. 16 to 22°F is right. Lower with good airflow: coils, fan speed, voltage.
- Clamp the compressor and fan amps and compare to the data plate.
- Voltage at the unit with the compressor running: under about 108 V and it will struggle, trip, and die young.
- Power off, discharge, test the capacitor sections against the label (within about 6%).
Numbers
- Temp drop (delta-T): ~16–22°F after 15+ min running
- Run capacitor: replace if more than ~10% under its label
- Voltage at startup: should stay above ~108V
- Low voltage limit: about 108 V running
- Capacitor tolerance: within about 6% of the label
Shop tip
A cap labeled 45/5 µF means 45 for the compressor, 5 for the fan. Test both sides.
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.
Keep going on air conditioning
Learn it
Prove it
- AC hums and trips
- AC runs but barely cools
- AC ices up and barely blows
- AC kicks off on hot afternoons
- AC does nothing, thermostat is blank