FixGrid Academy — Field checklist
RTU Not Cooling
Light commercial rooftop packaged unit · Intermediate
Rooftop running, space warm. Prove airflow and heat rejection before you touch a gauge port.
1Safety first
- Open and lock out the unit disconnect; tag it.
- Confirm dead with a non-contact tester before opening panels.
- Rooftop safety: fall hazards, hot surfaces, spinning fans.
- Gloves, eye protection, and secured panels in wind.
2First checks
- Confirm space temperature against setpoint with your own instrument.
- Check the thermostat is calling and the unit responds.
- Verify blower, condenser fan, and compressor are all running.
- Measure return-to-supply split — a low split tells you a lot.
3Narrow the system
- Split it: indoor airflow vs heat rejection vs refrigerant/compressor.
- Inspect filters and the evaporator coil face for loading.
- Inspect the condenser coil for grease, cottonwood, and debris.
- Check economizer/damper position and belt condition where fitted.
4Highest-probability tests
- Replace loaded filters and clean the evaporator face, then re-measure split.
- Clean the condenser coil and verify fan speed and amps against nameplate.
- Check compressor amps against nameplate for a mechanical problem.
- Only then consider refrigerant charge — with a clean, correct airflow baseline.
5Common outcomes and watch-outs
- Most common: capacity loss from restricted indoor airflow plus a fouled condenser.
- Rooftops usually carry two faults — fixing one guarantees the callback.
- Prove the repair with a post-clean split and space pull-down.
- Leave the facility manager a filter and coil-cleaning interval in writing.