RTU Not Cooling

Scenario briefing

The call

A facility manager reports the rooftop packaged unit serving a retail space is running but not cooling. Fans and compressor activity are audible on the roof, yet supply air is not cold and the space temperature is climbing through the afternoon.

There is no alarm or fault code on the thermostat or the unit control. Staff have already set the thermostat lower with no change.

Work the sequence: confirm the complaint with temperatures, split indoor airflow from the outdoor section, then inspect and test the high-probability items. Refrigerant is the last conversation, not the first.

1. 1Filters / indoor coilFirst stop. Loaded filters and a dirt-blanketed evaporator kill the supply-air split while the unit keeps running happily.Commonly fails: Filters load up and collapse; the indoor coil face then loads with the dirt that gets past them.Symptoms on the call: Poor cooling, a weak temperature split, and sometimes a frosted indoor coil.Often misdiagnosed as: Reading low suction from starved airflow as low charge and adding refrigerant.Rule out first: Replace the filters, clean the coil face, and re-measure the split before anything else.Expect to see: Good: clean filters with a solid return-to-supply drop across the coil. Bad: collapsed filters, loaded coil, or coil frost. Upstream: a slipping belt or failed blower starves airflow even with clean filters.
2. 2Supply / return airflow pathCheck the return path and dampers too — a blocked return or closed zone starves the coil the same way a dirty filter does.
3. 3Condenser coil / condenser fanSecond stop. Rooftop grease, cottonwood, and a lazy condenser fan drive head pressure up and capacity down.
4. 4Compressor sectionLast suspect. A hot or short-cycling compressor is usually reporting an airflow or heat-rejection problem upstream.
5. 5Thermostat / unit controlVerify the call, staging, and any economizer position against what the unit is actually doing before you condemn hardware.