RTU Fan Runs but No Cold Air: What Maintenance Should Check
If the rooftop fan runs but supply air is not cooling, the airflow circuit is alive but the cooling stage may be locked out, not commanded, or unable to operate. Gather evidence before calling service.
Likely causes
Do not jump straight to the most expensive possibility. These are the common places to start:
- Cooling stage not commanded
- Compressor lockout
- Economizer problem
- Airflow issue
- Refrigeration or electrical fault
| Possible cause | How to check | Routine first move |
|---|---|---|
| Cooling stage not commanded | Measure room and supply-air temperature | Correct basic airflow problems |
| Compressor lockout | Confirm cooling demand | Document timing and temperatures |
| Economizer problem | Inspect filter condition | Escalate sealed-system or electrical diagnosis |
| Airflow issue | Listen for short compressor cycles | Correct basic airflow problems |
| Refrigeration or electrical fault | Record faults and outdoor conditions | Document timing and temperatures |
Tools and preparation
Bring these
- Flashlight
- Clipboard
- Exact model manual and phone for documentation
- Air-temperature probe
Before touching anything
- Confirm the exact location and complaint.
- Photograph labels, settings, fault codes, and the original condition.
- Determine whether the failure affects one item or several nearby items.
- Know how to safely isolate the system if the task requires it.
Step-by-step checks
Measure room and supply-air temperature
Use an air-temperature probe to record room, return, and supply readings after stabilization; an infrared gun measures surfaces, not the air stream.
Confirm cooling demand
Confirm cooling mode, setpoint, schedule, and operator-visible demand; a running fan alone does not prove compressor operation.
Inspect filter condition
Isolate equipment before filter access and check size, loading, and bypass gaps; reinstall all panels before operation.
Listen for short compressor cycles
From normal safe access, note compressor starts and stops without opening energized compartments; repeated short cycles require service.
Record faults and outdoor conditions
Record fault codes, outdoor conditions, and equipment tag. Give these observations to HVAC service and avoid repeated resets or refrigerant adjustments.
Routine fixes that make sense here
- Correct basic airflow problems
- Document timing and temperatures
- Escalate sealed-system or electrical diagnosis
Common mistakes
- Replacing parts before proving the failure.
- Changing several settings or components at once.
- Failing to photograph the original condition.
- Ignoring whether nearby equipment has the same symptom.
- Crossing into electrical, refrigerant, life-safety, structural, or plumbing work beyond training.
When to stop and hand it off
- Compressor or refrigerant testing is required
- Repeated faults occur
- Unit makes abnormal grinding or electrical sounds
Frequently asked questions
What should I check first?
Start by deciding whether the problem is local to one fixture or piece of equipment. Then begin with the simplest safe observation: measure room and supply-air temperature.
What is the most common mistake?
Replacing parts before proving the failure. BuildingWrench recommends documenting the symptom, changing one thing at a time, and testing after each change.
When should maintenance stop?
Stop when the repair crosses your training or site-safety boundary. In this case, one clear handoff point is: compressor or refrigerant testing is required.
What should I document?
Record the exact location, model or asset tag, what the equipment was doing, what you checked, what you changed, and whether the fix held through repeated testing.
Related BuildingWrench guides
BuildingWrench provides general educational information for commercial-facility maintenance. Manufacturer instructions, applicable codes, site procedures, and required trade qualifications always take priority.
Tools that fit this task
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Record location, readings, defects, responsible person, and follow-up date.