Why Is My Rooftop Unit Running But Not Cooling?
A rooftop unit that runs without cooling usually has a failed run capacitor, a tripped breaker or high-pressure switch, a refrigerant leak, or an economizer stuck open on outside air. Start at the disconnect and the capacitor, which are the cheapest and by far the most common. A unit short-cycling on a 95°F afternoon is often fighting a condenser nobody has washed.
What your rooftop unit is doing, and how long you have
A rooftop unit that runs but does not cool splits quickly into electrical, airflow or refrigerant. Start here.
- Call today Space temperature climbing with the unit running
- Stop using it Unit trips the breaker or a high-pressure switch repeatedly
- Call today Fan runs but the compressor never starts
- Call today Compressor short-cycles every few minutes
- Call today Air from the vents is moving but not cold
- Monitor it Cools in the morning, loses ground each afternoon
- Monitor it Condenser coil visibly dirty or blocked
A rooftop unit that stops cooling can make a dining room unbearable fast — and in a kitchen, heat buildup affects both staff and food safety. This page covers the most common causes of rooftop unit cooling failures, what to check before calling, and when to book a service call.
Why a rooftop unit runs without cooling
A rooftop unit (RTU) that runs but does not cool usually has one of these root causes:
- Failed capacitor — the start or run capacitor fails, preventing the compressor from starting. The fan runs but no cooling occurs.
- Refrigerant leak — a slow leak reduces the charge. The unit runs continuously but cannot satisfy the thermostat.
- Compressor failure — the compressor may short-cycle on thermal overload or fail to start. Often preceded by a capacitor issue.
- Dirty condenser coils — dust and debris on the outdoor coil restrict heat rejection, reducing cooling capacity.
- Stuck economizer or damper — if the economizer stays open on a hot day, the unit pulls in outside air instead of cooling return air.
- Tripped breaker or blown fuse — a power interruption or electrical fault can take the unit offline entirely.
What can be checked from the ground
- Check the thermostat — is it set to cool and the setpoint below room temperature?
- Check the breaker panel — is the breaker for the HVAC unit tripped?
- Listen to the unit on the roof — is the fan running? Can you hear the compressor start?
- Check the condenser area — is the coil visible and clean, or blocked by debris?
- Check the economizer — if the unit has one, is it stuck open on a hot day?
When it needs someone on the roof
If the breaker is on, the thermostat is set correctly, and the fan runs but the unit still does not cool, the issue is likely a capacitor, a compressor, a refrigerant charge, or a damper problem. These require a technician with electrical test tools and refrigeration gauges. Book a service call before the dining room becomes uncomfortable or the kitchen overheats.
How a technician approaches a rooftop unit
Most rooftop no-cool calls resolve at the first two steps, which is why they come first.
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Check power and the capacitor
A failed run capacitor is the most common cause of a fan that spins while the compressor hums and stops. It is cheap, it is fast to test, and it fails more than anything else on the unit.
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Look at the condenser coil and the clearance around it
Rooftop coils collect everything the roof does. A coil packed with cottonwood or grease from a nearby hood exhaust cannot reject heat, and the unit trips on high pressure on the hottest afternoon.
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Verify airflow across the evaporator
A loaded filter or a slipping belt starves the coil. Low airflow can freeze the evaporator into a block of ice, which then reads as no cooling and hides the original cause.
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Check the economizer position
An economizer stuck open pulls hot outside air in while the unit tries to cool it. It is a control fault that looks exactly like a capacity fault from inside the building.
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Take pressures last
With power, airflow and outside-air position confirmed, pressures separate a leak from a restriction from a failing compressor — and a leak gets found rather than topped up.
Who you’re asking
This is written from the work Pierre Refrigeration Services does every week in commercial kitchens across Metro Atlanta. Owner Don Pierre has spent 20 years in commercial refrigeration and HVAC and established the business in 2014. Refrigeration and HVAC are handled by the same company, which is why these guides treat a fault as a building problem rather than one trade’s problem.
Related questions
- Why does it only fail on the hottest days?
- Because that is when the unit has no capacity to spare. A dirty condenser, a weak capacitor or a slightly low charge all work at 80°F and give up at 95°F.
- Is short-cycling damaging?
- Yes. Every start is the hardest moment in a compressor’s life, and a unit cycling every few minutes is doing that hundreds of times a day. Short-cycling is a symptom worth acting on quickly.
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