What is wrong? › Commercial HVAC
A dead unit is usually power: breaker, roof disconnect or a tripped safety
Half of these are a breaker or a disconnect and cost nothing. The other half need a technician the same day if you are trading in the heat.
What is probably happening
- The breaker for the unit tripped in the electrical panel, or the fused disconnect on the roof next to the unit is off or has a blown fuse.
- A safety switch in the unit tripped: a high pressure switch on a hot day with a dirty coil, or a float switch in the condensate pan.
- The thermostat lost power or its schedule put the unit in unoccupied mode.
Do this in the next five minutes
- Check the thermostat is on, set to Cool, and the schedule is in occupied mode.
- Go to the electrical panel. Find the breaker labelled RTU, AC or roof unit. If it is tripped, reset it once.
- If it trips again, stop. Leave it off and call. A breaker that trips twice is protecting you from a fault.
- Do not go on the roof yourself to check the disconnect. Ask the contractor to check it first thing.
- Move heat-sensitive stock, close blinds, and tell staff to keep doors shut while you wait.
Do not
- Do not reset a breaker more than once. Repeated resets on a shorted compressor can start a fire.
- Do not climb to the roof without fall protection. OSHA requires employers to protect workers at 4 feet or more above a lower level, and that includes you.
- Do not open the unit's panels. There are 208 to 480 volt terminals inside.
Who to call
A licensed commercial HVAC contractor
If the breaker holds and the unit stays dead, the fault is inside the unit or its disconnect. If the breaker keeps tripping, the contractor needs to find the short before an electrician can help.
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What it should cost
$100 to $250 service call, then $100 to $250 per hour
Diagnostic visit and labor for an electrical or control fault; parts extra
Source: Angi, HVAC repair cost, updated August 2026. View source
National published range, not a quote for your job. The full cost table for this trade is on the commercial hvac page.
Ask this contractor, on the phone, before anyone comes out
Tap each one as you ask it.
- Can you get to the roof today, and is the diagnostic fee credited against the repair?
- Will you check the disconnect, fuses, contactor and capacitor before anything bigger?
- If the breaker is tripping, will you measure the compressor before telling me it has failed?
- What is the unit's model and age from the data plate? Please photograph it for me.
End the call if
- A replacement quote for a unit nobody has opened yet.
- A technician who resets the breaker, leaves, and charges for the visit with no written finding.