A rooftop or split commercial AC unit that hums but does not cool is often an electrical problem—not an automatic compressor replacement. At A/C Dr. Naz we test contactors, capacitors, and 3-phase power before recommending major parts on Puget Sound buildings.
We are EPA Universal certified for commercial HVAC/R. This guide is for commercial properties only (packaged rooftops, splits on shops and restaurants)—not residential furnace or mini-split service.
Electrical checks before “bad compressor”
- Contactor — pitted or burned contacts, coil not pulling in, incomplete 3-phase pass-through.
- Capacitors — start/run caps below spec cause hum-without-start and overheat.
- Voltage and phasing — imbalance or a lost leg on 3-phase gear can destroy a compressor in minutes.
- Controls and safeties — high-pressure lockouts and time delays can mimic a dead unit after a power blip.
When the unit hums but does not start
Power is present, but the compressor cannot come up to speed. Common causes: failed start capacitor, seized compressor, or contactor not delivering all phases. Do not keep resetting—repeated attempts heat the windings. Shut down and schedule diagnostics.
When compressor replacement is actually needed
If windings test failed, the compressor is mechanically locked, or acid/contamination is confirmed, replacement may be required. Proper recovery, evacuation to manufacturer spec, and correct charge matter in Seattle’s wet climate—moisture left in the lines shortens the life of the new compressor.
Service hubs: commercial HVAC repair, rooftop unit repair, and commercial air conditioning repair.
Call for commercial AC diagnostics
Same-day diagnostics may be available when capacity allows. (425) 535-8990 with the unit nameplate and whether the problem started after a storm, power event, or gradual warm-up.
Why does my commercial AC unit hum but not start?
A humming unit often has power at the contactor but not enough torque to start—failed start capacitor, seized compressor, or contactor not passing all legs on 3-phase equipment. Shut down and call for diagnostics before forcing repeated starts.
What causes a commercial AC compressor to fail?
Many failures trace to electrical problems first: pitted contactors, weak capacitors, voltage imbalance, or single-phasing on 3-phase equipment—not sudden mechanical wear alone.
What is single-phasing on a 3-phase rooftop unit?
Single-phasing means one of the three power legs is missing at the compressor. A failed contactor or loose connection can let the motor hum without starting and overheat quickly.
Related guides
- Commercial HVAC static pressure testing
- RTU high-pressure lockout guide (head-pressure trips—not hum/no-start)
- Technical FAQ index
- Contact A/C Dr. Naz