WiremanWireman · Rooftop unit won’t start: troubleshooting for electricians · 2023 NEC ·
Rooftop unit won’t start
Three-phase power, control transformer, phase monitor, then the safety chain to the contactor.
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How it works
A packaged rooftop unit runs on three-phase power (often 208/230 V or 460 V) with a control transformer making 24 V for the controls. Many have a phase monitor that opens the control circuit on a lost or reversed phase. The cooling call passes through a chain of safeties (duct smoke detector or fire alarm shutdown, condensate switch, high- and low-pressure switches, freeze protection) before it reaches the compressor contactor. Scroll compressors must run in the right rotation.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What does the unit do?
- Completely dead, no control powerTest: Measure all three phases at the disconnectExpect: All three readings within 10% of nameplate voltage and close to each other.
- A phase missing on the line sideLost phase in the building supply
- Line good, a phase missing on load sideBlown fuse or bad disconnect
- All three goodTest: Check the control transformerExpect: Rated primary voltage and about 24 to 28 V on the secondary.
- Primary good, no 24 VControl transformer or control fuse
- 24 V presentDoes the unit have a phase monitor, and what is its indicator showing?
- Showing a faultPhase monitor tripped
- Normal, or no monitorTest: Trace the call through the safety chainExpect: 24 V present all the way to the contactor coil, and the contactor pulled in.
- No call from thermostat or controlsNo cooling call reaching the unit
- Smoke detector or fire alarm shutdown openSmoke detector or fire alarm shutdown
- Pressure or freeze switch openA refrigerant safety is open
- Condensate switch openCondensate overflow switch tripped
- 24 V at the coil, contactor doesn’t pullContactor not pulling in
- Controls powered, nothing starts on a callDoes the unit have a phase monitor, and what is its indicator showing?
- Showing a faultPhase monitor tripped
- Normal, or no monitorTest: Trace the call through the safety chainExpect: 24 V present all the way to the contactor coil, and the contactor pulled in.
- No call from thermostat or controlsNo cooling call reaching the unit
- Smoke detector or fire alarm shutdown openSmoke detector or fire alarm shutdown
- Pressure or freeze switch openA refrigerant safety is open
- Condensate switch openCondensate overflow switch tripped
- 24 V at the coil, contactor doesn’t pullContactor not pulling in
- Blower runs, no coolingTest: Trace the call through the safety chainExpect: 24 V present all the way to the contactor coil, and the contactor pulled in.
- No call from thermostat or controlsNo cooling call reaching the unit
- Smoke detector or fire alarm shutdown openSmoke detector or fire alarm shutdown
- Pressure or freeze switch openA refrigerant safety is open
- Condensate switch openCondensate overflow switch tripped
- 24 V at the coil, contactor doesn’t pullContactor not pulling in
- Compressor runs loud, no coolingScroll compressor running backward
- Completely dead, no control powerTest: Measure all three phases at the disconnectExpect: All three readings within 10% of nameplate voltage and close to each other.