Electrical, Controls, and Troubleshooting
Domain focus
This domain tests electrical safety, control sequence, and diagnostic logic for heat pumps: 24-V circuits, contactors, capacitors, defrost boards, sensors, reversing valves, and error codes. Expect questions that give a symptom and ask for the next measurement.
Core rules
- Control voltage is typically 24 VAC; line voltage is 208/230 V single-phase or 208/230 V three-phase in larger equipment.
- Measure voltage across a switch or coil to prove open or closed: a good closed switch has near 0 V across it, while an open switch shows source voltage across it.
- Capacitors must be checked with power off and discharged; use a meter with a capacitance function or compare microfarads to nameplate.
- Defrost controls use outdoor coil temperature and time; a stuck defrost board can cause a cold indoor coil, auxiliary heat, and high bills.
- The reversing valve is energized in cooling on most residential heat pumps (O terminal) and de-energized in heating.
Worked example
A heat pump in heating: indoor fan runs, outdoor fan and compressor are off. At the outdoor unit, 24 VAC is present between R and C, but 0 V across the contactor coil. The high-pressure switch is in series; measuring across it shows 24 V. That voltage appears across the open switch, so the switch is open. The contactor coil is not receiving voltage; do not replace the coil. Find the cause of high pressure, such as a restricted outdoor coil, failed fan, or overcharge, before resetting.
Traps
Jumping out safeties, diagnosing by parts replacement, using a non-contact tester as the only voltage check, and misreading a capacitor's stored charge.