Heat Pump Components and Electrical Controls
Heat Pump Components and Electrical Controls
This domain tests identification, function, and electrical verification of heat pump components: compressor, reversing valve, coils, metering devices, accumulators, defrost controls, contactors, relays, capacitors, thermostats, pressure switches, and lockout boards. Questions often ask what a component does, how it is wired, and what symptom a failed part produces.
Core rules
- The reversing valve is a pilot-operated solenoid. The solenoid is energized in cooling on most residential models, but verify the manufacturer sequence. A failed solenoid or stuck valve changes mode, not capacity alone.
- The compressor is protected by internal overloads, high/low pressure switches, and sometimes a lockout board. Crankcase heaters keep oil warm and reduce liquid migration; they must be energized during the off cycle.
- Run capacitors shift phase for compressor and fan motors. Start capacitors are brief and usually switched out by a potential relay or PTC. Always discharge capacitors before testing.
- Defrost boards use time/temperature, demand, or pressure sensors. A bad outdoor thermistor can cause long defrost or no defrost.
- Low-voltage controls include the thermostat, transformer, contactor coil, relays, and safeties. A low-voltage short often blows the transformer fuse.
Worked example
A compressor hums and trips its internal overload after a few seconds. The run capacitor is rated 35 microfarads; a meter reads 18 microfarads. Replace the capacitor with the same microfarad rating and equal or higher voltage. Low capacitance reduces starting torque, causing high current and overload trips. Check the start components and compressor terminals before restarting.
Traps
Do not replace a compressor before checking the capacitor, contactor, and voltage balance. Do not assume the reversing valve solenoid is energized in heating. Do not leave a capacitor charged. Do not bypass pressure switches or lockout boards to make a unit run.