In some heat pump defrost-control schemes, which component monitors conditions to start defrost?

• A. Expansion valve
• B. Blower motor
• C. Electronic board
• D. Compressor

Answer: (C)

Defrost starts when the system’s sensing and control logic detect frost building up on the outdoor coil and determine it’s time to melt it. The electronic board acts as the brain here, gathering input from sensors such as outdoor coil temperature (and often other signals like ambient conditions or a frost-detection routine) and using a defrost algorithm to decide when to begin. When it decides defrost is needed, the board initiates the defrost sequence, which may involve changing valve states or energizing heaters, and it ends the cycle once the frost is cleared. The other components don’t perform this monitoring: the expansion valve simply modulates refrigerant flow, the blower motor handles indoor air movement, and the compressor drives the refrigerant cycle.