Initiation of defrost in a heat pump system can be done by which of the following methods?

• A. Timed sequence
• B. Temperature
• C. Humidity level
• D. Electronic board

Answer: (B)

Defrost is started because frost forms on the outdoor coil, and the system needs to melt that frost to restore efficiency. The primary cue for triggering defrost is a temperature reading from a sensor on or near the outdoor coil. When the coil temperature drops low enough to indicate icing, the control initiates a defrost cycle, reversing the refrigerant flow to heat the outdoor coil and melt the frost. Some systems may also use a timer to schedule defrosts, but the actual initiation is still driven by the temperature condition, not a humidity reading. Humidity level isn’t a direct trigger, and the electronic board is the worker that carries out the defrost sequence once the temperature condition is met.