What are the three actions that occur when a heat pump is cycled into defrost mode?

The correct answer accurately describes the function of a heat pump during defrost mode in a way that addresses the need to manage frost and ice that can build up on the outdoor coil. When the heat pump enters defrost mode, the reversing valve switches to cooling mode. This change allows the heat pump to temporarily operate in a way that warms the outdoor coil, melting the ice or frost that has formed.

Simultaneously, the outdoor fan is shut off to maximize the heat transfer to the coil from the refrigerant, which allows the heat to effectively work on thawing the iced-over surfaces without pulling in cold air. Engaging the auxiliary heat is also part of the defrost process to support the heating needs of the indoor space while the heat pump is not fully operational in its heating cycle, maintaining comfort for the occupants.

This combination of actions ensures that the heat pump can effectively manage frost, preserving its efficiency and functionality while still providing adequate heating to the indoor environment.