During what operational mode would the indoor coil of a heat pump act as a condenser?

The correct choice is that the indoor coil of a heat pump acts as a condenser during the heating mode. In this mode, the heat pump extracts heat from the outside air, even when temperatures are low, and transfers that thermal energy to the indoor space. The refrigerant, having absorbed heat from the outside, flows into the indoor coil where it releases that heat into the interior environment. This process effectively transforms the indoor coil into a condenser, allowing the refrigerant to cool down and transition from a gas to a liquid as it releases its heat, contributing to the desired indoor temperature increase.

In contrast, during cooling mode, the indoor coil functions as an evaporator, absorbing heat from the indoor space to cool it down, while the outdoor coil acts as the condenser, releasing the absorbed heat. In defrost mode, the heat pump temporarily reverses this process to melt frost off the outdoor coil, but it’s not the indoor coil acting as a condenser in this case. Standby mode refers to a state where the system is not actively heating or cooling, so the coil would not be functioning in its heat exchange capacity at all.