During the heating cycle, what function does the outdoor coil serve?

In a heat pump during the heating cycle, the outdoor coil acts as the evaporator. This means that as the refrigerant circulates through the coil, it absorbs heat from the outdoor air, even if the air temperature is quite low. The refrigerant inside the coil evaporates as it absorbs this heat, turning from a liquid into a gas. This process is essential for transferring heat into the indoor space, which is the primary function of the heat pump during heating.

Understanding the role of the evaporator is crucial as this is where the heat exchange occurs in the heating cycle, effectively gathering warmth from the outside environment and preparing the refrigerant to be compressed and sent indoors where it can release this heat to warm the interior spaces. This concept is central to the operation of heat pumps, distinguishing them from traditional heating systems.