What is the purpose of the defrost cycle in a heat pump?

The purpose of the defrost cycle in a heat pump primarily involves addressing the accumulation of ice on the outdoor coil. As the heat pump operates in low outdoor temperatures, moisture in the air can freeze on the coil, leading to reduced efficiency and airflow. The defrost cycle temporarily reverses the refrigerant flow, directing warmer refrigerant to the outdoor coil, which helps to melt any ice that has formed. This process is essential in maintaining system performance and ensuring that the heat pump continues to effectively extract heat from the outside air, even in cold conditions.

While improving system efficiency is certainly a benefit of the defrost cycle, it is not the cycle's primary purpose. Similarly, noise reduction is not directly related to the defrost cycle, as it specifically addresses ice formation rather than sound levels. Melting ice on the indoor coil does not pertain to the heat pump's operation during heating mode, as ice tends not to accumulate there. Thus, the correct answer focuses on the critical function of maintaining the outdoor coil's performance through the defrosting process.