What is the purpose of defrost mode in air-to-air heat pumps?

The purpose of defrost mode in air-to-air heat pumps is specifically to reverse operation and melt frost that accumulates on the outdoor coil. During cold weather, moisture in the air can freeze on the coil, which reduces the unit's efficiency and heat transfer ability. When the system detects a drop in performance due to frost buildup, it automatically switches to defrost mode. In this mode, the heat pump temporarily operates in reverse to heat the outdoor coil, causing the frost to melt and drip away, thereby restoring efficient operation and maintaining heating effectiveness.

This operational adjustment is crucial for ensuring that the heat pump continues to function optimally during colder temperatures, preventing performance degradation that can occur due to ice buildup. The other options do not accurately reflect the specific function of defrost mode; thus, they do not provide the correct purpose of this operation within the heat pump system.