Under normal operating conditions, the auxiliary heat is brought on automatically by the thermostat when the indoor temperature drops during heat pump operation. There are also times during cold, wet weather when the outdoor coil may ice up and your heat pump will go into a defrost cycle. This is nothing more than reversing the process back to cooling mode. Cooling mode makes the outdoor coil hot and melts any ice. The defrost cycle should only last a few minutes and then return to heating mode. During the defrost cycle, your comfort system is in cooling mode and the supply air is cool. To offset this cool air, the auxiliary heat will be energized during defrost. A mist or fog may be visible from the outdoor unit during defrost.