A heat pump is essentially an air conditioner with a few extra components added. Unlike a standard AC unit, a heat pump utilizes a reversing valve and a defrost cycle. When a standard AC unit is running, the indoor coil is cold, and the outdoor coil is warm. The reversing valve “reverses” this process, flip flopping the two coils – now the indoor coil is warm, and the outdoor coil is cool. When ice builds up on the outdoor coil, a defrost cycle is used to reverse the warm / cool coil temporarily to melt the ice off the outdoor coil.