When your mini split heat pump goes into defrost mode, the reversing valve inside of the outdoor compressor unit is energized, and this switches the system from heating to air conditioning mode. The outdoor coil will then become hot, and the indoor coil will become cold. Both of the fans will shut off and allow the outdoor coil to melt any accumulated ice. A cloud of water vapor may be seen rising over the outdoor unit and a "whooshing" sound is sometimes heard as the refrigerant reverses direction. As the frost is melted from the coil, you may notice water around the base of the outdoor unit. This is also normal. When the built in microcomputer senses that all the ice has melted, the heat pump system will switch back into heating mode and the fans will come back on. This whole process can take from five to fifteen minutes depending upon conditions.
How often this defrost mode happens depends on different factors. Your ductless heat pump has a built in microcomputer that monitors the outdoor temperature and refrigerant pressures and will switch the system into defrost mode more often in colder temperatures than in warmer ones. Here in the Pacific Northwest, with our milder winters, it may not happen as often as in colder climates, so it may take you by surprise when it happens. Don't attempt to interrupt or override this function and never try to pick ice off of your outdoor unit with a sharp object. Both of these things could damage your unit or refrigerant coils.
If you notice your outdoor unit is severely iced up in the winter, it is possible it is not defrosting properly or may have other operating problems. In this case, give us a call and we can check the system for proper functioning.