[DRH]

Frank - We had two Mitsubishi Mini-Split units installed in 2013. In cold weather when the units are in heat mode, the fins on the outside unit (compressor) can accumulate frost due to the extremely low temperature of the coolant running through them.

However, the units are programmed to sense this frost buildup on the compressor fins and to automatically go into a "defrost" mode that melts the accumulated frost.

During the defrost mode, the outside unit runs but the fan in the inside unit does not run. The system circulates the coolant, extracting some heat from the air in the house and using it to defrost the fins on the outside unit. (The fan on the inside unit does not run during the defrost mode so as not to blow cool/cold air into the house.)

On our two systems, we can tell when they are temporarily running in defrost mode as we can hear a "gurgling" sound coming from the inside units. The defrost mode only lasts for a minute or so, after which the system reverts back to the normal heat mode. On very cold days the units can go into temporary defrost mode a number of times during the day.

If this is not the way your system is operating, then a call to HEP is in order, if you have not already done so.

Please let us know how you make out.