this morning it was 23 degrees F. The indoor unit was on and delivering heat. I looked out at my outdoor unit and the fan was not running but the compressor was. I panicked and pulled the quick disconnect. Later I put it back in and the unit ran normally (fan on, compressor on, etc). I did not feel excessive heat from the compressor but I did see what looked like some king of vapor?smoke coming out, just a little. Did not smell at all.
Question. Is that an efficiency thing for a outdoor heat pump unit to cycle off the fan? I'm a commercial HVAC guy and have zip experience with heat pumps other than theory of operation.
Best Answer
When heating below 32 °F or 0 °C frost forms on the outdoor coils of a heat pump - and that can happen when the air temperature is above that point, if the coils are that cold to collect heat from cold air.
The controller has some logic which either detects or assumes frost formation and kicks into defrost mode to melt the frost so that airflow is restored. Depending on the unit it may engage resistance heating while doing that.
Water vapor ("steam") would be an expected side effect from that process.