@CPow, to understand the heat pump you must understand first how a regular air conditioner works. What an air conditioner does is it uses a compressed refrigerant to capture heat inside the house and carry that heat in the refrigerant gas to the outdoor unit where is, it is expelled into the air. What a heat pump does is have a unique ability to change the direction of refrigerant so that it actually flows backwards, this captures the cold air and carries the cold air outside and gets rid of it.
The primary heat comes from the outdoor heat pump unit. There is secondary heat available if you’re indoor air handler has electric heat strips installed in it. This is called auxiliary heat. a heat pump is functioning as a heater, often it builds up ice on the outdoor coil. When this ice is detected, a defrost controller activates and causes the refrigerant flow to go into air conditioner mode long enough to pull enough heat out of the house to melt the ice on the outdoor coil. Once the ice is gone, the defrost control board returns control to the main control board and it goes back into heating mode. When the defrost control board takes over, it sends a signal to the indoor unit to activate the auxiliary heat and to slow the blower down to low speed. Your Auxiliary/Emergency heat strips need to be tested. Additionally, the basic Nest Thermostat does not support Emergency Heat function. Your auxiliary heat is the W2/AUX terminal.
AC Cooling Wizard
NestPro, Google Pro, Mechanical Engineer and HVAC service company owner.
