Not exactly. When the outside coil (the condenser coil) frosts up (because the condenser coil is cold in heating mode), the unit will sense this and go into defrost mode. When in defrost mode, the system will reverse into cooling mode. The fan on the outside condensing unit will stop. In cooling mode (aka air conditioning), the outside coil will heat up to melt the frost. Since the unit is now in air conditioning mode (the inside evaporator coil is now cold), there will be cold air coming from the vents. The auxiliary heating strips (resistance strips in the air flow that get hot because current is flowing through them) will be turned on to heat the cold air until the defrost cycle is complete - typically a couple of minutes. Once the defrost mode is complete, the system will reverse back to heating mode and the fan on the condensing unit will start again and the auxiliary heating strips will be turned off. It is easy to tell when the unit goes into defrost mode as the outside fan will stop and the auxiliary heating light will be on.
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Originally Posted by mvivanco
I had my HVAC orientation yesterday. The service guy told me the Aux Heat comes on when its cold outside and there is frost on the unit outside. It puts the Aux heat on to defrost the coils. It is an electric heat source and not as efficient so it isn't really intended to be used or long term heating of your home.
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