With a heat pump system your outdoor unit will operate normally around 35 degrees or above[depending on the insulation, age of home, and age of equipment]. Your outdoor heat pump operates for both heating and cooling. In the heating season you will normally operate on HEAT mode (on the thermostat). When temperatures begin to decrease, you will notice AUX or auxillary will light up on the thermostat.
WHAT IS AUXILLARY HEAT: Auxillary heat is the staging of the heating elements of your indoor furnace. Auxillary heat comes on either when the heat pump struggles due to extremely cold temperatures (actual temp 2 degrees lower than set point temp), or if you raise up the heat setting by more than 2 degrees.
On the thermostat display, next to the heat icon (letting you know system is on) you will see AUX. AUX shows you that your auxillary heat (indoor furnace is on). Temperature from vents will be 30 degrees higher than normal due to heating elements of the furnace running.
You will notice AUX heat is turning on when the temperatures from the registers feel hot. The air from the registers will feel hot, then cold, then hot again – this is telling you that your outdoor heat pump is struggling.
As each home is different, you will want to pay attention to not only the outdoor temperature, but the aux heat, and if your heat pump runs continually. When these conditions are all in play, you will know it is time to switch over to emergency (EM) heat.
Emergency heat means you are heating your home with only the heating elements from the indoor furnace, the outdoor heat pump is locked out (no longer running).
You make this switch manually on your thermostat. When temperatures are consistently higher, you will want to turn back to regular heat mode [this will be HEAT on stat] running through the heat pump.
As a homeowner you want to be aware of what is “normal” operation for your system.