The Emergency Heat setting on your thermostat is exactly what it sounds like: an emergency setting. It will help keep your home at a safe temperature when it is dangerously cold outside, but it will also use a lot of power and make a noticeable difference on your power bill.
Heat pumps work by pulling in and condensing heat from the environment outside before forcing it through the vents of your home. They do become less effective on the coldest days of the year, such as those we have experienced in the last couple of weeks.
While a heat pump is pulling in air from outside, the equipment can begin to frost. The unit is equipped with heat coils (similar to what you would find in a space heater) that come on periodically — usually for no more than 30 minutes at a time — to defrost the heat pump. You will know this is happening when you see the “Auxiliary (Aux) Heat” or “Emergency (EM) Heat” light or indicator on your thermostat.
When you switch to emergency heat, you are using these heat coils constantly. In practice, it’s a little like trying to heat your whole home with a space heater and a fan. If your regular heat pump is not keeping your home warm, you may need to call a licensed HVAC specialist out to check the unit sooner rather than later.