Aux Ht or Auxiliary heat is the backup heat that turns on to help the compressor for first stage maintain your temperature setting. You know if the auxiliary heat is running when you see "Aux Heat On" in the thermostat's display. During this time both your Auxiliary heat and your compressor may be on at the same time. The thermostat will only run the Emergency Heat when the thermostat’s system switch is moved to the Em Ht position. When the thermostat is in this position the compressor does not run, only the Emergency heat runs. It is common that the Emergency heat and auxiliary or backup heat is the same source of heat (electric heat strips or fossil fuel).