What is the difference between E and Aux?

Aux or Auxiliary heat is the backup heat that turns on to help the compressor for first stage maintain your temperature setting. During this time both your Auxiliary heat and your compressor may be on at the same time. When the thermostat is in Emergency heat 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).

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