What do Aux, Emer, and Malf mean on a heat pump thermostat?
Aux or Auxiliary Heat is the second stage of heat in the Heat mode. This is energized when the Heat Pump is not keeping up with the thermostat setting. It is normal operation with most heat pumps for the thermostat to call for auxiliary heat. If the auxiliary heat and emergency heat are using the same back-up heat source the thermostat may also show an Emergency Heat indication whenever the Auxiliary heat is on. Emer or Emergency Heat occurs when the thermostat is switched to Emergency. The thermostat bypasses the Heat Pump and uses the back-up heat. This is useful if the pump is not operating correctly or if the temperature outside is so cold that the pump is not economical to run. If the Auxiliary Heat and Emergency Heat are using the same back-up heat source the thermostat may show the Auxiliary and Emergency Heat indication at the same time. Malf or Malfunction indicates the heat pump system is sending a signal to the malfunction terminal on the thermostat marked “L”. It does not i
