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Thread: Aux and Emergency heat pump wires for new Honeywell thermostats

  1. #1
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    Aux and Emergency heat pump wires for new Honeywell thermostats

    I'm using three RTH9580WF thermostats with Gas/Electric units. My son just purchased two for use with his heat pump systems. His heat pump has separate wires for Emergency heat and AUX heat. The new Honeywell thermostat only has a single terminal. What should he do?

    If Emergency and AUX both energize the auxiliary (resistance) heat, what's the difference between energizing the emergency and AUX wires? If you energize Emergency, does that activate a relay to turn off the compressor?

    Phil


  2. #2
    HVAC Thermostat Specialist

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    If the system is dual fuel and has a separate fossil fuel kit then you MUST use a thermostat with separate aux & E terminals. If you do not then it is possible that the you might end up operating both aux and HP at the same time which can ruin your HP.

    The other solution is to use a thermostat designed for dual fuel which contains the programming to operate as a fossil fuel kit internally. This type of thermostat typically requires professional installation to remove the fossil fuel kit and outdoor thermostat.

    If his system is an all electric HP system then he can combine the aux & E wires at the thermostat aux terminal.

  3. #3
    Junior Member

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    Thanks for the reply. My son's system is all-electric (heat pump/resistance AUX). With an all-electric system, what is the difference between AUX and E?

  4. #4
    HVAC Thermostat Specialist

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    aux runs strip heaters with the heat pump running. E only operates the strip heaters (for use when HP is broken).

  5. #5
    Junior Member

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    "aux runs strip heaters with the heat pump running. E only operates the strip heaters (for use when HP is broken)."

    Does energizing Aux trigger a relay that runs both the strip heaters and the HP? In other words, what happens if you energize Aux without energizing the normal HP-run wire?

    My son found a message somewhere that said Aux turned on only half of the strip heater and E turned on all of it. Is there any truth to that?

  6. #6
    HVAC Thermostat Specialist

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    yes and no. It depends specifically on your make and model of air handler and the model number of the heater kit installed in the air handler.

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