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Community and Q&A

Thermostat – Heat Pump / Aux

ktcstl | Posted in Mechanicals on

Hi! I put a central heat pump system in a house that is now a rental. I want to make sure the renters know how to set up the thermostat over the winter (st louis, mo) with the AUX unit. 

The house has:

Trane XR14 2-Ton Heat Pump– 14 SEER Efficiency, 410a Refrigerant
Trane GAF2 Air Handler with electric auxiliary heat
And I’m attaching a photo of the thermostat. 

My question is: If the AUX is chosen (in the winter of course), does it first use the heat pump, and only the auxiliary heat if it needs to? OR does choosing Aux mean it only uses the Auxiliary heat?

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  1. walta100 | | #1

    The first question is who is paying the electric bills?

    Yes, if set to AUX the system will use 4X as much electricity and create much more revenue for the utility.

    Does the thermostat have an outdoor sensor? Some do and can be programed to lock out the strip heat above a temp you can set. I live near St Louis and my lock out is set at +7°F. Your mileage will vary and setting the thermostat to AUX would bypass the lockout.


    1. ktcstl | | #2

      The tenants pay the electric bills - but I want to help them make sure they aren't too bad!

      The thermostat does not have an outdoor sensor to my knowledge.
      I'm not sure I understand this part of your note - "can be programed to lock out the strip heat"
      The thermostat can be programmed to not allow "AUX" to turn on if it is above the temperature set?

      Then, if changing it to Aux bypasses that - does that mean the Heat setting can automatically turn AUX on as needed?

      I'm just looking for whatever is the most automatic setting, and what can save on using AUX.

  2. walta100 | | #3

    When the thermostat is set to HEAT most thermostats will turn on the strip heat while defrosting and anytime the room temp gets more than one full degree from the set point or whatever number the designer chose. It would not be uncommon for a set back programed into a thermostat make the strip heat come on so it is generaly best not to run them with a heat pump.

    When the thermostat is set to “AUX” the system will not turn on the outdoor unit and heat the house with the strip heat only.

    “The thermostat can be programmed to not allow "AUX" to turn on if it is above the temperature set?”

    Since your thermostat doesn’t know the outdoor temp, it can’t have the feature I was describing.
    If you continue to use AUX and strip heat interchangeably, we all will be confused and the thread will become unreadable.

    AUX is a poorly selected name the designer of your thermostat chose for what most thermostats designers call “Emergency Heat” or “E Heat”. The fact that the designer made such a poor choice makes me question all the choices made in the design of the thermostat. I doubt it would last very long on my wall.

    YES, some thermostat can be programmed to not allow strip heat to turn on if it is above the set outdoor temperature in heat pump mode.

    I think the best way to run a heat pump is to set the thermostat to one temp at the beginning of the heating season and don’t touch it until the end of the season.


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