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Thread: Upgrading from Honeywell to Nest 3rd Generation (heat pump related)

  1. #1
    Junior Member

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    Upgrading from Honeywell to Nest 3rd Generation (heat pump related)

    Hello, glad I found this forum and hopefully someone can shed some light on my issue!

    I currently have this thermostat installed and wired as shown at the link below:

    I have a central A/C, heat pump and gas fired furnace on a single thermostat in my house. Now according to this wiring I am using the following letters: AUX, S1, S2, R, O/B, Y, G AND C. I am going with the O/B because I have a heat pump with the gas furnace as backup (is this correct?). I know not to use the S1 and S2 wires, so those are a non issue.

    When I input this information into the compatibility checker on the Nest website it tells me that it is compatible, but to have a professional install this. I don't see why I would need a professional install if I have already found the wiring for the professional guide here:

    Now I have found that reading through the professional instructions that all I would need to do is enter professional mode, and chose dual fuel heating, with the heat pump as the primary and the gas fired forced heat as the backup for the really super cold nights. Would this be correct?

    Thanks in advance for any help you can provide!

  2. #2
    Junior Member

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    The S1/S2 are for an outdoor temperature sensor. Before you use the Nest, you'll need to determine what controls the defrost on your heat pump (heat pump board, stat, or both). This is probably why they tell you to have it professionally installed.

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