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  1. #1
    Junior Member
    Join Date
    Oct 2012
    Posts
    2

    Nest 2.0 With Heat Pump & Aux Heat

    I have a Lennox heat pump with electric emergency/aux heat, I also have a humidifier. I'm getting an E33 error, and Nest is telling me that I can only have one auxiliary heat wire. The diagram on Nest indicates that W1 and W2 are Aux heat, and OB is "heat pump". From what I've been reading, W1 is primary heat, W2 is Aux heat and O B is for the reversing valve.



    I've wired my new Nest 2.0 as follows:

    C = Transformer Common
    RC = Transformer hot (24Vac)
    Y1 = Cooling
    W1 = 1st stage Heating
    W2 = 2nd stage Heating (electric coils)
    O B = Heat Pump Reversing Valve
    G = Fan
    * = Humidifier
    Y2 = NC
    RH = NC

    I have an extra wire that instructed the old thermostat when it was time for service, this wire is not connected.

  2. #2
    HVAC Thermostat Specialist
    Join Date
    Feb 2012
    Posts
    395
    The primary heat on your system will be the heat pump. Y1 will be the compressor. In the heat mode for most heat pumps the reversing valve is de-energized. One option would be to just remove and tape off the W2 wire. Depending on where you live you may not even need the second stage of electric heat. A second option would be to remove both the W1 and W2 wires and installed a short wire on the W1 terminal. Join the short wire from W1 to the wires that were landed on W1 and W2 behind the Nest thermostat with a wire nut. The wire nut idea is needed because the Nest thermostat press connectors are not designed for two wires.

  3. #3
    Junior Member
    Join Date
    Oct 2012
    Posts
    2
    Thanks for the helpful reply! After a nights sleep to think about things and another review of the Lennox schematics, as well as reading your post, I think I'm starting to get a picture of what's going on.

    Here's how I'm hooked up now:

    Y1 = the compressor (same as before, just misunderstood what that wire did)
    O B = the reversing valve (same as before)
    W1 = the electric aux heat (same as before, but I thought it was primary heat before)
    W2 = Disconnected
    * = the humidifier (same as before)

    My system now appears to be working, at least it's heating and cooling the house. Further testing this weekend should reveal whether I'm using Aux heat all the time, or heat pump.

    According to the Lennox schematics, what I had connected to W2 was an "emergency heat" relay that is energized by an outdoor thermostat. It looks like this relay turns on the electric aux heaters when it gets cold outside. I don't entirely understand what the distinction is between "emergency heat" and "aux heat", when they are the same electric heater coils.

    On the Nest setup menu for the * terminal, it appears that I have to make a choice between this emergency heat relay or my humidifier. At the moment, I have the humidifier hooked up and W2 is disconnected. I'm guessing that the Nest will know when to use Aux heat based on the outdoor temperature that it gets from the internet.

    I sure would like to get more information on when and how the Nest uses the Aux heat. It can get cold here in Cincinnati, so I need it sometimes, but I don't want to use straight electric heat unless I have to. I don't mind it taking longer to warm the house, if it saves energy. The old thermostat ALWAYS used Aux heat to warm the house initially, then heat pump to keep the house at temperature, unless it was too cold outside.

  4. #4
    HVAC Thermostat Specialist
    Join Date
    Feb 2012
    Posts
    395
    The Nest thermostat will energize the auxiliary heat when ever the indoor space temp is 4F below the heating set point.

  5. #5
    HVAC Thermostat Specialist
    Join Date
    Jul 2012
    Location
    Oklahoma City
    Posts
    380
    Nest isn't the thermostat to own if you want configuration options. The ecobee which costs about the same money lets you set the minimum run time of the heat pump before the AUX strips come on AND the maximum temperature lag. You can set the ecobee not to use AUX heat until the heat pump has run for 30 minutes (adjustable) and/or it's 5 degrees (adjustable) below setpoint. Ecobee can also be configured not to run AUX heat unless the outdoor temp is below a value you select. However ecobee isn't as pretty at the Nest and doesn't have the autolearn feature.

 

 

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