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Thread: Nest Gen 3 Learning Thermostat to Control Damper

  1. #1
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    Nest Gen 3 Learning Thermostat to Control Damper

    Everything connected fine and worked fine...until the middle of the first cold night and the upstairs was blazing hot and the downstairs cold. I then figured out what the two extra wires were for. The old thermostat used the B terminal and a common to open a duct damper when the thermostat was set for HEAT. To get things going, I connected the damper wires to R and C (hot and common) on the furnace control board. The damper opened and everything controlled fine afterwards. I have an A/C unit outside and a gas furnace in the attic. All though my solution works, it is not ideal.

    I think I should be able to add a DPDT 24VAC relay to the system as I have shown on the attached diagram below. From what I have read the Nest with the O/B connected will automatically enable "heat pump" mode.

    My goal is to not have the damper opening and closing every time the thermostat calls for heat (otherwise I would just hook the damper power up to the W-heat terminal).

    Can anyone give me advice or recommendations on this suggestion?

    Thanks in advance,
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  2. #2

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    First your idea should work. You would need to reverse Y and W on the relay as W should come from the NO contacts since you will set the O/B to B (powered in heat mode).
    You may need a 3 pole relay as when Nest thinks it is a heat pump it will activate the fan (G) with a heat call. Depending on your furnace it may then run at a different speed with heat.

    I would rather not change the Nest's logic. Either put a toggle switch at the furnace or use a latching relay instead (W to latch, Y to unlatch).

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