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Thread: One Nest, two air handlers

  1. #1
    Junior Member

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    One Nest, two air handlers

    Here is my current setup:

    Hydro air system. Single stage heating and cooling.

    Air handler 1, A/C compressor 1, Boiler 1 -- split into two zones using a zone control panel on the air handler
    Air handler 2, A/C compressor 2, Boiler 1 -- not not split in to multiple zones.

    Both AH 1 and 2 get hot water from Boiler 1 -- handled by a control panel on boiler to control appropriate HW circulator.


    With the above, I have three zones and two different air handlers - A/C compressors.

    Right now I have three thermostats, one for each zone.

    Two of the thermostats are actually in the same living space. What I would like to do is to replace the two thermostats (going to different air handlers) with one -- the Nest



    How would this be wired?

    Currently, each thermostat has: R,W,Y,G -- I have an extra wire for 'C' but not currently hooked up.

    To make things more interesting, I have a Bryant Humidifier on air handler 2. Not connected to thermostat.

    Anyone up to the challenge?

    Thanks in advance.

  2. #2
    HVAC Thermostat Specialist

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    Quote Originally Posted by Cold_in_CT View Post
    Two of the thermostats are actually in the same living space. What I would like to do is to replace the two thermostats (going to different air handlers) with one -- the Nest

    How would this be wired?
    Would you want the new single thermostat to control the two air handlers as single stage or two stage? What about the third thermostat? Will you be leaving the third thermostat as is?

  3. #3
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    Hi -- Thanks for getting back to me.

    I would like to have the single stat handle it as a single stage. The area that is controlled by the two thermostats I want to combine is very open -- the air ducts from handler one do the east side of the living space and the other handles the west. I currently set both to the same temp and the heating/cooling in the space is pretty even. Not sure why the house was designed this way, but the two stats are within 3 feet of each other.

    The third stat I would leave as is. It is handling a zone in another living area that is closed off by a wall/door from the area handled by the two stats I want to combine.

    I had not thought of a two stage setup. Being new to HVAC terms, I'm not even sure that that means. Would that be a setup where the second one would kick in if the first does not heat/cool sufficiently?

  4. #4
    HVAC Thermostat Specialist

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    I think because the two unit serve the same space it would be better to have them run together. You could have first stage Y1 energize one unit and Y2 energize the second unit but this may cause the the space temperature in the space to be uneven. Either way in order to control both unit from one thermostat you will need to install a thermostat wire between the two air handing units and install three relays at the air handing unit that is connected to the new thermostat. Will it be posable to install a thermostat wire between the two air handing units?

  5. #5
    HVAC Thermostat Specialist

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    Does the systems with 2 stats have a zone control module and motorized dampers for each side of the house? Seems like a waste of a zone control system the way it's described. You could completely remove the zone control system if both zones are serving the same space.

  6. #6
    Junior Member

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    To HVAC:

    Yes -- it will be easy to run wires between the two air handlers. And I agree, that running together would make the heat/cold in the area more even. The goal is to have one Nest control both as though it was just one unit.

    Servicetech -- One air handler has motorized dampers -- one for sending heat/cold in the common area served by both air handlers and one for sending heat/cold to the closed off room. So I can't remove the zone control. The other air handler services only the common area. And I agree that this is a wacky design but it is what it is.

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