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My home has a honeywell thermostat (THM5421C)! It has 3 wire: DATA,R and C. Humidifier is Aprilaire 700 Series and Furance is Armstrong A97USMV!

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furnace side:

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The wire connection as below:

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Is it compatible with 3rd generation nest?

My guess wire connection as below:

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But still not sure:

  1. P1 - OB and P5 - * has been connected to the same wire! I marked it as blue line! How should I fix it?
  2. P9 point (to humidifier) still not connected, where should I connect P9?
  • Can you post photos of the furnace end of the setup? – ThreePhaseEel Nov 24 '18 at 0:24
  • @ThreePhaseEl Thanks, furnace side picture updated! – lucky1928 Nov 24 '18 at 0:53
  • Can you verify which wires from the wall go to which terminals on the thermostat? – mattdm Nov 24 '18 at 1:01
  • @ThreePhaseEel Update the connection! – lucky1928 Nov 24 '18 at 1:48
  • Can you post a photo that shows the end of the jacket of the cable between the thermostat and the interface module? – ThreePhaseEel Nov 24 '18 at 2:01
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You'll need to pull a new thermostat cable from the thermostat location to where the interface module lives in order to use a Nest

Your existing thermostat uses a data-link interface + 24VAC to minimize wires; as a result of this, you have the interface module you see to convert from the data-link to standard HVAC control I/Os. Since your existing cable only has the four wires, which is insufficient for your current system (and probably has something to do with why you have the thermostat you do), you will need to replace it with a fatter thermostat cable (18/8 minimum, 18/10 or even 18/12 recommended) in order to install a Nest. When you install the Nest, by the way, the interface module will simply get replaced with wire nuts -- your system beyond the interface module is conventional, with 1 cooling stage (Y), two heating stages (W1 and W2), and a single fan stage (G), as well as a humidifier connection (HUM). The other side of the humidifer should be connected to the C wire at the "grand junction", while O/B should be left disconnected entirely -- it's only used on heat pumps.

  • Great, Thank you very much for your kindly help! I am wrong after double check the connection and updated the diagram with correct connection setup. would you please help detail how to wire it if I pull a new cable? – lucky1928 Nov 24 '18 at 19:50
  • I have attached a wire diagram in my post but still need your help to figure out how to fix the connection! :-). I just guess it should connect that way, I know it's not correct way! Please help comment! – lucky1928 Nov 25 '18 at 4:02
  • @lucky1928 what make and model is your humidifier? – ThreePhaseEel Nov 25 '18 at 4:18
  • Furance is armstrong A97USMV,humidifier is Aprilaire 700 Series – lucky1928 Nov 25 '18 at 15:15

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