Installing an ecobee4 thermostat. Have a 2 stage furnace with ac. Had a Trane furnace installed couple years ago and just noticed it's jumpered between W1-W2. Old thermostat wire was only 4, apparently HVAC guy didn't want to pull a new one and just did this for the timed second stage. I will be running new wire and want it to run truly 2 stage. Question I have upon looking at everything is as I understand it the Y should be connected from thermostat, furnace control board, and outdoor ac unit. And the other ac wire connected to the B/C on the furnace, which is also going to be connected to the new ecobee. This isn't how it currently is though. Currently the Y on the thermostat goes directly to the other wire on the ac unit not connected to B/C. Furnace board has no Y connected to it. Now everything works fine, ac in summer has been fine. Is this how I should run the new ecobee4? Does the Y on the furnace not need to be used? Thanks.

  • What model is the furnace? Nov 18, 2017 at 16:11
  • New issue. Ecobee was not detecting w2 wire when going through setup. Gave option to manually add it so I did. Now I can go into test equipment setting on ecobee and test both stages. Both work, but I don't notice any change in sound of furnace such as fan speed changing. Is this something I would notice? I'm guessing since it runs in both stages everything is working correctly. Just confused as to why ecobee would not auto detect w2 wire like it did all the other 6 wires.
    – Tag0304
    Nov 19, 2017 at 2:26
  • @ThreePhaseEel Trane XL80
    – Tag0304
    Nov 19, 2017 at 2:27

1 Answer 1


Most all the newer furnaces there will be a 'Y' terminal at the furnace. However, this 'Y' terminal does nothing at the furnace location. On the board, it is a connection point only. Essentially, the Y signal originates at the thermostat, travels on the yellow wire, passes straight through the furnace out to the starting contactor at the A/C condenser. That contactor at the condenser also needs the B/C from the furnace. These High tech thermostats will also need a common, B/C wire as well as the normal control wires.

  • Not always -- my understanding is that gas furnaces use their Y terminal connection to drive the blower relay during calls for cooling...let me check the (dead) control board sitting on my desk tho. Nov 19, 2017 at 4:11

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