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I am installing an Ecobee3 and have found that, while the common wire exists, it is not connect to the furnace. The wiring schematic for my specific furnace can be found in this PDF

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There is no control board on this furnace so identifying the connections aren't as simple as they could be.

On my furnace, the current wiring is as follows:

  1. The 3 white wires from the low voltage field connection are joined with the single white wire going to thermostat.
  2. The green wire is connect to green going to thermostat.
  3. The red wire is connected to red going to thermostat.
  4. The blue wire is connected to yellow wire going to thermostat.

Here is a picture of the current wiring. current wiring

Do I connected the unused blue wire going to the thermostat, to the blue wire coming from the furnace? If so, what do I do with the yellow wire? Do I join all 3?

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    What's the wiring look like at the thermostat? – Tester101 May 2 '17 at 11:20
  • Wiring at thermostat has all five wires (red, yellow, green, white, blue). – ChemE May 2 '17 at 11:54
  • Right, but how are they connected to the thermostat? – Tester101 May 2 '17 at 12:11
  • Red, yellow, green, white were connected to their corresponding terminals. I can't remember if there was a jumper for Rh and Rc when I removed the old one. The blue wire was unused on old thermostat but, as it is needed for Ecobee, I hooked it up. However, as you can see from furnace picture, it is not wired to anything. – ChemE May 2 '17 at 12:58
  • It looks like the blue wire is connected to the other side of the transformer, so there should be 24VAC between red and blue in the plug (in other words, blue is your C wire). However, blue being connected to yellow going to the thermostat seems strange. Yellow is normally used for the Y terminal, which is for cooling. Do you have A/C? What are the terminals labelled on the old thermostat? I know I just said all this based on wire colors, but wire colors aren't always used consistently and can't be relied on. – gregmac May 2 '17 at 14:04

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