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No c wire ecobee 3

I’m trying to change my thermostat to Ecobee 3. The old one had 4 wires in the wall but only 2 are used. See photos. 1 to w and 1 to r. From what I understand I need a c wire from a 24v transformer. One option I saw is to buy a plugin 24v transformer and use that to connect 2 wires 2 c and rc. Will that work? I can’t run the new wires so there is an outlet next to the thermostat which I guess I could use to plugin the transformer into and then have the new wire go up the wall into the thermostat. Not pretty. I’m wondering about the 2 extra wires in the wall that weren’t used. Are those the same 2 wires connected to the 24v box in my other picture? Are they being used? Is that what my old thermostat wire is connected to? Can I use those wires as is? Or can I disconnect them from the 24v transformer and connect them to a plugin transformer in the boiler room? Thanks for the help


  • Herrick
    Herrick Member Posts: 5
    Does the yellow and green go to the furnace from the transformer, and the black and white go from the furnace to the thermostat? If there is an extra wire from the furnace to the thermostat that is not connected you could use that as a C wire.

    Yes, for the heat connect the W and Rh, the new transformer to Rc and C. (Dual transformer setup, the ecobee will not connect Rh and Rc internally)
  • albert0000
    albert0000 Member Posts: 12
    I’m not sure about which wires go to the boiler. I think the 4 wires from the thermostat go the 2 transformers you see in the picture. Maybe an older thermostat used all 4 wires. I’m not sure about which wires go from
    The transformers to the boiler
    HVACNUT Member Posts: 4,659
    Those are not your thermostat wires. The black and white look like a direct 24v. That would obliterate the thermostat.
    Post some pics of the boiler or furnace. From about 10 ft away. Get some electric controls too.
  • ratio
    ratio Member Posts: 3,235
    It's a little confusing looking. Has furnace been replaced in the past?
    The yaller & green are connected directly to an existing 24v transformer, but with the bare ends tucked back into the wall there's a good chance that the transformer had been shorted out & no longer works. That the black & white connect to the furnace is proven by the fact that it works, so those two wires must follow the high voltage back over to the furnace. Less than optimal, but not as uncommon as one might hope.
    If the ecobee can operate from a separate power source than the furnace (look for 'two transformer operation' in the wiring guide, use the new transformer as the 'cooling' transformer), you should be good to use the green & yaller, possibly needing to replace that transformer. Note that you should observe polarity with the new transformer to avoid potential issues (pun intended!)—check for bonding on one side of the new & furnace transformers, make sure that unbonded side goes to the 'r'/'rc/'rh' terminals.
  • Alan Welch
    Alan Welch Member Posts: 255
    It was common years ago to run an extra set of wires to provide a 24 volt power source for the original clock thermostats. If there is no power from the transformer it is probably dis- connected in the junction box behind it.