I want to upgrade from a battery powered Honeywell thermostat to an ecobee3 lite. This is for heat only.

My current Honeywell has two wires: Red and White. Missing is the C cable. There are enough wires in the cable in the wall they are just not connected.

The transformer on the boiler has the following terminals:




Currently the red cable from the thermostat is connected to G. The white cable from the thermostat is connected to Y.

Question: Where do I connect my 'c-cable' ? Is this a question of just connecting that one cable or would the entire transformer need to somehow re-wired ?


I did the same upgrade, but my old thermostat, YWGR were connected to the corresponding letters on the furnace. The blue wire was not connected on either sides. I connected the blue wire to C on both ends.

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  • thanks for sharing but I think you have a different system. mine looks nothing like yours and (sadly) my thermostat cable labelling don't correspond to the transformer they are connected to. As indicated above there is no 'c' terminal on my boiler. Jan 18 '19 at 22:56
  • can you add few pictures from both ends? When you say the "red cable from the thermostat" I am not clear whether you refer to the red wire or the wire (irrespective of color) connected to the R letter on your thermostat.
    – Quoc Vu
    Jan 18 '19 at 23:21
  • themostat: the red (color red) wire is connected to R, the white (color white) wire is connected to W boiler / transformer red wire is connected to G. white wire is connected to Y. Jan 21 '19 at 18:37

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