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So I am trying to wire a Honeywell lyric thermostat and I tried following videos but got a little stuck. So I have wires R, W ,Y , G coming to my original thermostat. I bought an 24V external transformer to power the thermostat. I have two wires coming from the external transformer. Where should the wires go on the new Honeywell lyric?

thanks for the help

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    Is there not a way to route a C wire from your existing system? Can you post photos of your thermostat wiring at both ends? – ThreePhaseEel Mar 4 '17 at 19:26
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Connect one end of the transformer to Rc on the thermostat, the existing R wire to R on the thermostat, and the other end of transformer to C on the thermostat. Also make sure to set the switch under the R and Rc terminals to the "R/Rc" position.

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    I would expect a new transformer to provide an isolated 24V output, with neither output bonded to ground, i.e. both ungrounded wires. If both the new transformer and the old one are not isolated, that's just asking for crazy trouble. – Harper Mar 4 '17 at 7:35
  • This will work if you are only controlling a heating system. If you're controlling heating and cooling, you might get some strange results when the thermostat calls for cool. – Tester101 Mar 4 '17 at 19:11
  • I've uploaded an image to chat, that highlights the circuit using an external transformer when in cooling mode. – Tester101 Mar 6 '17 at 16:35

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