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I am upgrading my thermostat to a Honeywell Wifi one, and would like to add a common wire. My current furnace controller has 3 blue wires at the 24 v end of the transformer. Which of these (blue) should I use to connect my blue C wire?

Circuit diagram below:enter image description here

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You can connect the C wire to any of the blue wires highlighted below, as they are all connected.

enter image description here

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  • As the chassis is also connected to equipment ground, would a connection to a close plug be valid? (Assuming a continuous path from plug ground to breaker box to furnace). Just wondering as my basement ceiling is finished and its not easy to route the c wire.
    – HerrBag
    Commented May 5, 2015 at 15:20
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    @HerrBag Would it work? Probably. Is it the proper way to do it? Probably not. Technically speaking, I'm not sure grounding one leg of the transformer is "proper". It seems a lot of furnace manufacturers do it, and I'm sure it's more convenient for them to do it that way. However, if you're using the chassis to complete the circuit, you're using the chassis as a current carrying conductor. While the current will be very small, it's still not the right way to do it.
    – Tester101
    Commented May 5, 2015 at 15:48
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Looking at the diagram I have to ask why you need to add a common if your replacing an existing t-stat.

If theres no feature you need to add or problem you need to fix, you should continue to use the same configuration.

Otherwise use the chassis ground. There should be a blue wire bolted to the frame. Its normal to use the chassis as a ground/neutral as it will keep the chassis neutral preventing a "Hot Skin" condition.

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