I have a hot water boiler and separate AC system and bought a wifi smart honeywell thermostat to get a C wire- installed a 24 volt transformer that has two wires if one goes to the C where should the second wire land?

  • I'd suggest checking the wiring diagram and instructions that came with the thermostat. If your asking for help understanding those diagrams and instructions at the very minimum you need to give us the exact model number and/or a link to the online manual. Does your thermostat support this configuration with a second transformer? It may or may not.
    – Tyson
    Nov 20 '16 at 16:37
  • The Thermostat is a Honeywell 9580WF, Nov 20 '16 at 17:09
  • Does the thermostat control both heating and cooling?
    – Tester101
    Nov 20 '16 at 22:24

The two new 24V transformer wires will be R and C, but wait. We don't know which one is which.

It's a 50% chance, so guess. Hook up New R Only -- do not hook up New C. Now, measure voltage from New C (which you haven't hooked up yet) to W, Y and G. You should see near 0V, or 24V if the system is cycled on. If you see any voltages over 24V, particularly if you see 48V, then you guessed wrong. Reverse New C and New R.

Once you have them identified, use colored tape to mark R as Red and C as Blue.


I just installed a Honeywell Lyric on my hot water boiler system. I did not purchase a separate transformer, but connected to an existing transformer. One wire goes to the C and the other goes to Rc.

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