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I bought a new WiFi tstat as a replacement. The tstat controls my second floor hot water baseboard heat and my central air conditioning. I've located all the wires....the ones coming up from the boiler, and the ones coming down from the ac unit in the attic (which also has wiring from the outside compressor). I have attached pictures. When I pulled the current tstat off the wall, I noticed in addition to the 3 other wires in use coming out of the cable, there is a blue wire coming out that is unused and wrapped around the sheathing.

The red, green, and yellow wires that come out of this air conditioning cable go to the respective Rc, G, and Y terminals on the current tstat. I followed the cable up to the air cond unit and saw that the blue wire is wrapped around the cable at the unit too. So how do I get this blue wire connected so I can use this new WiFi tstat?

There are other cables up at the air cond unit shown in the marked up pic attached.: - the gray cable is coming in from a flood detector in the air cond unit's tray. - the other white cable is coming in from the compressor outside.

The other red and white wires seen at the tstat (W and R) are obviously the wires for the gas boiler/heat.

  • 1
    Have you seen this question? The photos you've provided are not helpful, as they don't show the transformer, or give any indication as to which wires come from it.
    – Tester101
    Commented Mar 15, 2014 at 17:51
  • There is no transformer that I can see. Is there any chance its inside the ac unit? I tried to upload other photos but the site is giving me issues.
    – Evan
    Commented Mar 16, 2014 at 4:50
  • There's a transformer somewhere, unless your house runs on 24V or the A/C uses line voltage control circuits. It might be time to contact an HVAC tech. There are capacitors inside the condensing unit that could kill you if you touch the wrong thing, don't go poking around if you're not sure what you're doing.
    – Tester101
    Commented Mar 16, 2014 at 19:08
  • Ok. Its sounds like its time...what are questions to ask or ways to find a reputable HVAC that won't try and sell me services or labor that I don't need.I'm in northern NJ. Thanks.
    – Evan
    Commented Mar 16, 2014 at 23:52

1 Answer 1


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Connect the blue wire from the thermostat cable as shown above, then connect the other end to the C terminal of the thermostat.

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