I replaced an existing Honeywell programmable thermostat with a new Z-wave enabled CT101-L thermostat yesterday. All went well, and the C, W, Y, R and G wires found their proper target connectors. The thermostat's working fine, and I'm able to control it remotely. (Haven't tested air-conditioning on account of the deep freeze here in Minnesota, but I'm assuming that's OK).

My question is about an unused brown-colored wire tucked back into the wall. This loops back on itself, and wasn't connected to anything on the old thermostat. I'm not sure where / how to trace this back to the furnace.

What is this wire? Is it an outside temperature / humidity sensor (which would be more than awesome), or just something that's a standard part of the cable (which probably is also good, so I can probably add said sensor). Or does using it somewhere unlock some more thermostat goodness?

2 Answers 2


It's likely not connected to anything. If you go to the furnace, you'll likely see the cable that goes to the thermostat. You'll also likely see the brown wire either wrapped around the cable, or just not connected to anything.

  • So is this something that I can use to connect, say, an external temp / humidity sensor to the thermostat?
    – alt
    Mar 5, 2015 at 19:44
  • Yes. If it's an extra wire in the cable, you can use it for whatever you want (within reason).
    – Tester101
    Mar 5, 2015 at 20:10
  • What Tester Said - at the end of the day a wire is just a wire. Color coding is a guideline to help you know what to connect it to but you don't actually HAVE to go with color coding. You'll just be a horrible monster if you don't because people coming behind you won't know what your wiring is doing. :) Jun 6, 2018 at 14:05

The brown wire is for the emergency heat available on some HVAC models. I have a heat pump and have a brown wire.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.