The attached picture shows my current wiring. Notice there are two thermostats. The top one (Carrier, with four conductors) controls hot air heating. The bottom one (with two conductors, old mercurty style) controls radiant floor heeting. Although it's hard to reach behind the drywall, I believe the six conductors come from the same wire.

The part that concerns me is that the top termostat actually controls two zones. Does that mean I have a total of 3 zones (counting the radiant floor)?

In any case, I'm willing to combine the two Carrier zones as long as the radiant floor is on a separate zone. Unless it's very cold outside (<20F), the radiant floor is a sufficient source of heat.

Any hints on how to proceed would be greatly appreciated!

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Edit: other end of the wires enter image description here

  • Can you post photos of where the other end of this cable terminates? Dec 10, 2016 at 5:07
  • 1
    Can you post a photo of the rest of the board diagram/instructions please? Dec 10, 2016 at 5:45
  • You mean zoom out on the second picture?
    – Wynne
    Dec 10, 2016 at 5:48
  • That might do, yes Dec 10, 2016 at 5:49
  • You likely need two Nest's. Call Nest customer service they are very good at advising in cases like these
    – Tyson
    Dec 10, 2016 at 14:12

2 Answers 2


You're a bit stuck here

The Carrier Infinity system isn't a standard "on-off" control, unlike conventional thermostats. Instead, it uses a serial connection between the thermostat and the controlled devices (zone controller + furnace in your case), which the Nest hasn't a clue how to speak.

Your best bet for integrating more sophisticated automation into your system would be to use some sort of gadget with a RS-485 interface to speak the Carrier Infinity protocol.


Whoever wired up your system should be shot for that rats nest in the second picture. But yes you are stuck with the Carrier set up unless you revamp the controller. If you do I'd recommend Taco or Tekmar and don't hire the same guy who did the Carrier. Sorry but I'm a control wiring HVAC guy and I get a little worked up when I see a messy box.

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