Help wiring a Nest thermostat in an unusual situation

Question

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!

Edit: other end of the wires

Can you post photos of where the other end of this cable terminates? — ThreePhaseEel, Dec 10 '16 at 05:07
Can you post a photo of the rest of the board diagram/instructions please? — ThreePhaseEel, Dec 10 '16 at 05:45
You likely need two Nest's. Call Nest customer service they are very good at advising in cases like these — Tyson, Dec 10 '16 at 14:12
If it wasn't for the special communication protocol, as pointed out by ThreePhaseEel, it might be possible. You could use the radiant heat as the primary heat source, and connect the hot air system as a second stage or emergency heat. — Tester101, Dec 10 '16 at 14:29

score 1 · Answer 1 · answered Dec 10 '16 at 05:57

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.

score 0 · Answer 2 · answered Feb 18 '19 at 05:17

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.

Help wiring a Nest thermostat in an unusual situation

2 Answers2

You're a bit stuck here