I am trying to add a nest thermostat. But their support desk is not able to answer a simple question properly and keep asking me to buy a nest pro service. Right now my nest is connected without a c wire (basis their advise) and is working ok. But I would hate to not use a C wire that I feel probably is already existing.

enter image description here enter image description here I do see a blue coiled wire on either side tucked in (either side as in at the old thermostat and the furnace). But the wire is NOT connected currently. There is an additional red wire that’s on the “comm” of the furnace. I asked them if I can plug this blue wire which is currently running unconnected from my thermostat to the furnace and connect to both end of the nest and the furnace comm. which would mean though that the furnace comm will have two wires (there is already a red wire in the comm of the furnace and then there would be this new blue one) and the Y at furnace also has two already connected wires: one yellow and one white. Can one of you good Samaritan’s see from the connections if it’s ok to connect the blue wire currently unconnected to the nest and to the furnace and is it ok to have two wires going into the comm at furnace.

By the way what are all the other wires If anyone can throw light on the same.

  • Hello, and welcome to Stack Exchange. If it's all working, what's your concern? – Daniel Griscom Nov 25 '18 at 17:28
  • @ Daniel... the concern was that we don’t use HVAC all year long. So my assumption is that come summer when I use the ac only few days in the 5-6 month period, the nest will struggle to keep its battery charged. (I believe that either the heater or the ac has to be running for the nest to draw out power for charging.) – Leon Nov 25 '18 at 19:46

Go for it

The terminal strips on HVAC control boards are designed to clamp two wires under the same screw, 1 on each side. So, simply make sure the ends of the unused blue wire are stripped of insulation (and strip them if they aren't), tuck one end under the COMM screw on the furnace control board on the side opposite the existing red wire under that screw (you'll need to loosen the screw then tighten it back down to do this), and then you can install your Nest, using the blue wire as the C wire for the Nest.

As to the other wires in Y and C

The other wire in Y and the existing wire in C go off to your outdoor unit -- they carry the signal from the thermostat to the outdoor unit to turn the compressor on. (In fact, one of the great hints for people with A/C who can't find their C wire is to look for where the air conditioner wires join in -- one of them will connect to the Y wire from the thermostat, and the other will always be connected to C.)

  • Thank you threephaseeel. Also based on the image any idea where is the current red comm wire going into. I see two red wires so am assuming the comm red wire at furnace is not the one that’s coming out at thermostat end. So where and what is that wire for. And what are the two wires at the Y for. One am assuming goes into thermostat. But what about the other wire in the Y... where does that go. – Leon Nov 25 '18 at 17:32

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