Questions tagged [thermostat-c-wire]

The "C" or Common wire provides a way to get constant 24VAC power to the thermostat, now required by many newer thermostats with WiFi and/or touchscreens.

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2answers
68 views

Adding a C Wire for my new thermostat

I just bought a thermostat that needs a c wire. I ran a 18/5 wire, but where do I connect the c wire to? Here is a picture of the back of the board.
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51 views

Wired “like” a dual transformer

I'm trying to update to a smart thermostat and my current one is wired like a dual transformer but I only have a single hvac system in the basement. Any idea why? My plan is to move the blackwire ...
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upstairs furnace stopped working after connecting c-wires to both HVACs for ecobee 4 downstairs

My AC/heater for upstairs rooms used to work. But after connecting the c-wires to both HVACs in the attic it can no longer be started with the LED controller. I probably should have connected only one ...
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39 views

Thermostat/Furnace Wiring Confirmation

Like so many others, I am looking to install a new wifi thermostat with an older furnace. I've searched across the Internet and I have a pretty good idea of what is needed and why; however, I have a ...
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38 views

Where to connect a C wire for my smart thermostat

I am trying to connect the c wire but I'm at a crossroads of sorts. I have a dual zone so there are 2 thermostat wires come into what looks like a relay box ( see pictures) I have located the ...
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HVAC: Nest - Yet another 'C' wire question

I have a dual transformer system. At the AC/Blower unit, there are 'R_c', 'Y' and 'G' wires; there's 0VAC between the 'Y' and 'G' wires, and each shows 27VAC to the 'R_c' wire. At the boiler, there ...
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34 views

Red wire in place of C-WIRE?

I just moved into a new home and was trying to connect my Ecobee thermostat. I know it requires a C-wire. I found the wire wrapped around the others in the previous thermostat, which didn't require ...
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1answer
29 views

Connecting thermostat common wire to air handler or boiler?

we are planning to install a Nest learning thermostat in our house. On first installation attempt the device notified us that there was no power from the Rh or Rc wires which from my understanding is ...
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32 views

How to add a C wire to an old Lennox attic furnace GS810-105N

I bought a smart thermostat for my house in hopes of cutting some bills a little. I read up on them, I thought, thoroughly and figured I could handle the task. However, upon climbing up in the attic ...
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1answer
58 views

Where to connect C Wire to1990 Weil Mclain Steam Boiler

I am attempting to run a new 18/5 line in order to hook up an Ecobee 4 to my Weil Mclain boiler, as my current thermostat only has a R & W wire. I am having trouble figuring out where exactly to ...
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36 views

Where do I connect C-wire to Burnham boiler?

I'm adding a smart thermostat to one zone of a two wire, oil fired boiler system. I've roughed in a new wire and could use some help determining where to terminate it at the boiler. The wiring is far ...
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36 views

Need help with weird (?) wiring to thermostat and condensate pump

I've searched a lot and can't quite find what how to solve this one. I installed an ecobee smart thermostat, which didn't work with 5 wire connection so I switched to their PEK module with only 4 ...
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How do you install a thermostat if you don't have Y or C wires?

My thermostat is a LuxPro LUX PSP511LCa. I looked behind the thermostat and I see the following: A red wire connected to RC A jumper connecting RH and RC A white wire connected to W A green wire ...
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50 views

Adding a C wire and fixing White vs Red on my Furnace

TLDR: I think I need to flip R and W wires, and to splice the blue (C) wire to the brown wire coming of the transformer. Is this correct? Long version: Like many others on here, I'm trying to add a ...
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41 views

Secondary C Terminal on Furnace Circuit Board?

I was trying to install an Ecobee thermostat, and there was no C wire installed. When I went to install the power extender kit, I saw the standard C wire terminal was broken out of the black plastic ...
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99 views

Where to add a c-wire on my Goodman furnace?

I'm looking to upgrade my old thermostat and need to run a c-wire. Where would be the best place to tap into?
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58 views

Where should I connect my C-wire on my old Goodman unit (GMP075-4 rev 13)

I want see if someone help me on hooking up this new Honeywell thermostat with Wifi I just purchased. I have an old ac unit and I only have 3 connections to my circuit board which does not have an ...
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56 views

Replacing old thermostat and two unidentified wires

I'm replacing an old thermostat from the early 90's with a new wifi thermostat. When removing the old one, it only used R,G,Y,W wires, and the new wifi thermostat requires a C. After fully removing ...
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78 views

Only getting 15-16 volts from Furnace R and C

I recently purchased a smart thermostat. Because it needed a common wire and I only had a 3 wire setup, I ran a new thermostat wire to my furnace. However, I now get only 15 volts (inconsistently, ...
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55 views

Replace thermostat wiring from my Carrier Weathermaker 8000 to my Nest thermostat

I'd like to replace the wiring from my Carrier Weathermaker 8000 to my thermostat to add a C wire. I assume I need to turn off the Carrier first, but there is no power switch that I can see on the ...
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1answer
66 views

Where does my C wire go in this splice connection?

I'm looking to install the Ecobee3 Lite thermostat. My current thermostat has a blue C wire running to it, so my understanding is the PEK doesn't help because that is used if you have no C wire. On ...
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179 views

Wifi Thermostat, No C wire on HVAC unit. No control board? Condo

Looking to install Ecobee thermostat. Only 4 wires coming from unit. Ecoboo has power extender kit but there's no control board on the HVAC unit, so I can't locate a c wire. Any help would be ...
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50 views

Is the PY4G HVAC unit compatible with Nest Thermostat?

I recently bought a Nest Thermostat and I've had nothing but trouble with it. The main symptom is the fan and the AC compressor won't run at the same time. Power only comes through the green wire, not ...
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1answer
59 views

Where do I put the C wire in this setup

Got me a wefe thermostat. Old thermostat used 4 wires but the blue C was stuffed in the wall. I have read many forums and nobodys setup really matches mine. This what I see at the furnace and what I ...
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96 views

Can I pull a common wire off an evaporative cooler transformer relay?

I have a Nest thermostat currently controlling a LP furnace. The old thermostat didn't have a common wire connected, but there was a an extra wire in the bundle and a common terminal on the furnace ...
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86 views

Need a common wire

So, I recently moved. In my old place, I used my Nest without a common wire and never had an issue. However, in my new place, my Nest thermostat was working fine for about 20 days and started losing ...
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76 views

Another “How do I add ”c" wire to boiler for Nest

I have tried searching this forum and others, but I could not find anyone with a situation like mine. I thought it better to ask here, rather than damage something. I have a Lennox wall mounted ...
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45 views

Thermostat wire does not go to Furnace

My thermostat wires go to a control panel heat pump system which go to the furnance , i need to wire a c wire but not sure how to go about this with the york control panel
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1answer
43 views

How do I determine where to connect a new C wire inside my furnace when no control panel or wiring terminal exists?

I am replacing my existing thermostat with a new smart thermostat that requires a C wire connection and the old unit did not. I intended to utilize the unused Blue wire which is not connected on ...
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1answer
45 views

Identifying C wire

thank you in advance. I have been struggling to find a proper way to install my ecobee. It is again 'C wire' issue. My old thermostat uses 4 wires (red, yellow, green, white) and does not use c wire. ...
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958 views

How many wires should be in a new thermostat cable?

When roughing in thermostat wire, how many pair wire would you use to cover most thermostats? I'm asking because keep seeing questions about C wire and wifi.
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60 views

Where to install C wire?

I need to run new thermostat wire since I don't have a C wire to power my smart thermostat, but need some guidance on where to hook it up. I have five wires at my thermostat, two going back to my ...
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168 views

I have an unused C wire hooked up to thermostat and have no idea where to connect it to the furnace for power

I know this question gets asked all the time but I've spent hours staring at forums as well as my furnace and at this point I'm desperate. I just bought a nest thermostat that needs a C wire ...
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1answer
36 views

Old furnace with extra wire, but no c terminal Janitrol, 19 year old furnace

I've searched multiple threads regarding this, but I didn't manage to find any that seemed to match my use case. My board doesnt have a C terminal, and it doesn't have the other terminals connect to ...
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61 views

Can I use this White Wire for a thermostat C wire?

An AC-only machine cools a storage facility: no heat. The 30 year old thermostat is to be replaced with a wifi thermostat, which requires a C-wire to power the electronics. There are 4 wires: Red (...
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1answer
51 views

Furnace with no “C” connection

I have a mobile home furnace. This furnace only has a 4 wire connection terminal, G,W,R,Y. I also have a condensing unit outside with the 24 volt 2 wires coming to the furnace. I know I connect one to ...
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71 views

How to connect a new C wire

I have a 2 zone, single stage, heat only system with 2 wires running to each thermostat. I'm looking to install 2 Ecobee's; I have a second Honeywell AT72D1683 that I can install if needed and have ...
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392 views

Thermostat Wiring. 2 wires connecting to Y terminal

Did some more findings at furnace level (see furnace board image). The furnace board Y, W, G, and R terminals, ALL have a pair (2) of wires connected to them, instead of a single wire, making it a ...
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60 views

C Wire in Air Handler

Can someone please help me find where to connect a C wire for my smart thermostat? Here are some pictures with descriptions. It is difficult to get to the 24v connection in the back. My schematic ...
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2answers
84 views

No C terminal on furnace to connect a wifi thermostat

I have an new Emerson thermostat and would like to connect a c wire so it always has power. I have an extra wire that's not being used, but I don't have a C wire terminal on my furnace. I'm wondering ...
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170 views

Thermostat Upgrade, furnace C-wire reading low voltage

OK so here is the scenario. Upgrading thermostat from dumb to smart (Ecobee). I've done 2 others in my house that required re-wiring so I have a good sense for what should be done but I'm stuck. My ...
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401 views

What terminal to use on AC for C wire / nest dual transformer install

So i purchased a nest gen 3 and proceeded to install it. This is on a dual transformer system, separate boiler and AC system. The good news was i had a unused G wire on the Heat side that I was going ...
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44 views

C Wire to a Dual Zone House

Trying to connect a Nest thermostat in a dual zone house and I need to add a C common wire. There is an extra wire at the thermostat but there is no voltage on it. My current thermostat does not use a ...
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1answer
184 views

Connecting Common Wire from thermostat to Air Handler

My apartment has a central heating/cooling HVAC system consisting of a gas-fired hot water heater which feeds hot water to a First Co air handler for heating, and a rooftop compressor which feeds ...
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1answer
227 views

Thermostat C Wire (Possibly Unconnected)

I purchased a Honeywell RTH6500 Wifi Thermostat, which requires a C wire. My old thermostat showed a connected blue C wire. However, it was battery-powered. If I removed the battery, the thermostat ...
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68 views

Connecting Common Wire to Control Board

I have a heat pump and want to add a thermostat that requires a common wire. I have read through many threads that show how to add a wire but I already have a few extra in the bundle so its not so ...
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25 views

CT80 Thermostat Wiring Diagram Selection

Which terminal should the brown wire be connected to the RadioThermostat CT80? The original thermostat was wired as depicted: The brown wire is connected to terminal E (not F). As I understand it: ...
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773 views

Where to attach the c-wire inside Goodman GMP100-4 Furnace

As a relatively new home-owner, I recently came across the whole C-wire issue for the first time, having had to recently replace an old thermostat, and there is no C-wire connected inside the furnace. ...
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130 views

need to connect a c wire to my furnace/air conditioner

I installed a nest with 5 wires, but the blue wire is not connected to anything on the 25-30 year old furnace. We tried connecting the blue wire to what we thought was a common terminal, but the ...
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64 views

Where to connect Blue C Wire at old furnace?

I have looked at other responses and each layout is a little different. Looking to connect a Smart Thermostat and need to know where to connect the Blue wire at the Furnace. See the attached picture....