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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How to add C wire to a very old 2-wire Lennox G16 Gas Furnace?

I recently bought a Sensi Thermostat with Wifi to replace my vintage one. I thought it would work with my 2-wire heat only system because it has batteries , but found out that using the Wifi feature ...
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Where do I attach C-Wire in this Rheem RBHA-24J11NUBAI system

I'm installing an ecobee lite thermostat to this system and the c (blue) wire was hidden inside the wall, tracing the c wire inside the air handler I see that is not connected. I have tried connecting ...
Damian H's user avatar
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24v Doorbell Transformer for Smart Thermostat

I have searched for this topic and the previous answers are not conclusive to me so I will try again with my specific situation. Thank you in advance for your help! Current situation: I have a 16V ...
HongVani's user avatar
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Ecobee Thermostat / Common Maker / Gas Valve

I have a two-wire thermostat and an old forced air gas furnace in my house. When you open up the cabinet, inside you find a Dayton 2E228 Gas Valve and it is wired to an external 24VAC Transformer ...
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Connected C-Wire (previously unused) to thermostat, but still no power?

In search of help with hooking up a Nest Thermostat E. I have seen several threads seeking solutions for similar issues and have troubleshooted with their proposed solutions with no luck. I am ...
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Nest, Blue Wire, W1 and W2

I just got a Nest E thermostat and I'm trying to hook it up. There is a blue wire that I had assumed was the "C" wire, but looking at the furnace, it doesn't look like that's the case, as the blue ...
Pete's user avatar
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"C" wire in wall! Where does it go in air handler/furnace?

Got a wifi thermostat. Behind the old thermostat was a blue "C" wire. The other side of the blue wire in the equipment room isn't plugged into anything. I need to get the 24v common hooked up to this ...
user3330299's user avatar
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Nest Thermostat Wiring for Baseboard Heaters

I'm using a Nest Thermostat to power baseboard heaters (240VAC). I have 4 wires that I have access to: 2 wires: Red/Black from a transformer which outputs 24VAC 2 wires: 24VAC from a thermostat ...
Nick Lang's user avatar
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Added a new 5th wire for my wi-fi thermostat, now where to connect it?

4 wires at thermostat: R G B W (red green blue white) 5 screws at furnace: G C W Y R Single-stage gas furnace with A/C unit The home's previous owners kind of did everything in a very make-shift way. ...
Donna_At_Home's user avatar
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Installing of Honeywell Wi-Fi Programmable Thermostat

I need to install new wifi thermostat but I don't have the C wire. There are G, W, Y and R wires attached to my old thermostat. There is also one blue wire not connected on the wall.
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Ecobee SmartThermostat - Blue connected to W at furnace, what to do?

Trying to install my new ecobee SmartThermostat. Furnace is a Trane heat pump, 20 years old from when the house was built. When I looked at the existing thermostat, it has RGWY. So I thought I needed ...
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3 answers
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How to add Thermostat C Wire to Furnace TH/TR and Transformer

I am trying to figure out the proper wiring for a C wire for my thermostat. My 24v transformer and the current thermostat wiring at the furnace are shown in the pictures below. Can I wire a thermostat ...
Chris Peterson's user avatar
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Where do I connect my unused C wire to my Goodman Heat Pump?

I understand similar questions have been asked but I have been unable to extrapolate what has worked for others to my own system. I am trying to upgrade my thermostat to the Ecobee Lite. I have an ...
crsexto1's user avatar
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Existing thermostat wire colors - no white?

Looking to switch from an old Honeywell CT87A thermostat to a smart thermostat, probably something requiring a common wire. There are 2 thermostats (one on each floor) in a 2 zone forced hot water oil ...
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Connecting C-wire to Nest Thermostat for Lennox HVAC

My Nest thermostat has been running fine for quite a while, but recently it started complaining about low battery and wi-fi gets shut down due to power issues. I've been looking for the answer, and I ...
scifi88's user avatar
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Adding a C Wire using a 24V AC Transformer to a 2 Wire Heating System for Nest

Continuation from Can I use any of my existing oil control terminals (TTFFAA) for a C wire? EDIT: Wanted to finally say this is SOLVED with help from @JPhi1618 (see conversation below). If you have a ...
Teuszie's user avatar
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Fast Stat Common Maker Setup on 2 Wire Boiler

I'm trying to wire up my ecobee in a new house that only has 2 thermostat wires (R, W). I bought a Fast Stat, but I'm unsure how to set it up as I have a complicated setup. There are two aquastats (...
nos9's user avatar
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Low voltage over C wire at thermostat

Similarly to this question of the same name, I am reading low voltage over the R,C wires at the thermostat. Unlike that question, however, the voltage is unusably low. I am only reading 2V over R,C ...
Drew's user avatar
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Adding Venstar Add-a-wire ACC0410 to HVAC with Nest Thermostat E

I purchased a Nest Thermostat E and found that I don't have C-wire. With the research, including adding venstar to HVAC and Youtube video by Venstar, I tried to add c-wire to my Nest Thermostat E with ...
Jong Lee's user avatar
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Why is Thermostat Y wire connected to C terminal on furnace?

I have a Nest E thermostat, and like many others have lost wifi and getting low battery because I have no common wire connected. After looking at my old Lennnox 78ugf3-75-1 furnace, I noticed that my ...
DJ2904's user avatar
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No C terminal on furnace. Workarounds? Possible the AC?

I'm Trying to connect my C wire to install a new thermostat but the furnace doesn't have a C terminal. In my readings on this I found that because I have an AC,I can connect my C to one of the wires ...
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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 ...
Brad Drake's user avatar
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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 ...
Chris's user avatar
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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 ...
Matt's user avatar
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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 ...
Greg's user avatar
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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 ...
Karl's user avatar
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Nest E Thermostat Power

My thermostat is in the basement, connected to a Honeywell R845A to run a circulating pump for heat. I bought an Nest E for the main house and it works great. I want to replace the thermostat in the ...
Tstatguy's user avatar
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Can I use the extra thermostat cable as C cable for wifi?

I just purchased an ecobee4 and I found that I need C wire for wifi which is not connected to my current thermostat. However, there 2 extra wires I found in the wall. Could you use one of them for the ...
Sam's user avatar
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Setting up an ecobee and need to adjust HVAC wiring

My HVAC control board has two bundles of wires. One has 5 wires which runs to the thermostat but the 5th wire which I want to use as C is currently unused. The second set runs to the unit outside (...
Fletcher Moore's user avatar
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1 answer
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Add c wire to furnace for smart thermostat

I have an older furnace and am looking to add a c wire for a smart thermostat. Currently behind my thermostat I have red/black wire, however, there are 3 additional wires that are not connected. At ...
user65052's user avatar
2 votes
2 answers
13k views

Wifi Thermostat to Reznor Garage Heater. No "C" wire connection on terminal strip

I just had a 75000 btu Reznor UDAP garage heater installed. Can I run a "C" wire for a smart thermostat if I don't have a C connection on the terminal strip? There is a C connection on the circuit ...
Glen G.'s user avatar
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Can I use a WiFi thermostat without a C wire?

I have a very old Honeywell programmable thermostat. I am going to replace it with a White-Rodgers UP500W Emerson Sensi WiFi Thermostat. I just want to know if the WiFi unit will work without a C ...
qinking126's user avatar
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1 answer
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Is it possible to add a "C" wire to my electric heater?

I bought a Wifi Thermostat but my old thermostat did not have a Common Wire. I can't use any of the swapping the G for the C solutions because I have an electric heater. I was told by Honeywell that I ...
Kenny's user avatar
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3 answers
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Where should I connect the C wire in a Janitrol furnace?

I currently have a Honeywell Thermostat with 4 wires going to a Janitrol Furnace. I am trying to install a Honeywell WiFi Thermostat RTH6580WF and I'm missing my C wire. My bundle has 8 possible ...
Gary P's user avatar
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Where to connect a c-wire on a furnace with existing two wire mercury thermostat

My existing thermostat is an old mercury two wire device, heat only. Looking to upgrade to a better model, but I need to run a c-wire. I don't know where to connect the wire at at the furnace, any ...
Joshua Lake's user avatar
2 votes
1 answer
98 views

DUAL Nest Power Connector wiring - separate heat/cool thermostats

My house has separate thermostats for heat and cool, each located in a different part of the house. It's a single-zone forced air system handling both heat and cool. The existing thermostat wiring ...
DougSlug's user avatar
2 votes
1 answer
234 views

Where to connect a common wire to a Burnham alpine Furnace

Like many on this site I am trying to install a smart thermostat. I have 3 wires, 24V Red, white, and blue at the thermostat, but only 2 down at the boiler. My boiler is a Burnham Alpine model number ...
Paul's user avatar
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2 answers
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Trying to figure out which is the C wire to use for my new Honeywell Wifi Thermostat?

I am trying to figure out which one of these 3 blue wires is the correct one to use as my C-wire. I have a reg Honeywell thermostat but recently bought a WiFi Honeywell thermostat. When I pull ...
Donald M's user avatar
2 votes
1 answer
235 views

No voltage on old thermostat wiring

I'm trying to add an amazon smart thermostat. I don't have a C wire, but there is a spare black wire behind the thermostat. I put my multimeter on 200 VAC and expected to see 24 VAC on the red wire. I'...
bonhoffer's user avatar
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Advice on how to wire a 4 wire RGWY wifi thermostat to an old hvac with no c wire?

I have a 4 wire RGWY coming out of the wall to my old thermostat with no additional wire available for c wire. I was going to use the green one instead as I saw a tutorial online, but when I got to ...
Bryce Fager's user avatar
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3 answers
1k views

Where do I connect my common wire (C wire) for smart thermostat?

I've been running 2 Nest thermostats (1 upstairs, 1 downstairs) for years that didn't require a common wire. I'm replacing them now with Ecobee3 Lite thermostats. Both thermostats control heat from ...
eman3gt's user avatar
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2 votes
1 answer
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How to get the C wire for Nest thermostat on Burnham series 2 gas boiler

I am trying to setup a smart thermostat for my boiler. Can you help me see where I can extract the C wire out from my boiler. Thanks
Phuong Le's user avatar
2 votes
2 answers
2k views

Honeywell 5-wire zone valve wiring

TL;DR Is a jumper wire necessary between TH/TR and the left-hand END SWITCH? If so, what does it do? As requested, here is my best approximation of a wiring diagram: As it is now (note the missing ...
Nigel Stackhouse's user avatar
2 votes
4 answers
2k views

Where can I install a C-Wire for Weil-McLain gas boiler? (EGH 115, 4 Series, Natural gas)

I want to install a C-wire so that I can use a WiFi Thermostat. Behind my current thermostat there are only 2 wires (red and white) There are 2 wires tucked and unused, (green and blue) I had tried ...
user2285215's user avatar
2 votes
1 answer
470 views

HVAC: adding a C wire for Nest thermostat

Just installed a brand new gas boiler, want to keep using my existing Nest thermostat (they are compatible according to Nest). Current system only has two wires: Rh and W Nest is designed to steal ...
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1 answer
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New ecobee3 lite, have heat but no cool option

I recently installed a new ecobee3 lite. When I completed the setup process, I found that there was no option to enable cooling. My original setup had both heat and cool options. I believe the option ...
BurgerKing's user avatar
2 votes
1 answer
533 views

Is this wiring dangerous? Smart Thermostat C-wire AC adapter twisted together

Is this wiring scenario dangerous? Is it likely produce sparks or damage my HVAC system? Please read the description and see the image below. Description I didn't have a C-wire for my Emerson Sensi ...
InteractiveRob's user avatar
2 votes
1 answer
202 views

Emerson Sensi Touch C wire help needed

I am heating with a fuel oil boiler in the basement and a Unico air conditioning unit in the attic. I some help getting past the c wire issue. Air is using yellow, green and red-c and heat is using ...
Robert Kessler's user avatar
2 votes
1 answer
328 views

Wiring HVAC for smart thermostat

I'd like to install a smart thermostat in place of my old mercury coil thermostat. The old thermostat controls both heat and AC - it uses only 4 wires, the new clearly requires 5. Wiring to the ...
Code Magician's user avatar
2 votes
1 answer
173 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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