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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3answers
3k views

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 ...
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1answer
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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 ...
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1answer
66 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 ...
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1answer
64 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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1answer
139 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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2answers
695 views

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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2answers
212 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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1answer
591 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
284 views

How to fish thermostat cable using an existing cable

So I have an existing thermostat cable on my 2nd floor that only has 4 wires, and I need a 5th wire to use as the common wire. Can I use the existing cable to fish or guide a new 6 wire cable through ...
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1answer
101 views

Nest thermostat to Kelvinator furnace

My old thermostat had 4 wires. I replaced with a Nest thermostat. At times the AC condenser is running but the furnace blower is not running. Found other with this and they said need to add common ...
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2answers
8k views

No C (Common) terminal on Furnace.

I am trying to hook up a new wi-fi thermostat. I currently have 4 wires running, but there are many unsued that are terminated in the jacket of the wire. No issues there. However, when I ...
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1answer
226 views

Where to connect c wire

I’m installing a new WiFi thermostat that needs the c wire. I ended up running new thermostat wires that contained the common wire but now I’m stuck on where to connect it to the furnace. Can ...
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1answer
176 views

How should I hook up my WiFi thermostat given my current wiring situation?

I've upgraded a few thermostats before, and they have been quite straightforward; i.e. take pictures and connect the wires as the the original. However, this is my first attempt at a "smart" WiFi-...
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1answer
3k views

Honeywell Zone System Wiring

I have a Goodman HVAC system that is split into three zones. I wanted to upgrade to programmable thermostats, so I picked up three Honeywell RTH8580WF. Reading through the descriptions I noted they ...
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2answers
1k views

Help wiring a Nest thermostat in an unusual situation

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 ...
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1answer
993 views

Want to add a WIFI thermostat, but no 'C' wire… only 2 wires to each thermo

i have a 5 zone radiant floor, oil fired heating system. Each thermostat only has 2 wires running to it. i have a TACO SR506 switching relay that receives the wires from each thermostat and controls ...
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2answers
470 views

How can I add a C wire with this unusual installation?

I recently got a new Honeywell WiFi thermostat, which requires a C-wire. I have 5-wires going to my current thermostat, with only 4 being used. So I thought adding the 5th as my C-wire was going to ...
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1answer
843 views

Can I connect an additional wire to HVAC system where all the terminals are full?

I'm installing a new thermostat (ecobee Smart Si) and it requires "C" wire. I checked before I bought, and saw an unused blue wire behind my current thermostat. However after connecting everything the ...
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3answers
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Where's the C terminal on my boiler control?

The new thermostat I purchased doesn't run on batteries, and needs 24 volt power. I have only a red and a white wire connected between the boiler and the thermostat, but there is also a third, unused ...
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0answers
39 views

Connect “C” wire on boiler model number 1135-601

I've read several threads here and on the other websites but still can't figure out how to add C wire to my boiler wiring. I would like to replace my old mercury thermostat with Nest E or any other ...
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1answer
154 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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0answers
612 views

C-wire on Weil-McLain CGA Series 2

I have an old thermostat with only 2 wires connected to my Weil-McLain CGA Series 2 boiler. I would like to replace it with a Nest thermostat. I need to add a C-Wire in order for it to work. Luckily, ...
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3answers
1k views

Wiring for ecobee3 lite

I recently purchased the ecobee3 lite and am now stuck trying to install them. I have 3 Honeywell thermostats I am looking to replace and my heating system is oil/hot water. My home was built in 1970 ...
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2answers
279 views

Lennox G23Q3/4-100-2 And Ecobee4

**Pictures attached to go along with my rambling explanation I have my Lennox G23Q3/4-100-2 natural gas furnace and my central air system that has been working since 1995 on a basic programmable ...
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0answers
962 views

Honeywell WiFi thermostat jumper loop separate ac and gas furnace

I need to validate my understanding for the installation of a WiFi thermostat that requires 24volt continuous power to operate in a constant state. I am DIYer who is computer literate. Like a number ...
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0answers
2k views

Is Nest 3rd Gen Thermostat compatible as a replacement for Honeywell TH3110D1008

I have an Ultra-Zone EWC-ST-2E controller unit for my gas AC unit at home (heating and cooling). It is set for two zones. Where the first zone has to be on (heating or cooling) in order to enable to ...
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0answers
1k views

Where to connect the c wire for a gas HW boiler (heat only) w/Wi-Fi Thermostat

I recently purchased a new Thermostat with Wi-Fi capabilites for use in one zone of a two zone baseboard hot water heating system. I pulled the existing thermostat (stat) to look at the wiring and ...
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1answer
6k views

How can I add a “C” common wire to this system? [duplicate]

I would like to add a "C" / Common wire hookup to my hvac system and get a fancy new thermostat. I've read How can I add a "C" wire to my thermostat? and still have some questions. First of ...
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5answers
4k views

Can I provide a secondary power source for my thermostat?

The thermostat at my home is set up with only the 24 power and return through the relay. I would like to add a 'smart' thermostat, however the line is fairly well buried in the wall/basement ceiling ...
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2answers
6k views

How do I connect a WiFi thermostat to my furnace's primary control?

I would like to install a Honeywell WiFi controlled thermostat to connect to a Honeywell R7184U primary control. My question concerns the "C" wire. The R7184 has the terminals labeled 1, 2, 3 and 4 ...
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97 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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4k views

Is the transformer bad on my Central Air Conditioner?

Need some help diagnosing a problem. My very old central AC unit isn't working. It's just AC and about a year ago I connected a C-Wire to the air handler when swapping the thermostat for a wifi ...
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2answers
5k views

HVAC wiring for wifi thermostat installation (Ecobee) gas furnace + AC

I'm trying to determine whether I can install a wifi thermostat (Ecobee or possibly Nest) with my existing gas furnace (dual stage) and electric air conditioner, ideally using existing wiring but able ...
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6answers
15k views

I am installing a Honeywell RTH6580WF with a TACO SR504

The current installation does not have a C wire. So I have run an extra wire and am now confused as to where to connect it. There is a 24VAC connection and a common connection. Comments say that ...
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1answer
296 views

Adding C wire to WiFi thermostat

I’m trying to add c wire to furnace to run a WiFi thermostat but don’t know where to hook up c wire to get power to turn on thermostat i have 2 extra wires just don’t know where to put one them to get ...
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3answers
4k views

How do i hook up a new 5 wire cable to an existing 4 wire furnace

I would like to replace the existing thermostat wires on my 2000's era janitorial furnace with the upgraded 5 wire setup that will allow fan control of my Honeywell wifi 9000 thermostat. I was going ...
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1answer
2k views

Could I use this Speaker Wire for my thermostat?

I had this wire left over and thought I might as well use it if I can. I think it will work but I just wanted to make sure! So can I use this speaker wire for the Rc and C terminals on my thermostat? ...
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1answer
257 views

Where is the C wire in this tangle of wires?

I have looked and searched, but still iffy where to connect the c wire coming from the thermostat. I appreciate any help.
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1answer
79 views

Nest thermostat e heat link replacing the old danfoss

I really need help with replacing this current danfoss thermostat with a new nest thermostat e heat hub. I'm in the UK. Below is the new heatlink just need to know what wires I need to wire and what ...
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1answer
123 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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1answer
62 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
2k 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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1answer
158 views

C-Wire identification

I am interested in replacing an older thermostat with a modern networked 'smart' thermostat \ humidity monitor. That being said, does the photographed thermostat have a C-wire? The thermostat ...
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1answer
400 views

Can I just extend this blue wire in furnace for Nest thermostat?

First time doing this, don't want to burn house down. The brown bundle is coming from thermostat, the white bundle I suspect is coming from A/C unit. Do I follow these steps? Find a scrap piece of 18 ...
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2answers
5k views

Test voltage for common on HVAC wiring

I'm working on adding a smart thermostat, and as many older setups have it, my current thermostat doesn't have a 'C' wire. However, if I pull the wires further out from the wall, the feed from my A/C ...
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2answers
110 views

Need to identify a common wire on my old HVAC

For Nest Thermostat 3, I am trying to identify a common wire on my old HVAC. The diagram can be found on the link below:
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2answers
100 views

Gas Furnace only, is Y wire needed?

I have force air (propane furnace). I would like to upgrade to a new "smart" thermostat, but they didn't run a C (common) wire which is required for most of these units to run properly. I do however,...
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1answer
1k views

Wiring a new thermostat without a C wire, extra wires from furnace

I have checked other posts regarding this and don't see a solution yet. I have purchased a new thermostat that requires a c wire. My old thermostat's c wire didn't work so I went ahead and added a ...
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1answer
994 views

Wiring C-wire in old Rheem

looking for some help with an old Rheem 1005 HVAC system. I’ve scoured the Internet for days, so this is a last ditch effort at self-help. I’m installing an ecobee to an existing 4-wire system that ...
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1answer
137 views

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 ...

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