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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1answer
322 views

What are these extra wires behind my thermostat?

I an hoping to install a new thermostat that requires a C wire. I pulled off my old thermostat, an old round dial type, and it was of course only connected to four wires. The four wires are the only ...
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1answer
2k views

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 ...
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182 views

Connect C wire to Carrier board

I got Carrier board HK61EA010 and extra C-blue wire that is not connected to the board. I wanna use this wire to power WI-FI thermostat. I cannot find the manual for this board. Does anyone know where ...
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610 views

how to connect C wire to Weil-McLain EG-65 Series-5 gas steam boiler

Which of the Weil-McLain EG-65 transformer terminal should I use for a thermostat common wire? (I am trying to connect a nest learning thermostat to that boiler and it needs a common wire). Thanks.
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916 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
392 views

Installing a smart thermometer (ecobee4) can I move the wires around on the HVAC?

I want to install an ecobee4. I want to install it with the C wire so I do not need to use their kit. It seems to me that I should be able to just move the wires around on my hvac. Here is a diagram ...
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2answers
10k views

Less than 24 volts on the thermostat C wire

We have a brand new air handler at work, but the thermostat is mounted on a wall next to a server/computer room which is always warmer than the rest of the office. I want to install a thermostat that ...
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1answer
467 views

Install Common Wire (unable to find terminal or 24 VAC)

I bought a Nest 3rd Generation Thermostat. Live in a 2 family home, where my floor/basement has the boiler room/furnace. I have a Laars Mini Therm Furnace Model JVT. There's a thermostat upstairs as ...
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1answer
522 views

How do I wire a thermostat with an external 24V transformer?

My current wiring has 5 wires, R, Rc, Y, G, and W wires. No C wire. I've added an external 24V transformer and connected one wire to the C connector but don't know what to do with the other. If I ...
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2answers
33 views

Home thermostat woes

I tried to switch to a new smart thermostat in my home. When I was adding it, something happened. The new thermostat didn't show any power (And yes, the C wire was plugged in). Not knowing what else ...
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3answers
4k views

How to move from RH/RC thermostat to C thermostat in AC only system?

I see this is a pretty common question, but the answer seems like it might be dependent on your thermostat and air handler, so I'm posting with my specific questions. I'm upgrading from a White ...
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3answers
2k views

Wifi Thermostat wiring

Replacing 2 Hunter thermostats with a Honeywell RTH9580 Wifi. Neither unit has a C wire - I'm comfortable with the upstairs high efficiency unit, but the downstairs older unit has thrown me for a loop....
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175 views

Confirming C wire in my thermostat

can someone confirm that the blue wire is the "C wire"? What are the other wires (yellow, light pink, brown, and orange) that are not connected to anything?
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786 views

Need Help on Adding a Common Wire for Nest

I recently purchased a Nest thermostat. After couple weeks of usage, the Nest is constantly shut down due to low battery. I search online that adding a common wire is the go solution for this. However,...
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1answer
725 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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701 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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2answers
406 views

can I use an unused y2 or w2 aux wire and connect to the C terminal

I got a new ecobee thermostat that requires a c wire. my current thermostat has 4 wires plugged in, and there are 2 extra wires not being used coming out of the wall one blue labled y2, and a brown ...
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1k views

Old oil furnace with 2 wire thermostat, installing new wifi and need C wire. How can I wire with my current stack switch?

I'm trying to wire a new wifi thermostat and only have two wires, R and W. The stack switch (transformer - pictured) only has two wire slots as well. What is the easiest way to add a common C wire? ...
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1k views

Can I use the Y wire in place of the C wire for my new thermostat?

I'm looking to wire a Honeywell Lyric T5 to my furnace (I do not have an air conditioner, I do have a humidistat). My current wiring at the thermostat is G,W,Y,R. At the furnace it's G,R,C, and 2 at W,...
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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
239 views

Wiring new 'c' wire to heat pump system. Do I need to connect at both air handler and condenser?

I am planning on installing a new WIFI thermostat (Honeywell RTH6580WF). I have a heat pump system (outdoor condenser unit and air handler in attic). Old thermostat did not have a 'c' wire, but it ...
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88 views

Why doesn't my thermostat work?

I replaced old thermostat with Honeywell wifi and added C wire from furnace to new thermostat. I left the jumper wire on Rc and R in place and wifi would not come on line. Checked C wire and have ...
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1answer
129 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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1answer
12k views

Nest thermostat not turning on heat

I have an old gas furnace/boiler for hot water baseboard heat and I'm trying to switch from a 2 wire mercury bulb thermostat to a Nest Thermostat "E" (the new white faced one). There are 5 wires in ...
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1answer
197 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
469 views

C Wire for Thermostat

I am upgrading my thermostat (Have AC & Heat) and found that it doesn't have the "C" wire that I need to power my new thermostat. The furnace does have a terminal for C, but only condensation pump ...
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3answers
13k views

How can I wire my WIFI thermostat to control my millivolt fireplace?

I have a gas fireplace that has an millivolt on/off switch on my wall (and a remote that includes flame on/off, a countdown timer and a thermostatic flame control). I want to be able to program ...
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3answers
343 views

Can I add a “C-Wire” to boiler with this device?

https://www.amazon.com/FAST-STAT-Common-Thermostat-Extender-Connection/dp/B01N6ERM04/ref=pd_sim_60_4?_encoding=UTF8&psc=1&refRID=D3D121DG0B4379V2TRC6 On the surface it appears that it will ...
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1answer
323 views

Connect C wire Lennox G16

I bought a new thermostat that needs the c wire. Old thermostat wasnt using the blue c and was stuffed back in the wall. Come to find out the blue c wire is not connected to the furnace. Trying to ...
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2answers
509 views

Max Current Supplied by Thermostat Wire

So I want to run a Raspberry Pi 3 off of thermostat wire. My question is, will the thermostat wire supply enough current for the raspberry pi (2.5 amps)?
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1answer
232 views

WiFi Thermostat - C Wire

I am trying to install an Ecobee 3 thermostat to replace the upstairs one I have now but the 3rd wire isn't connected to anything on either end. I added the basement thermostat just to complete the ...
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341 views

NEST thermostat for hydronic floor heating: the three wire setup

I'm trying to install a 2nd generation Nest thermostat for my hydronic floors. So it's only meant for heating and not for cooling. Someone else removed the old thermostat a long time ago so I don't ...
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2answers
642 views

How to hook up thermostat C wire?

I would appreciate a sanity check/advice from this community. I plan to install a Wifi thermostat (likely a Honeywell) that requires a C wire in a house built in 2005. It's propane forced air plus ...
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2answers
22k views

Where do I connect my C wire from my thermostat when there are two transformers?

I have a Laars gas boiler that has 3 heating zones. I want to replace the old mercury honeywell tstat on the 1st floor (heat only) with a newer honeywell RTH9580 wifi model. I've taken pictures of my ...
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2answers
278 views

I have older HVAC unit (heat pump) and looking for the C wire to hook up to Smart Thermostat

I spent 2 hours with Thermostat vendor, Ecobee and they couldn't help me. I have R,G,Y,B and the white lead is not connected. I need some guidance on how to get a Common wire. I tested voltage and R-G,...
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2answers
6k 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 ...
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1answer
255 views

No C for new thermostat

I bought a new thermostat, and I need a C connector, but its missing on the unit. There is no C what so ever, but I do have a B and O connector, maybe I can use one of them for the C. Please see the ...
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1answer
17k views

Add C wire for Thermostat to Goodman furnace

I have read most of the posts here and have a specific question about my install. In the picture you can see the 3 blue wires coming off the 24v side of the transformer. One goes to the side and than ...
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2answers
307 views

C wire thermostat assistance

I am trying to install a new wifi stat. The wiring seems to all be there but apparently not connected correctly. So far I have tried using the green wire from the furnace, the blue and the white from ...
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2answers
7k views

Can you terminate more than one common on the furnace C terminal?

TL;DR: Can you terminate more than one common on the furnace C terminal? Background: I've linked a diagram of how my non-programmable thermostat is currently hooked up. I want to replace it with a ...
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1answer
692 views

Can I buy a 24v transformer with a dedicated common c-wire feed?

I have an oil furnace that feeds hot water to baseboard in four zones in my house and also to a hot water tank for tap water. There is a 24vac transformer with two output wires that feed power to the ...
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361 views

3 wire heating system, has 3 unused wires with 16v, can any be used as a common wire?

I hopefully have a quick question. I am thinking of getting a Ecobee or Nest smart thermostat, but doing my checks/research before getting one for this house. I rent, so I can only do minimal changes. ...
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1answer
3k views

Where to connect c wire in furnace

I have 5 wires running to the thermostat from the furnace. The blue wire is not connected on either side. I'd like to connect it as my c wire for a smart thermostat. On the furnace I see W R Y G and ...
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1answer
202 views

Unsure where to find “C” connection on furnace

Another "don't have a C wire for thermostat" question. Been following along, and if I could identify the C connection at the furnace, I'd be all set. Unfortunately, it's not simply marked C or common. ...
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204 views

Thermostat for my furnace has no power during fan-off delay?

I have an older Tempstar 5000 furnace and a newish Honeywell wifi thermostat. The furnace works fine and the thermostat appears to be wired correctly (4 wires including C wire) but for some reason, ...
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1answer
88 views

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 (...
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1answer
162 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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5answers
44k views

Adding a C wire to a new Honeywell WIfi Thermostat

I am trying to install a new Honeywell touchscreen WIFI thermostat and need to add a C wire to my terminal inside the furnace (using the help found at How can I add a "C" wire to my ...
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1answer
779 views

Needing Help with new thermostat install (C Wire)

I am in the process of installing a new thermostat that requires a c wire. I checked my thermostat to see if it has a c wire running to it and it does not. After doing lots of googling and watching ...
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1answer
223 views

Red Wire labelled as 'Common' in HVAC

I'm looking to replace my old thermostat with a wifi one. Looking at the wiring on my HVAC, I can see red and white wires connected (matching what's connected to the back of the existing thermostat). ...