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
702 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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210 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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556 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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585 views

Can I put the unused blue wire in the C port, while still having the existing wire there?

I am installing a Honeywell Wi-Fi thermostat. I do not have a C wire, but there is an unused blue wire in the thermostat wiring, and also in the furnace wiring. The videos I've watched say that I ...
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1answer
1k views

Install New Lyric T5 on second AC, first one went great, this ones confusing, help!

Ive got 2 thermostats hooked up to 2 heat pump style AC units at my house. Over the weekend I impressed myself and installed a Lyric thermostat to replace a broken thermostat (on the inside of my ...
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714 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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1k views

WIFI thermostat no C wire

I hooked up a Honeywell WiFi thermostat last fall. There is no C wire in the furnace & the Honeywell rep suggested I wire up a small separate transformer with new wires going to the C and RC ...
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1answer
957 views

How can I tell if I have a C wire?

Below is a photo of my current thermostat's wiring. I have a few questions: Why is the RC and RH are bridged with a black wire? I have an unused blue wire. Is there a way to find its function? Could ...
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3k views

Trying to figure out 'C' wire

I'm looking to upgrade my programmable thermostat to a wifi thermostat. I purchased a basic Honeywell wifi thermostat but upon installation could not find a 'C' wire. I started to investigate more ...
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1answer
6k views

Connected C-Wire to new Smart Thermostat and Furnace board and still no power

I need help. I've been surfing the internet and have had no luck finding answers to my situation so am hoping for some guidance. I've seen other questions here answered but none that suit my issue so ...
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1answer
2k views

Where to add C wire on this air handler?

Thanks for helping me here. I am adding a new wifi thermostat to my system but need to add a C wire directly to the air handler (system is a First Co air handler with hot water boiler for heat and ...
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2answers
18 views

how to add c wire to heat pump ac unit (only have wires with nuts)

I can’t figure out which is the C wire or how to make a C wire and how to connect it to my ecobee 3lite Power extender kit. My old thermostat has wires G,Y,RH, W/E, and O/B plugged in. No "C"...
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98 views

installing ecobee SmartThermostat with Voice Control on older home

I recently purchased the most recent ecobee Smart Thermostat with Voice Control. The existing thermostat needs to be updated. However, I'm not sure if I'm able to install this without installing the ...
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79 views

No C wire on furnace

I’m trying to connect a C wire to my Nest Thermostat. However, I’m confused on where to connect it on my furnace seeing that there’s no marked slots on my old furnace. Any help would be greatly ...
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1answer
101 views

Low Voltage C-Wire at Thermostat

Looking to install an ecobee thermostat and decided to measure my common wire before unhooking anything. I tested the R, C wires and see they are only reading 14v, whereas R,Y, and R,W are reading 28v....
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43 views

Add “C” wire to NEST

I have a NEST thermostat. My heating and cooling system is in the attic.I need to add a "C" wire to the control board, but I can't find out where I need to open the unit in the attic to see where the ...
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1answer
60 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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1answer
81 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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49 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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1answer
66 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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1answer
87 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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1answer
115 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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657 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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1answer
223 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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1answer
87 views

Can a C wire be added to an old Teledyne Laars JVT-100C boiler?

Good morning. I have an old Teledyne Laars JVT-100C Boiler, that only has R,W,A terminals. is there a way to add a C wire to this boiler in order to power my thermostats? Thanks in advance...
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1answer
902 views

How to connect c-wire to Burnham Series 2 boiler?

I am trying to install my ecobee 3 light but the previous thermostat did not use any c wire. Fortunately, there was an unused wire in the wall and I would like to set it up as a c-wire. The problem is ...
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77 views

Installing a Honeywell Wifi thermostat and need a C wire connection [duplicate]

My current thermostat is wired as follows and the wires are from my Separate heater and separate AC air handler: W - From my heat R - From my heat Rc - From my AC Handler Y - From the compressor G -...
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291 views

Zone Controller T-Stat Wiring w/ 24V transformer, isolation relays for 2 zones

I have a new Navien SmartZone+ 6-Zone Control for my boiler-based heating system. It is great, except it provides no power on C. I realize the internet has researched the heck out of adding a C ...
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1answer
115 views

Install Hive Thermostat - C wire power

I am installing a Hive Active thermostat. My previous thermostat was battery operated and did not use a C wire. However, I found a 5th, blue wire, behind the thermostat so I used that as the C wire ...
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1answer
221 views

Finding the c-terminal on my furnace

I have a two wire furnace. I’ve been doing a lot of research but haven’t found anything super similar to what I have at my house. I’m trying to find where to connect the c-wire. I have a couple of ...
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1answer
172 views

Smart Thermostat without C on Heater

I want to hook up my Honeywell rth9580wf to a natural gas garage heater that only has R, W, and G (not in use). It's currently hooked up to a standard battery powered Honeywell thermostat. I've read ...
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1answer
2k views

Taco SR503 switching relay and common wire?

I just bought a home with a three-zone radiant heat system controlled by Nest thermostats. There's only two wires (red and white) going to each thermostat. This works, but is problematic — in the past ...
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1answer
470 views

Adding Unused C-Wire for Nest Thermostat

I'm having an issue with my NEST thermostat short cycling my air handler fan while in running AC. This is a known issue, and the fix seems to be adding a common wire to the NEST. My thermostat is ...
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1answer
2k views

Where should I connect a C wire on my Goodman PGB030075-1 furnace?

Is this where I can piggyback for c wire on Vine thermostat? Thanks
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1answer
47 views

HVAC Adding a Common

Got a new Honeywell WiFi thermostat and need to add a common. Currently the 3 common wires per the diagram (BL, BK, GY) are wired to one side of the AC Unit. Is there an issue if I piggy back a ...
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1answer
593 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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1answer
1k views

Add-A-Wire - COMMON / C Wire to a 3 Wire Thermostat

I am battling a Nest and losing at every turn. I have a 2 zone system and was able to install a Nest on the 1st floor by repurposing a wire as a COMMON wire. Unfortunately, the 2nd floor only has 3 ...
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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
4k views

How do I connect the spare C wire to the old Lennox system (model # lennox g12q4-110)?

I am trying to install my Ecobee3 now. After finishing the connections, Ecobee did not power up. I did some reading and I believe the C-wire, though it is available is not connected to the power at ...
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503 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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1answer
912 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
444 views

Connect Ecobee3 thermostat - no c wire but c terminal occupied

I am trying to install a new Ecobee3 wi-fi thermostat to replace my old mercury thermostat. When I pull off old thermostat there are only 4 wires (red, white, yellow, green and a blue tied around). ...
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1answer
390 views

Finding the “C” Wire on old Honeywell L8048G Controller/Relay

Need to add a "C" Wire for an older Honeywell L8048G Controller with built-in 24V transformer in order to connect an Ecobee3 thermostat. Attached is a copy of my current wiring diagram (pre-Ecobee3 ...
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1answer
259 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
803 views

What wire should be used for modern thermostat

I may (or may not) be pulling new wires to support current-generation thermostats.... Upgrading the thermostats themselves seems a good time to upgrade the wire too, though I think I could get by on ...
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204 views

Where do I connect a C-wire on my Honeywell TrueZone board?

My Honeywell TrueZone 311 board does not have a terminal for the blue wire. Where does it go? I plugged in the wires for the dampers and the thermostats, but the five wires from the furnace to the ...
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1answer
407 views

Old oil forced air heat only system (R/W wire only) - Is adding a C possilbe to use Wi-Fi

This system is from the 60's era - the furnace has a two T connectors for the R and W (thermostat) and two F connectors - one labeled "Thermostat Load 24VAC 0.2AMP, the other for the Burner Motor ...
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304 views

Wifi Thermostat C Wire Help

Looking to hookup a wifi (smart) thermostat that requires common constant low voltage power. The hookup of the old thermostat didn't use the blue wire as it was shoved into the wall but I traced it ...
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1answer
1k views

Installing an Ecobee3 - Add C Wire with PEK

I have another post with this question (Adding a C wire) but thought I'd clear up any confusion with this new post and I'm adding a diagram also. The diagram shows my current thermostat wiring and not ...
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1answer
249 views

Smart Thermostat C-Wire wrong voltage

I'm looking to upgrade the thermostats in my apartment to ones which I can control programmatically over WiFi. I opened up one of the old thermostats and discovered I have the classic "C-wire" ...

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