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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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Apart from a thermostat, where else can a C-Wire go?

As part of the home work before buying a smart thermostat (yes, because of the standard C-Wire issue!), i noticed that my current old thermostat have only four wires (No C-Wire), but my furnace does ...
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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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414 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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312 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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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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Why isn't the “C” or Common wire working on my Nest thermostat?

I Have connected the Blue wire to the C terminal on a 3rd Generation Nest and to the Blue wires on the Transformer....... But the Nest Thermostat does not show the Blue wire connected on the Equipment ...
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ecobee3 thermostat on old 2-wire boiler

Situation: 2 lugs on boiler control, one I'll call "red" or "R" that comes from the built-in transformer with ~14v and one I'll call "white" or "W" which, when jumpered to R, turns on the boiler. ...
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254 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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2k 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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How do I hook up Honeywell wifi stat to 2 terminal boiler pump control with second transformer?

I hooked up a Honeywell RTH6500 stat to a Taco boiler pump control, a 2 wire system. It also will work for a Honeywell TH6000 series wifi stat. This only works to control the boiler pumps only - no A/...
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877 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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429 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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585 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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1answer
1k views

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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1answer
160 views

Honeywell Wi-Fi Thermostat

I need to hook up a C wire to power a wi-Fi thermostat. I have an extra unused blue wire, but I don't know what to hook it up to at the furnace. I don't have the traditional wiring R W RC G Y C on the ...
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Does it matter which 24v line I use to splice a common wire from?

I have a RayPak boiler that currently has a W and R wire only. In wall i have a 5 connector thermostat cable so I have three extra wires I can connect to. I'm a little unclear if it matters if i ...
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648 views

Honeywell thermostat hookup with No C wire with wires RC, RH, G, W, Y currently used, how How to hookup external wall adpater transformer

How Do I hookup Honeywell RTH8500 thermostat with No C wire with wires RC, RH, G, W, Y currently used? I have an external wall adpater transformer but don't know where to connect it to. I have ...
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1answer
914 views

C wire and smart thermostat (with pictures)

I'd like to install an ecobee3 thermostat, but I'm not entirely sure about my wiring. The current thermostat seems to only have a W and RC wire coming into it. There is also what appears to be a blue ...
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1answer
554 views

Wiring a C (common) wire to a Triangle Tube Prestige Excellence 110?

Wiring noob here. Can someone give me step-by-step instructions for where and how I should attach a C (common) wire to a Triangle Tube Prestige Excellence 110 boiler for a ecobee3 thermostat? I think ...
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199 views

How to wire new Honeywell smart wifi thermostate( RTH9590WF) to replace older Honeywell thermostat using only 3 wires

Old thermostat was only using 3 wires, connected to W, G and R-Rc jumper. no C wire. Does have extra wires not being used at each end. Big question is what to I connect to at the furnace for a power ...
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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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376 views

Need help with wiring new Thermostat. Original 3 wire to Sensi Emerson

Hoping to get some help with this. The current old thermostat has a R (red wire) ,G (black wire, switches to blue at the furnace end) , and W (white wire). I noticed at the furnace side the white ...
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2answers
142 views

Smart wall heater thermostat line voltage from high to low.

Cadet's blog provided an approach on connecting a smart thermostat to a Com-Pak wall heater by connecting it to a relay/transformer. They have recently updated and changed their post to not recommend ...
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684 views

How to add C-wire from Laars mini-term

Folks, I am having problems connecting my Nest thermostat to a boiler. Given Laars Mini-term JVS series with Temperature Control PCB Module (pictured below) When I short Com call and Heat call wires ...
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143 views

Adding C Wire to thermostat

I bought a wifi thermostat and tried to use G instead of C since that's what the manufacture suggested if no C. Was able to on the thermostat end but how do I add it on cooling/heating system end. ...
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1answer
233 views

Installing an Ecobee3 - Wiring Question

This shows what my wiring looks like in the living room and garage. I had a Honeywell Thermostat that I want to replace with an Ecobee 3, is it possible to use the C terminal for this? The red wire ...
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1k 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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146 views

C Wire in HVAC Unit

I am looking to install a new style thermostat in my apartment and it requires a C wire. I do have an extra wire run from my HVAC unit to where my thermostat is. But when tracing it back to my HVAC ...
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2answers
2k views

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 ...
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2answers
153 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
334 views

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. ...
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323 views

Installing new thermostat without a C wire. Can I use my heater/R wire instead? Random 13V as well

I'm installing a thermostat, but I don't appear to have a C wire. My old thermostat had R/RH,Y,G,and W. It's an AC system that was setup before I got here. The actual AC unit is a long way away and ...
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1answer
297 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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C wire for Ecobee3 thermostat in 2 zone heating and A/C

My previous post was deleted by the Community. I'm incorporating very helpful comments from Stack users here in this updated entry. I have hydronic baseboard heating that uses a Weil McClain oil ...
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1answer
3k views

No C wire connection on old Goodman furnace

I have read for hours, and tried figure this out myself, but I need help. I'm trying to connect the Blue C wire from the tstat, without a C terminal at the furnace. From what I have read, it seems ...
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1answer
255 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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2answers
603 views

Can I use the unused blue wire as a C wire?

I'm installing a wifi-capable thermostat in my house which for obvious reasons requires a C wire. The old school mercury thermostat I'm replacing didnt have a C wire connected but I've checked the ...
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1answer
1k views

Multiple C wires? [duplicate]

I recently received a wifi enabled thermostat which I thought I could install easily since I saw an unused wire (Blue) which I assumed would be the C wire behind the current thermostat. When I went ...
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1answer
995 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
376 views

Finding “C” wire on old heat pump HVAC unit

I am wanting to replace my old round mercury filled Honeywell thermostat with a new wifi smart Honeywell thermostat, the only problems is I don't have a "C" wire on my current thermostat and need help ...
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572 views

Adding a C wire

I know this question has been asked multiple times and I'm going to try some of the methods I have read in other threads but wanted to ask this question since I didn't see anyone else's wired the way ...
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0answers
1k 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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1answer
663 views

Honeywell VR8300A3500 Single Stage Heat Gas Valve to Ecobee3 with C wire

Hey everyone have a quick question regarding a honeywell VR8300A3500 Single Stage Heat Gas Valve and the introduction of a Ecobee3 wifi enabled thermostat with a "C" wire. just want to make sure i am ...
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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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1answer
203 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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2answers
8k 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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0answers
807 views

Thermostat will not power on with C wire, but it has voltage

I recently purchased a Honeywell wifi thermostat and tried to hook it up as a replacement for an existing battery powered unit. I have a typical forced air HVAC setup with gas furnace (no heat pump), ...
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2answers
573 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
3k views

How can I connect a WIFI thermostat to a Honeywell R845A switching relay?

I'm trying to install a wifi thermostat and it won't power up. The old thermo was battery operated 2 wire but there were spare wires. At the R845A relay, red and white whites connect to terminals 5 ...
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496 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 ...