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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363 views

Help me to find C wire on A/C side

I replaced my old AC only thermostat with Honeywell RTH6580WF smart Thermostat. Also replaced existing 3 terminal wire with 4 for C wire hookup. TSTAT specs h t t p yourhome.honeywell.com/en/products/...
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Adding a C wire to thermostat with oil berner and AC

I have a oil burner for baseboard heat and forced air AC that is controlled by one thermostat. There are a total of 5 wires coming to the thermostat 2 wires from the boiler and 3 wires coming from the ...
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883 views

Where to connect C-wire on old furnace (diagram attached)

I've got an old steam furnace and would like to add a common wire. The wire is already ran, I just have no idea where to connect it. I thought it would go to TR but that didn't work. Thanks in ...
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212 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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218 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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891 views

How to add a 'c' wire common wire to this system

I have a galaxy cast iron gas boiler and i'm trying to determine where to attach the 'c' wire on the furnace for my new wifi thermostat. The cable is an 18/5 but is only using 2 of 5 wires currently. ...
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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 ...
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505 views

Pull 7- or 8-conductor thermostat cable?

I've got 4-conductor cable now, but am looking to put in a smart thermostat (ecobee3). It requires a C wire, or the use of their adapter kit, so I'm thinking of just pulling new cable. 7-conductor ...
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421 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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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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321 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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Is voltage too high in C wire going to thermostat?

I've managed to add a C wire to my thermostat. I then decided to check the voltage between the R and C wires, and find that it read as 27.8 volts, not the 24 volts it was supposed to be. Could this ...
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857 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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399 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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289 views

How do a replace an old baseboard thermostat with a wifi thermostat?

I don't know if this is a dual pole or single pole thermostat (n00b here, don't know what I'm talking about). I don't even know what voltage this is (240 or 120). But here's a photo of the wiring: ...
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2k views

Can I use a separate 24V transformer with an Ecobee 3 and an Aube relay?

I've purchased an Ecobee 3 Lite smart thermostat which requires 3.5 VA to operate but I'm using an Aube RC840T-240 electric heating relay with 24 V transformer which only provides 1.2 VA. Can I ...
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330 views

Help me to find C wire

GV1 - ZONE 1 VALVE GV2 - ZONE 2 VALVE Y1 - YELLOW WIRE 1 Y2 - YELLOW WIRE 2 R1 - RED WIRE 1 R2- RED WIRE 2 TT1 - ZONE 1 THERMOSTAT TT2 - ZONE 2 THERMOSTAT R RED WIRE W WHITE WIRE B BLUE WIRE BOILER ...
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1k views

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 ...
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4k 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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2answers
461 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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2k views

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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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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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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144 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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14k 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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26k views

HoneyWell Wifi Thermostat and C wire

I just bought the HoneyWell Wifi thermostat and so wanna use it. However my old Thermostat was battery powered and does not use the "C" wire. I opened up my panel and found that there were three ...
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411 views

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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489 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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4k views

Connection for C wire on Mastertrol Mark V zone control panel

I have spent hours searching the web for information on this 20+ year old zone control panel, which supports 3 zones of Heat and A/C. The thermostat I'm replacing (on Zone 3) connects to T4/T5/T6 as ...
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1k views

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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161 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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666 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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389 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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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
236 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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3k 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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3k views

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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343 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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328 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
318 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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17k views

Wireless Thermostat C-Wire Substitute

I've read How can I add a "C" wire to my thermostat? and checked out the Venstar Add-A-Wire Accessory. I've watched Honeywell's YouTube video on "DIY Wi-Fi Thermostat Substitute G-Wire for C-Wire," ...
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263 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
628 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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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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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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597 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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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 ...