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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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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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373 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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289 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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1answer
638 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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334 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
216 views

C Wire placement in HVAC?

I'm installing a new thermostat and the previous one did not have a C wire connected. Tracing in back to the HVAC, I can't see a C output connection? Based on the wiring diagram (and reading other ...
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1answer
2k 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
368 views

Old AC and Gas Boiler wiring for new thermostat

I need help connecting a new 'C' wire for a Honeywell RTH6500WF thermostat. As you can see the heating is an old boilder with a simple pump to circulate the hot water to the baseboard radiators. The ...
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1answer
193 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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92 views

Smart Thermostat Wiring

All, First-time homeowner here. I recently bought a WiFi enabled thermostat which requires a C wire. My old unit did not need this, but I did have the extra wire in my wall to hook up to the unit. ...
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190 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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2answers
252 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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1answer
83 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
261 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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1answer
153 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
684 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
1k views

Adding a C (common) wire to my Goodman (Janitrol) furnace

I have a Goodman Janitrol furnace and I am needing to add a C (common) wire, for the newer thermostats. I would like to avoid the substituting G for C, and just run an additional wire. I have read ...
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2k views

Isolation relay for Ecobee smart thermostat: NC or NO?

I've been looking for "isolation relays" to install an Ecobee 3 smart thermostat on my 2-wire T/T furnace (replacing a Honeywell round thermostat). I know I want to use the 24VAC transformer/relay ...
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764 views

question regarding c wire and a nest

I know this issue has been beaten to death but i am stumped and nest support has no clue how to help me. I thought i had the issue resolved but that was wishful thinking. Here is my issue: I have a ...
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58 views

Need c wire from Team air handler

Help- I have a new wifi thermostat and need to run c wire to unit from the air handler . There are a bunch of wires but no labels . I reviewed the wiring diagram and looks like the blue wire (way left ...
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1answer
541 views

Can I run a C wire from Fan Relay

We've just moved into a new home with an heat only, oil, forced hot air furnace. The current thermostats are very old and we'd like to update them to something like a nest. The furnace is currently ...
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1answer
177 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). ...
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2answers
790 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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2answers
185 views

Lennox G16Q3-50 Furnace with AC, Door Interlock Switch and Main Circuit Breaker

If the door of the Lennox G16 is removed, thus the door interlock switch unpressed and system off. Does this mean that I only need to shut off the AC power from the outside breaker panel and leave the ...
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302 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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8k views

Can't get the AC compressor to turn on after switching to Lyric T5 Thermostat

I swapped out my old thermostat with a Lyric T5 around Christmas time. I live in an older house so wasn't sure I could make it work. I opened my old thermostat and saw that there was a blue wire ...
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275 views

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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1answer
416 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
571 views

Thermostat wiring with external transformer

So I am trying to wire a Honeywell lyric thermostat and I tried following videos but got a little stuck. So I have wires R, W ,Y , G coming to my original thermostat. I bought an 24V external ...
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1answer
358 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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2answers
2k views

“C” wire in wall! Where does it go in air handler/furnace?

Got a wifi thermostat. Behind the old thermostat was a blue "C" wire. The other side of the blue wire in the equipment room isn't plugged into anything. I need to get the 24v common hooked up to this ...
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1answer
260 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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1answer
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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1answer
305 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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1answer
353 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
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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2answers
1k views

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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2answers
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
396 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
294 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
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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1answer
1k 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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2answers
2k views

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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1answer
253 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
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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365 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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1answer
816 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
396 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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1answer
543 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 ...