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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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

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
18 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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174 views

How do I connect a honeywell wifi thermostat with no C wire at the furnace?

I purchased a Trane XV90 furnace in 2001 & I'm upgrading the thermostat to a Wi-Fi model. This thermostat needs a wire labeled C from the furnace & of the 4 wires coming from the furnace none ...
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1answer
160 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 and 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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352 views

Want to add a WIFI thermostat, but no 'C' wire… only 2 wires to each thermo

i have a 5 zone radiant floor, oil fired heating system. Each thermostat only has 2 wires running to it. i have a TACO SR506 switching relay that receives the wires from each thermostat and controls ...
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51 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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180 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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7 views

C wire hook up when c wire is occupied on furnace only, what do I do? [duplicate]

Originally my old thermostat had red white green yellow and a spare blue the new thermo connected the same way but I added blue to the c terminal... On the furnace there are red white green yellow in ...
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1answer
346 views

Need help adding a C wire for a wifi theromstat

With the increasing popularity of wifi thermostats I know this question has been asked many times, however I'll ask it again fr my specific situation. How do I add a "C" wire to my existing air ...
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296 views

C wire for thermostat

I have a boiler system that uses 2 wires to the thermostat. I am going to buy a new thermostat that needs a C wire. There is a spare wire not being used and it is not hooked up to anything. How can I ...
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638 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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752 views

How can I add a C-wire to a Venstar T5800 thermostat?

I am trying to replace my existing thermostat. My old thermostat has: Rh, W (coming from the heating unit). Rc, Y, G (coming from the AC unit). On the Venstar T5800, I can map as follows: Rh+Rc -...
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28 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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72 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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35 views

2 Common wires to 1 Thermostat

I am consolidating 2 separate thermostats, one just for heat and one just for AC, into one to do double duty. If each has its own common wire, is it safe to hook both up to the same terminal on the ...
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4answers
30k 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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115 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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2answers
234 views

Where do I connect a C wire in a Rheem Furnace REAB 1415J? [duplicate]

I'm upgrading the thermostat with a Honeywell wifi one. I found all cables at the thermostat end, but can't figure out where to connect the C wire to the furnace (Rheem REAB 1415J).
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119 views

Yellow wire connected to C terminal, blue wire connected to Y terminal at furnace

I'm trying to install a new smart thermostat the requires a C wire. My old thermostat operated on batteries. The pre-existing wiring system of the old thermostat looked normal (ie. red wire to Rc, ...
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3k views

Can I piggyback a thermostat with a C Wire hookup on a furnace?

I'm trying to install a new thermostat that needs a C wire hookup. There are currently 4 wires running from the thermostat to the furnace, none of them are a c wire. When I look at my furnace itself ...
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95 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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130 views

Swapping out Nest for Ecobee - c-wire issues

We are trying to replace our nest with an Ecobee (with remote sensor capability). We have an existing Nest thermostat which was installed on our boiler/radiator system last year. Back then, we had ...
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483 views

What if the furnace doesn't have a C wire connection

I have a 25+ year old York Forced Air electric furnace, want to hook up the new Honeywell WIFI stat, there is no C wire connection in the Furnace itself, and some instructions have indicated using the ...
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900 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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168 views

Wifi thermostat with c wire

I tried upgrading my existing thermostat to an HP wifi thermostat. The non wifi thermostat has a blue wire connected to the C terminal so I thought I was all set. I swapped the wires over to the wifi ...
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1answer
127 views

I changed my thermostat, not my heat does not work… help!

I know I made a mistake, but here's the storey. When I was hooking up the new thermostat I did it with live wires, and I used metal pliers (metal touching metal). The wires may have also touched ...
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431 views

Honeywell WiFi thermostat newly installed--not working

I just replaced a 2 wire thermostat with a wifi one that required a c wire. There was an unused blue wire which I attached to the C connector on the transformer at the furnace. At the thermostat, the ...
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0answers
14 views

WiFi thermostat won't run. Not sure where to connect C wire [duplicate]

I just replaced a 2 wire thermostat with a wifi one that required a c wire. There was an unused blue wire which I attached to the C connector on the transformer at the furnace. At the thermostat, the ...
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5answers
267k views

How can I add a “C” wire to my thermostat?

So, the spiffy new wi-fi thermostats want power on the C wire from thermostat, but the old thermostat doesn't have a C wire at it. What's the easiest way to add the C wire/24 volt to the thermostat ...
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2answers
315 views

Can I connect the R and C wires directly to the HVAC transformer?

I have a Rheem furnace and a Ecobee3 smart thermostat. I'm having a problem where the Ecobee3 randomly loses power and reboots. I've traced this back to the control board (IFC) inside the HVAC unit. I ...
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168 views

Where to connect the c wire for a gas HW boiler (heat only) w/Wi-Fi Thermostat

I recently purchased a new Thermostat with Wi-Fi capabilites for use in one zone of a two zone baseboard hot water heating system. I pulled the existing thermostat (stat) to look at the wiring and ...
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1answer
366 views

Can't find the circuit board of my HVAC system

I'm trying to install a wireless thermostat for my church building's HVAC system. Currently there is no C wire, so I tried to see where the circuit board was so I could install one. I couldn't find ...
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2answers
1k views

Can I use a WiFi thermostat without a C wire?

I have a very old Honeywell programmable thermostat. I am going to replace it with a White-Rodgers UP500W Emerson Sensi WiFi Thermostat. I just want to know if the WiFi unit will work without a C ...
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1answer
3k 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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214 views

How do i connect a Commmon wire to Heat Controller Inc air handler?

I swapped out a 24v Honeywell Thermostat for a NEST Thermostat. I followed the Manufacturer instructions for install. Everything was working for a week or so. I recieved an error(pictured below) ...
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217 views

Where is the C wire in this tangle of wires?

I have looked and searched, but still iffy where to connect the c wire coming from the thermostat. I appreciate any help.
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37k views

How do I identify the C terminal on my HVAC?

How do I identify the C terminal on an HVAC, furnace, or boiler? If the terminals don't have a screw labeled C what should I look for on the wiring diagram? What steps should I take if a wiring ...
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1answer
116 views

How to hook up C wire to Crown air handler

Sorry for the newbie question, but I would like to hook up a C wire so that I can get a Nest, Ecobee, etc. My current thermostat is just a regular battery operated Honeywell and looking at the ...
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1answer
298 views

Add a c-wire to 25+ year old Rheem furnace

I've looked at just about every post on adding a C-wire to power a WiFi thermostat but remain uncertain on how to proceed. The current installation on a Rheem RGAC-080A is two wire as here: XFMR - ...
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1answer
82 views

How to wire a C-wire from a humidifier to a Carrier furnace?

I need guidance on a wire. One wire connects the humidistat to HUM. Another wire should go from the humidifier to COM24Vac. My COM is taken as you see in the picture. Is it an issue to wire the second ...
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2answers
861 views

No electricity going to thermostat

Our old thermostat is working but I want to change to a new one (nest thermostat). I have two exact HVAC unit at home. I change one it works but when I change the second one it doesn't work because ...
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1answer
952 views

How many wires can be connected to the C terminal at the furnace?

I'd like install a new Wi-Fi thermostat. The old one didn't have a C-wire but luckily there is an unused wire on both sides (one coming out of the wall and one at the furnace not connected yet), so I ...
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1answer
210 views

New Thermostat Ecobee3 to Old Galaxy GG-75 boiler - Needs common wire

I would like to replace my old thermostat with a new Ecobee3 thermostat. HVAC system is a Slantfin Galaxy GG75 boiler - single zone. (No air conditioner.) Current set-up has only a red and white ...
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99 views

Need a furnace/air conditioner C wire, but can't make permanent modifications

I live in a rented apartment, so I can't do any permanent modifications to fix the problem. I have a basic WiFi thermostat that requires a C wire, and the heater/air conditioner is just old enough to ...
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2k 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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1answer
740 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
436 views

G to C trick not working for cooling. Honeywell RTH9080WF

My electric furnace has no c terminal. Furnace is heat & cool both. (Lennox Electric Fan coil Model CBS18-21-3P). I installed Honeywell RTH9080WF. I switched G to C on my thermostat and got the ...
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2answers
2k 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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230 views

No C wire terminal on my electric plenum but there is on my furnace. How do I solve?

I had a brand new furnace installed recently but also now picked up a wifi thermostat. The problem I now encountered while installing the thermostat is that my system also has an electric plenum as ...