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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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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21 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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181 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 ...
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31 views

Installing c wire for Ecobee3 Thermostat in a 2 zone configuration

I have hydronic baseboard heating that uses an oil boiler by Weil McClain. It has two zones, one for the first floor and one for the second. I'm unfamiliar with A/C terminology, so I'll describe it ...
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358 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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26 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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21 views

Adding a “C” wire to an A/C unit with no circuit board [duplicate]

I have an old thermostat with only 4 wires and no "C" wire or Comm wire. I started fresh and bought 18/5 wire and wired everything up to my old thermostat still leaving out the C wire.Everything is ...
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181 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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1answer
349 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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298 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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98 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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20 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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649 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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763 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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99 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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194 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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38 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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134 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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187 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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32 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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238 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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86 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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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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48 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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45 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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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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264 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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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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112 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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150 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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499 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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919 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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173 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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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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442 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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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
273k 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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339 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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169 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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397 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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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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2answers
222 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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222 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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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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123 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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320 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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84 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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874 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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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 ...