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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Do I have to run an extra wire all the way to my new thermostat, or just to the “router”?

I've got an older home (plaster walls) and an older furnace, It's a Peerless. My current thermostat is located on my main floor and only has 2 wires to it, red and white. I should mention that my ...
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515 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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114 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 ...
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1answer
78 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
2k views

How to connect Nest in Europe

I found a question regarding the Nest thermostat here. But that is only a question "Will it work?". Thanks to that I know it WILL, the problem is...HOW. I know the wire naming conventions are ...
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11 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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173 views

Trying to figure out 'C' wire

I'm looking to upgrade my programmable thermostat to a wifi thermostat. I purchased a basic Honeywell wifi thermostat but upon installation could not find a 'C' wire. I started to investigate more ...
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482 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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576 views

Where is my common wire on the unit?

I have 2 systems in my house, my heat is gas hot water radiator heating and A/C is a separate newer system. I'm trying to figure out if I can run a C wire off air handler from the A/C system to run ...
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532 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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534 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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How do I install a WiFi thermostat with a Well-McLain steam boiler?

I have a Weil-Mclain 150000 BTU steam boiler, and have only 2 wires connected to my thermostat, but would like to add a new Honeywell WiFi thermostat. I have no C wire, no A/C, or any other type of ...
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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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231 views

How can I tell if C wire is active and connected to Thermostats?

We are trying to install a smart thermostat (Ecobee 3) so we are in need of a C wire from our boiler. We see a green wire in the wall, but it's not connected to our thermostat currently since it ...
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270 views

Wireless Transformer no C-Wire

I'm trying to install a wireless thermostat but i ran into issue putting in an external thermostat. I have a 2 Stage Cool / 1 Stage heat system. I have the Red 24V from Air Handler connected to R. I ...
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76 views

Trane XV80 missing “C” wire at thermostat

I have a Trane XV80 with a 2 stage motor. I'm replacing a Trane TCONT803AS32DAA thermostat with a Honeywell wifi thermostat thinking it would be an easy switch out. However, the older thermostat had ...
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287 views

Honeywell 6580 thermostat installation

I'm attempting to use Honeywell's G-wire for C-wire substitution instructions. At the thermostat I've got a Y, W, R and G wire, so no problem there. The instructions then say to go to the furnace ...