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 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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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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Can you terminate more than one common on the furnace C terminal?

TL;DR: Can you terminate more than one common on the furnace C terminal? Background: I've linked a diagram of how my non-programmable thermostat is currently hooked up. I want to replace it with a ...
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3answers
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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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1answer
1k views

Can I connect two wires to a single screw terminal in my HVAC system?

I am installing a new honeywell WiFi thermostat but the old battery operated thermostat had batteries and no "C" wire. I look at both the thermostat and the air handler and found an extra un-used blue ...
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1answer
258 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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3answers
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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
157 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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1answer
4k views

Where do I connect my C wire from my thermostat when there are two transformers?

I have a Laars gas boiler that has 3 heating zones. I want to replace the old mercury honeywell tstat on the 1st floor (heat only) with a newer honeywell RTH9580 wifi model. I've taken pictures of my ...
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2answers
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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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2answers
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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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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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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 ...
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1answer
431 views

Where do attach C wire on my boiler?

I need to add a C wire to use my wifi thermostat. I have a 2 wire thermostat now. There is a third wire in the wall which runs to the basement where it is cut off. I can easily extend it to the ...
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1answer
218 views

Is it okay for wires at the C terminal to touch?

I just purchased the Honeywell wifi thermostat but my existing thermostat does not have a C wire. So the instructions given suggest using the G wire as the C wire and at the furnace to move the G ...
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4answers
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How can I use a separate transformer to provide a C wire to my thermostat?

I Purchased the Honeywell WiFi Prog Thermostat to replace my Round Digital Honeywell Thermostat. I have separate Heating and Cooling (Heating is a Weil McLain Boiler for Hot water baseboard and Hot ...