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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154 views

Zone Controller T-Stat Wiring w/ 24V transformer, isolation relays for 2 zones

I have a new Navien SmartZone+ 6-Zone Control for my boiler-based heating system. It is great, except it provides no power on C. I realize the internet has researched the heck out of adding a C ...
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195 views

Can I just extend this blue wire in furnace for Nest thermostat?

First time doing this, don't want to burn house down. The brown bundle is coming from thermostat, the white bundle I suspect is coming from A/C unit. Do I follow these steps? Find a scrap piece of 18 ...
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39 views

How to identify a c wire?

I have 3 wires coming out from the wall at my new house (red, white, blue). It is a water boiler heat only system. The tstat was already removed unfortunately. I thought the blue wire was the C wire, ...
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32 views

Four thermostats and three zone valves

I want to ask HVAC experts how to troubleshoot my floor heating system. I have a Lynx-85a boiler, pumping hot water into three zones, one per room and three zone valves. For some reason, original ...
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83 views

No C terminal on furnace

This old Lennox furnace has no c terminal. I can run a wire directly from the furnace to the thermostat. Question is, which terminal do I hook it to?
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83 views

Thermostat has two wires going into C terminal

Trying to help my dad install his new Nest thermostat - for some reason his thermostat has two wires going into the C terminal. I've searched and cannot find any other question about a setup like ...
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32 views

AC Common wire disappears

On my furnace there is a wire that is labeled C/Common. But at my thermostat there is no common wire. Wires on furnace control panel: W - White Y - Yellow G - Green C - Common Wires at my thermostat: ...
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27 views

What are the functions for the wires beige, green, orange, black, yellow, red; when wiring up a thermostat for One's central A/C

I want to change (digital) thermostat and replace it with (non-digital) non- programmable thermostat. The thermostat I'm wanting to use only has spots for 4 wires. Which leaves 2 remaining wires. ...
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3k 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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1answer
742 views

Taco SR503 switching relay and common wire?

I just bought a home with a three-zone radiant heat system controlled by Nest thermostats. There's only two wires (red and white) going to each thermostat. This works, but is problematic — in the past ...
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75 views

C wire Hooked to Compressor but terminated at Thermostat

I'm looking to install a thermostat that requires a "C" wire. See the picture below of the terminal inside the furnace one of the blue wires appears to go to the cable that goes outside to the ...
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1answer
110 views

nest 2nd gen, no common wire/terminal in the fan coil unit

I'm trying to install an older nest in my condo but it immediately complains that there's no common wire attached (error e24). I opened up the fan coil in my condo and there is no specific terminal ...
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24 views

Need C wire for Nest thermostat with zones. From zone panel to thermostats?

If I have a C wire connected from the furnace to the zone panel (but not to the thermostats), do I only need to run new C wires from the zone panel to the thermostats?
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How do i hook up a new 5 wire cable to an existing 4 wire furnace

I would like to replace the existing thermostat wires on my 2000's era janitorial furnace with the upgraded 5 wire setup that will allow fan control of my Honeywell wifi 9000 thermostat. I was going ...
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65 views

Install Hive Thermostat - C wire power

I am installing a Hive Active thermostat. My previous thermostat was battery operated and did not use a C wire. However, I found a 5th, blue wire, behind the thermostat so I used that as the C wire ...
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294 views

Nest E Thermostat Power

My thermostat is in the basement, connected to a Honeywell R845A to run a circulating pump for heat. I bought an Nest E for the main house and it works great. I want to replace the thermostat in the ...
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144 views

Finding the c-terminal on my furnace

I have a two wire furnace. I’ve been doing a lot of research but haven’t found anything super similar to what I have at my house. I’m trying to find where to connect the c-wire. I have a couple of ...
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thermostat does not accept C wire connected to AC unit

I have three identical thermostats, model Radio Thermostat CT101. Two of them have C wires connected to the burner and work fine. The third one controls a heating zone and also central AC. It has R ...
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60 views

How do I create a C wire? [duplicate]

My furnace is old and only has 4 wire connections, but I require a C wire. How can I create one based on this diagram? I do have 5 wires running between the furnace and thermostat.
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54 views

No 24VAC between R and C on Rudd R801 Control Board

I’m replacing a older programmable thermostat with Ecobee 4 which is to control a Ruud R801 HVAC unit installed last year. I have access at the thermostat to a wire (orange) that goes to the C ...
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2answers
2k views

Test voltage for common on HVAC wiring

I'm working on adding a smart thermostat, and as many older setups have it, my current thermostat doesn't have a 'C' wire. However, if I pull the wires further out from the wall, the feed from my A/C ...
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1answer
2k 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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219 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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1answer
315 views

How to wire isolation relay to create C-wire

I'm trying to follow the ecobee instructions here: https://support.ecobee.com/hc/en-us/articles/227874527-ecobee-thermostat-installation-with-an-isolation-relay Which uses a wiring diagram as such: ...
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1k views

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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152 views

Connect C wire to Furnace

I have a Lenox Merit system in the house. I am trying to install Nest E thermostats and want to do it with a C wire. I noticed an unused Blue Wire (pic attached) at the thermostat location and then in ...
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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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97 views

Do they make 24v transformers that have individual terminals for up to 4 thermostats? [closed]

I have two ecobee 3 thermostats sharing a 24v transformer. I'm about to buy two more. Do they make 24v transformers that have separate pairs of terminals for each thermostat? This is not a question (...
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4answers
4k 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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1answer
250 views

Where is the C wire terminal on my boiler

I am trying to connect the T5+ smart thermostat to my oil based furnace. I see there are only two used wires at the thermostat R And W but 3 additional cores available. My boiler is a old Weil McLain ...
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3answers
2k views

How do I connect a C wire to an Utica PEG112CDE steam boiler?

I want to upgrade my old mercury thermostat that I currently have to a newer programmable thermostat. This little DIY project is for my first late 1920's home that I recently purchased. I currently ...
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1answer
605 views

Is it possible to add a “C” wire to my electric heater?

I bought a Wifi Thermostat but my old thermostat did not have a Common Wire. I can't use any of the swapping the G for the C solutions because I have an electric heater. I was told by Honeywell that I ...
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349 views

Where should I connect my C wire on Goodman GMP075-3 Rev.B

I am trying to install an ecobee4 to replace a digital battery powered thermostat. I removed the existing thermostat and found that there was a spare blue wire that was wrapped around the others that ...
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3answers
5k views

Where's the C terminal on my boiler control?

The new thermostat I purchased doesn't run on batteries, and needs 24 volt power. I have only a red and a white wire connected between the boiler and the thermostat, but there is also a third, unused ...
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3answers
638 views

Wiring for ecobee3 lite

I recently purchased the ecobee3 lite and am now stuck trying to install them. I have 3 Honeywell thermostats I am looking to replace and my heating system is oil/hot water. My home was built in 1970 ...
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2answers
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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1answer
4k views

Where to connect c wire at furnace for Honeywell Wi-FI thermostat

We are installing a new honeywell wifi thermostat have an extra blue wire in our bundle. We aren't sure where to connect the blue wire on the furnace board. Click for larger view
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100 views

Need to identify a common wire on my old HVAC

For Nest Thermostat 3, I am trying to identify a common wire on my old HVAC. The diagram can be found on the link below:
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4k views

Where do I connect a C wire in an old Furnace?

I am hoping to be able to upgrade to a smart thermostat but unfortunately my furnace doesn't have the "easy" option with an obvious C terminal. My furnace is OLD but it actually has the full 5 ...
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67 views

Gas Furnace only, is Y wire needed?

I have force air (propane furnace). I would like to upgrade to a new "smart" thermostat, but they didn't run a C (common) wire which is required for most of these units to run properly. I do however,...
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2answers
756 views

How to add C-wire from Laars mini-term

Folks, I am having problems connecting my Nest thermostat to a boiler. Given Laars Mini-term JVS series with Temperature Control PCB Module (pictured below) When I short Com call and Heat call wires ...
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1answer
149 views

How to identify the C terminal on my HVAC [duplicate]

As you can see I have a Y G W R/2 C/3 D/1 terminals on my HVAC. I am planning to change my thermostat to one that requires a c line. There are 2 spare lines available, however I am not sure which is ...
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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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2answers
173 views

Where do I connect a C-wire on my Honeywell TrueZone board?

My Honeywell TrueZone 311 board does not have a terminal for the blue wire. Where does it go? I plugged in the wires for the dampers and the thermostats, but the five wires from the furnace to the ...
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88 views

Just another C wire problem

Sorry for the long post but if you can make it to the end, I would greatly appreciate it. I have what I believe to be an uncommon situation that I haven't been able to solve. I have a thermostat that ...
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2answers
3k views

Is the transformer bad on my Central Air Conditioner?

Need some help diagnosing a problem. My very old central AC unit isn't working. It's just AC and about a year ago I connected a C-Wire to the air handler when swapping the thermostat for a wifi ...
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1answer
67 views

Nest thermostat to Kelvinator furnace

My old thermostat had 4 wires. I replaced with a Nest thermostat. At times the AC condenser is running but the furnace blower is not running. Found other with this and they said need to add common ...
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39 views

Wifi thermostat

Can I connect C-wire to the empty terminal(red arrow)
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1k views

Where to connect c wire in furnace?

I'm trying to figure out where to connect a C wire from a thermostat to my furnace. I found this old post for the same furnace type (Trane XE70), but it seems as though a red wire goes to the terminal ...
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1k views

Why does my Nest thermostat report error e195 when I try to use the air conditioning?

I had a Nest E installed in February and it has worked really well until it was time to turn on the AC. Now I get random e195 errors (no power detected to the R wire). The most likely answer from my ...