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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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How to fish thermostat cable using an existing cable

So I have an existing thermostat cable on my 2nd floor that only has 4 wires, and I need a 5th wire to use as the common wire. Can I use the existing cable to fish or guide a new 6 wire cable through ...
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Nest E without common wire stuck in purge loop as soon as the Rheem heat furnace ignites

I have a honeywell hz432 and just installed a few NEST thermostats with a common wire and they seem to run fine. I added another NEST E thermostat, but this one didn't have a common wire, and when it ...
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25 views

Heatpump Wiring Color Convention

A heatpump thermostat is to be upgraded. I am unsure of: Which color wire is the Common (thought to be BLUE) Where should the brown wire be connected? (B node?) Is the 'B' node in the photo ...
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How to add a C-wire to Weil McLain Boiler

I am having trouble installing a new wireless thermostat to my existing Weil McLain boiler. My existing control wire utilized an Rh and W on the thermostat, but utilized the Y and G terminals on the ...
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Using a ground wire as a thermostat C

Just theoretically, A C wire is just a return for 24v transformer right? So if you were in a situation where you have access to the slab, or just a standard outlet or switch receptacle could you wire ...
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17 views

Thermostat with an external transformer

I am trying to hook up a Vine TJ-610B-W WiFi thermostat to my heater. The original thermostat had only 2 wires, hooked to R and W, none hiding in the wall. I plan to use an external transformer for ...
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28 views

C wire - thermostat - not providing power

I'm replacing my old Honeywell thermostat with a new Wifi thermostat. My old/current one is battery powered and doesn't use the C wire. I found the C wire behind the plate. I connected the c wire to ...
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51 views

C-wire on Weil-McLain CGA Series 2

I have an old thermostat with only 2 wires connected to my Weil-McLain CGA Series 2 boiler. I would like to replace it with a Nest thermostat. I need to add a C-Wire in order for it to work. Luckily, ...
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32 views

Help adding C wire to furnace - Heat only system

Looking to add C wire into my furnace. Heat is currently running on two wires with older thermostats. Luckily the in wall wiring has three wires. Any help to point out the C wire for my setup? Or ...
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1answer
60 views

Can't get wifi thermostat to work with gas boiler

I bought a Sensi WiFi thermostat and I have been trying to install it. I can't quite figure out how this wiring should work. Here is a link to pictures of the boiler and the current thermostat. The ...
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1answer
33 views

How to connect c-wire to Burnham Series 2 boiler?

I am trying to install my ecobee 3 light but the previous thermostat did not use any c wire. Fortunately, there was an unused wire in the wall and I would like to set it up as a c-wire. The problem is ...
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65 views

Adding C-Wire for Nest to Triple Aquastat Relay L8148J

Need some extra eyes and advice on how to add a common wire to our current boiler setup. We have a Triple Aquastat L8148E that has 2 wire that connect to our thermostat. Like many before me, I'm ...
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30 views

C-Wire identification

I am interested in replacing an older thermostat with a modern networked 'smart' thermostat \ humidity monitor. That being said, does the photographed thermostat have a C-wire? The thermostat ...
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1answer
21 views

how to connect C wire for ecobee lite to Weil-McLain EG Series-5 gas steam boiler

I want to upgrade from a battery powered Honeywell thermostat to an ecobee3 lite. This is for heat only. My current Honeywell has two wires: Red and White. Missing is the C cable. There are enough ...
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28 views

Installing a Honeywell Wifi thermostat and need a C wire connection

My current thermostat is wired as follows and the wires are from my Separate heater and separate AC air handler: W - From my heat R - From my heat Rc - From my AC Handler Y - From the compressor G -...
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Using T-Terminal for C-Terminal on Lennox Furnace — Thermostat will not power

Furnace: Lennox Pulse Thermostat: Honeywell RTH6500WF I have the similar setup as the person in the below link Can I use the T terminal in my furnace as the C for a Wifi Thermostat? When I ...
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how to communicate thermostat to turn on wood heat system when during heat pump defrost cycle

I have physically disconnected the strip heaters in my air handler but the fan still runs which I do not want. I have a wood boiler I want to run in place of the aux heat Can I disconnect the white ...
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22 views

How should I wire my thermostat considering the existing adapter device?

Currently 18-4 is running from furnace to thermostat. I should be able just to use the T terminal to get power but there's some kind of funky adapter on it. Any know what the adapter is for and how ...
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Can I use an available wire as a C-wire by adding it to an unoccupied 24v terminal?

I want to add new thermostats in my home. Luckily, the wiring to my thermostat has an unused wire in the bundle for the required "C-Wire". It has R,G,Y,W, and an unused blue. The thermostat wiring ...
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1answer
34 views

Can't find HVAC C terminal

Could any of these be the c terminal on my hvac unit? There's already a c wire there, just can't figure out where to connect it.
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21 views

How should I connect a new Ecobee4 thermostat?

I'm wanting to hook a Ecobee4 thermostat up. On my furnace I have 5 terminals R with a red wire attached, W with a white wire attached, Y with 2 black wires attached, G with a green wire attached and ...
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Fan Blows Constantly Even With C Wire

Nest stated that my furnace only set up (we have an evap cooler) would work with their Nest E product. I self installed only two wires, R and W. The heating works; however, the fan always blows ...
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1answer
38 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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1answer
51 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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31 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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26 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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22 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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26 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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0answers
25 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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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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1answer
22 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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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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31 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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1answer
31 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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1answer
57 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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1answer
59 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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1answer
55 views

Smart Thermostat without C on Heater

I want to hook up my Honeywell rth9580wf to a natural gas garage heater that only has R, W, and G (not in use). It's currently hooked up to a standard battery powered Honeywell thermostat. I've read ...
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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
279 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
26 views

C wire in wireless Thermostat Transofrmer question

I have a rheem handler with a 46-23115-02 transformer. I do not have a C-wire connection that I can see. I have a 5 wire cable... so I do have a blue wire available that I can connect as the common ...
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1answer
92 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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1answer
265 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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1answer
40 views

“C” wire on thermostat

I have an electric furnace that has power, fan, electric heat, but I do not have a "C" wire. The thermostat wiring has several unused wires that are routed to furnace. It is a matter of applying ...
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1answer
71 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
52 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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1answer
117 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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0answers
92 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
398 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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1answer
183 views

Connecting Wood/Electric Furnace to Smart Thermostat

UPDATE: I spoke with an ecobee tech and they pointed me to an alternative solution. If I get an adapter called a "Fast-Stat Common Maker", all I need to do is figure out where to connect the C-Wire ...