Thermostat C Wire: Everything you need to know about the “common” wire

Back in the olden days, thermostats were simple on/off devices that didn’t need their own continuous power supply.  Modern thermostats with Wi-Fi and backlit display, by contrast, need a steady supply of juice.

The C wire, or “common wire” enables the continuous flow of 24 VAC power to the thermostat.

Technically speaking, power flows from the R (red) wire, but not continuously (not on its own, anyway). To make it continuous requires a common wire to complete the circuit.  When the circuit is complete, 24V energy will flow continuously.

If you’re considering purchasing a smart thermostat, you’re probably thinking of doing the installation yourself.  After all, if you’re able to change a light switch or receptacle, you’re skilled enough to install a smart thermostat – assuming your system already has a C wire.

If your system has a C-wire, it might be in use or just tucked away behind your current thermostat.

If your system doesn’t have a C-wire, you’ll need to run a new cable from your furnace to your thermostat to install most of the modern smart thermostat models.

Thermostat wire is just a big spool of bundled, color-coded wires. The “18” refers to the gauge and the “5” refers to how many individual wires are inside the cable.

Thermostat wire comes in various weights.  If you have heat and AC, you’ll need 18/5.  If you’ve only got heat, 18/3 is sufficient but you may want to run 18/5 anyway for a bit of future-proofing.

Closeup of 18/5 wire. Pull back the plastic sheath to reveal the individual wires. These are what gets wrapped around your thermostat’s terminals.

Identifying Your Thermostat’s Wires

There are two ways to find out what wires your HVAC system has.

Method 1: Look at the wires behind your thermostat

Detach your thermostat from the wall and look at the wires connecting to it. If you’ve got a wire connected to the terminal labeled “C”, you’re (probably) good to go with a smart thermostat installation.

If you don’t see a C-wire here at your thermostat, don’t assume you don’t have one just yet. It might be stuffed into the wall, which is what some installers do when the C-wire is present but not needed.

You can also look inside your furnace (see method 2). If you find a C-wire in your furnace, then the other end is probably stuffed into the wall behind your thermostat.

Method 2: Look at the wires inside your furnace

Disable power to your furnace and pop the cover off. (The ease of doing it this way will vary by furnace and installation.)

Look for a row of screws labeled R, C, W, W2, G, Y/Y2, like so.

Looking inside my 2010 Rheem gas furnace

As you can see in the photo above, this furnace does not have a C-wire hooked up. To make this HVAC system compatible with a smart thermostat, it would need either:

  • a new bunch of wires run between the furnace and the thermostat, or
  • the Venstar Add-a-Wire, or
  • a smart thermostat like the ecobee3 which comes with a Power Extender Kit for systems without a C-wire

If the thought of poking around in your furnace or thermostat makes you uneasy, consider hiring a professional for the installation of your smart thermostat. 

There are no standards for wire color! Any wire can be used for any purpose.  A previous owner or handyman may have been “creative”, so what you find behind your thermostat might vary from what you read about online or here. These photos are from my own home, but every home is a unique snowflake.

Here are common uses for wire colors:

  • Blue or BlackC – Common wire, may be unused by your existing thermostat. Enables continuous power flow from the Red wire.
  • Red – R – 24VAC power from the furnace’s transformer
  • Red – Rc – 24VAC (dedicated to heat call)
  • Red – Rc – 24VAC (dedicated to cooling call)
  • Green – G – Fan
  • White – W – Heat
  • Yellow – Y – Air conditioner

Do I Really Need a C Wire?

Yes, you should install a C-wire if you are upgrading your thermostat.

Nearly all modern thermostats, smart or not, require a power source, and that doesn’t look likely to change any time soon.

Nest Owners Not Exempt

The popular Nest thermostat claims to work without a C-wire, there are some caveats. Without a C-wire, the Nest gets its power from your heating or cooling system… assuming it’s running. When it’s not running, the Nest still needs to get power. The Nest will “pulse” the heat wire, turning on the furnace to pull a bit of power to keep itself going.

In some systems, this is unnoticeable, but in others, the furnace responds as if being told to turn on and then immediately off.

Read one frustrated Nest owner’s review of his experience with the “pulse” problem here.

Nest’s manufacturer updated its literature to warn that the Nest may be incompatible with some single cycle, no-C-wire systems, but the reality remains that outdated wiring is going to become more and more of a pain in the butt to deal with.

Future Proofing

Most states have outlawed the old fashioned “mercury blob” thermostat. Even the most no-frills thermostats on the market today still need a power source. Adding a C-wire, either through new wiring or an adapter, ensures compatibility with new thermostat technology.

Don’t do the “fan wire” trick

It is true that you can re-purpose the fan wire as a power wire, but then you (and future homeowners who live in your home) won’t be able to manually turn the fan on. If you’re going to spend a couple hundred bucks on a thermostat, spend a little more for an adapter or a technician to do the job right. 

What to do if you lack a C-wire

  • Hire a pro to run new wires between your furnace and your thermostat (or do it yourself)
  • Grab a Venstar Add-a-Wire, which adds a 5th wire to your 4-wire setup
  • Get a smart thermostat that’s designed to work with systems lacking a C-wire, like the ecobee3 (it has an adapter for C-wireless systems) or the Emerson Sensi (in many systems, it doesn’t need a C-wire at all)
This inexpensive adapter by Venstar can add the wire you lack.

Further Reading

193 replies on “Thermostat C Wire: Everything you need to know about the “common” wire”

That is a great question! I don’t think it’s very common. But HOW uncommon is an excellent question (one I was all too happy to spend 30 minutes researching)…

I can’t find a single survey of wiring configurations in U.S. homes (letalone worldwide), but we can probably make some educated guesses based on what we know about thermostats and housing in the U.S.

Programmable thermostats exist in less than 40% of U.S. homes, that much is well known. Homes without would have no reason to run a C-wire, so we’re already down to 40% of the U.S.’s housing stock having any plausible reason to run this wire. (This diagram summarizes a thermostat survey’s findings:

Now, we can also take into account that “setback thermostats” (or just “digital thermostats”) weren’t really a thing until California decided new construction should have them in the 1970s, amid the oil crisis. Still, it took the rest of the country another two decades to catch up. Programmable thermostats weren’t the law of the land until 1995.

So then I wonder, how many homes are built after 1995? Probably not many, indeed, it looks like about 33% of the homes still standing in the U.S. today were built in the 1980s, 1990s, and early 2000s. More specifically, 16% were built in the 1990s and later (“later” being capped at 2005, the study I’m referencing is getting old:

So 16% of homes built after the federal mandate to use a programmable, and only 40% have a programmable (and even programmables don’t require C-wires)…. that’s not a whole lot.

Anecdotally, my 1977 home on the west coast has a programmable thermostat but no C-wire. My sister’s 1983 home in the Midwest is also C-wire-less. Nest forums and hvac forums seem to be full of people reporting a lack of a C-wire in their home.

With ALL THAT in mind, I’m guessing the C-wire is in less than 20% of U.S. homes. Heck, even 20% sounds optimistic. I don’t know if I’d quote me on this, this is just speculation, Googling, and some loose estimates. I’ll continue to think about your question, though, and I’ll come back and give an update if I find an answer!

Saying that the C wire is the power source, or supplies the power is misleading.

The C wire is simply the “common” side of the transformer 24V output (there are two wires coming out of the 24V transformer, “hot” and “common”). Just tap into the “common”.

The R (red) wire is really the power source.

The C wire closes the circuit and allows continuous flow of 24V energy to power the thermostat.

Hi Rich,

Thank you for your careful explanation and thoughtful comment. I write this site for a general “big box DIY” audience, so I try to keep it from getting too technical, but I love your explanation so much I think I’ll do an edit to clarify the purpose of the C-wire.

Hi M.B.,

I’m actually a DIY consumer. I do a lot of my home improvement because I enjoy researching, understanding and saving money. I installed several Nest and Honeywell RTH9580. I also have an ecobee (in San Diego, SDG&E installs them for free). I totally agree with your reviews of the 3. I have not tried the Lyric yet.

When I first started this, air-conditioning was a little foreign to me. In my condo, the AC is one of those that is in the ceiling and there is no panel with “C” clearly marked. I had to research and figure it out.

I found a lot of info out on the internet lacking and confusing to DIYs, but some sites like yours are very useful. Thanks.

This is exactly what I was looking for. I keep reading that the C-wire provides power for the thermostat but my electrical background knows common as ground (I know, big discussion of ground vs common). This explanation validates why my C-wire is connected to the grounded side of the 24 volt transformer. Thanks for the explanation.

I need the C wire for a thermostat that communicates with a wireless router. My old oil furnace and thermostat only have two wires between them, R and W. Since there is no existing transformer, I will need to rig one up (probably in a j box near the thermostat, spliced off a nearby outlet or something) So one of the 24v wires will go to the C terminal on the thermostat, but then the other to which terminal? R? Thx.

Did you get a solution for this? I am also confused at this point. I hooked up an external power transformer.

I can hook one wire of the 24VAC to C and the other wire to ?

More detailed explanation
OLD Thermostat: G – G wire, W – W wire, Y – Y Wire, Rc + Rh (R wire with jumper between Rc and Rh)

New Thermostat : needs a C wire.

Due to old heater panel, I could not find the C wire to hook up. So, hooked up external transformer and connected to C and Rc. Removed jumper between Rc and Rh as suggested by Bill Lee below. Heater works. A/c does not work.

Please help Bill Lee.

Your Rh and W terminals control your heat. The C and Rc terminals are what powers the thermostat. The remaining terminals should (I am not an a/c technician so I am using deductive reasoning here) control the a/c. I would start by removing the Rh and W wires and see if the a/c works. Be sure all remaining connections are tight. As long as you connected the correct wires to the corresponding terminals it should work. Is there a heat / cool switch option on the thermostat that you are forgetting to switch over for the a/c to turn on?

I am having a similar issue. I only have 2 wires R and W. I plugged these wires into the R and W terminals. I used an external transformer and ran those wires to C and RC. My furnace has a heat and cool switch. The heater works fine. When I switch the furnace switch to cool it runs continuously Even if the thermostat is on cool, there is no response. The old thermostat looks like it probably either completes a circuit or grounds it. Two pieces of metal touch, and it stops. I have tried to plug the W wire into every available terminal to see if it would work. Nothing worked. Now I’m confused. Clearly, the “C” wire issue is not my issue. So now what? I don’t understand why the new thermostat will not complete or grounds out the circuit like the old one. Should I remove the switch from the furnace?Thanks.

Bill lee is incorrect. Rh stands for Red-Heat and Rc stands for Red-Cool. If you only have a single red wire the jumper between Rh and Rc are required for some heating and cooling hybrid systems depending on your thermostat.

Yellow/Y is the AC compressor.
White/W is the Heating element
Green/G runs the fan.
Blue or Black/C should be a ground and will be used by both Rh and Rc, and apparently your thermostat.

Sounds like the transformer is not supplying power to Rc Vamsee

Nathan, if you only have red and white wires, your system is only set up for heat. The cool switch on the furnace is manual for a reason, and it is for running the fan when the furnace is off.

If you want to run the cooling fan on the furnace using your thermostat you can run a wire to bypass the switch, and connect it to Yellow/Y or Green/G depending on your needs.

Hello guys. Great info here. I have a question, I want to install the ecobee4 and I have just 2 cables from my old mercury thermostat. For what I understand, please correct me if wrong,
Rh has 24V and close the circuit with the W correct? Then the thermostat act like a dimmer?
Then C will power my thermostat with 24v too, correct?
Then, can I jump or Feed my C connector from my Rh cable?
Is this possible?

Great site and good questions!
as a Electrician and HVAC/R technician from north of the border I see a lot of US products up here so this site caught my interest.
There was a question about older equipment not having a “C” terminal only R and W, if your planing on using a thermostat that requires a C and a second transformer as a power source to get your “C” (common wire) then please be careful, if you mix phases on you 120 volt power source (Phase A one and Phase B second) between the heating appliance and the new thermostat you will find a lot of factory smoke leaving the transformer(s),
Instead power your thermostat with the second transformer and use that power to switch a relay coil, then use the dry normally open contacts of the relay to make the circuit between R and W of the heating appliance to isolate the two power sources yes it costs a relay but that’s much cheaper then a mistake and finding you now have equipment damage..
Better yet you can buy a thermostat that uses battery to switch the contacts and thus no second transformer the only downfall is the battery changes every couple years on that.
Good luck to the DIY folks just remember not all appliances or thermostats work the same so take time to read the product literature.

If my boiler’s 24v transformer is powered by a given circuit, I could wire an extra 24 VAC transformer to that same circuit and not worry about phase, right?
and, what’s “factory smoke”?

“Factory Smoke” is the special smoke that is installed during the manufacturing process, if you let the smoke out-it will not work. The device must be sent back to the factory to have the smoke put back in:)

Small clarification. The power coming into your house has two phases and a neutral. From phase A to phase B is 240 vac. This is used to power your electric oven, electric dryer, etc. Phase A to neutral is 120 vac. Phase B to neutral is 120 vac. His caution is if the circuit at the boiler is A/neutral and the thermostat is B/neutral, it can cause problems. (What he calls Factory Smoke is normally referred to as Blue Smoke in these parts. If you fry a circuit, the blue smoke escapes.)

Sorry Dave but residential power here in Canada is single phase. I have a hunch that this is true through out North America.
Power comes from a single phase transformer with a center tapped ground and is referred to as the neutral. The power is supplied over two legs. Legs A & B if you will. The voltage between A & B is nominally 220 volts or 120volts to the neutral.

I have a Honeywell my wire colors are Blue, Red, Green, Brown and Black. I see none of these with those colors, My connectors are as follows from right to left B-O-G-Y- Aux-Not Used- R- RC that are joined with the bridge. Can anyone please tell me where I need to connect my wires on this chart? Any help wold be greatly appreciated

Thank you!! It IS confusing when everything I read talks about the C wire providing power to the thermostat. It doesn’t! The C wire simply completes the circuit which DOES provide the power. Very helpful information!

Outstanding comment. This was the EXACT technical information I was searching for regarding the real meaning of the “common” wire. Thank you!

On a wire schematic, yes the 24v red wire is hot.

But remeber that its power source is alternating current.

So hot & common are swapping 60 times every second when power is present & the circuit is closed.

But yes, I know what you are saying. & you are correct. Thanx for setting the record straight.

This is not correct. 110V / 120V is AC, the 24V is DC. It is connectted to a trasformer which converts the the AC to a lower voltage DC, just like the numerous “wall wart” power adapters for device chargers, cables boxes, TVs, etc.

I’ve a drawer full of “wall warts ” .. most output somewhere between 5 and 40 volts DC (rectified AC. ) Many others output AC. All depends on the appliance to be powered.

BUT, you are correct in saying that “transformers” do not convert AC to DC. Many “wall warts” include a rectifier so they output DC..the transformers considered for heating systems are normally 24volts AC. SO, you normally could not just power up your 24VAC thermostat with a typical 24VDC wall wart. you would need to get a 24VAC transformer. (but you knew that… I just wanted to clear it up for those who did not)

You are 100% correct. There is no conversion from AC to DC. The voltage is stepped down from 120 to 24 volts. Just like the transformer on the pole outside your house steps down from 7,620 volts to 240 volts for your house panel.

Not on my Rheem furnace. 24 volts ALTERNATING is fed to the C and R terminals on my Honeywell Lyric thermostat.

YOU ARE WRONG. a 40VA 24v transformer supplies 24 volt AC power, which is being converted from the initial 120V house supply.

Thanks for this info, adding my comment because Google brought me here and this might help others. I found a blue wire hanging out behind my old dumb thermostat and I suspected it was the infamous “C” wire, and it turns out it is. It’s like the thermostat mfg’s are scared to go into any detail and most general info says the common wire supplies the thermostat with power. So I got my multimeter out and touched the red lead to the blue wire and black lead to ground and got 0 vac, so I was really bummed out. Then my Google foo kicked in and brought me here! So I tested again, this time touching the red lead of my multimeter to the red wire and the black lead to the blue wire and what do you know, I got 24vac!


John, there is no polarity in measuring AC voltages. Most likely you didn’t have a good connection the first time you tested. Or perhaps you accidentally had the meter in the DC range. Swapping leads when measuring AC voltages won’t make a difference.

Read John’s post again. He said that he, “touched the red lead to the blue wire and black lead to GROUND.” The 24 vac output of the step-down transformer in a furnace is not tied to “ground”, so it is considered to be “floating”. The only way you can measure a 24 vac output is across the two secondary terminals. This is different from something like a wall outlet, where the neutral and ground are fastened together at the “service equipment”, and both are connected to ground. Household wiring is not floating, but rather, is considered to be earth referenced.

My furnace bonds all common leads to the furnace frame assembly. Look at your transformer you will see on many that one terminal is labeled “load” and the other is labeled “C” for common on the 24vac side of the transformer. Your “C” wire only provides a path to complete the circuit back to the power supply transformer which may terminate on the frame,transformer or a dedicated terminator on the control board or wired to but mounted separate from control board

I know this thread is old, but important since people (including me) still visit this site.
Many people have that misconception about electricity. Actually the “C” wire is just as important as the “hot” wire. The neutral on your home wiring will kill you faster than the hot wire if you find yourself in the circuit.
Only saying this so DIY folks always put safety first. Turn off power before attempting any wiring project including “low voltage”. In the right conditions–heat, humidity, the amount you sweat–6 milliamps will kill you.

There is no way 24 volts will kill you. Why scare DIY s? A neutral wire will not kill you either. “In the circuit” by definitions means you have to be touching a hot and neutral (or ground) at the same time.
Yes, 6 milliamp at 10,000 volt can give you a thrill. I have experienced that on an old picture tube type TV. I am still here.

Old TV technician.

As a qualified electrician, I heartily agree. I have had many more SURPRISES from the neutral wires. The voltage that’s connected to a load allows current to flow through the device and that voltage is present on the neutral. We don’t normally expect to have voltage on a neutral and become complacent while working on the circuitry.

The voltage applied to the circuit forces current through the device, and the entire supplied voltage will appear across all device(s) in the circuit that are in parallel. As long as the neutral is tied to ground and is intact there will be no significant voltage on it. If, however, a break appears in the neutral before the point where it is tied to ground, that entire applied voltage will now appear across the break. If you bridge that break with your body, you’ll get shocked, the severity depending on your impedance vs the impedance of the device you’re now in series with (voltage divider).

No! In a typical four-wire system the 24vac is supplied by the R wire. Inside the thermostat are three switches/relays. If the thermostat says heat is required, it allows the 24vac to go back on the W wire. If cooling is needed, 24vac is sent back on the Y wire. If the fan should be running, 24vac is sent back on the G wire. If no heat, cooling, or fan are required, there is no power flowing through the thermostat. If you have a five-wire cable, the C wire (of whatever color) is a usually a common that allows 24vac to be flowing (therefore used) by most smart thermostats.

There are two white wires attached to the common side of the transformer and no space to connect the “C” wire. Can I splice into 1 of the white wires? (Nothing to do with white t-stat wire which connects to a brown wire at unit)

As it’s 24 AC from a transformer, power “from” is not completely wrong or right.
But as common is using the same side of transformer as the relay sinks the powers, you could consider it ground and is not power “from” then.

Agreed! Some “C Terminal for Thermostat with WIFI” sites are SOOO confusing. It’s really very simple. My Goodman furnace has a control board with no C terminal. All I did was connect the unused blue wire from the thermostat control wire bundle to the blue bundle at the 24V transformer (be aware, there is another similar looking wire bundle next to the thermostat control wire bundle, but only 2 wires are used, black and white. This wire bundle goes to the outside compressor to control it). After hooking up the new thermostat, connecting the same blue wire (now at the thermostat end) to the C connector of the thermostat. Works perfectly.


So does the common (c) wire from the transformer go to the C terminal in Nest thermostat? And does the hot (r) wire from the 24v transformer go to the R terminal in the nest thermostat? My gas stove only Ned’s 2 wires to turn it on when the thermostat allows it. Those 2 wires are white and blue.
What connections would be made at the Nest?

Hi, have you ever encountered old thermostats functioning with a 1 single wire? I’m looking to replace my old thermostats with programmable ones and i’m facing the issue of having only 1 wire! Any ideas?

Hey there, Dan. Sorry for my slow reply, your comment got buried as spam and I just found it. 🙁

Have you tried the compatibility checkers on the manufacturer websites?
Here’s Ecobee’s:
And Nest’s:

I tried both with putting in just an R wire (which is what I’m guessing you have) and they both ended with a request for me to email in a photo. I suspect you’ll need to run an additional wire, but I’m not sure. Do you just have a heating system? Most WiFi thermostats need a C-wire, so at the bare minimum I think you’d need to add a C-wire.

If you do resolve this, either through adding more wires or choosing a particular thermostat that works with your 1-wire setup, come back and let us know how it goes! Good luck.


My house was built in 1942 and still has the original furnace. It’s an old octopus, gravity style furnace that once burned coal and was converted over to oil sometime in the 60’s or 70’s. I’m very tech savvy and really want to install a wifi thermostat but my furnace is so old I don’t know how to install a C-wire. As far as I know there is no control panel. Pic’s can be found here: Do you have any advice on how I can go about installing a C-wire?

Hello there, Aaron!

That’s an lovely old beast of a furnace you’ve got there! I can’t say I’ve seen many of these – some are as old as the late 1800s and even the youngest ones are nearly 60 years old. I’m sorry, but yours is beyond my expertise when it comes to adding a C-wire. I have a few things you might try (sorry if you’ve tried these already):

1. Online compatibility checkers:
Ecobee3 compatibility checker
Nest compatibility checker

2. Try calling the manufacturer of whichever WiFi thermostat you’re interested in. They all like to claim widespread compatibility (90-95%!) and they’ve been good at answering my questions in the past.

3. Here’s a thread I found from someone trying to hook up a Nest to a gravity furnace and some of the things he ran into:

4. Try a pro – there’s probably an HVAC company in your area that has seen this before that could at least advise you over the phone (“nope, not compatible” or “yeah, but you need X Y and Z…”).

Of course, there are plenty of thermostats that don’t require a C-wire that are compatible with gravity furnaces (which is probably what you’re replacing…), but I don’t know how to advise you on installing a C-wire into a system without a control panel to work with. If you have typical wires behind your thermostat, those wires must go somewhere. I just couldn’t tell you where, or how they interface with your furnace.

Sorry to be of such limited help. If you do figure it out – or find out that your system is simply incompatible with C-wire needing WiFi thermostats – do come back and give us an update!

Good luck!

Thanks for the quick response. I think your suggestions are a good starting points. I recently got married and we purchased our first house this year. We, eventually, plan on replacing the furnace but it’s not financially feasible right now. I attempted to hire an hvac company to come clean the furnace and the guy took one look on the furnace and was completely baffled. He had never seen one before and had no idea how to clean it. We have a more knowledgeable tech coming in a couple weeks. I’ll ask him if a c-wire can be added. I’ll update you with any results.

Thanks again

As promised, here’s an update on my furnace. I never actually pulled the cover off my thermostat to check whether it had a c-wire or not. I just assumed it didn’t based on the fact the furnace is over 70yrs old. To my pleasant surprise, the hvac tech looked the furnace over and said it already has a c-wire. Now I just need to figure out which wifi thermstat to get. I wish they weren’t so expensive.

My thermostat has 3 wires red white and green am I missing wires? The only way I can use furnace is to combine red wire with white help!!!!!!

Sorry about the slow response, sounds like you have a simple heating thermostat with a fan function,
for ease of future conversations use the marked letter designations rather then colors as not everyone uses standard colors.
i am assuming its a mechanical stat not electronic but both can work similar in operation.
the R terminal on the thermostat usually connects to a red wire from the furnace which is the 24 volt power wire from the furnace transformer marked R on the furnace board.
the W terminal (often the white wire) will be the thermostats output for a heating call to the furnace sending the 24 volt power back to the furnace to its W terminal. this allows a automatic control of the furnace to turn the fan on when the heat exchanger is warm enough.
the third terminal should be G (often the green wire) which is the Fan output from the thermostat and connects to the G terminal of the furnace to control the fan directly and bypass the furnace automatic fan function.
it is very rare to find a three wire thermostat with a C or common connection so i feel confident enough that you have a simple heat only thermostat with fan function.

if you are planning on using a electronic thermostat with these functions it will require a battery powered thermostat to allow relays to turn on and off for heat and fan, otherwise it will have to be a energy scavenging thermostat which gets its power by pulsing the heat function on when it needs to regenerate its power,
the only other option is to pull more wires from the thermostat to furnace if you want to use a powered thermostat. (meaning it uses R and C to complete a 24 volt power circuit)

Similar situation here, except my green wire is disconnected at the thermostat & appears to be floating on the terminal block in the old furnace to which it’s connected. I suspect I can simply move the free green wire over to the common side of the 24V transformer to make it a C-wire with heat only function for an ecobee? The furnace is a circa 1970 chrysler airtemp with a 4-terminal block labeled “general control”. The existing thermostat is a 2-wire setup, “R” is connected to one of the 2 “THERMO” terminals on the “general control” block, “W” is connected to 1 side of the 24V transformer, one terminal of the 24V terminals on the “general control” block is connected to the other side of the 24V transformer, and the spare thermostat green wire is connected to the other “THEMO” terminal (which is jumpered to the non-connected “24V” terminal) on the “general control” block. The more I look at this setup, the more I suspect the “R” & “W” wires are also swapped; which a dumb thermostat wouldn’t care but an ecobee might?

I have a boiler with two wires, R and C coming from the transformer inside the boiler. they run to the valves and tie into the thermostat wires. I ran a new wire with 5 wires. Just seeing if and how I can wire the thermostat so it functions. I need a constant 24v circut and one to switch the boiler on an off….

I am installing a Honeywell Model # RTH8580WF WiFi thermostat for someone who only has two wires running from R and W on their existing thermostat to TT on the triple aquastat on their boiler. There is nowhere to attach a C wire to on the boiler side. Any suggestions on how to proceed? Could I install a 120v to 24v transformer and if so how would it be wired? I know the low voltage transformers I’ve seen have a R and C terminal. Any help would be appreciated.

Like Bill, I have the very same situation. And it seems there is no answer anywhere online. I have managed to get the thermostat to power on, since there is 24vac existing in the 2 wires at the thermostat. I jumped the W wire to the C terminal on the new TS. I programmed my new thermostat but as soon as the boiler fires up, the TS shuts off and power resets. So, close but no cigar.
I too have looked for this infamous “C” wire on the boiler end, but there just isn’t one.

Well, I installed a 120v to 24v transformer which has a R and a C terminal an ran an additional thermostat wire from the transformer to Rc and C on the thermostat. I removed the jumper between Rc and Rh and the transformer powers the thermostat without problems. I then wired TT on the aquastat on the boiler to Rh and W on the thermostat. Unfortunately the thermostat is not sending a heat call to the aquastat when the heat is turned up. I assume the thermostat may be bad. Ad the package appeared to be opened already when we first opened it. I advised my friend to return it and we will try a new one tomorrow. Wish me luck.

Success! I spent about $20 on a 24v transformer, 4×4 electric box to mount it to, 2 – 3/8″ stress reliefs and about 8″ of BX wire. I wired the transformer off the boiler’s maintenance switch and ran an additional 2 wire thermostat wire from R and C on the transformer to Rc and C on the thermostat. Making sure to remove the jumper between Rc and Rh I then wired Rh and W to the remaining two wires running to TT on the aquastat. The thermostat is now working perfectly.

Thanks to Bill Lee post. I have successfully installed my new Honeywell Wi-Fi Smart Thermostat, RTH9580WF.

I replaced an older Honeywell model that only used 2 wires (R & W, no C wire) and was battery operated. I replaced the old wires with new 18 gauge 6 wire (overkill but for the future), using the old wires to pull the new wire through the walls (please don’t try if you are not sure what you are doing). My first attempt I connected the R & W and also connected a third wire to the C pole on the Transformer that is connected to the boiler to the C position on the Thermostat. The Thermostat powered up but a few days later when the house temperature dropped below the desired house temperature and the demand for heat was initiated the thermostat would recycle. I googled for a solution and came across Bill Lee post. I then connected a fourth wire to the R side of the transformer and connected that to the Rc on the thermostat. Removed the jumper in the thermostat between the Rh and Rc. Below is the finished wiring:
R (from transformer) —–> Rc (on Thermostat)
C (from transformer) —–> C (on Thermostat)
R (the old R from previous connection – Thermostat) —-> Rh
W (the old W from previous connection – Thermostat) –> W
The thermostat works great now and no longer recycles and the house is maintaining the desire temperature(s).

Fixed! I figured it out!
Not sure if anyone even cares anymore. But if you do, let me know. I will post the solution.

Congrats, Jared. Glad you figured it out. I wish more people were willing to offer solutions. I pulled out a lot of hair trying to work this one out. Now I know and will be able to implement it in my business in the future.

What if you lose current to the thermostat while installing it? And the breaker did not trip? I replaced an old one with a digital. The display came up and I was able to use the screen and the fan worked. But the heater did not. So I removed it to double check the wiring. When I put it back on, I got no current at all to the stat. I even tried putting the old stat back on. And it no longer works either.

did you ever find a fix for this? I had the same issue today.. but no i have no power to the AC nor the thermostat.

Because the thermostat draws power from the furnace I would ensure that the furnace itself has power. Look for 1) a switch box in the power line going into the furnace 2) a black toggle switch on the service/control panel of the furnace itself and/or 3) a 3 or 5 amp fuse on the furnace’s circuit board

On my Rheem furnace I had a similar problem. I found that there was a inline fuse holder on the transformer output. The 2 amp fuse was inadvertently blown because I failed to shut off the power supply switch to the furnace before removing the old thermostat.

I got a nest as a housewarming present. Worked great for 5 days without the blue common wire installed (directions said to use the same wires as the old therm.) woke up one morning and it was 82* in the house set to 67 and still running. Wouldn’t turn off even at the nest. Had to use the breaker to shut it down… Called nest support and he said to hooked up the common… No luck. Said it must of been a bad nest. Got a new one… Worked for about 5 hrs then the same thing happened… Please help!

I’m upgrading from a Honeywell Mercury thermostat to a Honeywell wifi 8580. My old thermostat only used 4 wires, the white,green, yellow and red. My old thermostat didn’t require continuous power so the blue,C wire wasn’t connected. However, their was a blue, C wire in the wall, so I figured was all set. I followed all the instructions and connected all wires correctly. Once I powered on my furnace (Ruud Silhouette II) my new wifi thermostat doesn’t turn on. Any ideas as to why I’m not having any luck?

I have the same setup. I was upgrading from an older Honeywell thermostate to a wi-fi enabled thermostat. Only needed 4 wires in the old model. And there is a spare (blue) wire not used.
I know I need to connect the blue wire to a C terminal at the furnace side. But I am hesitant because there are already 2 other wires (both black) connected on the C terminal at the furnace. One wire leads to the AC unit, which I suspect powers the central AC. The other black wire I’ve yet to figure out where it leads to. Is it okay to connect a third wire to power the thermostat?

I have the same setup. I was upgrading to a wifi Honeywell .

did you solved the problem ? I am also stuck there?

I had the exact same thing – one wire on the C post seemed to run to something on the AC side – maybe the condenser outside? The other wire ran to a whole house humidifier that was attached on to the return line of the air system. I added the blue as the c – 3 wires on one post, and so far, so good. Just give a gentle pull after screwing them down to make sure they won’t slip out.

Josh 1 wire provide power to AC, if one more wire is connected to C you need to check with the manufacturer because if we connect 3 wire on same terminal we will not get proper voltage and either one of them will not work

Check for the blue wire if it is connected to C terminal on furnace if it check for the voltage between R and C requires 20-30Vac

I currently have a circa 1976 simple mercury bulb thermostat (2 wire) and would like to upgrade to a programable one, the rub is that it is a high rise condo with a centralized boiler feeding hot water baseboard heating to the units. I was looking at the Nest, but will it work with this sort of setup?

You NEED a third or possibly fourth (depending on your boiler/furnace) wire. Being in a high rise you would likely have trouble running new wires. Unless you are able to run the additional, necessary wire(s) then you will be unable to install the Nest.

Yes, you can make it work. Technically, it is possible to run the Nest as is, but you’d be better off buying a $6 24 volt DC adapter on Amazon and plugging it in to a A/C outlet in your apartment. You do NOT need access to the furnace to run the extra power supply, it just looks nicer. If you’re still interested, I’d be happy to provide photos and detailed instructions.

I would appreciate photos and instructions. I have oil heat with R and W wires running to current thermostat and want to change to a Honeywell 9000 WiFi stat. I plan to purchase a 120v to 24v AC adapter from Amazon and running additional wire to my new stat. You mention a 24v DC adapter??? Am I wrong in believing this should be 24v AC?

I am trying to wireup a nest thermostat to an old furnace without a common . I only have a white and red wire going to my furnace. I ran new wires though not sure where I should hook up my white, common wire ? Can I put the red and white wire in the same place ? My furnace only has those two spots for wires that I can see. Above the red wire it says pg. Above the white wire says th. Sorrta lost.

You need three wires. If your furnace or boiler only has two terminals, see my solution above. You will need to purchase a 24 volt transformer and wire it to a 120 volt line somewhere near the boiler/furnace. Run an additional two wires from the transformer to the thermostat. The original red an white wires should be connected to R and W on the thermostat. The two new wires should be connected to Rc and C on the thermostat and to R and C on the transformer at the other end.

Be careful if you tap in the furnace control panel to get the C … I say smoke coming ouit of a board… My fault… I connected the C and a 24 volt line to the Nest in addition to W1 and RH… The old thermostat is back on…

Hi Bill – thanks for your posts. I have a Nest and 3 wire bundle. The zone relay on the furnace end uses only the R and W wires, but the relay circuit board has a power tap with a 24v terminal and a com terminal. Any chance I can wire this successfully without using a fourth wire to connect to Rc as you have described? That would mean pulling new cable which I’d like to avoid. Thanks!

I said that wrong I mean should I hook my blue wire to the red wire for my common so the circuit is complete or does it need its own place to hook up?

No. Tying the red and blue wires would only power the thermostat when there was a call for heat. The C terminal needs to be connected to a C terminal at the other end. If your boiler/furnace does not have a C terminal then you need to install a 24 volt transformer and wire it as the way I said earlier.

I’m installing an Emerson Sensi thermostat. The Taco zone relay for my boiler has both a terminal marked “com” and immediately adjacent to it is a terminal marked “24v.” Can you tell me what the difference is between these two terminals? To me they sound like they serve the same purpose. Thanks!

Mark I have the same question (also a Taco zone relay). The 24v power tap has 2 terminals in one block, one labeled 24v and one com. Did you get an answer to your question? I assume the 24v is positive, the com negative, but since there is only one C terminal on my thermostat I’m confused about which of these to connect to.

My system has an “extra” blue wire. I think it might be a “c” wire, but not sure. Is there any way for me to test this prior to buying a wifi thermostat?

Have a nest with a three wire system and ofcourse it is pulsing when it first starts to run, I originally had a (add a wire) but it dropped the voltage just enough that the nest would not recognize the power as 24v. I have a 110 plug in the wall just below my nest, can I just wire in a 24v transformer from it to give my nest the 24v common its looking for. Stringing a new furnace wire is not an option.

My old thermostat has 8 wires, all hooked up. New one doesn’t have instructions for that. What do I do?

I have a new home with a Payne forced air furnace. I replaced the cheap thermostat with a Honeywell th9320wf5003. All wires were hooked up on the thermostat to their corresponding colors including the Blue Common wire. The thermostat looks like at times it’s not getting enough power because it reboots at random times. At least when I explained it to Honeywell that is their thought. When I look at how the furnace is wired I have the following wires on the following terminals.
G Terminal – Green Wire
C Terminal – Blue Wire and Two White Wires
W Terminal – White Wire
Y Terminal – Yellow Wire and Red Wire
R Terminal – Red Wire

I recently bought a nest to replace my Honeywell thermostat, but when I take the thermostat off, I just saw a Red “R” wire and blue “C” wire. what can I do?

You need to buy a 24 volt power supply to power the thermostat. Connect the + and – wires from the power supply to your new thermostat.

[…] Modern thermostats come with the function to switch the fan mode, that is, you can either keep it on ‘on’ or on ‘auto’ apart from other settings. If the HVAC is cooling unevenly, then it would be a good idea to switch the fan to ‘on’. The reason behind this is that on ‘auto’ the fan is working only as long as the main unit is, whereas in the ‘on’ mode, it runs continuously. When the house is big, all you need is circulation. So if you the fan keeps on running, then it will keep circulating the air to maintain an even temperature. […]

I tried this “fan on” continually and it doubled my electric bill. Went back to “auto” and saved money.

I have a 5 wire system/cable but the 5th wire is wrapped up and not being used on either end (thermostat or blower/furnace. Can I hook up the spare wire to the C on the panel of the furnace/blower and then have the correct C wire? I see speak about 24v transformer…if I have a C labeled spot on my furnace will that automatically be supplying 24v…or would it need an inline voltage transformer? And finally which wires would I test with a voltage meter to double check I’m getting 24v…the C wire and which? Thanks, great write up.

My house had old wire (No C wire) Which is the common wire…or known as ground. I installed a crude programmable battery operated thermostat years ago. Now That theyre wifi they need a constant source of power. I had an electrical outlet just a couple feet from my thermostat. I used a meter and verified that my unit is commonly grounded with my house (of course it should be) I simply ran a single wire from the earth ground off that plug with a small wire and up the wall to the new thermostat. Keep in mind the red is 24VAC control. Your new thermostat already has a rectifier built into it.

I’m looking to do the same thing. Instead of trying to run a C wire all the way downstairs to the basement/furnace, is there a way I can just connect it to a power outlet that is about a foot below my thermostat?

I did the same thing except I completed the circuit using the white (neutral) wire found inside a switch box that is 2 feet from my thermostat. These neutral wires are what return all of the AC circuits in your home – the ground wires are only there in case of a power surge or other emergency. Tying into the ground wire could get you electrified plumbing or other consequences. I have the same question as the other guy – why doesn’t everyone do this? Well it’s kept secret because building codes don’t let you “pull a neutral” unless it’s on the same circuit (and furnaces are required to have a dedicated circuit hence you’re required to only get your neutral there) but when you consider that only a trickle of 24v current is required to power your thermostat you probably won’t kill yourself if you forget to turn off your furnace circuit next time you service your switch box.

I have a furnace that has a white wire connected to the c connection inside the furnace. At the thermostat the white wire is W. I also have a yellow wire free at thermostat and furnace. Can I connect the yellow at furnace level with the white already connected and use the yellow at the thermostat for the C connection?

Hi Jim that white is providing power to AC unit so do not make any changes on C terminal as white on TT is W wire for heating. And yes you can use that yellow wire along with white to C terminal on furnace

At the beginning of the summer, I installed a Z-wave thermostat. I have only 4 wires that were marked R, W, Y, and G. There was no Common wire. The good news is that it worked perfectly all summer. But now it is heating season…
Evidently I mistakenly connected the W wire to the C, and this allowed power to the thermostat for the display. Again, everything worked. Until it was time for heat, and then the heater would not come on. I moved the wire from the C terminal to W, and now it will heat but the thermostat display does not come on. I guess I have two questions:
1) why did this work at all?
2) Since it did work, why not just jumper a wire between the W and C terminals?


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Your excellent common wire color table has an error: The Red 24VAC wire for dedicated heat call (Rh) shows an “Rc”, not the “Rh” you intended.

Blue or Black – C – Common wire, may be unused by your existing thermostat. Enables continuous power flow from the Red wire.
Red – R – 24VAC power from the furnace’s transformer
Red – Rc – 24VAC (dedicated to heat call) —-> this should have the “Rh” label
Red – Rc – 24VAC (dedicated to cooling call)
Green – G – Fan
White – W – Heat
Yellow – Y – Air conditioner

Thank you SO much for this article. I wouldn’t probably have tackled the circuit board addition of the C wire if you hadn’t given me this info. I really appreciate you putting this together!

[…] Best wifi thermostat reviews | ultimate resource , Corrected. letting ! glad thermostat wire article helpful .. Best Wifi Thermostat Reviews | The ultimate resource for …Smart thermostat guide – wifi-enabled thermostat reviews, The ecobee3 hands smart thermostat today. innovative motion-detecting sensors (1 included thermostat unit) . Smart Thermostat Guide – Wifi-enabled thermostat reviews …Thermostat wire: , Saying wire power source, supplies power misleading. wire simply “common” side transformer 24v output ( . Thermostat C Wire: Everything you need to know about the … […]

I installed my wifi thermostat using the techniques mentioned in this article. I have one question. My thermostat has a “circulate” settings for the fan. The fan only is supposed to come and circulate the air 35% of the time. When I set the fan to that setting the fan comes on to circulate the air only but the heat comes on as well. Is that possible?

this has to be the best site to understand the “c” wire issue i am trying to figure out, and yet can’t seem to apply it to my situation.

i have a oil boiler in basement, thermostat in 1st level and ac central air blower in attic.
my thermostat has R & W going to the T & T terminals on the Aquastat boiler. the 3 going to the central air are Y,Rc,G. at the CA blower the Y (goes to a lot of other white wires), the Rc goes to a red wire of a wire coming from the CA unit wire with no other connections to this. G goes to a gray wire again a solo connection.
So my connections seem to be 2 wires at boiler to tell it to come on and off (W&R) 3 wires to Central Air (Y,Rc,G) which seems to be on, off, fan. i am trying to hook up a honeywell smart wifi thermostat TH9320WF, and i do have the Honeywell wiresaver module too THP9045.

would like to add the info of the bundle of wires coming from the central air if this slightly helps my cause and your knowledge of the subject.

on the fan coil (control) I have these connections.

R (Rc on therm)
G (G on therm)
W2 (Y on therm)
W3 (Y on therm)
E (Y on therm)
C (goes to a red wire on Cent Air compressor in yard, the white wire goes to the bundle of other white wires all connected to the Y wire from thermostat)

This red wire “C” i just mentioned in the attic on the central air blower is this wire needed to power my HW thermostat? thank you.

I purchased a new thermostat that I plan on installing tonight. My question is related to the power though. Is it completely necessary to turn off the furnace power and breaker switches tied to the thermostat? My home is old and unfortunately the breaker box did not label which each switch controls. Let me know when you can. Thanks!

[…] Installation is easy if you already have a C-wire in place. If you don’t, an adapter is included and a few more steps to do. Either way, the instructions are easy to follow and you don’t need any specialized tools. Unlike its competitor the Nest Learning Thermostat, the ecobee3 doesn’t rely on “power stealing” to make up for a lack of a C-wire (read more about that here on ecobee’s site), they just include a power extender kit to make up for the missing wire. Read more about the C-Wire here. […]

Hello Bill,

Sorry for adding more questions your way but I enjoyed reading the previous ones you helped trouble shoot.
We recently bought an allure eversense wifi thermostat and when we uncovered our programmable thermostat it had two wires a “w” and an “r”. Can we still intall the new or do we need to add other wires? I am afraid of burning the unit. Please help. Thank you!

Connecting new HoneyWell thermostat to old Lennox furncase. The old thermostsat had a C wire connected but probably not used – it was battery powered. The C wire is connected to the cabinet (ground) at the furnace. I am not sure the transformer return is conncted to the cabinet (probably not) – see wiring diag at

When I attach new thermostat it does not power up. I think the blue wire in the diagram comes out as the red wire to the thermostat. Should I disconnect the C wire from the cabinet and connect it to the yellow wire on the transformer?

Hi all,
This site is really helpful (I think) but I want to make sure I am not getting myself into trouble.
I have a two wire (R W) right now, but the builders helpfully installed a multi wire cable despite not using most of them. So I think running a C wire should be as simple as attaching my Blue at both ends.
The contact panel where my R and W terminate has a point marked C; but it in turn connects to my Honeywell S87B in a port marked “Valve”.
That S87B does have something marked as 24v which is actually connected to my White wire.
Does this seem right? Is my C the legit place to connect?

I think you’re fine connecting the blue wire at both ends, but I’d recommend calling Honeywell. They have really helpful customer service that seems to be able to handle any wiring configuration, and it’s extra peace of mind!

Something doesn’t sound right to me (as you have described it) . The wire connected to R on your thermostat should be the 24 v (and usually Red). W (usually white) activates heat. The C wire is the common which is the return to the opposite side of the transformer from which the 24v wire comes, completing the 24 v power for the thermostat.
I don’t know anything about your furnace but I just hooked up 3 wifi thermostats on old furnaces at my church and that is what I learned from my research and experience.

My dogs chewed up the wires outside of the ac unit.
Out of the fan comes out a blue and yellow wire. Out of the wall comes out a red a white wire. Which wire would go with which wire?

I have 2 wires going to my thermostat. Using the voltmeter, I get 28V and when the heater turns on, they are connected, so it shows nothing. Does this mean I can jump one of these wires to create a common wire? If so, how do I know which one is the common and which one is the hot wire?

I am interested in installing a smart thermostat but lack the c wire at the thermostat. I have 2 thermostats. One for heat (Nest that I installed because it had the C) and one for A/C. The A/C thermostat only has an Rc, Y, and G wire. The Ecobee has a PEK kit that you can install at the boiler to effectively create a c wire there. I may be losing my mind but on my boiler, which is only 5 years old, there is no clear indication of what wires are what, so I’m stuck. Any help?

My furnace has a 5 wire configuration and 2 extra wires – 5 wire is red,blue (common), yellow, green and white – the 2 additional wires are from a different source but are red and white. I am installing a Honeywell EIM and it is not powering on. The common and red from the 5 wire source are not showing power on a volt meter. I checked breakers, the furnace shows it is powered. Configured the EIM exactly as the wires were configured in the furnace.

Any thoughts? I’m sure it is simple but it is not coming to me.

I have a blue red white and green wire with a jumper wire.I can’t figure out where these wires go..I have G,RC/RH,W/B,Y/O,Y1 on my new thermostat

thought id share….I have no C wire, and its just a milivolt heat gas fire , which just closes the contact for heat. i first found a 24volt DC power(110 to 24 VDC, i had one spare but they are cheap on amazon, less that $10 for plug in 1 Amp wall type.. yes DC power works fine just to power the thermostat) and connect to RC to +24V and C to 0V . Removed the link between RC to R (important) . Connect the existing milivolt connection from gas fire to R and W and hey presto , works a dream . . with power supply near thermostat and run power to thermostat (hid wires to rear of thermostat through drywall) . use existing 2 wires for heat contact that were already on old Mercury switch. . Thanks for all the information on your site , very useful(this is how i worked it out) . using RTH8500 honeywell wifi .


Thanks a lot for writing this article. I replaced my old thermostat with a Nest 3rd generation. I hooked everything up as instructed but could not get Rh to have a higher voltage. It was reading around 2V for VOC. I called Nest support, but they were not able to help. I almost had to call an HVAC technician to come out before reading your page. It turns out the old thermostat had all the wires connected, but the furnace only had W and R wires connected. I connected C, G, and Y to the appropriate connectors on the furnace, and voila!… i got heat! And my VOC for Rh went up to around 32V. So thank you so much for this article!


I have a two wire boiler (quite old and dated) that is hooked up to a basic thermostat in my family room. Recently we installed an attic based A/C system (Air handler in the Attic with the condenser outside). The Thermostat for this is upstairs on the second floor. If I wanted to upgrade my thermostat in my family room to run both systems, is this possible? Where would the C wire need to come from? my boiler or the Air Handler?

This site is incredibly helpful. Bought Honeywell RTH6580 WiFi thermostat for a place an hour from home. I knew I was in trouble when I opened the thermostat and realized I only had a mechanical switch with just a two-wire thermostat wire, red and black wire (on a heating system only). I closed it back up, and began research. I do not know what I am going to encounter when I look at the furnace circuitry, but I am going to armed with 18-5 wire. I intend to test new thermostat at the furnace with the solutions that you have given me before I try to pull the new wiring. Do not know if I will need a separate transformer, but this site has done a wonderful job for preparing me for what I am up against. Thank you, thank you.

After reading many of the ‘posts’ here, I seem to be one of few looking to go the ‘opposite’ way for wire connection.
We own a house in which we have my almost 80 yr old mother-in-law living in by herself, which has a combination heat/ac Carrier system.
It has a Carrier digital thermostat that my mother-in-law has continuous problems figuring out how to set it. Through her attempts in ‘playing’ with the settings – especially for heat – we’ve had to call for service twice when the system failed to operate correctly.
Without getting into any further detail, we decided to switch the thermostat to an ‘old style’ manual one – one that has no digital operation, and very simple heat-off-cool controls ….. along with the temp control.
I purchased a Honeywell CT31A non programmable therm. (very basic) before knowing how many wires the existing therm. was connected with – which turned out to be 5.
The ‘new’ Honeywell would not accept 5 wires so I called the company, and was advised that therm. was not compatible.
He gave me a model CT51 which he said could be used. I went online when I got home, and ‘googled’ that particular model and in looking at the wiring diagram, it also shows a 4 wire hookup. In reading further, it said NOT to connect the “C” wire (which happens to be the BLUE), and I also learned this is the “common”.
Is this something you would agree with, and could I be confident in wiring this in this configuration (less the C wire) with the results being that of working properly.
Looking to get this replacement completed before the next need for heat (LOL).
Thanks for any feedback you can send my way.
With Regards, Tommy E

I know this is an old post, but thought I’d try to get a little info. I have a Honeywell wifi thermostat set up. I have a generator wired to power several circuits in my house in case of power failure (wired the right way as to avoid arguments). The power circuit powers the furnace but not the condenser due to the generator not being strong enough to power it anyway. My first power failure resulted in everything working fine but the wifi thermostat was not powered. Does this power originate from the condenser power?

I’m upgrading from a Honeywell 4 wire (R, W, Y, G) thermostat to a Nest 3rd gen. Old thermostat didn’t require continuous power so the blue,C wire wasn’t connected. However, their was a blue, C wire in the wall. How can I tell if the C, blue wire, is connected to my new Goodman GMSS96 gas furnace without climbing into my cramped attic and trying to figure out if it’s connected there?

Sensi needs a C wire. Also the Venstar product you mentioned above is not for adding a C wire – says so right in the manual and on Amazon.

Greetings. Very informative post. I have been investigating this because a painter damaged our thermostat (obsolete, works for now, long story). I wanted to go smart thermostat, and like an idiot, didn’t get the Ecobee unit. Honeywell

I have found my carrier system to be even weirder than one might expect.

*Carrier Infinity 80 2-stage with humidifier, air filtration, AC unit, Infinity Control Thermostat SYSTXCCUID01-B drives these.

Carrier uses a proprietary ABCD junction block (Block PL7 in the circuit diagram) for the thermostat.
A-Green= Data A to Green wire
B-Yellow= Data B to Yellow wire
C-White= 24VAC (common) to white wire
D-Red= 24VAC (hot) to red wire.

In the wall is the fifth, blue wire, unconnected.
At the furnace end, also unconnected, I am unsure where to connect it at the furnace control board. ABCD block at either end has only 4 connection points.

Furnace Control Board Wiring diagram doesn’t seem to clearly indicate the attach point at the furnace end.

Well… qualifier, there is a screw-terminal block (W2, Y1, DHum, G, Com 24V, W/W1, Y/Y2, R) on the board /screw labeled Com 24V. I suspect that is the proper point, but don’t know if the ABCD connection disables the screw-terminal block. It would seem dumb if they put it on there but proprietary ABCD disables it, but you never know.

Any recommendations?
The experiment starts…

I have a Carrier system and I wish to replace the failing thermostat with a smart thermostat. Carrier uses a 4-Wire ABCD connection.
A = Green = Data A
B = Yellow = Data B
C = White = 24Vac (common)
D = Red = 24Vac (hot)

This is the labeling (and wire color matches, yay) at both thermostat and the furnace. Both ends use a 4 pin connector (thermostat and Furnace Control Board). Lucky me, the Blue C wire is present in the cabling. Unconnected at both ends, natch.

As I understand it, connection at a smart thermostat in this case is simply, blue wire to the C terminal. Furnace end is my question… there on the FCB is a full screw-terminal strip (W2, Y1, DHum, G, Com24v, W/W1, Y/Y2, R) none of these presently have connections because, ABCD connection is for the established thermostat cabling. In other words, the FCB has a dedicated terminal, rather than use the screw terminals.

So, finally the question…
Do I simply take the unconnected Blue C wire, and land it at the Com24V screw terminal?

I assume the first step is to check between Com24V and R for voltage. Sound correct? If that is live, would there any reason to NOT just make the Blue C wire connection there, then connect C wire at the thermostat? (I have the schematic, and I see no other landing point for C.) Logically, this makes sense to me. I just don’t know enough about the FCB to be 100% certain.

My Carrier installer wants nearly $1000 to install the replacement thermostat. It seems with my approach, I should be able to do this.

Opinions? Ideas?

I have a white Rodgers 1f89-211 thermostat and have 8 wires. Black (not used on original and thermostat worked), brown (not used on original and thermostat worked), blue (common), red, yellow, white, orange, green wires what do these wires connect to please. I have terminals C, L, W2, E, b/o, g, y, R

I am trying to hook up a Honeywell RTH 221B thermostat to my furnace. This is new work and there are no previous wires. The furnace panel has the typical Y W R G C terminals. The thermostat has; O/B Y RC R G W terminals. I only need heat, no AC, heat pumps, etc. I just need the furnace to keep us warm. We are 90 miles south of the Canadian border in MN.
My question is: From the transformer terminals W & C, to which terminals do W & C connect at the thermostat? There is no “C” on the thermostat.
By the way; The Honeywell 800 number and the help on the other end barely speak English. It is NOT customer service, but more like a Joke!

Hello I have a ritetemp thermostat and I am installing a Honeywell basic digital thermo, I followed the instructions carefully but when I finished I turned on the AC and the heats started to come out .. I noticed on my old thermostat that there was a jumper from the Y to the W and the white wire was going to the W2 which there’s no W2 on the new thermostat .. also in the manual it says not to connect the C wire to the new thermostat … need some help

The thermostat originally in my old house (heat only) had 4 wires, 2 of which powered a clock. I replaced that with a more advanced one 20 years ago by not using the two wires that ran the clock (labeled W and RC). The other two wires are connected to terminals labeled W and R). That thermostat (battery powered and programmable) works fine. I want to install a Honeywell Wi-Fi thermostat. Which of the two currently unused wires do I connect to the C terminal (W or RC)?

My thermostat is three floors away from my boiler, and so running a new cable is a significant effort. It is surprising that Nest doesn’t sell a device that sits at the boiler and repurposes the two conducts for both power and data simultaneously.


I installed a Nest on an old Lennox furnace. When I removed the old thermostat, I noticed I did not have a C wire. Everything works but when I turn on the A/C, it only stays on for about 2 minutes and looking at the Nest, the VIN drops to ~4. Question being, am I able to use the Add-a-Wire to get the C wire connection?

You need to by a 24VAC 1A power supply. Connect one wire to C, the other to RH (with the R wire from the heater). This solved my exact same problem.

I have one thermostat that controls my heat and AC. I bought a Honeywell RTH6580WF and a 24 volt wall adapter because I don’t have a C wire. I have W and RH wires from my boiler and G, Y, and RC wires from my AC. I know the RH goes into RC on the thermostat and the RC goes into R on the thermostat. I see that the 24 volt adapter should plug into C and R. Not sure exactly what to do from there. Could really use some expert advice.

This post was quite helpful when I had to install my ecobee4. My wiring had 5+ wires going to the thermostat initially, however only 4 were in use (R, Y, G W). It turned into a simple exercise of connecting the “blue” wire and providing the C wire as needed. My problem was that there was no terminal on the 1999 vintage control board but after a bit of research was able to determine I can just wire into the blue coming from the 24v transformer.

This post would have answered all my needs with something like “the C wire is normally connected to the transformer and provides ground” (if that’s indeed what it does).

My old thermostat ( old mercury Honeywell) has the following wires: blue to Y, green to G , red to RH and white to W. It also has a bridge from Rh to RC, another bridge from RC to C and yet another from a second C terminal to the Y. But I don’t see a wire going directly to the C wire. So I guess I don’t have a real C wire.

On the old thermostat only three wires are used: Green on G, Whiteon W, and Yellow on a terminal marked A. There is a jumper between Rh and Rc. How do I hook up the Honeywell with no red wire?

I purchased a NEST Gen 3 over 12 months ago and tried to get it to work with just the 2 wires off the heater – I had the same problems that everyone on this thread has mentioned – so I packed it away. Last week it got the better of me and I tried again. This time I purchased a $29 POWERTEC 24VAC 1A BARE ENDS POWER SUPPLY from JAYCAR Electronics (Australian). I connected one wire (doesn’t matter which one) to the C, and the other to RH with the R from the heater (twisted them together). that is now 3 “ports” on the base plate in use – C – W1 – RH. Plugged the 24VAC power supply in to the 240V point, connected the nest, and it automatically detected the change in power. I have no problems with it at all now. To hide the power supply, I extended the wires by 2 meters (making it 3.8m) and plugged it into an available powerpoint in the roof, and hiding the wires down the wall cavity.

The specifications of the Powertech power supply is: Voltage input: 240VAC, 50Hz, Voltage Output: 24VAC, Current: 1000mA.

I bought a Lyric T5 to use with a Baxi Duo Tec 28 GAC gas boiler.
Anyone knows if there´s any way to get these two working toghether?????


You can also wire a 24v transformer near your thermostat and connect to the Rh + C terminals. Basically duplicates what the furnace would be sending. The Rh terminal gets used for both power and control.

I bought a Simple thermo and like alot of people here only have two wires, R and W. I ran a 24V power supply by my furnace and hooked one side into the blue in the wire bundle going to the thermo and the other side in the RH. It powered up but the heat will not shut off. Just continues to run. Any ideas.

Hi all,

I am trying to install a Nest thermostat. I currently have an old Honeywell, which is powered by 4 AA batteries.

Once I remove the face, I see 5 total wires. Black wire connected to the “R”, white wire connected to “W”, gray wire connected to “G”, and a black wire connected to “Y”. The red wire coming out of the wall is not connected to anything.

Is it likely that the red wire is a common wire?

Any suggestions on where to connect each wire on the Nest? I’ve heard horror stories of people frying their furnaces because they wired it wrong. I don’t want to do that.

I have 2 thermostats – one runs heat, the other air….I want to hook up a nest to the air. I have 3 wires that run the air thermostat – white, red green….hooked them up accordingly and all was fine. Afte about 5 hours the nest throws a e195 error – basically a common wire error which I do not have. Anyone know how I can fix this?


I have a Rheem system with a Rheem 500 series thermostat. I bought a Nest Learning thermostat to upgrade. The Rheem thremostat has the following 4 wires with these labels, “R=red wire, C=white wire, 1=blue wire and 2=green wire”. The Nest thermostat has the R&C but where do I set the 1&2 wires in the Nest?

My old thermostat has just 2 red wires that are quite thick. How do I determine which wire does what and whether I’ll ever be able to install a modern thermostate?

I have an old Luxaire electric furnace from the early 70s. The previous owner added a basic Honeywell digital thermostat. They connected furnace R -> thermo W, and furnace W1/W2 -> thermo R. Pff. It works fine as I suspect the heat call just completes the circuit, so the swap doesn’t matter. But I found out the hard way that if the batteries in the thermo die, no more heat (and hence frozen pipes).

The furnace does have a C terminal, so I plan to use it for a new Nest I’m going to install. I’m going to use a voltmeter to test across R and C for 24v first though. The furnace also has an unused G as well, so I’m going to hook it up too. Why not. 🙂

Opinions welcomed.

I had a c wire hidden in my wall and connected it to the c terminal on my nest thermostat. I then connected the c wire to the control board on my HVAC unit. When I flipped the power back on my nest said that there was no voltage detected in my c wire and won’t work. How do I fix this?

I have an old mercury switch thermostat and a heat only system. I want to install a wifi thermostat. I have confirmed I have a 4 wire cable. On the thermostat the blue wire is connected to the Y terminal. At the furnace it’s not connected. I’m assuming that’s because I don’t have AC so my question is, couldn’t I just use the Blue wire as my C wire?

yes — that is what it is usually used for. but you’ll need somewhere to connect it to at the furnace too (not just at the thermostat).

I have a Lyric Round controlling a NG forced air/ac and desire it to control a millivolt NG fireplace. I understand that the Lyric does not have “ dry contacts “ so I am choosing to include a 24vac relay driven as stage 2 output configuration on w2. Two wires to relay one on w2 and other one on “R” at the Lyric? Relay output to fireplace will include parallel switch to maintain heat source during any power outages. Great posts!

I have a boiler for heat and a separate air handler for AC. I have no C wire. I want to add a 24v power supply so the geo can work as if it had a C wire. I have an outlet in the closet beind the tstat.

My question is do I connect the 24v to C and Rc, leaving the Rc wire from the AC connected as well or do I leave the Rc wire from the AC disconnected and use the Rc/Rh jumper? I’m case it matters, my boiler has 2 other old mercury Tstats on 2 other zones. My AC has no zones.

Question: 20 yr home 5 yr. Hvac. Typical control board inside furnace and second panel on wall next to system, thermostat Bryant T6 connected with 1 ea. Red/White. My thoughts are the second panel sorts out the functions/colors. To replace the failed thermostat with simple less expensive non program or any, yet retain upgrades like 2 stage. What requirements are needed within the Thermostat. Thanks

Hi. Finally after 3-days searching I’ve found mote than interesting post. Thank You for that! 👍 Unfortunately I’m stuck with my wiring. I have 4 wires but with labels: W C G Y 🙄 No red one here. What can I do in this situation. Can I have smart thermostat too? Can anyone help…?

My White Rodgers thermostat does not have a place for C wire to go. That is, no place labeled C. Where do I connect it?

For older homes with only 2 or 4 wire set up, A way to cheat and not have to do a major rewire or give up, many homes were extensively wired with telephone jacks in many rooms, that now lay unhooked and idle. They might be able to be repurposed to provide a common wire if they are in the vicinity, without running and entire new thermostat wire.

I have a two wire forced air heat only system. The next works fine after restarting but in a day or pulses. I have bought a c wire power adapter. Question is when hooking up adapter. Do I still connect the power from the heater or do I leave the power unhooked and let the adapter supply the power or do I hook all wires up. Any help would be awesome. The thermostats works well once restarted but otherwise every other day have to restart so I think it’s a power issue. But there again I don’t know if I need to leave the power wired hooked up if I have the adapter connected.

I have experience with some thermostats that require power but use no C wire. For instance, the Honeywell T8775 is a digital device but runs with just two wires. When there is no call for heat it allows a small current to flow that it can use to keep itself alive. If you put a current probe on the wire you can see this as a fairly complex current waveform. I think they assume the small current is not enough to energize the downstream device. Different thermostats seem to allow varying amounts of current to flow with varying waveforms, but I do not have a survey of all such devices, or know how much current is the max any one would assume they can allow to flow without making it look like a call for heat. A really comprehensive standard for what is workable in the industry would be nice to see. I work on specialty controls that tap into these signals so it is of high interest to me.

I have experience with some thermostats that require power but use no C wire. For instance, the Honeywell T8775 is a digital device but runs with just two wires. When there is no call for heat it allows a small current to flow that it can use to keep itself alive. If you put a current probe on the wire you can see this as a fairly complex current waveform. I think they assume the small current is not enough to energize the downstream device. Different thermostats seem to allow varying amounts of current to flow with varying waveforms, but I do not have a survey of all such devices, or know how much current is the max any one would assume they can allow to flow without making it look like a call for heat. A really comprehensive standard for what is workable in the industry would be nice to see.

Great advice… Happy Holidays
Recently went through this with my nest / blue wire. Hints the reason why I’m here
I had the problem fixed 12/24/22 and was trying to see if me not connecting the blue wire was a result of my contactor going out ? It was stuck in the closed position making my heating element randomly come on Thank God nothing caught fire and was a easy fix tech connected the blue wire on both sides and replaced my contactor P282-0421 🙏 Everything worked before the common blue wire was connected and after… only difference no more contactor clicking and random heating without the blower being on
Nest really needs to do a better job warning the consumer and spreading awareness

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