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No C-Wire? Venstar Add-a-Wire adapter has you covered

Problem: Your HVAC system lacks a C-wire (common wire) for your new WiFi thermostat

Solution: Install a Venstar Add-a-Wire adapter!

Venstar Add-a-Wire Adapter: a Simple Solution to a “Common” Problem

Short a wire?

If you are lacking the (ironically named) “common wire”, also known as the C-wire, you might be pleased to learn that there’s an adapter for that.

The Venstar Add-a-Wire is an inexpensive, DIY-friendly alternative to running completely new HVAC wires.

Venstar ACC0410 Add-A-Wire Accessory for All 24 VAC Thermostats (4 to 5 Wires), White
From the manufacturer: In applications where additional wiring cannot be run to your programmable digital thermostat, the Add-A-Wire accessory can be used to add a wire to the thermostat.

Venstar Add-a-Wire on

Why add a wire?

In short, Wi-Fi thermostats need more power to run that always-on Wi-Fi connection. The C-wire a common wire that delivers a continuous flow of electricity to your thermostat.

Some manufacturers, like the Nest, claim that their thermostats work without a C-wire, drawing power from the heating wires instead. Unfortunately for many homeowners, the outcome is an intermittent WiFi connection or unwanted furnace cycling.

Read more about the C-wire here, including why Nest owners aren’t exempt from needing one.


If you have a low voltage HVAC system, you can skip some of the hassle if you select a thermostat that works without a C-wire or ships with an adapter.

  • Ecobee thermostats, including the ecobee4, ecobee3, ecobee3 lite all come with their own Power Extender Kit that you can set up yourself inside your furnace
  • Emerson Sensi is a low-power smart thermostat designed to work without a C-wire in systems that include both heating and cooling

All ecobee thermostats ship with a super handy “power extender kit” (PEK for short). In the photo below, an ecobee3-style PEK (in my hand) and an ecobee4-style PEK (in the background). They are functionally identical.

ecobee3 power extender kit (in my hand) and ecobee4 power extender kit

If you’re not sure what you have or what to do with your wiring, check out our full guide to the C-wire.

Unboxing the Venstar Add-a-Wire

In the box:

  • Venstar Add-a-Wire unit with wires
  • Additional wires
  • Installation guide
  • Two twist-on electrical caps

Venstar Add-a-Wire Installation

Adding an adapter is not difficult: if you’ve ever installed a ceiling lamp or replaced an electrical outlet, you’ve got the skills and tools for adding this adapter.

Most of the work is in twisting wires together and matching colors. Installation takes first-timers about an hour, start to finish. Instructions are included with the adapter.

The furnace end of the Venstar Add-a-Wire adapter installation process.

Bring along:

  • Phillips screwdrivers
  • Electrical tape
  • Needle nose pliers
  • Drill / drill bit
  • 2 wire nuts (included)

You will need access to both your furnace wiring and your thermostat wiring. You’ll be installing new wire parts at your furnace and behind your thermostat.

This five minute video made by Venstar’s manufacturer demonstrates the installation process. If you are considering buying this adapter, watch this video first to see the full scope of the project:

Expect a little ambiguity as your furnace panel will invariably vary from the one shown, but anyone savvy enough to install the thermostat itself should be able to handle this project.

We highly recommend the Venstar Add-a-Wire adapter. It’s much cheaper and easier than running new HVAC wiring, can be done by almost anyone in about an hour, and well-reviewed by other buyers on

See the Venstar Add-a-Wire on

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46 replies on “No C-Wire? Venstar Add-a-Wire adapter has you covered”

Suppose you have a gas furnace, heat only, with 4 zones and no c-wire. Is the only solution to run new wires? I assume you can’t use 4 of the “add a wire” devices, just one?

[…] Honeywell claims it works without a C-wire, but reviews around the Internet say otherwise. The included battery is an AAA size Lithium battery, but if yours arrives drained (like many do) you’ll need to replace it. And keep replacing it, if your home lacks a C-wire. For best results, we recommend installing a C-wire or a C-wire adapter. […]

I have three wires in my existing progammable thermostat and want to upgrade to a wifi thermostat. I have wires connected to W, Y and R (linked to Rc). Will Venstar Add-A-Wire Adapter work to give me a C wire connection to power the wifi thermostat?

Hi Stephen,

Good question! I know Venstar markets their product as a way to add a 5th wire to a 4-wire system, but on page 3 of their manual pdf they do show it as being used to add wires to a 3-wire system (look for the “add cooling to a system that already has heating” diagram). However, all these diagrams assume there’s a blue wire, which you don’t seem to have (unless you do and it’s connected to one of your other-letter terminals).

Sorry I can’t give you a clear answer that’ll get you on your way. The Venstar company makes thermostats (in addition to the Add-a-Wire) and their support might be familiar with a greater variety of wiring setups. Their support email address is:

If you do resolve it, please come back and let us all know how it goes! Good luck.

Thanks for your posting. I have four thermostats for four zones. Only one of these controls the AC and it has a blue wire. However, I won’t be using this location for the WIF thermostat, but one that only controls heat and has the three ires I noted above. In fact, I want to install two WIFI thermostats at different zone locations. I’ll email Venstar with this info.

So I just hooked up my new Nest Thermostat (2nd Gen) and got rid of my old “stupid” thermostat. Love the Unit so far but I believe I may be one of those people that need a “Common” wire set up. I only have a heating unit at our home and it was set up to only use the heating unit and has no access to fan controls either. Very basic but has worked fine until I hooked up the Nest. I only have a 2 wire system at the moment and use “W” & “RH”. My unit powers up and is able to control the heating but my issue now is the fact that when I go to set the Nest @ 70 degrees and my house is currently @ 61 I cannot keep the unit running consistently as I now get “Delayed for 2min. 30 seconds” on my Nest display. So basically now the unit will heat up, fuel system will shut down and enter it’s “cool off” period and before the “delayed” time is over the system shuts down only to re-light the pilot and start up again and do this process over and over again until my desired temperature is reached. Can this adapter work for me and help me go from 2 to 3 wires and if so would this fix my issue?

I only have 2 wires running from the Furnace as far as I can see. Looks like garage door wiring actually. The unit is newer and whoever installed basically cut the old wire at the wall and used the existing 2 wire to splice into to run a few extra feet.

Thanks for your time.

Though a bit more money, I decided to try the Nest. The website and compatibility check said it would work without a “c wire”. I confirmed this by calling their toll free number. I installed two of these yesterday and they seem to be working fine.

Your two wire set up should also work. Call their toll free support number.

I don’t know your heating system details, but my oil furnace cycles on and off as it is heating the baseboard hot water. When the hot liquid reaches a certain temperature the furnace cuts but the liquid keeps circulating until the set temperature is reached. If the liquid cools but set temperature not reached the furnace ignites again. This cycle continues until the set temperature is reached.

Your best bet is toll free support.

I, too, only have 2 wires (and no common wire). What I found is that it ran fine, for a few days, then my furnace started acting funny, e.g., when Nest was in “heating” mode, my thermostat would sometimes stay off, and just make clicking noises. Other times, the fan would turn on and off, every 2 seconds, over and over and over.

After doing a little research on Nest’s site, I found a page that says that the above 2 problems are likely caused by an incompatible furnace motherboard. Since the Nest is stealing power from the motherboard, to charge itself, some motherboards start acting strangely.

The only solution is to wire new furnace wire (that includes a common wire). No way to avoid it.

They usually deal with using electricity for transmitting energy.

You should not have any problem. Nissan is click here expected to carry this tool as it enables
the electrician to make the best decision to ensure the promotion. While buyingelectrical appliances
people need to make sure to establish a business. There are two individual 120V wires that provide the best electricians with
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commercial contracting.

[…] The exceptions, of course, are heat-only and cooling-only systems. It is not possible to run the WiFi features of the thermostat on AA batteries alone (unless you like changing batteries every couple hours). If this describes your home’s HVAC setup, you will actually need to run a c-wire or consider a solution like Venstar Add-a-Wire. […]

Not that I know of, unfortunately. Wifi is too power-consuming to get by on battery alone in the long run.

emerson sensi. I tried the nest and they lyric, both would not work. I have a 4 wire system, but no common wire. The nest would tell me there was no power from my red wire and I later found my furnace is incompatible even though it has the wires needed. Bought the emerson and haven’t had any issues.

Lux Geo
Powered one with batteries
And one with usb
I have a red and white wire set up
For 2 homes
Both work fine
Great and reasonably price
Plus good rebate offers

Wifi needs electricity
Power sharing or stealing is not dependable
Reason it can run on batteries
No fancy screen

You need to make it very clear these are only compatible with 2 wire systems by adding a power supply or replacing 2 conductor cable with 3 or more conductor. I can’t return this expensive RTH9580WF thermostat!

so your saying the be star will only work on thermostats that only have 2 wires hooked to it?

I do not have a furnace only a monitor propane wall heater. Used the G wire to hook up my Emerson sensi. Wifi worked for two hours and then stopped. Will this device work on a monitor heater? Didn’t seem to see wire to hook to when I took the cover off.

[…] Honeywell claims it works without a C-wire, but reviews around the Internet say otherwise. The included battery is an AAA size Lithium battery, but if yours arrives drained (like many do) you’ll need to replace it. And keep replacing it, if your home lacks a C-wire. For best results, we recommend installing a C-wire or a C-wire adapter. […]

[…] One caveat: Like other smart thermostats, the Smart Si requires a C-wire. If your system lacks a C-wire (and it’s good to figure this out before surprising someone with a smart thermostat of any brand), you’ll need to buy a Power Extender Kit from ecobee ( link) or a Venstar Add-a-Wire adapter. […]

I only have 2 wires for my thermostat that is in a 1987 double wide. Will the constant work with 2 wires?

Hows it going my furnace doesnt have a C spot at all would there be another spot that would be the same? I have a T spot!! Thank you!!

Hi Ryan, did you ever get this resolved? I’m currently having the same problem! I have two T terminals on my furnace. Ran new wire from the thermostat so I just need to get the C-terminal added to the furnace somehow…

I have an old fireplace that works on 24v thermostat and there are only 2 wires behind it (R+W). If I manage to get the C wire by buying a generic 24V AC/DC adapter and have it connected as a replacement for the C wire, will it work with Ecobee or Nest?

My furnace control board has C-common connection which used for another device power source but not the thermostat . Will that be ok if I use 2 Common wire for one connection on the board.

Great info. I bought a wifi thermostat from that actually claimed on the website for the exact unit I ordered that it did not require a C wire, only to find that the instructions unequivocally state: you need a c wire. They took the units back (I got 3) and should have charged them for wasting my time.

They say “depends on which sensicomfort unit you bought” – and that is to say, if you bought one from since they do not sell units that work without batteries without the c wire – for reasons you explain quite clearly.

I wish I had seen this post – it’s bookmarked now and now I think I will revisit the wifi thermostat option (which I had given up on).


Can I add a C wire with the VENSTAR add a wire kit if I don’t have a Y wire ? I just have a R, W & G wire.

Can you show or send me a wiring diagram ?

I purchased a Venstar add a wire unit with diode and a Honeywell RTH 9585 WF thermostat. Installation seems straightforward. Currently I have a 4 wire bundle connected from my thermostat to my HVAC without using the C . However, I also have another 2 wire ( red connected at R and white connected at Y) . Do these two wires stay connected as they currently are AFTER I connect the add a wire accessory. Plan to connect 4 existing wires: Thermostat: red to R, white to W, green to C, blue to blue(add a wire diode). HVAC: red to R, white to W, green to C, blue to blue ( add a wire unit). Add a Wire green to G , Add a Wire yellow to Y at both the thermostat and HVAC. I believe the two wire line goes to the transformer. Look forward to your response. Furnace is American Standard , Freedom 95, single stage.

Hey Jordon,

So I see you mentioned that I could use the Venstar C-wire to power my Smart Ecobee 4, but I don’t see the wiring details.

My issue is that I don’t want to run the PEK wire from my furnished basement through the utility room. Apparently, there is a lot of cost and hastle.

Can I use the following two wire switch with my Ecobee 4 with Alexa?
Venstar ACC0436 2-Wire Kit for all 24VAC Thermostats
Learn more:

I added a Honeywell RTH6580WF to my home recently. I discovered no c wire. Without knowing what all it involved, I found the blue wire tucked into the wire bundle as it had been cut off since not in use. I stripped it and applied it to my thermostat. Still no power. I then went down to my furnace to see the blue wire was not hooked up either it was just wrapped around the same wire bundle going to the thermostat. I stripped it and hooked it to the C terminal on the furnace. Now the C terminal had a white and brown already attached going to the AC. But you can hook more wires to the C. You can hook the 3 right into the terminal or you can bundle them together with a 4 wire called a pigtail then you only have one single wire hooked to the C terminal. User preference. So don’t just assume because you found the blue wire in your wall or wire bundle behind your thermostat that its connected to the C terminal in your furnace. You will be best to take the panel off your furnace , find if a C wire is connected and determine the color while making sure it’s the wire bundle going to the thermostat.

If I install the add-a-wire kit, do I need to attach to furnace, side or can I attach it out side next to thermostat, asking this because I live in an Apartment, and can’t get to furnace its in the ceiling, and I don’t have a C wire, other than something extra on wall, is it possible?

I have a Nest thermostat. I do not have a C wire but I do have 5 wires, Y1/G/Rc/Rh/W1. Which model of the Venstar do I need to hook up my C wire?

My old thermostat is 4 wires and then I put nest e thermostat but the heat it doesn’t work after that I bought venstar adapter ang follow the diagram but still not working.

You’ll help yourself enormously get a response if you took a photo of the heating unit’s end connector where the thermostat wire connects to the heating unit’s control board. See if you can get yourself a smartphone picture and add it here, blowing away any dust first so any letters or words can be seen and please disconnect the power first while you delve inside.

The answer depends on what is seen inside at the connector.

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