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

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 Amazon.com

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.

Alternatives

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 Amazon.com.

See the Venstar Add-a-Wire on Amazon.com


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33 Responses

  1. […] out our review of the Venstar Add-a-Wire, which easily adds a 5th wire to your 4-wire […]

  2. […] 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. […]

  3. […] 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 or a Venstar Add-a-Wire adapter. […]

  4. Stephen Smith says:

    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?

    • M.B. Grant says:

      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: thermostatsupport@venstar.com

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

      • Stephen Smith says:

        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.

  5. Jordan says:

    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.

    • Stephen Smith says:

      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.

      • Dimitri says:

        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.

  6. next says:

    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
    a certain amount of experience for certain kinds of Oregon contractor work mostly
    commercial contracting.

  7. Mike says:

    In AC systems with plenty of life ahead of them, a repair would make
    the most sense.

  8. […] 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. […]

  9. Joe says:

    I have a heating system with just a W and R wire, can i use this to add a C wire for a wifi thermostat?

  10. Peggy says:

    Has this product been tested by Underwriters Lab or a similar group?

  11. Richard says:

    Is there a wi fi enabled thermostat that can operate solely on battery power?

    • M.B. Grant says:

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

      • Richard Huttas says:

        Thanks much for your reply.

      • Chris says:

        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.

    • Cheyenne says:

      The IRIS system does. Purchase at Lowes

    • Niki says:

      Yes
      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

    • Gary says:

      In fact there is! The Lux / GEO comes with 2 AA Lithium batteries. http://www.luxgeo.com

  12. John Higley says:

    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!

  13. April says:

    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.

  14. […] 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. […]

  15. […] 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 (Amazon.com link) or a Venstar Add-a-Wire adapter. […]

  16. Bryan searcy says:

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

  17. Ryan Shong says:

    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!!

    • Molly says:

      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…

  18. David says:

    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?

  19. Kody Pham says:

    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.

  20. Aaron says:

    Great info. I bought a wifi thermostat from sensicomfort.com 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 sensicomfort.com 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).

    Thanks!

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