Smart Thermostat Without C Wire [How to install it]

Imagine buying a new smart thermostat, patiently waiting for that Amazon box to be delivered. You open it up, read the instructions, maybe watch a video that tells you how to install the device.

You remove your existing thermostat to find that things aren’t quite right – you don’t have a C-wire. A C-wire or common wire is a power supply required for your smart thermostat to work correctly.

You can learn more about the C-wire and why it’s important in our article here. Unfortunately, most of the older thermostat models didn’t require one, so many homes do not have a C-wire installed.

Almost all Wi-Fi thermostat no c wire models require a common wire. But, just because you don’t have one does not mean you will never be able to get a smart thermostat.

Many buyers wonder if they can install a smart thermostat without a C wire. There are several ways to install programmable or Wi-Fi thermostats without a C-wire; it takes some additional work. We’ll walk you through your options below.

How to Install a Smart Thermostat Without A C-Wire

The first thing you want to do is check for a C-wire by looking at your old thermostat.

  1. Detach your thermostat and take a look at the wires connected to it
  2. If there is a terminal labeled “C,” then you have a C-wire
  3. If there is not, go to your furnace and check the source

Sometimes your furnace has a common wire that wasn’t needed during installing your existing thermostat so that the wire can sometimes be hidden in the wall. If there is no C-wire on your furnace, then take a look at these simple workarounds that will teach you how to install a smart thermostat without c wire.

Get a Wi-Fi Thermostat that Works Without A C-Wire

The simplest solution would be getting a smart thermostat that doesn’t require a common wire to work. The following thermostats work without a c-wire.

Ecobee4 and other Ecobee Thermostats

The Ecobee4 is our pick for the best thermostat with remote sensors. Ecobee smart thermostat without c wire comes with a “power extender kit” to replace the C-wire.

This Ecobee power extender kit will work with Ecobee4, Ecobee 3, and Ecobee 3 Lite thermostats. Installing the power extender kit is very simple and took us about 10 minutes during our testing.

All you must do is connect your existing wires for your furnace into the power extender kit and then wire the extender to your furnace. Ecobee has a great video below that shows how easy this is to achieve.



Check out our review for more information about the Ecobee4 thermostat with remote sensors.

Google Nest Learning Thermostat

The Nest Learning Thermostat is our pick for the best smart thermostat. In most cases, a Nest Thermostat no c wire can work without a common wire. Without a common wire, it charges its internal battery using power from the heating and cooling wires.

Some HVAC systems, such as micro-controller-based systems and certain gas valves, will require a C-wire to work with Nest Learning Thermostat. You can read more about Nest thermostat’s C-wire compatibility on their website.

For more information, check out our in-depth review of the Nest 3rd Generation Learning Thermostat.

Lux Geo thermostat without a common wire

We reviewed some lesser-known models in our guide to the best wifi and programmable thermostats.

A critical model to take note of is the Lux Geo Smart Thermostat. Lux Geo is very likely to work with your system when nothing else does. It is affordable and can run on AA batteries.

With batteries installed, it does not require a C-wire or any adapters to power the thermostat. Also, it can work with two-wire systems, which are incompatible with any form of a C-wire replacement.

If you want to get a thermostat that is not mentioned above and requires a C-wire, you should consider hiring a professional who can run a C-wire from your furnace to your thermostat.

It’s possible to do this work yourself, but home electrical work is not for the faint of heart. If you have never done something like this, it is best to leave it to someone who has.

A quick visit to Amazon’s Home Services center makes it easy to find someone to install the thermostat for you, including your new common wire.

Venstar Add-a-Wire Adapter

If running a common wire is out of the question, and you’re installing a smart thermostat that’s not listed above, you have one other option for installation.

The Venstar Add-a-Wire is a DIY-friendly alternative that allows you to add additional wiring in systems where new wiring can’t be run. This accessory is effortless to install, and it is much easier (and cheaper) than running a completely new wire.

Adding this adapter is not that difficult, even if you are not experienced. The installation for our test took less than an hour. Most of the work is in twisting wires and matching colors—the included instructions for the adapter to make things easy.

Here is what you’ll need to install a Venstar Add-a-Wire adapter:

  • Screwdrivers
  • Electrical tape
  • Two-wire nuts (included)
  • Needle nose pliers
  • Access to furnace and thermostat wiring

Venstar put together a short video to show how easy it is to install the unit.

Keep in mind that your furnace panel will likely be different from the one shown in the video, but the process remains the same.

Substitute a G-Wire for a C-Wire

A C-wire provides constant power using the furnace as a power source. The G-Wire connects to the fan, and you can easily substitute it for the C-wire.

This solution works the best if you have a forced-air, gas, or propane heating and cooling system. While it’s a decent substitute as a power source, this method will remove the ability to control your fan.

The fan will run when your system does, so the trade-off is you lose some functionality compared to your current HVAC configuration.

To see how to substitute the G-wire for the C-wire, check out this two-minute overview on how to do it.


Add a Transformer

Thinking about adding an external transformer may have you running for the hills. Wiring an external device into your system will allow you to get a Wi-Fi thermostat that needs a C-wire to work correctly.

We think installing a transformer is probably the most overkill of all your options so that we won’t go into much detail here. But for some people, it may be their only option.

For more information, check out this guide over at The Balance.

Wi-Fi thermostat no c wire: Conclusion

Keep in mind that, unless stated otherwise, the Wi-Fi thermostat you want to get requires a C-wire.

It is highly recommended to check your HVAC system and see if you have a C-wire before buying a new thermostat. If you don’t have it, then pick one of our solutions that will work the best for you.

Even if you are a newbie to installations, don’t panic. It is not as complicated as it seems, and the step-by-step video instructions we included are straightforward, and anyone can follow them.

Smart Thermostat Without C Wire: Amazon Best Sellers

Save 57%Bestseller 1
Smart Thermostat for Home, WiFi Programmable Digital Thermostat for Heat Pump, Energy Saving, C-Wire Adapter Included, DIY Install, Voice Control, Black
1,263 Reviews
Smart Thermostat for Home, WiFi Programmable Digital Thermostat for Heat Pump, Energy Saving, C-Wire Adapter Included, DIY Install, Voice Control, Black
  • 95% High Compatibility: This thermostat comes with a C-cable adapter and is compatible with heating and cooling systems, cooling-only systems, heat pump systems and more. It does not work with heating...
  • Easy to Install: installation of the thermostat takes less than 30 minutes. A user manual is included, simply follow the step-by-step installation guide.
  • Remote Control: easily control your thermostat wherever you are, as long as your phone has the Smart Life App.
Save 16%Bestseller 2
Emerson Sensi Wi-Fi Smart Thermostat for Smart Home, DIY, Works With Alexa, Energy Star Certified, ST55
  • PRIVACY PROTECTION*: Sensi won’t sell your personal information to third parties
  • SAVE ABOUT 23% ON HVAC ENERGY*: The ENERGY STAR-certified Sensi smart thermostat can help you save energy with features like flexible scheduling, remote access, and usage reports
  • EASY DIY INSTALLATION: Use the built-in level and step-by-step app instructions for a quick installation. Works with HVAC equipment found in most homes. Common wire (c-wire) is not required in most...
Save 19%Bestseller 3
Google Nest Thermostat - Smart Thermostat for Home - Programmable Wifi Thermostat - Charcoal
  • ENERGY STAR certified smart thermostat for home that helps you save energy and stay comfortable.Connectivity : Wi-Fi - 802.11b/g/n 2.4 GHz, 802.11a/n 5 GHz Wi-Fi., Wireless interconnect : Bluetooth...
  • The Nest Thermostat is designed to work without a C wire in most homes, but for some systems, including heating only, cooling only, zone controlled, and heat pump systems, you’ll need a C wire or...
  • Nest Thermostat turns itself down when you leave, so you don’t waste energy heating or cooling an empty home. Lock feature: No
Save 44%Bestseller 4
Sensi Touch Smart Thermostat by Emerson with Touchscreen Color Display, Programmable, Wi-Fi, Mobile App, Easy DIY, Data Privacy, Works Alexa, Energy Star Certified, ST75 - Black, C-Wire Required
  • PRIVACY PROTECTION*: Sensi won’t sell your personal information to third parties
  • EASY DIY INSTALLATION: Use the built-in level, illuminated easy-click terminals and step-by-step app instructions for a quick installation; Works with the HVAC equipment found in most homes. Common...
  • SAVE ABOUT 23% ON HVAC ENERGY*: The ENERGY STAR-certified Sensi smart thermostat can help you save energy with features like flexible scheduling, remote access and usage reports
Bestseller 5
Honeywell Home RTH6580WF Wi-Fi 7-Day Programmable Thermostat with C-Wire Adapter
  • This bundle includes Honeywell Home RTH6580WF Wi-Fi 7-Day Programmable Thermostat and C-Wire Power Adapter.
  • EASY OPERATION AND INTUITIVE. Big, backlit digital display and easy controls make for a user-friendly experience.
  • SMART HOME INTEGRATION. The Wi-Fi 7-Day is compatible with voice assistant devices like Amazon Alexa, Google Assistant, Microsoft Cortana, and more.

As an Amazon Associate I earn from qualifying purchases. Last update on 2023-09-30 from Amazon / Images from Amazon Product Advertising API

  • Updated October 29, 2021
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