How to Install a WiFi Thermostat Without A C-Wire

Imagine buying a new WiFi 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 that is required for your smart thermostat to work properly. 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 and that is the reason why many homes do not have a C-wire installed.

Almost all WiFi thermostat 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 wifi thermostats without a C-wire, it just takes some additional work. We’ll walk you through your options below.

 

How to Install a WiFi Thermostat Without A C-Wire

The first thing you want to do is check for a C-wire by looking at your old thermostat. Detach your thermostat and take a look at the wires connected to it. If there is a terminal labeled “C” then you have a C-wire.

If there is not, go to your furnace and check the source. Sometimes your furnace has a common wire that wasn’t needed during the installation of your existing thermostat, so the wire can be hidden in the wall sometimes.

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 a C-wire.

 

Get a WiFi Thermostat that Works Without A C-Wire

The simplest solution would be getting a WiFi 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 best thermostat with remote sensors. Ecobee smart thermostats come with a “power extender kit” that can be used as a replacement for 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 complete.

 

 

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

 

best thermostat with remote sensor

 

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Nest Learning Thermostat

The Nest Learning Thermostat is our pick for best smart thermostat. In most cases, a Nest Thermostat can work without a common wire. Without a common wire it charges its internal battery using power from the heating and cooling wires. With 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.

best smart thermostat

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Lux Geo thermostat without a common wire

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

Lux Products GEO-WH Wi-Fi Thermostat

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If you would like to get a thermostat that is not mentioned above, and which 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 for the best to leave it to someone who has. A quick visit to Amazon’s Home Services center makes it easy to find someone who can 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 very easy 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 makes 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 is very likely to be different from the one shown in the video, but the process still 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 tradeoff 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 WiFi thermostat that needs a C-wire to work properly.

Of all your options, we think installing a transformer is probably the most overkill, so 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.

 

Conclusion

Keep in mind that, unless stated otherwise, the WiFi 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 hard as it seems, and the step-by-step video instructions we included are very simple and anyone can follow them.

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