How to install Nest Thermostat on an old furnace with no C wire?

You know, we move quiet often. Every time we move into a new home, this is the first thing we do. We upgrade the old thermostat into our Nest Thermostat to get all the smart features, even if its a rental home. Having smart thermostat saves energy, convenient to program & control via phone and saves money in course of time. These wifi thermostats come with easy installation guide. Hence, its fairly easy to swap the old one with smart thermostat and replace it back to old one when we move out of the rental home. So far its been a easy swap in our previous homes. But in this home, we have a very old furnace. So, it did not have a ‘C or common‘ wire. In this blog, I show you How to install Nest Thermostat on an old furnace with no C wire.

Disclaimer : We only own Nest thermostat. so this blog is going to be only pertaining to it. And we are not sponsored by them. Also we are not electrical experts of any sort. We found this solution to make our smart thermostat work and wanted to share it to help anyone who is in the same boat with no C wire from your furnace.

What is C- wire?

A common wire (C-Wire) provides Wi-Fi thermostats continuous power by connecting it to a heating and cooling system. C-Wires are required for installing most of the smart Wi-Fi thermostats in the market. Old thermostats with small display does not need constant power. Instead they only use AA or AAA battery. So, if your house has an old furnace, they may not have wired with C wire.

Step – 1: Turn OFF power

You have to first turn off the furnace power from the main panel. Make sure to test using a pen tester.

Step – 2: Identify the wires

Remove the existing thermostat to expose the existing wires and identify the colors. In my previous house, we had the exact wires that we need to install nest. i.e. yellow, green, blue, red, and white. So, we installed the nest as per the below diagram and it worked great.

But in this house, we had more than 5 wires in different colors. Refer below image. So, next we checked the furnace panel to identify the connectivity of each color to the furnace’s circuit.

Turns out we had an old furnace. From the image, you can see there are only four wire – an R wire (for power), a G wire (for the fan), a W wire (for heat) and a Y wire (for A/C) configured correctly to the screw connector. But screw connector for C wire is missing. There is a C indicated below the 4 screw terminal set. From the manual, I understood that its the power wire for the outdoor condenser unit.

Step – 3 : Try these options

With some research, we found on internet the following three options to install a Nest thermostat with no C wire:

OPTION – 1: Use an extra, unused wire as a c-wire

As you can see, we have lot of unused wires. But there is no screw terminal to connect the C wire. The C terminal below dind’t seem like we can add one wire to it. May be you can, but we did not know how. So, this option was ruled out for us.

OPTION – 2: Using your thermostat’s C-wire (fan wire) as a C-wire

We tried this. But our nest did not accept this configuration. Not sure why. It kept on indicating ‘No power’.

So, we went for the third option.

OPTION – 3: Installing separate 24Volt Power Adapter

This one worked guys!!! We bought this power adaptor from Amazon that is supposedly meant to work for all smart thermostats. This adaptor has a plug in transformer that plugs into your standard 120V outlets and converts to 24volt AC current. It has long cable that enables you to install the cable along the wall and connect to the electrical outlet inside or outside. Also it has two wire terminals.

Next, close up the furnace cover and head to the thermostat location. Group the needed wires and tuck in the remaining wires safely.

Install the thermostat base and pull all the wires out including the two white wires from power adaptor.

Then wire them to respective terminal like in below image. One of the wire should be connected to C terminal and other to Rc. Both can be used interchangeably, it does not matter which one you use.

It worked!!! 🙂

This option worked out great. The only draw back is the ugly cable running to the power outlet. You can either hide them behind some furniture or route the wire inside the wall.

Hope this article was helpful to install Nest Thermostat on a very old furnace with no C wire. Please remember that we are not experts in such electrical works. We researched a lot and found this option that worked for us. If your furnace wire configuration is different or not understandable to you, then call your local electrician for the works. Do not risk. Ignorance can result in accidents that you do not want to involve yourself or your loved ones in. So, please be careful.

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