How To Install A Smart Thermostat Without C Wire

By Steve Hall

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Are you preparing to install a Smart Thermostat and only have two wires (a red and a white) and no c wire or common wire? Then this article is the easy answer to the problem that no one seems to be talking about.

When my wife and I recently purchased the four Ecobee Smart Thermostats that now reside in four zones of our 2700 square foot house we had no idea that there was one potentially problematic (yet common) installation problem if you do not have a c wire.

The following are the steps we took and how to install a smart thermostat without c wire (or common wire). In our example, we used an Ecobee thermostat during this install.


If your furnace does not have a C terminal then according to Ecobee you will need to use their PEK or Power Extender Kit. Which looks something like this.

If you contact Ecobee they will tell you that you need to have a c wire in order to get your thermostats up and running and that includes the PEK.

Even though at the time when we bought ours Home Depot didn’t say anything about needing one. In fact, they had a sign right in front of the smart thermostat display saying “No c wire needed for installation”.

I have seen on the forums where a lot of people are complaining about this. So I did my research and have crafted a simple way of doing the install yourself without any need for adding a c wire to your existing system and it is TOO SIMPLE.


I suppose I should preface this by saying a couple of things.

1. – I am not a certified electrician or plumber and that if you do this, you are doing this install at your own risk.

2. – My heating system is a forced hot water boiler system and I am using Ecobee thermostats so your results may vary depending on the type of heating system you have in your home.

Now that I have that out of the way, let’s begin. First of all, you will need to have your smart Thermostat completely unboxed and ready to go.

Remove your old thermostat. If you only have two wires, like me, then you will see a red wire and a white wire. Personally, I prefer to shut off the breaker to the particular thermostat that I am working on before going any further.

screwdriver install smart thermostat


Once you have turned off the power to that smart thermostat, then remove the red and white wires from the backplate. Typically you will find that they are held in by small Philips head screws.


Remove the backplate from the wall. Usually, this is held in place by two to four screws into the wall. If you have Sheetrock, be careful not to tear it to ribbons when removing the old backplate.



At this point, we are ready to install our new wifi thermostat. Now, here comes the answer to the problem.

You will need to buy a 24 Volt c wire Power adapter from Amazon (I haven’t found them in local stores personally). Check the current price on the ones that I personally purchased over here on Amazon.

They are relatively inexpensive and typically these can be purchased for under $30. Many of these come with a rather long power cord, some are 25 feet or more!

Cut down the wire to the intended length and strip off the ends of the wire to a very short length. Then you will want to run the two wires that come from the AC adapter as well as your red and white wire coming from the wall up through the center of the round backplate before attaching the backplate to the wall.

As you can see in my picture from the one I installed in my finished basement it doesn’t need to be very long if you are near a power outlet. I know it looks messy, but this can easily be remedied with a little wire molding to tidy it up.



The wiring part is easy as pie. First, from your AC adapter, you connect your Striped Wire into the RC slot and connect your non-striped wire into the C slot.

Now connect the red wire from the boiler into the RH slot and your white wire into the W1 slot.

Double-check your connections and then finish up by affixing the backplate to the wall.

At this point, you should be able to pop on your Ecobee thermostat and see it power right up with no problem.


A little bit of a side story on this. We didn’t realize that it was this simple, so we brought home four thermostats from home depot on a Saturday morning and returned them that same Saturday night.

Because we figured we were just out of luck altogether. But I was determined that there MUST be a fix for the problem of not having a c wire to the Ecobee thermostats in my house without spending an arm and a leg to have an electrician come in and rewire things. So I googled for hours and found all kinds of other problems but not this one in particular.

I really hope this helps someone else out before they make the mistake of returning these thermostats. Also, if you are just starting out on your home automation journey, then you may want to check out my article 11 Tips For Smart Home Beginners.

Is there a WIFI thermostat that doesn’t require C wire?

I keep hearing stories about the Nest smart learning thermostat and the Ecobee thermostat working without a c wire out of the box. I have owned several versions of both of those thermostats and I have NEVER once been able to get any smart thermostat working properly without a common wire.

This could be HVAC system dependent. I really don’t know. But I have never been able to replicate those results with either of those thermostat brands personally.

I know that in my search when I bought my Ecobee thermostat’s at Home Depot they claimed right on their signs that these will work without a c wire. I suppose that depends on your HVAC system as I mentioned before.


No matter which Ecobee you have, wether it’s the Ecobee3, Ecobee3 Lite or the Ecobee4. If you are having issues connecting or staying connected to your wifi. It could be one of a few things. So you’ll want to ensure that you follow the steps below to make sure that everything is working properly.

  1. Make sure your router is powered on and that the Ecobee is connected to the 2.4 Ghz band.
  2. Make sure your Ecobee Thermostat’s firmware is up-to-date by going into MAIN MENU > ABOUT > VERSION
  3. Make sure that the encryption type on your router is set to WPA/WPA2-AES. Ecobee no longer supports TKIP encryption due to some security flaws.
  4. If your signal strength is too low on the ecobee then it may be having a problem in general. You can check it by going into MAIN MENU > ABOUT > WI-FI. If you see a signal strength lower than 75% that could be your problem.

Ecobee actually discussed this issue on their website and offer some helpful troubleshooting tips as well.

How do I install a Honeywell thermostat without a C wire?

The video below will help you install your thermostat if you do not have a c wire. But you do have a gas, oil or propane conventional forced-air system and a wire labeled G. If you use this option, the G wire will be substituted for your c wire at both your thermostat and HVAC system. This will remove your ability to turn your system fan on, however the fan will still run when your system runs.


Once you have smart thermostats in your home it is impossible to go back. It is absolutely a lifesaver, especially if you own a second home or if you rent an apartment out to someone and want to be able to control the heat.

I live in New England and just took a trip to Disney with my wife and daughter. It was so nice to be able to just schedule the thermostat differently while we were away and then have it automatically scheduled to heat up the house before we came home!

No one wants to return to a cold house in New England in the middle of winter after enjoying 8 days in sunny Buena Vista, Florida!

So there you have it, all you need is a 24 Volt AC adapter for each of your Ecobee thermostats and you are good to go. I’m no math whiz, but it makes so much more sense to me to do this rather than calling an electrician!

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Steve Hall

Steve Hall is the founder of this website, a certified technology professional and a smart home tech enthusiast fascinated by the power of connected devices.

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