A C-wire, also known as a ‘common wire’, allows the continuous transfer of 24V power to a thermostat. It is the wire that runs from the HVAC system to the thermostat. You can easily see it if your system has a C-wire.
A lot of traditional and non-connected thermostats do not require a C-wire to operate. However, in the modern world, many Wi-Fi thermostats possess features like LED colored screens that need the device to be powered constantly. Thermostats get this constant supply of power through the C-wire.
When shopping for smart thermostats, you are bound to hear that a C-wire is required. The C-wire is important because it provides the power needed to operate a smart thermostat. In this article, we take a look at how to recognize a C-wire, and what to do if you do not have one.
How to Recognize a C-wire:
- The first thing you need to do is to confirm that you have a 24-volt low voltage HVAC system.
- Remove your thermostat and look at the wires attached to it. The colors do not have a specific meaning. For most thermostats, the C-wire is usually blue, but this is not the case for all devices.
- At the back of the thermostat plate, you will see a couple of letters. If there’s a wire going towards the letter ‘C’, your system has a C-wire.
- If you’re unable to identify a wire that looks like the C-wire, you can check behind your current thermostat again. You may see a wire that is not attached to any terminal. If the existing thermostat did not require a C-wire, it may be rolled up into the wall.
After checking your system you may find that you have a C-wire, or you do not have one. If you have a 24V low voltage system and a C-wire that’s pretty good! You can now choose the right smart thermostat for your system. On the other hand, if you have a 24V low voltage system but there’s no C-wire there’s still something you can do.
If you have a 24V low voltage system but no C-wire, you can choose to:
- Turn the spare wire at the back of the thermostat into a C-wire
Earlier on, while checking for a C-wire, you may have found a spare wire that is not connected to any terminal. You can transform this wire into a C-wire. All you need is to connect this spare wire to the ‘C’ terminal at the backplate of a smart thermostat.
Thereafter, go to the control panel of your HVAC system. Attach the same colored wire that you attached to the C-terminal of the thermostat, to the C-terminal of the control panel. Ensure that you put back the front cover securely. Some systems will not operate if the safety lock is out of place.
- Convert a G-wire into a C-wire
A number of thermostat manufacturers suggest the conversion of the G-wire into a C-wire. To do this, you simply disconnect the G-wire from its terminal and reconnect it into the C-terminal of both the thermostat and the control panel. This works but it also hinders your ventilation fan from working in manual mode.
- Get new thermostat wiring
Here, you will have to remove the current thermostat wiring that connects your HVAC system and your thermostat. Thereafter, you will have to connect new wiring through your floors or walls. Depending on the size of your home/building, this may be a quick job or a tiring task. You may need to hire a qualified technician to do the rewiring.
The advantage of doing this is that your HVAC system will now be compatible with almost any type of smart thermostat. This is actually the best step to take if you don’t have a C-wire.
- Purchase a Venstar Add-a-Wire adapter
If your HVAC system has 4 wires already and only needs a fifth wire to act as the C-wire, a Venstar Add-a-Wire adapter is the solution. It is cost-effective and operates well.
This device adds a C-wire to 24V low voltage systems. The only condition is that your system needs to have at least 4 wires. You will also need to open up your heating and cooling system and spend some time opening all the different parts. Follow the instructions for using the Venstar Add-a-Wire adapter according to your situation and attach it.
It is not hard to install the Venstar Add-a-Wire adapter. If you have ever reconnected an electric outlet or installed an electrical lamp, you can easily connect an adapter. Some tools you’ll need are: screwdrivers, a pair of pliers, a drill with a drill bit, electrical tape and two wire nuts.
After gathering your tools, access the wiring of both your heating/cooling system and the thermostat. From here you will connect new cable parts at the back of the thermostat and in your HVAC system.
- Select a thermostat that comes with an adapter
Ecobee products are suitable for you if you have a 24V low voltage system that lacks a C-wire. Ecobee thermostats come with an adapter that you can connect to your HVAC system to act as a C-wire. The adapter is small and you can install it easily.
Just follow the instructions manual to install the Power Extender Kit into your HVAC system.
- Select a thermostat that does not need a C-wire or an adapter
A great device to use is the Emerson Sensi. This low-powered smart thermostat does not need a C-wire. It works without a C-wire in heating and cooling systems. But if you need single mode like for heat-only, or cool only or heat pump without auxiliary heat systems.
The Honeywell Lyric Round Wi-Fi Thermostat is also a smart thermostat that does not need a C-wire to operate.
If your low voltage system has very few wires and you are unable to rewire it, you can try the LuxGeo. If your HVAC system is completely incompatible with other devices in the market, a LuxGeo may be the solution. It comes at a fair price and has great features.
The LuxGeo Wi-Fi thermostat is a great device for your home if your current thermostat wiring fails to work with most smart thermostats. You can run this device on AA lithium batteries and connect it to a two-wire HVAC system. It is simply a wonderful thermostat.
If you have a line voltage system and you want a Wi-Fi thermostat you can choose to:
- Get an adapter
If you own a line voltage HVAC system and you really want to connect it to a smart thermostat, an adapter is what you need. An adapter, for example, the Aube RC840T-240, lowers the system’s voltage. You can lower the voltage of your line voltage HVAC system by just installing the adapter.
After installing the adapter, you can then search for a low voltage smart thermostat that you like and use it with your HVAC system. This adapter, however, is very bulky. You’ll probably have to find a way to hide it in the wall. Additionally, you will require one for each of the line voltage heaters in your home.
- Purchase a thermostat that is made for a line voltage system
Wi-Fi enabled devices are not that many for line voltage systems compared to those for low voltage systems. However, if you own an electric baseboard heater there’s a Wi-Fi thermostat you can use. Caleo, which is made at CaSA, is a Wi-Fi thermostat made specifically for electric baseboard heating systems. It works with IFTTT and you can easily control it using your smartphone or computer by signing in on Caleo’s website (this is not an app but a mobile-friendly website).
This Wi-Fi enabled thermostat is great for those who own electric baseboards. However, it does not work with a system that uses a motor or a fan. It is incompatible with forced air heating systems; therefore, if your current thermostat controls a fan, Caleo is not for you.
For line voltage systems, you can also get the Stelpro KI STZW402WB+. Unfortunately, this thermostat is also incompatible with systems that use a fan. Furthermore, the Stelpro is a ZigBee (Z-Wave) smart thermostat. This simply means that the device is non-programmable by itself.
You will have to get a Z-wave hub or controller like Samsung SmartThings. Connect this Z-wave controller to the internet so that you can control the thermostat with your smartphone.
In the coming years, hopefully, there will be more options for line-voltage users. There aren’t that many smart thermostats suitable for line-voltage systems.
There are a lot of differences among HVAC systems and it is very likely that your heating and cooling system is not among those featured in this article. The best thing to do is to first establish the type of heating or cooling system you have and find suitable options from there. Do market research and establish which companies make devices suitable for your HVAC system and accessories.