In the past, thermostats didn’t require a continuous power supply of their own as they were merely on/off devices. However, modern Wi-Fi thermostats with backlit display need a steady source of power to function. Thus, for installing a Wi-Fi thermostat, you need a ‘’Common” wire or C wire to enable a continuous flow of power to the thermostat.
What is a C Wire?
A large number of homes, especially the older ones, don’t have an additional C wire which the modern thermostats need. The job of this C wire is to establish a connection between the furnace or air handler and the thermostat to ensure that they can communicate and use the features of the modern Wi-Fi thermostats appropriately.
The wires used in older homes usually have two or four strands, whereas, modern cables have five strands out of which, one is a C wire. When this wire is connected inside a furnace or air handler, it provides continuous 24vac power to the Wi-Fi thermostat.
If you have an older thermostat and are planning to replace it with a Wi-Fi thermostat, before browsing through the options, the first thing that you need to do is to check whether you have a C wire or not.
How to Find if I Have a C Wire or Not?
The process is pretty simple! Don’t disconnect any wires from your current thermostat and remove it from the wall. You will be required to remove the wall plate as well to reveal the connections. If you see that there are five wires, then you do have a C wire. But if not, then you will be required to run a C wire from your furnace or air handler to your new thermostat.
A C wire is usually blue in color, but can be of the different color as well. It is also imperative to mark the wires to their terminals before you disconnect them. If you don’t have a C wire, it is important to get one as almost all the modern thermostats require them.
But apart from running a new cable, which can be a lengthy process that will need expert help, there are many different ways in which you can resolve this problem. Some of the easiest solutions are mentioned below.
Buying a Wi-Fi Thermostat which doesn’t need a C Wire
Yes, there are some Wi-Fi thermostats without C wire. Two of the best examples are Sensi Wi-Fi Thermostat and ecobee3. Let us have a look at how these thermostats function without a C wire.
Sensi Wi-Fi Thermostat
The Sensi Wi-Fi Thermostat doesn’t need a C wire as it makes use of 2 AA batteries for backup power. These 2 AA batteries can keep the Wi-Fi thermostat functional, generally for up to a year or so, to ensure that you don’t have to get into the hassles of replacing the batteries now and then. Thus, with a Sensi Wi-Fi Thermostat, you can easily replace your previous thermostat on your own even if you don’t have a C wire.
However, if you do have a C wire, then it is recommended that you should connect the Sensi Wi-Fi Thermostat to the C wire to enhance the thermostats battery life and Wi-Fi connectivity. While the thermostat is compatible with the majority of the traditional and modern, combined cooling and heating systems, there are a few cool-only and heat-only units to which you can only connect Sensi Wi-Fi Thermostat with the help of a C wire.
If you are looking for a Wi-Fi thermostat without C wire, ecobee 3 is currently the best available option. Apart from the innovative features of this Wi-Fi thermostat, one of the best things about this unit is it doesn’t require a C wire. The thermostat comes with a Power Extended Kit of its own which eliminates the need for a C wire. This is the only Wi-Fi thermostat that comes with an additional power kit.
Installing an External Transformer
If you don’t want to buy a thermostat that doesn’t require a c wire, an ideal solution is to install an external transformer. While the process looks complicated, and appears as if you need to have professional knowledge about wiring to install an external transformer this is not true. It is a straightforward and quick process and at the end, you will be able to install a Wi-Fi thermostat that requires C wire in your home even if you don’t have a c wire.
Using Venstar Add-a-Wire Adapter
If you have a little knowledge about cables, then installing a Venstar Add-a-Wire Adapter can do the job for you. For installing this adapter, you will be required to access the behind of your thermostat and your furnace or air handler as well. While you might be required to spend about an hour for installing this adapter, it is still a pretty convenient and cheaper alternative to running a new C wire.
Fan Wire Trick
Yes, you can also re-purpose the wire of the fan as the C wire to power a Wi-Fi thermostat. But then you won’t be able to switch on the fan manually. This method is not recommended as it limits the functionality of your system. Any of the alternatives mentioned above is a lot better than repurposing your fan wire, so, prefer choosing them over this method.
The bottom line is, even if you don’t have a C wire, there are many different ways in which you can still install a Wi-Fi thermostat. You can choose a Wi-Fi thermostat with no C wire, or connect an external transformer or an adapter if you don’t have a C wire. But among all the options, it is better to go for a Wi-Fi thermostat which doesn’t need a C wire as with all the other methods, you will be required to spend the extra money and might need professional help.