One common problem when installing a smart thermostat is the lack of a c-wire in your home’s wiring. As more consumers install the device themselves, we suddenly need to become electrical experts. This quick guide will explain what a “c wire” is, what color it could be and how best to install one if needed.
Since the arrival of the Nest device, smart thermostats have been a hot item for homeowners. They help heat a home efficiently and can use clever features to control the temperature from anywhere and save money through smart budgeting.
Traditional HVAC wiring in homes uses four or more wires, one of which is called the “common” wire or c-wire. The basics layout of a typical home wiring system is:
The trouble is, the c-wire is not that common in many (older) homes, as a traditional thermostat didn’t require much power. Today’s thermostats come with Wi-Fi connections and high-resolution color touchscreens, demanding more power than older models.
They are more likely to require one for this purpose, either to complete the circuit that helps power the smart thermostat or to charge its battery.
Even so, it can be used by installers to provide power to the thermostat. With some smart thermostats not requiring a C wire, this is confusing for people.
Those who find their wiring is missing the c wire can use one of the several solutions to power their smart thermostat. Since most brands sell their smart technology without mentioning this on the box, it is easy to find out after you have purchased one.
The smart home thermostat market is booming with many new models available. The majority of them need a common wire to operate. Some of the leading models on the market can work without a c wire, but only on some HVAC systems, not all of them. These include the:
Even then, the Nest uses a battery that may degrade over time.
The Sensi recommends connecting to a common wire, to improve battery life and performance. With the ecobee, the company provides a common wire fix, offering a Power Extender Kit that is included in the package.
The simplest way to check if you have a common wire within your existing wiring is to open up the installed thermostat and take a look. Check for a terminal that has a “c” next to it. If this terminal is connected to a wire, then you have a common wire.
In some rare cases, if you can’t see the connections, then you may need to remove the thermostat behind the wall. Check there for a wire running to the “c” terminal to confirm if you have an existing c wire.
For homes that lack a common wire, a range of solutions are available, ranging from the practical to the rather risky.
The best all-round solution is to add a c wire to your home. When it comes to adding a c wire, you should use 18/5 gauge wire. If you only use the heating circuit, then 18/3 will do, but you might as well use 18/5 to cover yourself for future changes.
You can buy an add-on kit. Venstar is one vendor that offers a smart kit.
Having opened the thermostat, use the wire jumper in the kit to duplicate the wire suggested in the guide, allowing you to add a new wire with relative ease. If this sounds worrying, then a professional can install one for you quickly and easily.
This option is perhaps the simplest. Buy a smart thermostat that doesn’t require a common wire. The Nest manages quite well without one in typical homes, another example is the Honeywell Lyric Round 2.0. While the ecobee and Sensi recommend them, you can get by without one.
Other solutions include power stealing. This is where the thermostat leaches power from another wire when that’s not being used by the HVAC system and stores it in a battery for the thermostat to use.
The problem is that too much power stealing can damage the HVAC system, while if you steal too little, this will damage the battery and thermostat, making it a highly risky strategy.
Issues with poor battery stealing may show up through poor WiFi reception or regular loss of signal, or failure to switch on. This is not something you would expect from a smart device, so don’t risk it.
Finally, if you have a HVAC system that doesn’t use a fan, then you can borrow the G wire from the fan connection and use that as your new wire. In the video below you’ll find instructions how to do this.
However, if you ever decide to add a fan to the system, or someone else in the property in future does, then that does leave the system unable to operate such a feature. Considering that, it is probably best to get an expert in or use an adapter to make a more sensible change.
Building a smart home around an old property’s wiring can cause several issues, with this as one of the most common. As a rule, for your smart thermostat to function properly, they definitely require a common wire. Relying on a battery has proven troublesome to users in the past, and dubious workarounds are just that.
Use the best way to solve this problem based on your technical skills or get an expert to do it. That way your smart thermostat will have sufficient power to do its job and you can enjoy, smartly controlled, efficient heat and cooling when you need it.