Smart Thermostats almost always require a C-wire
The internet of things is on the rise and one of the easiest and most useful applications of this is the smart thermostat. Popular smart thermostats that come to mind at the Nest, Ecobee, and the Honeywell Lyric most of these and other smart thermostats that come with WiFi. Installing these thermostats can be tricky due to the C-Wire or Common Wire.
For thermostat installations you need to have a set of wires that run from your heating and/or cooling system to your thermostat(s). In instances where you just have a boiler or furnace without the ability to cool you see a thermostat with just two wires, red and white. If you have a cooling system with your heating system you'll most commonly find the thermostats using 4 wires. If you add a smart thermostat you're going to need a fifth wire, the c-wire. C-wires help delivery more power to the thermostat to power the Wi-Fi functions. Adding this wire can be difficult as it require you to run new wire your old wire only had the require wire for the thermostat. Thermostat wire is usually bundled together with several wires together in configurations of 2,4, or 5.
In newer homes you'll find that the thermostat may only need two wires but has more. This makes installing a smart thermostat easier as new wire won't be required.
The Nest thermostat in some circumstances won't need a c-wire to be installed, but one should always be installed to prevent against wear on your heating system. The Nest has an internal battery that runs the thermostat and it needs to be charged to function. Where a c-wire isn't giving it constant power the Nest will pulse the heating system on periodically to get the proper amount of power. In the winter months this isn't much of an issue in the Northeast because your heating systems normal use should be enough to keep the thermostat powered. During the warmer months this is an issue, your thermostat will be turning on the heat to keep it powered in the middle of the summer. This will burn additional fuel, add heat to your home, and could short your heating system electrical systems.
Before purchasing a Wi-Fi or Smart thermostat, check to make sure you have enough wires for you needs. If you have just heat you'll need three, heat and cooling you'll more than likely need five.