A common wire is needed to provide enough power for advanced functions in smart thermostats. There is a requirement for a C wire in smart thermostats. The thermostats are on low power and can be installed by anyone who wants to. It's important to keep in mind that some thermostats have more reliable performance without a C wire than others.

No C wire? No problem (mostly)!

Most older homes don't have a C wire. If you don't want to pay for an electrician or install the wire yourself, these smart thermostats without a C wire will still save you money. Some use batteries that you have to keep replacing, while others need a power extension kit that requires some simple wiring. Many of these are among the best smart thermostats, so you can be sure that a C-wire requirement doesn't mean better quality.

Since it's so affordable, the Nest Thermostat E is your best option if you prefer painless, battery-powered units. It has a lot of smart features like motion and multi-room sense. At this point, it's a fairly old thermostat, and the new Nest Thermostat has mostly been moved on to. The old dog has a few tricks that make it a better choice than the nest thermostat.

Whether your Nest or Nest Learning Thermostat will work with a Power Connector varies from home to home. For more consistency, consider ecobee instead.

An older Nest Learning Thermostat (3rd Gen) requires slightly more power than the Nest E and may be compromised if a C wire is not used. The low power issues that people had with the old Power Connector did not go away even after the new one was released. Whether a nest works without a C wire is dependent on where you live.

The ecobee SmartThermostat requires you to wire in a power extension kit to fix the missing C wire. The process should be easy because ecobee designed them with it in mind. It's the best smart thermostat for multiple zones and it's great for people with large homes. ecobee's offering is the king of smart thermostats and its price reflects it.

A simple Venstar kit can be used to install a new set of wires. Virtually any smart thermostat should work with your home's heating and cooling system.