Smart thermostats are usually easy to install, but it's important to know how your system is wired before you begin. Always make note of your wiring before you remove your old thermostat, and if possible, take a picture. Most of the latest devices come with pre-printed labels that you can attach to the wires to help identify them during the installation. You should also know what type of system you have as that often comes up during setup. Common HVAC system types include gas, oil, electric, propane, forced air, and radiant. Most smart thermostats work with multi-stage heating and cooling systems, but if you plan to control other components such as whole-house dehumidifiers, heat pumps, and ventilation systems, make sure you select a model that supports those devices. In fact, if you have a complex multi-component HVAC system, it's a good idea to have a professional HVAC technician install the thermostat for you to avoid damage to your system. Another factor to consider is whether your system contains a C (common) wire, which supplies constant power to the thermostat. Almost all smart thermostats require a C wire to work, but older houses with older heating systems usually don't have a C wire because older thermostats did not require one. Some smart thermostats come with a power extender kit that you can install yourself to supply power to the device. Other devices, including Nest thermostats, can be installed without a C wire, but steal power from the furnace control circuits to provide enough power to keep the internal battery charged. Although this method usually works without issue, it still might cause problems with certain HVAC systems and might actually cycle on the system to steal power. If you prefer not to take a chance on frying your furnace's circuit boards, hire a professional technician to run a C wire for around $150 or so. Between installation and the device itself, the cost of a smart thermostat can carry a significant premium over a traditional model. But keep in mind that it can save you money on heating and cooling costs over time, not to mention the flexibility it provides when you're both in and out of the house.