System Compatibility (what do need to know?)
Low voltage heating/cooling systems are the most common, requiring 24-volt power. All of the Wi-Fi thermostats offer here are compatible with low-voltage systems.
Direct line voltage systems use 110 VAC to 240 VAC power. This would include electric resistant baseboard heating systems or radiant heating. The thermostats offered here are not suitable for use with direct line voltage heating.
Millivolt systems are often natural gas wall or floor heaters that do not use any electricity. Only one of the thermostats is millivolt compatible.
Some thermostats need 24-volt power to be provided through a common-wire ("C-wire"). In some cases, a "C" wire is required, and in other cases it is only recommended. If a wire is attached to the "C" terminal behind the thermostat, you have a C-wire already installed. If a C-wire is not present, one can be installed, but professional installation is recommended.
Conventional one stage heating and cooling systems are either "on" or "off". All these Wi-Fi thermostats will work well with these systems.
Conventional two-stage systems have both low and high speeds. To tell if you have a 2-stage system, if the wiring behind your thermostat is attached to "y1" and "y2" terminals, you have a one-stage system. If the wiring is attached to "w1" and "w2" terminals, you have a multi-stage system. Most Wi-Fi enabled thermostats are compatible with two-stage systems.
Compatibility with heat pump systems that have auxiliary heat will vary by model.
Whole House Systems
Sometimes dehumidifiers, humidifiers, and energy or heat recovery ventilation systems that must also be controlled by the thermostat. The ability of thermostats to control these other systems will vary by brand and model. Often professional installation is recommended in these situations.
If unsure about compatibility, online compatibility checkers are available on many of the product pages, or consult with a qualified HVAC professional.
Smart Home Devices
These devices allow your home to become smarter, often allowing you to communicate with the smart thermostat in new and innovative ways.
Amazon Alexa and Google Assistant, for instant, allow you to change the temperature by talking, instead of having to use your smart phone or walk over to the thermostat. These usually require having an Amazon or Google device in your house, though the ecobee4 allows for voice control without integration with a third-party device.
Apple HomeKit, Wink, and Samsung SmartThings are hubs that facilitate the control of numerous devices from different brands using a common interface, including lights, thermostats, and appliances.