Nest and Ecobee can work with many common 24V heating and cooling systems, including some heat pumps
https://s3.us-west-1.amazonaws.com/home-fix-hub/why-your-thermostat-quit-when-it-isnt-even-the-thermostat.html
Nest and Ecobee can work with many common 24V heating and cooling systems, including some heat pumps, though certain homes may need a C-wire or add-on power accessory.