Nest and Ecobee can work with many common 24V heating and cooling systems, including some heat pumps

https://telegra.ph/Signs-It-May-Be-Time-to-Replace-Your-Salt-Lake-City-AC-09-04

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.