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

https://blogfreely.net/borianyvyf/ac-services-salt-lake-city-homeowners-commonly-need-over-time

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.