I have a smart thermometer that’s more than 30 years old. When the temperature drops below a preset level, the furnace come on and then shuts off automatically. It even has a switch for cooling so that the air conditioner comes on in summer. It’s amazing!
You can still buy “dumb” setback thermostats for $25 or so. They are just a timer that automates the process of turning down the heat when you’re sleeping or vice versa for a/c. Proven energy savings and no snooping.