What is a smart radiator valve and how is it different from a smart thermostat?

A smart radiator valve (smart TRV) replaces the manual knob on an individual hydronic radiator, letting you set per-room temperatures and schedules. A smart thermostat, by contrast, controls the boiler or HVAC system as a whole. You generally use smart TRVs in addition to (not instead of) a central thermostat, and they're most useful in homes with hot-water radiators rather than forced-air systems.