A thermostatic radiator valve (TRV) controls how much water is allowed to flow into a radiator, based on the room temperature. TRVs prevent rooms overheating by automatically turning down the radiator once the room has reached a set temperature. They’re an improvement over old-style manual valves, and should already be fitted to many modern heating systems. You’ll need compatible TRVs before you can install some brands of smart radiator thermostat.