Thermostatic radiator valve

A thermostatic radiator valve (TRV) is a self-regulating valve fitted to a hot-water heating system radiator to control the temperature of a room by changing the flow of hot water to the radiator. == Functioning == === Conventional wax motor TRV === The classic thermostatic radiator valve contains a plug, typically made of wax (forming a wax motor), which expands or contracts with the surrounding temperature.

Source: Wikipedia — Thermostatic radiator valve (CC BY-SA 4.0)