Honeywell Rondostat Radiator Thermostat Electronic For Home 5 to 30 °C ‎HR10

Easy Installation: No need to drain the radiator or touch the plumbing. It simply screws onto your existing M30 x 1.5 radiator valve in minutes. Save Energy & Money: Automatically lowers the temperature when you aren't in the room or when a window is opened, helping to significantly reduce heating costs.

eBay