A thermostatic radiator valve (TRV) is a self-regulating valve fitted to 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. This plug is connected to a pin which in turn is connected to a valve. The valve gradually closes as the temperature of the surrounding area increases, limiting the amount of hot water entering the radiator. This allows a maximum temperature to be set for each room. As the valve works by sensing the temperature of the air surrounding it, it is important to ensure that it is not covered by material (such as curtains). If the controller is removed from the valve, the valve turns on and the radiator will always be hot. Interaction with room thermostats Thermostatic radiator valve should not be installed in the same room where the room (air temperature) thermostat is installed. This is because in the case that the TRV set temperature is below the room thermostat set temperature, the TRV would shut off the radiator before the latter temperature is reached. The central heating boiler would continue to run in an attempt to reach the room thermostat set temperature, potentially heating the rest of the house to uncomfortably high levels if TRVs are not installed on the radiators in every room. If both TRV and thermostat set temperatures were set equally, unpredictable behaviour may occur with both devices attempting to control the room temperature. Therefore, in case of installing a TRV and an air temperature based boiler thermostat in the same room, the TRV should be set to a higher temperature than the room thermostat. Compared to manual control The replacement of a manual heating control with a conventional wax motor TRV has been estimated to save at least of CO per year (in 2011 for a British semi-detached house with 3 bedrooms and gas heating). They are also considerably cost-efficient, using heat only when needed, and can reduce heating bills by up to 17 percent a year. Electronically controlled variants As of 2012, electronically controlled TRVs have become more common, and some of these are marketed as smart thermostats or even smart TRVS. They frequently use electronic temperature sensing, and