Adjusting a thermostatic radiator valve (TRV). It is a relatively simple uncalibrated device in which the flow of water through a radiator varies with the expansion or contraction of a wax plug. They are used to control room temperatures according to choice and thus can promote energy efficiency. It is claimed that they reduce energy bills and CO2 emissions. They need to be sited with care and set to an appropriate thermostat setting. If set too low, it may give rise to condensation and a damp environment.