Thermostatic Radiator Valves
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 The purpose of these valves is to enable control of the temperature in individual rooms, turning off the hot water flow when the room reaches the set temperature. If radiators are nor fitted with these there is considerable scope to improve the efficiency of the heating system by fitting them. Unless there is a bypass system one radiator, usually the bathroom, should not be fitted with one otherwise when all rooms reach their set temperature the pump will be trying to force water through a blocked system.
 The valve is set by turning the control know to set the desired room temperature. Since rooms can take a long while to reach temperature setting these valves correctly isn't a quick process. The best way is to do it for each room over several days, before adjusting the valve make sure the heating has been on for a few hours to make sure it has stabilised. If the room is too hot screw the valve clockwise one mark and check the temperature again the next day. Unscrew the valve one mark if the room is too cold.
There are two main types of thermostatic radiator valve, the older uni directional type which must be fitted on the 'flow' side of the radiator and need to be the correct way round for proper operation (valves fitted the wrong way round can cause a knocking sound from the system) and the newer bi directional type which can be fitted either way round and are sometimes found on the 'return' side of the radiator. Left alone for a long time these valves can stick closed or partially open. It is a good idea once a year to unscrew the mechanism from the top and make sure the small pin controlling the valve moves freely up and down against it's spring which should hold the pin fully extended above the valve.
The pictures show a typical thermostatic radiator valve with and without the control mechanism which usually just unscrews. Note the set temperature markings on the control in the top picture. The centre control pin is visible in the lower picture.
Radiators fitted with thermostatic valves work best when there is free air circulation around them, for more information check out radiators in the miscellaneous section.
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