Thermostatic Radiator Valves
 Often referred to as TRVs 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 not 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.
 Setting Thermostatic Valves
The valve is set by turning the control knob 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.
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.
The temperature markings on these valves do not represent specific room temperatures because the temperature at which the valve operates depends on the air flow around the valve, the water temperature, the size of the room and the losses through walls, windows, doors, floors and ceilings. It is quite normal to have very different settings for the same temperature in different rooms because of this.
With some types of thermostatic valves opening them fully just means they switch off at a higher temperature rather than being fully open independent of temperature. This can cause a problem when balancing heating systems and it is usually safer to remove the adjustment mechanism when setting up and balancing the system.
More Than One Radiator In A Room
If there is more than one radiator heating an area there tends to be an interaction between them. To set the valves in this case start with them all on the same setting and then if the room temperature is not even try adjusting the one adjacent to the coldest part of the room. This can require a bit of experimentation. In practice it is easier to try and reduce the draughts and heat loss causing the imbalance in room temperature which will save energy and money as well.
Radiator Covers
Sometimes radiators are fitted with decorative enclosures which look nice but they do reduce the radiator efficiency quite a bit. Where these are fitted the thermostatic valve should be outside the cover to ensure adequate air flow and the correct operating temperature range of the control.
Types Of Thermostatic Radiator Valve
There are two main types of thermostatic radiator valve, the older unidirectional type which must be fitted on the 'flow' side of the radiator and need to be the correct way round for proper operation and the newer bi directional type which can be fitted either way round and are sometimes found on the 'return' side of the radiator. Unidirectional valves fitted the wrong way round can cause a knocking or loud buzzing sound from the system which usually occurs as the valve is closing.
Thermostatic Radiator Valve Faults
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. If the pin is stuck tapping gently with a hammer will often free it, if not then the only option is to replace the whole valve assembly.
Air Circulation
Radiators fitted with thermostatic valves work best when there is free air circulation around them, for more information check out radiator efficiency.
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