Practical Home Energy Saving
Updated: 31-Oct-2014
Central Heating Radiator

Thermostatic Radiator Valves

Thermostatic Radiator ValveIt is surprising how many different faults radiators and thermostatic radiator valves can suffer from but even more surprising is the fact that the majority of radiator faults can be fixed by just turning the radiator on and setting the thermostatic radiator valve properly. 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.

Thermostatic Radiator Valve - Control Mechanism Removed Setting Thermostatic Valves or TRVs

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.

Air Circulation

Often TRVs fail to work properly because either the valve or the radiator has poor air circulation. Radiators fitted with thermostatic valves work best when there is free air circulation around them and the radiator. The cause of restricted air circulation is often furniture such as settees or book cases in front of the valve and radiator, furniture should ideally be at least two feet away from the radiator and valve. Another common cause is things hanging over the valve or radiator blocking the airflow such as clothes or towels, removing these will make a big difference. Over time the head assemblies of these valves can become clogged with dust and dirt, regular vacuuming around the valve and the slots in the head assembly will stop this problem. The gap between the floor and the bottom of the radiator should always be totally clear of obstructions. For more information check out radiator efficiency.

Confusion between Room Thermostats and TRVs

While most homes have a wall thermostat for the central heating that switches off the heating once the room has reached the desired temperature it is possible to get thermostats which are programmable to cut off at different temperatures at different times of the day enabling the house to be cooler at night for instance. f a room thermostat is in a room with radiators fitted with thermostaticĀ valves you will need to decide which is going to control room temperature, since the room thermostat may also turn off the heating to the whole house when its set temperature is reached,. The best compromise is to set the thermostatic valves for slightly higher than the desired maximum room temperature and then the room thermostat to the desired room temperature, with programmable thermostats this will be the maximum temperature.


If thermostatic radiator valves are in any sort of draught they will fail to operate correctly giving variable room temperatures. Radiators are often mounted below windows which can be a major source of draughts, fixing these will not only enable the room temperature to be controlled more accurately it will also save money on the heating bills. Open doors are another source of draughts which means that draughts in adjoining rooms need to be identified and fixed too. For more information check out radiator efficiency.

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.

Manual Thermostatic Radiator Valves

There are two main types of manual 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. There are occasionally cases where even bi-directional TRVs can cause a knocking sound from the system, if this happens the only option is to reverse the valve. It is possible to get valves with a flow reversing collar which can be set after the TRV has been installed. The temperature is set by rotating the control knob - clockwise for lower temperatures.The important thing is to follow the manufacturers instructions when fitting these valves.

Programmable Thermostatic Radiator Valves

More expensive than the manual type they are a great way to achieve zoned heating without modifying the pipe work. They fall broadly into two classes: locally and remotely controlled. The local control ones apart from having adjustable temperature have a motorised valve which is controlled by a battery powered programmable time switch to turn the radiator on and off. The remote ones connect to a central controller either via wires or using a wireless signal. The temperature is set in a similar way to the manual TRVs.The important thing is to follow the manufacturers instructions when fitting these valves.

Which Way Up Should Thermostatic Radiator Valves Be Fitted

The advice from most manufacturers (but not all) is to fit them horizontally on the grounds that it will minimise the effect heat from the radiators have on the valve. In practice most valves end up mounted vertically and from experience appear to work just fine. Mounting vertically instead of horizontally is likely to affect the adjustable temperature range but as long as they can be set to the required temperature that shouldn't be a problem. More importantly see the point about air circulation below. TRVs should not be fitted in tight corners where there will be little air flow or in draughts.

Thermostatic Radiator Valve Faults

Room Is To Hot Or Too Cold

Radiator Always On

Radiator Always Off

Thermostatic Radiator Valve Maintenance

Apart from the very obvious checking for leaks or seepage from the joints occasionally most people forget there is another thing that can cause thermostatic valves to misbehave. It is important to stop the head assembly vents getting clogged by dust and other objects which reduces the air flow within the valve. When ever a room is vacuumed and at least every couple of months use the vacuum cleaner to suck out any dirt from the vents in the head assembly of the valve.

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.

Often forgotten is the need to replace batteries regularly for programmable TRVs - they should last around two years but this will depend on how they are programmed.

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