Thermostatic Radiator Valves – How Do They Work?

08 May. 2019

Written by Neil Marsh MCIPHE RP RHP EngTech

Common Misconceptions: 

There is a common misconception that TRV’s (Thermostatic Radiator Valves) control the temperature of the radiator, this is not true. TRV’s measure the temperature within the room they are located… Once the required setting is achieved, the TRV will shut the flow of hot water off to the radiator, therefore causing it to cool when the central heating is in use.

How TRV’s Work:

Radiators, towel rails and other components of a central heating system that make use of TRV’s as controls will modulate in temperature while in use if the room that it is in temperature matches that of the TRV’s setting. This is part of the normal working operation. As the room begins to cool, the flow of water will be returned to the radiator as the TRV opens up and the radiator will begin to heat up again. The radiator will transmit this heat back into the room and process will repeat.

TRV Settings:

TRV’s are typically labelled with numbers 0 to 5 rather than temperature settings. Some incorporate a frost setting, which usually has a picture of a snowflake to indicate this particular setting. Setting “3” will usually be in the 19-23DegC range, although, please keep in mind that TRV’s are usually at lower/ground level, and this will often be the cooler part of the room as heat rises. Generally, rooms are warmer at a higher level.

The frost setting is designed to allow a flow of water when a low room temperature is reached. This is often set at 5DegC by the manufacturer. Should your heating system incorporate an electronic frost stat or other frost protection features, the system could be designed in such a way that would cause the boiler to fire and heat the central heating circuit to prevent freezing within the property should the cold weather trigger the frost stat.
By not turning off radiators completely, and setting a TRV on the frost setting, you are not preventing the flow of water to the radiator when the temperature drops below the frost temperature on the TRV, which is commonly 5DegC. Should your frost stat engage, and providing it is correctly installed, the warm water from the boiler would flow to all of the radiators where the temperature in the room where 5*C or less, or to rooms where TRV’s are set to higher temperatures to heat them and prevent freezing within the property. This not only protects the central heating system but can protect other aspects of your home such as plumbing, fish tanks, storage tanks and other water containing equipment, which could be damaged due to water freezing.

More information about how central heating systems work can be found by clicking here!

TRV’s, A Low Cost Way To Save Money:

A TRV is one of the most cost-effective controls you can install on your central heating system to increase its efficiency. In addition to this, it makes it easy to isolate radiators in rooms that you do not wish to heat.


Generally, TRV’s are very robust and will last many years. Some may stick as they age and/or as heating systems become heavily contaminated with sediments like limescale or sludge, however, more often than not they can be freed easily and will provide you many years of service. 

It is essential to understand the devastating effects of central heating sludge and limescale can have on TRV’s and other central heating equipment. Click here for a great article about this!

FORTHRIGHT supply highly skilled heating engineers and plumbers in Gloucester, Cheltenham and the surrounding areas of Gloucestershire to undertake tasks such as installing and replacing TRV’s. If you feel that systems efficiency and overall operation could be improved, or your existing controls are ageing and are showing signs that they need to be replaced, do not hesitate to contact us.