Thermostatic radiator valve

1 How it works

Viessmann explains that inside the valve a pin is moved by expanding and contracting metal. As the room warms, the pin closes and reduces the water coming in. As it cools, the pin opens again¹. A plain manual valve has no sensing part and works like a tap¹. The valve at the other end of the radiator is the lockshield, which sets flow for balancing.

2 Using the numbers

Viessmann's advice is to pick a medium setting such as 3 and leave it, as the radiator then holds the room at that level¹. Baxi group's Main Eco Compact manual says fully closing and opening TRVs causes undesirable temperature changes, and to move them in small steps⁵. Ideal suggests starting low and turning up until the room is comfortable. It points out that a higher setting does not heat the room any faster⁶.

Viessmann says only that valves are usually labelled this way, so treat its 20 °C as a guide and not as an exact setting. For minimum room temperatures, see room thermostat.

3 Down or off in unused rooms

Sources disagree here. The government's Clean Energy campaign says to turn radiators down, not off, in rooms you are not using, as turning them off can increase the risk of damp and mould⁷. Some maker tips say otherwise, including Main's manual, which says to turn off radiators in rooms not in use⁸. This entry follows the government wording.

4 TRVs and the room thermostat

In England, Approved Document L says that when a boiler is replaced, each room should have thermostatic room controls. One accepted way is a room thermostat plus a TRV on every radiator outside that room, and it says TRVs should not be used in the same room as the thermostat³. Other routes are allowed, such as a thermostat for each room or zone, and some rooms and homes are excepted. Viessmann's page words this as every radiator must have a TRV¹, which is simpler than the document itself.

Viessmann's reason for keeping them apart is that the two could compete and overheat or underheat the space¹.

5 Sticking valves

Worcester Bosch's Greenstar 4000 manual advises moving all TRVs by hand every 2 to 3 months so they do not stick⁴. Viessmann says they can seize, particularly on an old radiator, leaving it cold⁹. Baxi and Viessmann's web pages describe taking off the head and working the pin, but no user manual read gives that as a homeowner step, so it is left to a heating engineer here. Vaillant says to keep valves fully open when bleeding¹⁰.