What do the numbers on a TRV (Thermostatic Radiator Valve) mean?

[Clive]

You may, or may not already know this, but it's a useful little guide to what TRV's actually do and how to use them properly.

I can't take credit for this report, although I don't know who actually wrote it. Obviously it relates to radiators in use with a standard boiler, but may also apply to when in use with a heat pump.

The numbers on a thermostatic radiator valve refer to the TEMPERATURE IN THE ROOM, not the radiator.

A TRV can detect the temperature in the room and will let more or less hot water into the radiator.
If the room is cold, the TRV will allow hot water into the room to heat it up. As the room gets warmer, it restricts the amount of hot water coming into the radiator. If the room temperature starts to drop, the TRV will allow more hot water in.

•Numbers & temperatures-
The numbers on the valve roughly correlate to the room temperatures below:

0 = Off

* = 7°C

1 = 10°C

2 = 15°C

3 = 20°C

4 = 25°C

5 = 30°C

In winter, you should really set your TRV to 2 or 3 and leave it. If you go into a room that's cold and the radiator is burning hot, leave it alone and don't turn it up to 5. Let it do it's job to heat the room.
(If you're away during winter, set your TRVs to * The rads will come on if the temp gets below 7°C)

If you turn a TRV up to 5, you are telling it not to stop letting hot water into the radiator until the room reaches a high temperature (around 30°C). You rarely need the room to be that hot and it also means you lose any savings you may have otherwise made on heating bills.