Radiator thermostat - boiling or cold, nothing in between?

[Wonderbug81]

Double radiator around a metre long with the dial style radiator thermostat with the 0-5/MAX options on it.

If I put it on three the entire radiator heats up, if I put it on 1 or 2, it's just cold rather than half heated up.

Both the radiator and thermostat are under 10 years old (they were put in by previous owners so don't know exact date but point is they are modern).

I have bled it as well (as shown by the fact that it's roasting when on)!

Anything I can? It's in a bedroom so it's just too hot to have it on early morning when the heating comes on but would be nice to have a little heat to take the edge off!

Unscrew the valve, see how loose the pin is, consider a drop of oil if it doesn’t move freely. We had one like this in an office, it was stuck, i whacked it, radiator then worked. You can also buy a new valve for about £10-15

Oh this is great. Will try it out.

A sharp whack with a small hammer or a spanner usually fixes them, I've got a couple that stick when the heating's off over the summer. They do fail and are fairly cheap, but you normally have to drain the system. An easy fix is to get it in the mega hot mode, then restrict the flow through the radiator using the valve at the other end - the old fashioned way!

Oh- I thought that was how they were meant to work! I thought if it was on a low number it came on, but switched off more quickly when the room was ‘tepid warm’, but on a high number it stayed on till the room was ‘toasty warm’

So we have one in another room and that was one has more or less hot water in it depending on the number setting.

So on 3 only half of it is hot at the top.

But it's a good point because I just assume that's what it was supposed to do...

I think the half hot one is the broken one. I think they should either be all hot or all cold depending on the number on the thermostat and the temp of the room. Like @Clarabellawilliamson said

Sorry what I mean is I get cold on 0-1, half hot on 2-4 (varying levels depending on number) and totally hot on 5 which is what I think should be happening with the radiator in my bedroom.

I still think it should be hot or cold not half hot. I think that's how mine work. They turn on if the room gets too cold and off when it hits the set temp.

Maybe ours is a different type of valve to yours?

Oh I see what you mean. Will investigate further!

I think your on/off radiator is probably flow balancing. You can look on youtube how to do it. You'll probably find limiting the first radiator on the circuit will improve the flow to that radiator and allow the heat control to be more linear.

The one that gets half hot at the top is probably blocked with sludge at the bottom.

The numbered valves, or TRVs, are like taps that let hot water flow into the radiator. They don't control what proportion of the radiator gets hot, it's an all or nothing arrangement. A radiator that doesn't heat up evenly, all over, needs air bled out of it, has sludge at the bottom, or both.

Thank you. The full radiator can definitely get hot all over (at 4 or 5) and when it's on 3, it's half hot and on 1-2 pretty much nothing.

The radiator in another room is similar except at 1-2 it can still have a little heat at the top.

As someone mentioned above, the system may need balanced, so this radiator receives its fair share of hot water from the boiler.