One radiator not heating up properly

[krav]

Hi there,

One of the radiators of our central heating system is not heating up properly, there is heating coming through the pipe at the bottom of the radiator but there is heat only up to about halfway up to the pipe running into the radiator and no heat in the radiator itself (see picture), heat gets up to the marked red circle, but no further.

The radiator giving the problem appears to be the last one in the circuit, the others seem to be working properly.

I have bled the air from the radiator as best I can, there was quite a lot of air in it initially and some black coloured water came out, but it has subsequently run clear and no more air comes out

In order to check that the problem was not caused by balancing, I closed all of the other radiators on the circuit, I then observed that the pipe became hotter but the heat travelled no further (i.e. remained in bottom half of the pipe only)

The pressure in the system is about 1.5-1.8 bar

Any ideas what might be causing thie problem?
I thought something in the radiator might be blocked (we live in a very hard water area) but the water seems to flow from the radiator bleed valve quite freely so that seems to suggest the water does flow into the radiator?

Any ideas appreciated.

David

 

[kevplumb]

are both the valves (top and bottom) open

 

[krav]

Yes, both valves were open.

After thinking a bit more about what I wrote I descided to inspect the top valve a bit more. I took the cover off and noticed that it seemed a bit seized, so I pulled it out with a pair of needle nosed pliers. No joy, so then I pushed it in and out a few times with the pliers and voila! The flow is now restored, so fixed

David