One radiator won't come on... ?

Have you tried bleeding the air out of it? - when you've bled it, you'll probably need to top up the boiler with the filling loop

If it were a vented system, I'd say that there was an air lock in either the feed or return pipe - but with a combi boiler, it's probably a pressurised system, so unlikely to have an air lock provided the pressure is correct.

So it's more likely that there is a blockage somewhere. Is the rad in question fitted with a TRV? If so, the 'blockage' could simply be that the TRV is stuck shut. Remove the thermostatic head and check whether the pin in the top of the bit of the valve still attached to the radiator can be pushed down, and springs back up when released. If not, it is stuck. Tapping the side of the valve with a small hammer will probably free it.

If it's got a TRV you may find that the 'pin' that controls the temperature has stuck in the down postion. Remove the control unit and give the 'pin' a tap to see if it'll pop up . WD40 can help too.

Hi, thanks for your suggestions, but...

I have bled the radiator - air came out, eventually followed by some water, so it appears to be filling up.

The radiator *does not* have a thermostatic valve fitted - just white knobs at each end (where the pipes are) at the bottom!

biskitz4cheez

IANAHE/P (I am not a heating engineer / plumber)

Try closing off a couple of other rads - it could be that they need balancing, and the heat is going through the others first

Reading that you have bled it and it hasn't got a TRV, has this rad worked in the past? If it has, what's changed? For example, is the control valve on this rad fully open? Has the lock shield valve on this rad been closed? Have the lock shield valves on any of the other radiators been opened? What happens when you turn the control valves of all the other rads off, does this one get hot? If yes, then the radiators could be unbalanced. If no, there could be a blockage assuming that the control and lock shield valves on this non-working rad are both open.

Where is this rad in relation to the pump? Is it the furthest rad away or is it in the middle somewhere?

Thank you for all your tips - I will try your suggestions. The radiator is probably one of the furthest away from the boiler. How do I tell which valve (left side or right side) is the controller rather than the one that lets water in - they both look the same!

The one for turning on and off should have a knob which you can turn by hand. The one for balancing the system (lockshield valve) should have just a square-ended shaft with a plastic cover over it. In order to adjust it, you need to remove the cover and turn the shaft with a small spanner.