One rad remains cold when CH is switched on

[Solar77]

Hi everybody.
I am a new member to this Forum and I hope you can shed some light on my CH problem.
My Ch system was working fine in June during a cold snap.
When the Ch was switched on in November all the rads worked except one which was and still is stone cold. I closed all rads to try and force the pump output to the cold rad with no success.
We called in a plumber who drained and flushed through the cold rad and it immediately heated up when re-installed.
Slowly the rad cooled down again and the plumber admitted defeat and left.
I can only assume that the hot water filled the rad (whilst being bled) and either the outlet valve is blocked or the outlet pipe is blocked. Both valves (non TVR) appear to be working although I haven't checked this physically.

The system is the vented expansion type

Any suggestions please
Thanks

 

[kbdiy]

The bucket test should confirm a blockage. With the heating off, remove the rad and open the flow into a bucket, then repeat for the return. They should be roughly equal in terms of quantity and pressure. If the return is significantly weaker, or non-existent, then you have identified a blockage.