intermittent hold/cold radiator

[phil1963]

I have 4 rads downstairs and 3 upstairs. The large one is the front room farthest away from the boiler sometimes heats to a normal temp and sometimes heats to a lukewarm/cool temp and on inspection the top half is warm and bottom half cool - which seems an odd way round. I have bled it and it makes no difference! If I turn the tap to the rad off for 10 minutes and then on again - hey presto it comes on at the normal temp. Is there a blockage?? The rad is only about 8 years old and does not leak and appears quite a modern, thin one with no rust at all.

 

[MANDATE]

sounds as though it is being starved of hot water. The longer pipe work will mean more resistance and the water will take the easiest path so you may need to open the lockshield valve a little and/or close the lockshield valves a touch on the other radiators in order to balance the distribution.
You don't state how the temperture is controlled. Room stat or TRVs
If TVRs have been set to higher temperture in recent cold spell it could explain the reason. ( I think )

 

[phil1963]

The temperature is controlled by a thermostat upstairs which is a silly plave to place one so I use the controls on the boiler to turn on the central heating whenever it is cold enough to require their use.