On the understanding that the radiator is part of a water fed central heating system
It appears you have air in the radiator and it will need to be bled out.
You will need a small bleed key (buy at ironmonger or DIY centre) on top right or left of radiator you will see a small square headed nut with cloth under back off /unscrew using the key and you will hear air escape then you will see a small quantity of water .
At this point tighten screw up. Top of radiator should now be warm.
Depending upon the heating system you may now need to readjust the system pressure on the expansion tank to about 1.5bar.
Maurice, March 2011
