if its the only rad that is not hot on your system, its probably thermostat or air lock. this can be sorted as above. feel the pipes comming to the thermostat and cut of valve both sides. if warm thats your problem.
how far away from the boiler is this rad? furthest away or off on a spur from main system suffer air locks.
other case is a sludge build up in bottom of rad, but then I would expect more rads to be the same. in this case its rad off.
nothing electrical or mechanical to break down ,
couple of things to consider.
flush the system every fewyears.
fit a magnaflux system unit,
fit a water softner.
fill with fernox.
the nature of the system makes rust and lime scale as would find in kettles and water and steel.
http://www.fernox.com/problem+solvin...heating+system
