Individual water filled radiators

[koiman]

I recently saw individual radiator heaters for sale. The basic system lets you fit a thermostat controlled mains heating element into the bottom of a radfiator, you then fill a rad with water and and use it as a stand alone heater any where it is difficult to get your normal boiler system water fed to,
My question is this, as the water heats it expands, in a normal system this would be taken csre of by an expansion tank or a pressure relief valve. how is the change catered for on an individual heater?

[wildwood]

Into or onto????

If it somehow clips onto the bottom of the radiator, the pipes would have to be disconnected so there will be pressure relief as you'd no doubt have to be able to periodically top up the water through one of the pipes.

I really can't see how even a handy handyman could actually fit an element into the radiator without sophisticated equipment.

Whatever, wouldn't it be a lot cheaper and easier to buy one of those oil filled column heaters?