Type 22 radiators work by filling up with hot water through a boiler or central heating supply. The water enters the double metal panel, typically manufactured using steel, and distributes heat via the convector fins incorporated within the design.
Whilst Type 21 convector radiators have the same dimensions as single panel Type 11 rads, they are able to deliver a higher heat output on account of their double panel and convector fin profile.
For a more detailed insight, explore our article, About (Type 22) double panel convector radiators.