The primary difference between Stelrad K1 (Type 11) and K2 (Type 22) radiators involves their physical dimensions, internal structure, and thermal output capacity. A K1 radiator consists of a single water-carrying panel paired with one set of convection fins welded to the back, resulting in a very slim profile that is ideal for hallways, bedrooms, or tighter spaces with lower heating demands. Conversely, a K2 radiator features two full water panels and two complete sets of convection fins, making it considerably thicker but capable of delivering significantly higher heat output. K2 radiators represent the most widely installed type in modern homes because they balance high thermal efficiency with manageable wall protrusion.