The primary difference between Stelrad K2 (Type 22) and K3 (Type 33) radiators lies in their internal construction, physical depth, and resulting thermal heat output. A K2 radiator features two distinct panels and two sets of convection fins sandwiched between and behind them, making it the standard choice for most residential heating applications. In contrast, a K3 radiator is built with three distinct panels and three sets of convection fins, significantly increasing the metal surface area available for heat transfer. While a K3 radiator projects further into the room due to its increased depth, it delivers up to fifty percent more heat output from the exact same wall footprint as a K2, making it ideal for hard-to-heat spaces or low-temperature heat pump systems.