The Stûv 21 uses the simple physical principle of natural convection:
the air circulates through the double wall (1) of the stove, gathers heat, rises and emerges through the special vents provided (2).
Part of the hot air can also be directed to a neighbouring room,
hence there is no need for a fan in the basic configuration.
The air required for combustion sweeps over the inside of the window (3), preventing the formation of soot deposits.