Abstract: A heat generator comprising a fuel supply chamber (7a), having at its lower end an inwardly tapered section (12), comprising at its lower end a burner plate (14) with mixing strips (33) and a combustion pipe (20), said tapered section (12) being provided with apertures (8) for primary air in its upper part, apertures (13) for secondary air at a level just above the level of the burner plate (14) and apertures (22) for tertiary air at a level just below the level of the burner plate (14) at the level of mixing strips (33). Preferably all air supplies are preheated. The combustion pipe (20) stands in closed connection with an outlet-pipe, preferably provided with an adjustable ventilator.