The invention concerns a hot air device for thermowelding membranes to surfaces. It includes an air inlet, a casing in fluid communication therewith, a combustion chamber mounted within the casing and spaced apart therefrom. The chamber has perforations allowing air to flow therein. The perforations allow some air to flow into the upstream section of the chamber producing an air-fuel mixture, and let the remaining air into the downstream section, thus shortening the flame within the chamber and producing hot air. The device includes an outlet portion for expelling hot air and not allowing a flame out therefrom, and a blower to force airflow from the air inlet through the chamber and out from the nozzle. The chamber may be pyramid or cone shaped and have a deflector to help direct air into the upstream section.