With direct-fired heaters, the flame comes into contact with the air while indirect heaters do not. Direct fuel-fired heaters are less expensive and more efficient to run and are designed to heat open or well-ventilated spaces such as construction sites, open-air buildings, warehouses, and large venues.
Indirect fuel-fired heaters draw air through intake vents which pass over the burn chamber where it is heated and then delivered into a building through the ducts. Indirect fuel-fired heaters are ideal for warming indoors and tight spaces with an exhaust outlet, job sites that can benefit from less humidity, and environments that require no combustion by-products in the airstream.