A burner consists of a cast-iron or aluminum pipe assembly with drilled gas orifices. The fuel-air mixing is controlled by perforated stainless-steel baffles attached to the pipe assembly. The burner can be placed in the outside airstream or in the outside / return mixed-air stream. But to prevent potentially hazardous combustion by-products of indoor-air contaminants, the burner is placed in the outside airstream only. The required airflow across the burner must be within a specific velocity range (typically 2,500 to 3,500 fpm) for proper combustion. For variable airflow units, a modulating bypass damper or profile plate maintains the airflow velocity within the specified velocity range. Otherwise, the burner can experience inefficient operation, generate odors in the occupied spaces served, and create an un-safe increase in certain products of combustion.

Fans typically are double-width double-inlet (DWDI) with forward-curved or backward-inclined blades. Fans can come with open drip-proof (ODP) or totally enclosed fan cooled (TEFC) motors. The most typical external static pressure in the 2- to 3-in.-wg range. Because most outside installed air handling units can operate below 0°F, fan motors and bearings are rated to handle lower temperatures. For applications that require supply-air temperatures higher than 110°F, the motors and fan bearings are rated for maximum design operating temperatures when placed in draw-through configurations.

Though simple in principle of operation, direct gas-fired AHUs have sophisticated burner-control systems. Unit sizes range from 1,000 cfm with a heating capacity of 80,000 Btuh to 100,000 cfm with a heating capacity of 15 million Btuh. Direct-gas-fired-AHU components usually include housings, supply fans, outside air intake hoods, intake dampers, filters and burners.

Direct

A direct gas-fired AHU has a gas burner installed directly in the outside makeup-air stream. The products of combustion (carbon dioxide [CO2], carbon monoxide [CO], nitrogen dioxide [NO2]) are discharged directly into the makeup air stream and supplied to the building area.

Indirect

An indirect gas-fired AHU has a sealed combustion chamber where all products of combustion are discharged through a flue to the outside environment and no products of combustion are discharged within the make-up air supplied to the building. Such equipment also frequently is utilized for recirculating-air applications.