Though simple in appearance, direct gas-fired AHUs have sophisticated burner-control systems. Unit sizes range from 1,600 cfm with a heating capacity of 94,000 Btuh to 120,000 cfm with a heating capacity of 18 million Btuh. Direct-gas-fired-AHU components usually include housings, outside-air intakes, intake dampers, filters, burners, and supply fans.

AHU housing. AHU housings are made of galvanized or aluminized steel. Access panels are provided to access dampers, filters, burners, and fans/motors. AHUs located outside should be provided with a sloped roof and standing seam joints.

Outside-air intake hood/dampers. Outside-air intake hoods or weather hoods need to be designed for an installation's specific location and should be provided with bird screens. Outside-air intake isolation dampers are provided to prevent cold air from dropping through a unit into a warm building and warm air from escaping through a unit during colder months, when it is not in operation. Low-leakage parallel-blade isolation dampers are preferable. The fan discharge is an alternative isolation-damper location.

Filters. A filter cabinet normally has 30/30 filters (with a minimum efficiency reporting value [MERV] of 8) in a V-bank configuration to increase filter area and reduce static-pressure drop across the filters. Higher-efficiency filters normally are not used.

Burners. A burner consists of a cast-iron or aluminum pipe assembly with drilled gas orifices. The fuel/air mixture is controlled statically by perforated stainless-steel baffles attached to the pipe assembly. The burner can be placed in the outside-air stream only or in the outside/return mixed-air stream. However, to prevent potentially hazardous combustion byproducts of indoor-air contaminants, the burner should be placed in the outside-air stream only. The required airflow across the burner must be within a specific velocity range (typically 2,500 to 3,200 fpm) for proper combustion. For variable-airflow units, a modulating bypass damper or profile plate must maintain the airflow velocity within the specified velocity range. If the velocity is not maintained within the specified range, the burner can experience inefficient operation, generate odors in the occupied spaces served, and create an undesirable increase in certain products of combustion.

Most manufacturers provide a standard gas train with their units (Figure 5). Direct-fired gas trains have a substantial turndown burner capability--typically, in the 20-1 to 30-1 ratio range, which allows operation with 4 or 5 percent to 100 percent of rated capacity. Most direct gas-fired AHUs have a supply-air temperature between 50˚F and 100˚F. Variables that determine which gas train is used include burner-heating-capacity rating (British thermal units per hour), gas-supply pressure, whether the unit is listed (Edison Testing Laboratories Inc. or Underwriters Laboratories Inc. [UL]) or unlisted, code requirements, and the owner's insurance carrier's requirements. Some insurance carriers require that specific gas trains be provided.

FIGURE 5. Standard gas train.

Fans. 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. Most manufacturers provide fans with a maximum external static pressure in the 2- to 3-in.-wg range. Because most supply-air temperatures are below 100˚F, fan motors and bearings are rated to handle lower temperatures. For applications that require supply-air temperatures higher than 110˚F, it is important to verify that motors and fan bearings are rated for maximum design operating temperatures when placed in draw-through configurations.