It is important to perform a detailed codes and standards search when considering a design utilizing direct gas-fired AHUs. There are two primary equipment standards and two primary codes that provide specific operating and application guidelines for direct gas-fired AHUs:

• ANSI Standard Z83.4, Non-Recirculating Direct Gas-Fired Industrial Air Heaters. ANSI Standard Z83.4 is the primary equipment standard for 100-percent outside-air direct gas-fired AHUs and includes limitations on combustion byproducts (CO less than 5.0 ppm, NO2 less than 0.5 ppm, and CO2 less than 4,000 ppm), gas-ignition control, combustion-air control, and flame safety.

• ANSI Standard Z83.18, Recirculating Direct Gas-Fired Industrial Air Heaters. ANSI Standard Z83.18 is the primary equipment standard for recirculating direct gas-fired AHUs and includes limitations on combustion byproducts (CO less than 25 ppm, NO2 less than 3 ppm, and CO2 less than 5,000 ppm), gas-ignition control, combustion-air control, and flame safety. The standard prescribes a limit on air-temperature rise based on the percentage of recirculated air.

• International Fuel Gas Code. The International Fuel Gas Code has separate sections to address non-recirculating direct-fired AHUs (Section 611) and recirculating direct-fired AHUs (Section 612). Both sections note that direct-fired AHUs cannot supply any area with sleeping quarters, makeup air must come from outside, motorized outside-air dampers/louvers must be proven open prior to burner activation, systems must be controlled properly to prevent building overpressurization, and burner controls need to have a burner purge cycle.

• NFPA Standard 54, National Fuel Gas Code. NFPA Standard 54 has similar requirements as sections 611 and 612 of the International Fuel Gas Code. Its requirements differ depending on whether a direct-fired AHU is listed (Edison, UL) or unlisted. Unlisted direct-fired AHU discharge-air temperature is limited to 150˚F.