Disadvantages of direct gas-fired AHUs may include:

High minimum makeup-air requirement. Facilities/processes that consistently require less than 20-percent outside-air makeup are not good candidates for direct gas-fired AHUs. The standard minimum outside-air percentage for direct gas-fired AHUs is 20 percent.

Unwanted humidification. When a direct gas-fired AHU is utilized, facilities/processes with low humidity requirements require additional dehumidification equipment to remove the resulting moisture from the supply-air stream, resulting in higher installation and energy costs.

Application limitations. NFPA Standard 54 prevents direct gas-fired AHUs from being used for sleeping quarters. It is important to remember that direct gas-fired AHUs supply products of combustion to facilities and that there must be a positive method of removing and diluting the concentration of these products, which includes removing air from each space being supplied.

Contamination. When using direct gas-fired AHUs, facilities/processes that are very sensitive to contaminants need to be evaluated carefully to determine whether the products of combustion are considered contaminants.

Potential odors. If a direct gas-fired AHU's gas burners are not tuned properly, facility occupants may smell unpleasant odors.