Advantages of direct gas-fired AHUs include higher energy efficiency, lower installation cost, free humidification, and inherent freeze protection. Disadvantages include potential space contamination and limitation to applications needing a minimum of 20-percent makeup air. It is important to evaluate all of the advantages and disadvantages for a particular application.

Freeze protection. A direct gas-fired AHU has no fluids to freeze. Electric resistant heaters and heat pumps also will not freeze. However, steam and hot-water coils are prone to freezing if not maintained and operated properly. At very low temperatures, the moisture produced within indirect gas-fired AHUs may condense and freeze within the combustion chamber. Direct gas-fired AHUs are a good choice for facilities that are used intermittently during the heating season.

Higher energy efficiency. Direct gas-fired AHUs are highly efficient because their burners are located in the supply-air stream, where all generated heat is transferred. Energy can be lost when radiant heat escapes through AHU casings or stoichiometric combustion--a process by which all fuel is burned ideally--is not realized. Typically, a direct-fired AHU is 100-percent-efficient with a 92-percent thermal efficiency, while an indirect-fired AHU has thermal efficiencies in the 76- to 80-percent range.

For example, let's look at an auditorium with an annual energy usage of 42,000 therms. An 80-percent-efficient indirect gas-fired AHU would produce an annual energy usage of 52,500 therms, while a 92-percent-efficient direct gas-fired AHU would produce an annual energy usage of 45,650 therms, resulting in an annual energy savings of 6,850 therms and an annual energy-cost savings of $6,850 (based on $1 per therm).

Although the example illustrates the energy savings of direct gas-fired vs. indirect gas-fired AHUs, direct gas-fired AHUs are attractive when compared with most other heating sources. For example, a steam heating coil is supplied by a boiler, has a maximum efficiency of 80 percent, and experiences distribution-piping energy losses. An electrical-resistance heater technically may be 100-percent efficient, but its higher electrical-energy cost usually makes it a less attractive option. Heat pumps are energy-efficient when the outside-air temperature is above 32˚F, but lose their efficiency as the ambient temperature drops below freezing because they must be supplemented with electrical-resistance heat at those temperatures. High-efficiency hot-water boilers have efficiencies equal to or slightly higher than those of direct gas-fired AHUs, but they experience distribution-piping-energy, pumping-energy, and boiler-radiant-energy losses.

Lower installation cost. Smaller (less than 5,000 cfm) direct gas-fired AHUs have equipment costs similar to those of indirect gas-fired AHUs. As their sizes increase, direct gas-fired AHUs become progressively cheaper. AHU equipment costs are lower for units with electric heat, hot-water heat, and steam because they do not have burner-management systems. Regarding auxiliary support equipment and required infrastructure, direct and indirect gas-fired AHUs only require gas piping. Steam systems require steam boilers, condensate-return systems, makeup-water systems, chemical-treatment systems, steam-distribution systems, combustion-air systems, flues, and gas piping. Hot-water heating systems require hot-water boilers, hot-water distribution piping and pumps, makeup-water systems, chemical-treatment systems, flues, and gas piping. Electric systems need only electrical service. Taking an AHU and all of its associated auxiliary equipment into account, a direct gas-fired AHU has one of the lowest installation costs.

Lower internal AHU static-pressure drop. The static-pressure drop across direct gas-fired burners is lower than that across indirect gas-fired burners, hot-water coils, and steam coils, resulting in lower fan energy usage and cost.

Free humidification. Moisture is one product of combustion that a direct gas-fired AHU supplies to a facility/process. If a direct gas-fired AHU has a firing rate of 1 million Btuh utilizing gas with a heat content of 950 Btu per cubic foot, then 1,053 cu ft of gas is needed per hour, supplying 97.7 lb of moisture per hour to the facility/process. When a direct gas-fired AHU is utilized, facilities or processes that require humidification during the heating season may not need additional humidification or may need only lower-capacity humidification systems, resulting in lower installation and energy costs.

Lighter AHUs. Direct gas-fired AHUs tend to weigh slightly less than the other AHUs previously discussed. However, this difference normally is not enough to make a substantial change in the supporting structure needed.