There are two primary gas-fired AHU types: direct gas-fired and indirect gas-fired. It is important to understand the differences between these two AHU types.

Direct gas-fired. 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], aliphatic aldehydes, and water vapor) are discharged into the makeup-air stream and supplied to the building or process. Figures 1 and 2 show diagrams of non-recirculating direct gas-fired AHUs, and Figure 3 illustrates a recirculating direct gas-fired AHU.

FIGURE 1. Constant 100-percent-outside-air direct gas-fired AHU.
FIGURE 2. Variable 100-percent outside-air direct gas-fired AHU.
FIGURE 3. Variable outside-air/return-air direct gas-fired AHU.

Indirect gas-fired. An indirect gas-fired AHU has a sealed combustion chamber in which all products of combustion are discharged through a flue to the outside environment and no products of combustion are discharged within the makeup air supplied to the building. Figure 4 shows a diagram of an indirect gas-fired makeup AHU for a system utilizing 100-percent outside air. Similarly, such equipment also frequently is utilized for recirculating-air applications.

FIGURE 4. One-hundred-percent-outside-air indirect gas-fired AHU.