Gas-Fired Pressurization Units move large quantities of air at low temperature differentials (usually 50° or less), which is a strategy to minimize temperature stratification in the large spaces they are employed to heat. The equipment is typically mounted on the roof of the facility or at grade on elevated supports to ensure that the supply air is delivered high. The high air delivery allows for a longer “throw” of the air, thereby requiring less equipment to cover the required floor area. The most common fuel is natural gas, however, units can be converted to burn propane. There are two primary categories of gas-fired AHUs: indirect-fired and direct-fired. The 80/20 design can vary the quantity of outside air from 100% down to 20%. The burner has a high turndown ratio and only outside air should be drawn across the burner. The discharge air volume is fixed and the quantity of outside air can be adjusted to maintain building pressurization. The makeup air unit configuration is ideal for supplying large quantities of replacement air for facility exhaust systems. Such systems can include paint spray booths and other industrial exhaust applications. These units supply a fixed amount of 100% outside air. Air Recirculation units recirculate a fixed 80% of the air while bringing in 20% outside air that passes over the burner. They supply a minimum amount of ventilation to spaces that do not require large quantities of outside air to maintain building pressurization. For higher pressure ranges, a backward curved centrifugal fan often is selected. This design offers pressures up to 20" water gauge. They also offer the benefit of having the shaft and bearings outside the fan, and they can be mounted on the roof with a weather cover.