Smaller building-specific boilers that only require high- and low-fire capability are normally installed with manual damper controls that are set up during startup and commissioning. The damper positions are set and locked manually to the gas valve position, restricting airflow during low fire to maintain combustion air and minimize NOx formation. At high fire, combustion is optimized to 3% to 6% excess air with fixed damper positions to optimize performance.
NOx emissions for normal burners range from 70 to 100 ppm in the flue gas. Low NOx burners are most often designed to 30 ppm, with ultra-low NOx burners targeting 9 ppm. Low NOx and ultra-low NOx burners are most often fit with flue gas recirculation systems, which bring flue gas from after the boiler or economizer back to the inlet of the forced draft fan. The decision to specify a low NOx versus ultra-low NOx burner should be made while determining the environmental permitting for the boiler. Ultra-low NOx boilers may be needed to meet federal regulations on emissions netting. Permit applications with the U.S. Environmental Protection Agency require emissions netting, which is the process of determining the net increase or decrease of hazardous air pollutants (HAP) possibly emitted from the new boiler compared to the actual emissions measured from the boiler being replaced. Data from the previous 10 years of operation is used to determine the actual HAP levels emitted by the boiler or source being removed. If the boiler being replaced has not operated or has been operated sparingly in the past 10 years, HAP netting may indicate the need for added environmental permit actions, including prevention of significant deterioration (PSD) review, public comment, and public review periods.