Common Boiler Excess Air Trends and Strategies to Optimize Efficiency

Boilers are among the most common heating equipment in commercial facilities, often consuming the majority of a facility’s fuel. The load on a boiler plant in a commercial facility constantly fluctuates depending on weather conditions, occupancy rate, and internal heat gains. Proper control of part-load operation can significantly affect energy use. Most boilers in medium-to-large commercial facilities are equipped with a single-point positioning control that has a shaft mechanically linking the fuel control valve with the combustion air damper. With this control, the air-to-fuel ratio is typically calibrated at high fire, or near 100% capacity. However, as the load on the boiler decreases and the fuel valve begins to close, the combustion air flow is decreased at a lower rate due to the non-linearity of the linkage between the fuel valve and the combustion air damper. This results in increased excess air levels at medium and low fire, yielding poorer efficiencies. While boilers could potentially have higher efficiencies at reduced firing rates, high excess air levels trump the efficiency gains at part-load and significantly lower the efficiency. Linkageless burner controls and O2 trim controls are much more energy-efficient combustion control packages. Both of these packages allow the fuel valve and combustion air damper to operate independently of one another. This paper presents case studies demonstrating trends in excess air over boilers’ firing range in commercial space-heating applications. Case studies include boilers with single-point positioning control and boilers with linkageless controls. For single-point positioning controls, the quantity of excess air rises with decreased firing rate, and the difference in excess air between high and low fire is typically between 40 and 80 percentage points. For linkageless controls, excess air remains relatively low throughout a burner’s firing rate.