Tramp Air Infiltration. While the furnaces are designed for minimum excess air
to maximize efficiency, they are often operated differently in the field. The oxygen
required for combustion is controlled with draft and burner register positions to the
desired oxygen and draft levels, typically between 1.5 and 2.0% O2 and a pressure of
0.1 in. of water at the arch. Since the furnace firebox operates under a negative pressure,
any opening in the furnace leads to unwanted, cold tramp air entering the furnace. This
lowers efficiency, as shown in Figure 9.16. The fuel consumption could increase 1–2%
from tramp air incursion.