The pressure inside the furnace changes according to the furnace temperature and combustion control. When the furnace pressure is low, outside air is sucked into the furnace, and when the furnace pressure is high, the hot furnace atmosphere is released out of the furnace through the gap (seal), which increases fuel consumption in both cases.
In other words, optimal control of the furnace pressure reduces fuel consumption.
We recommend control by dampers, which are relatively an easy way to save energy.