Furnace Pressure control 

A basic boiler has a steam water system and a fuel-air flue gas system. 

In the fuel-air-flue gas system, the air and fuel are mixed and ignited in the furnace. 

Air and fuel flow into the furnace and flue gas flows out. 

The force driving this flow is the differential pressure between the gases inside the  furnace and those outside the furnace. 

Furnace pressure is commonly referred to as draft or draft pressure. 

The draft is maintained slightly negative to prevent the combustion products and ash  from being discharged from the furnace into surrounding areas through inspection ports,  doors, feeders, etc. 

For greatest efficiency, the controlled pressure should be as close as possible to  atmosphere, thereby minimizing the ingestion of "tramp air" or excess air drawn through  the openings in the furnace duct work that cool combustion gases. 

Furnaces are classified by the method for moving air and other gases through the  system.