The chimney draught is what makes the flue gases exit the system to atmosphere. This makes it an essential part of the combustion process. Draught can be either positive, in the case of forced-draught burners, or negative, in the case of atmospheric burners. There are various factors that can affect draught, but testing of burners will be more interested in changes in draught, indicating problems with the stack or blower. Basically, atmospheric burners produce draught due the pressure difference caused by the height of the stack. In operation, this is increased by temperature effects from the combustion process, but there must be a certain draught present from the start. Too little draught will cause flue gases to exit into the boiler room, producing a risk of asphyxiation and reducing the efficiency of the burner. Too much draught will cause the heat to exit the stack without contributing any useful work.