This movement of combustion air and exhaust is called draft. In essence, it is the difference in pressure between the air/gasses inside the chimney flue and the outside air that creates this movement. Warmer, lighter gasses in the flue tend to move upward.

What is Draught in chimney?

Draught is the pressure difference which causes a flow of air or gases from one point to another point in the boiler system. … To remove flue gases from the system after combustion and the heat exchange.

What is a natural Draught flue?

An effective natural draught (or atmospheric) flue will rapidly establish draught conditions that allow safe elimination of products of combustion, and avoid excessive spillage of products of combustion into living spaces.