Chimney draft

Chimney draft is the force that causes air or combustion products to move through the chimney system. This draft arises as a result of the pressure difference between the inside and outside of the chimney.

Air is heated in a combustion device (such as a boiler, fireplace or hearth) and rises relative to the cooler air in the chimney and the outside environment. This temperature difference causes a pressure difference and creates a draft that pulls air and gases through the chimney.

Chimney draft depends on a number of factors, including chimney height, gas temperature, chimney diameter and smoothness, outlet location, local wind conditions, and other factors. Its size is expressed in Pascals (Pa) or in millimeters of water column (mm H2O).

There are various formulas and methods for calculating chimney draft that take these factors into account. For example, Bernoulli's equation, the relationship between the velocity and pressure of a fluid or gas, is often used to calculate chimney draft.

However, it is important to keep in mind that the exact calculation of chimney draft is a complex process and usually requires specialist knowledge and experience in the field of ventilation and heating systems. For accurate results and safe operation of the chimney, it is advisable to consult with an expert or an engineering company.