Chimney Draft & Height

Draft

• The chimney draft is pressure within the chimney in relation to several different factors. The draft is related to the height of the chimney, along with the pressure of the air inside and outside of the chimney, and the force of gravity acting downward on the air. The basic calculation for determining the pressure difference involves multiplying the height of the chimney times the difference in the inside and outside pressures and the acceleration of gravity, which is expressed as the constant 9.81 meters per second squared.

Pressure Differential

• The pressure differential is essential to the proper operation of the chimney. The air inside the chimney is lighter and more buoyant than the air outside, which is what allows it to rise. The pressure differential is related to the height of the chimney, but also to the absolute average outside temperature and the absolute average temperature inside the chimney. The atmospheric pressure is also factored into the equation. The equation used to determine the pressure differential does not provide an exact number, but is instead an approximation that assumes the mass of the flue gases and the outside air are the same.

Flow Capacity

• The flow capacity of the chimney is another important consideration related to both the draft and height of the chimney. The flow capacity is the ability of the chimney to get enough draft. Height and temperature all factor into the equation to determine the flow capacity, as do the cross-sectional area of the chimney and the discharge coefficient. The discharge coefficient is the amount of gas released from the chimney in relation to the mass flow rate of the gas.

The House

• The problem with determining an exact draft for a chimney is that each home can differ significantly. The air temperature and density in the home has an affect on the flow rate and draft in the chimney. Fans and open windows can complicate the matter. Air flow within the house also affects the chimney draft. Because each home is different, taking a cookie-cutter approach to determining the chimney height and calculating its draft is not likely to yield accurate results.