How to calculate stack draft
Hot flue gas inside a chimney is lighter than the outside air column, so it rises on its own. The pressure difference this creates is stack draft, and on naturally drafted boilers, furnaces and incinerators it is the only force pulling combustion air in and pushing products of combustion out.
The relation is draft = height × (ambient air density − flue gas density), converted to inches of water column by dividing by 5.202 lb/ft² per in. wg. Densities come from 39.68 ÷ absolute temperature in °R, where °R = °F + 459.67. Written out, this is the familiar 0.52 × H × P × (1/To − 1/Ti) form. Draft grows with a taller stack, hotter gas and colder outside air.
A 100 ft stack with 60°F ambient air and 400°F average flue gas gives a density difference near 0.0302 lb/ft³ and about 0.58 in. wg of theoretical draft. Real breeching friction, bends, dampers and heat loss along the run cut that down, so the effective figure at 80% is shown next to it.
Raising flue gas temperature buys draft but throws heat away, while running too cool invites weak draft plus condensation and corrosion in the stack. Pair this result with a flue pressure drop calculation, and where available draft falls short of what the appliance needs, bring in an engineer to size an induced draft fan.
Frequently asked questions
Draft comes from the density difference between outside air and flue gas. Warm ambient air is already light, so the difference shrinks and available draft falls. It is the classic reason naturally drafted equipment burns poorly in hot weather.
No. Theoretical draft assumes a frictionless flue. Bends, dampers and heat loss along the breeching typically leave only 70 to 80% of it available, which is why the effective figure is shown alongside. If that still falls short, an induced draft fan is the usual answer.