Natural Chimney Draft
Calculate the natural draft (depression) of a chimney, ΔP = 353 × h × (1/T_air − 1/T_gas), from the chimney height h (m) and the absolute temperatures of the outside air and the hot gases (K). The result, in pascals, is the pressure difference that 'pulls' the gases up and the combustion air into the burner, generated by the density difference between the hot (light) gases and the cold (dense) air — the chimney effect. Taller chimneys and hotter gases generate more draft. It is the basis of natural-draft furnace and boiler chimney design; insufficient draft requires fans (forced draft). Enter the height and the air and gas temperatures.
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Natural chimney draft
How does a chimney manage to 'pull' flue gases upward and draw air into the burner with no fan at all? Through the stack effect: the hot gases inside the chimney are less dense (lighter) than the cold air outside; this density difference, acting over the height of the chimney, creates a pressure difference that drives the gases upward — exactly like a bubble rising in water. This natural draft is ΔP = 353 × h × (1/T_air − 1/T_gas), where h is the chimney height (m) and T_air, T_gas are the absolute temperatures (K) of the outside air and of the flue gases (the 353 comes from atmospheric pressure and the gas constant of air). The result, in pascals, is the suction available to overcome the pressure losses of the system (burner, furnace, ducts, the chimney itself) and keep the gases flowing. The formula shows the two ways to increase draft: taller chimneys (a longer column of light gas) and hotter gases (a bigger density difference) — which is why old chimneys were so tall, and why draft drops when the gases cool down (an efficient economizer that cools the gases a lot reduces the draft, a side effect worth keeping in mind). Natural draft is simple and reliable (no energy, no moving parts) but limited and variable (it depends on weather and load). When it falls short — in large boilers, with heat-recovery and gas-cleaning equipment imposing high pressure losses — the answer is mechanical draft: forced-draft fans (pushing combustion air in, positive furnace pressure) or induced-draft fans (pulling the gases at the outlet, negative pressure), or both (balanced draft). Computing the available natural draft is the starting point for choosing between natural and mechanical draft and for sizing the chimney. Enter the height and the air and gas temperatures.
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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.