Calculate theoretical chimney draft based on the stack effect. Understand how temperature and height affect your wood stove's safety and performance. Proper draft is essential for ensuring all combustion gases are exhausted safely to the outside environment.
Enter the vertical height of your chimney from the stove collar to the cap, then input the outdoor air and flue-gas temperatures. Each 90-degree elbow in the run effectively subtracts about one metre of working height, so a 6 m chimney with two elbows behaves like a 4 m straight stack. The calculator rejects any configuration whose effective height falls below 0.3 m (1 ft), because below that point a reliable natural draft simply cannot form.
The tool applies the stack-effect formula deltaP = 0.0342 x 101,325 x effective height x (1/T_outdoor - 1/T_flue) with temperatures in Kelvin, returning the static draft in Pascals. For example, a 6 m chimney venting 200 C flue gas on a 0 C day produces about 32 Pa of draft; drop the outdoor temperature to -10 C and it climbs to roughly 35 Pa, which is why stoves draw so eagerly in deep winter.
Because draft scales with the temperature difference, the same stove that pulls 32 Pa at 0 C only manages about 30 Pa once the outdoor air warms to 10 C, and adding two elbows cuts that 6 m stack to around 21 Pa. This is exactly why a stove that smolders on a mild autumn afternoon roars on a freezing night, and why a long horizontal run starves the fire of pull. Aim for a comfortable middle band rather than the highest possible number: insufficient draft (under roughly 12 Pa) leaves sluggish, smoky fires, while excessive draft burns wood too fast and can over-fire the stove. If the effective height after subtracting elbows ever drops below 0.3 m (1 ft) the calculator returns no result, because no dependable draft can form there and smoke spillage or carbon-monoxide backdraft becomes a real hazard. The practical takeaway is to keep the flue as straight and as tall as your roofline safely allows, since taller stacks build more draft even though going too tall can cool the gases enough to encourage creosote.