Visualize how temperature and height affect chimney draft. Use interactive graphs to troubleshoot poor draw and optimize your stack design. Seeing the logarithmic relationship between temperature and pressure helps in visualizing the performance limits of your flue.
Enter the outdoor temperature, the flue gas temperature, and the number of 90° bends; the tool then plots theoretical natural draft for every chimney height from 1 to 12 metres. Each point evaluates ΔP = 0.0342 × 101,325 Pa × effective height × (1/T_outdoor − 1/T_flue), with both temperatures in kelvin, so the curve is a straight line whose slope is set entirely by the temperature pair.
Every 90° elbow subtracts one metre of effective height before a point is computed, shifting the whole line downward. Heights whose effective value falls below 0.3 m are omitted from the curve entirely rather than plotted near zero — below that threshold the model cannot support reliable natural draft, and charting a number would imply a working flue where smoke spillage is the realistic outcome.
Reading the chart: with 0°C outdoors and 200°C flue gas, a 5 m chimney develops about 26.8 Pa and a 10 m chimney about 53.6 Pa — double the height, double the draft. Re-running the same flue at a 25°C summer outdoor temperature drops the 5 m point to roughly 21.5 Pa, which is why marginal chimneys that draw acceptably in deep winter can struggle in mild weather.
Because in the stack-effect formula, height is the only variable changing along the x-axis; the temperature term (1/T_outdoor − 1/T_flue) is fixed once you set your inputs, so pressure grows in direct proportion to effective height. Changing either temperature rotates the line to a new slope, while adding elbows slides it sideways by consuming effective height, one metre per 90° bend.
The model enforces a minimum effective height of 0.3 m. With two 90° elbows, a 2 m chimney nets 0 m of effective height and a 1 m chimney goes negative, so the curve only starts at 3 m. Skipped points mark configurations where natural draft is physically unreliable and backdraft of combustion gases into the room becomes the governing risk, so the graph refuses to dress them up with a number.
Draft comes from the density gap between the cold outside air column and the hot gas column, captured by 1/T_outdoor − 1/T_flue in kelvin. Cooling the outdoor air from 25°C to 0°C lifts the 5 m example from about 21.5 Pa to 26.8 Pa with no change to the chimney itself. The relationship is reciprocal rather than linear, so each additional degree of flue temperature buys slightly less draft than the one before it.