Analyze flue gas flow and friction losses in your chimney system. Optimize diameter and height for peak stove performance and smoke prevention. Calculating friction loss is critical for ensuring your stove operates at its designed efficiency without spillage.
Enter the straight pipe length, inside diameter, flue gas temperature, gas velocity, elbow counts, and the flue material. The calculator returns pressure loss from the Darcy–Weisbach equation, ΔP = f × (L/D) × ρv²/2, with gas density taken from the ideal-gas law at your flue temperature: about 0.746 kg/m³ at 200°C, far lighter than room air.
The friction factor f adapts to the flow regime. From density, velocity, and diameter the tool computes a Reynolds number, then applies 64/Re for laminar flow below Re 2300, the Swamee–Jain approximation of the Colebrook–White equation for turbulent flow at Re 4000 and above, and a linear blend through the transitional band between them. Material enters through wall roughness: stainless liner 0.045 mm, smooth steel 0.046 mm, galvanized pipe 0.15 mm, and unlined masonry 3.0 mm.
Elbows count as equivalent straight length: each 90° bend adds three pipe diameters and each 45° bend adds 1.5, so two 90° elbows on a 150 mm pipe contribute 0.9 m. As a worked case, 5 m of stainless pipe carrying 200°C gas at 2 m/s loses only about 1.65 Pa to friction, while the identical run in rough masonry loses roughly 2.7 Pa — resistance that subtracts directly from the natural draft the chimney generates.
Wall roughness feeds the Swamee–Jain friction factor through the relative roughness ε/D. Unlined masonry at 3.0 mm is over sixty times rougher than a stainless liner at 0.045 mm; at the Reynolds number of the worked example, about 8,300 on a 150 mm flue, that raises the friction factor from roughly 0.033 to 0.054 — around 64% more loss over the same run.
Through the equivalent-length method: every 90° elbow behaves like three extra pipe diameters of straight run and every 45° elbow like one and a half. On a 150 mm flue each 90° bend therefore adds 0.45 m and each 45° bend 0.225 m before the Darcy–Weisbach loss is computed, which is why a pair of 45° offsets costs exactly what a single 90° elbow does in this model.
In a short, smooth, vertical run it is minor: the 5 m stainless example loses under 2 Pa while a 5 m chimney at a 200°C flue temperature can generate on the order of 27 Pa of natural draft. But loss grows linearly with length and with the square of velocity, so long horizontal connectors, undersized diameters, stacked elbows, or a rough masonry liner can erode a meaningful slice of the available draft. The Reynolds readout shows which flow regime produced the friction factor.