Chimney sizing

Draft ΔP ≈ 4 Pa/m of height (at Δt ~200K) minus bend losses (a 90° bend ≈ 1 m of pipe). Diameter by area: ~1 cm² of cross-section per kW for wood, gas — per boiler manual. Minimum height above the ridge: 0.5 m when <1.5 m from it, else turbulence. Insufficient draft = smoke in the room, excess draft = fuel burning up the stack.