Chimney Draft Calculator
Calculate the theoretical natural draft (pressure difference) produced by a chimney based on its height, flue gas temperature, and ambient air conditions.
Results will appear here.
Formulas Used
Air / Flue Gas Density (Ideal Gas Law):
ρ = P / (R · T)
where P = atmospheric pressure (Pa), R = 287.058 J/(kg·K) (specific gas constant for dry air), T = absolute temperature (K).
Chimney Draft Pressure:
ΔP = g · H · (ρambient − ρflue)
where g = 9.80665 m/s², H = chimney height (m).
Theoretical Flue Gas Velocity:
v = √(2 · ΔP / ρflue)
Conversion to mmH₂O:
ΔPmmH₂O = (ΔP / (ρwater · g)) × 1000
Assumptions & References
- Flue gas is treated as dry air for density calculation (conservative approximation).
- Ideal gas law applies: ρ = P / (R · T).
- Standard gravity g = 9.80665 m/s² (ISO 80000-3).
- Specific gas constant for dry air R = 287.058 J/(kg·K).
- Atmospheric pressure defaults to 101 325 Pa (ISA sea-level standard).
- Water density taken as 999.7 kg/m³ (~15 °C) for mmH₂O conversion.
- Draft is purely theoretical (natural convection); friction, bends, and stack effect losses are not included.
- Reference: ASHRAE Fundamentals Handbook, Chapter on Chimney, Gas Vent, and Fireplace Systems.
- Reference: EN 13384-1 — Thermal and fluid dynamic calculation methods for chimneys.