Draft Pressure Calculator

Stack effect calculations for chimneys, flues, and ventilation systems.

The Physics of Draft

A chimney doesn't pull smoke up—it pushes it. Cold, dense air outside creates higher pressure at the base than warm, light air inside. That pressure differential is your draft. Get it wrong, and the flue backs up. Get it right, and combustion flows clean.

Factory chimney emitting smoke into clear blue sky demonstrating stack effect principle

Cold ambient air density drives upward flow through hot flue gases. The greater the temperature differential, the stronger the draft.

ΔP = H × (ρ_out − ρ_in) × g

Where:
• ΔP = Draft pressure (Pa)
• H = Chimney height (m)
• ρ_out = Outside air density (kg/m³)
• ρ_in = Inside air density (kg/m³)
• g = Gravitational acceleration (9.81 m/s²)

Air density derives from the ideal gas law: ρ = P/(R×T). At standard pressure (101,325 Pa), dry air density varies inversely with absolute temperature (Kelvin). Source: Standard Atmosphere model, R_dry_air = 287.058 J/(kg·K).

Calculator

meters (typical residential: 10–15m)
°C (winter conditions create stronger draft)
°C (wood stove exhaust: 200–400°C typical)
Draft Pressure:
0 Pa

⚠️ WARNING: Insufficient Draft!
This configuration risks backflow. Increase height, raise flue temperature, or reduce cold air infiltration.

Why This Matters

Condition Minimum Draft Risk Below Threshold
Wood Stove ≥ 10 Pa Smoke spillage into room
Natural Gas Fireplace ≥ 5 Pa CO buildup, flame rollback
Industrial Flue ≥ 25 Pa Combustion instability

My own stovepipe runs 14 meters. In January, with -15°C outside and 300°C flue temps, I measure 22 Pa at the throat. Drop that below 10 Pa, and you'll see smoke creeping through the door seal. That's not theory—that's Tuesday morning.