Stack Effect Draft Calculator
Calculate the natural draft pressure generated by your chimney flue based on temperature differential, chimney height, and atmospheric conditions. Used by HVAC engineers, chimney sweeps, and home performance auditors.
Input Parameters
350°F
35°F
20 ft
0.80
Chimney Diagram
Temp Differential
— °F
Calculated Results
Draft Pressure
—
Pascals (Pa)
Draft (in. H₂O)
—
inches water column
Air Velocity
—
ft/min (estimated)
Buoyancy Force
—
lb/ft² differential
Draft Strength
0%
Weak
Good
Strong
Adjust the sliders above to calculate stack effect draft for your chimney.
Stack Effect Formula
// ASHRAE Stack Pressure Equation
ΔP = C × h × g × ρ_out × ( 1 − T_out/T_flue )
// Where: ΔP = draft pressure [Pa or in.H₂O]
C = flue efficiency factor [0.5–1.0]
h = chimney height [m or ft]
g = gravity 9.81 m/s²
ρ_out = outdoor air density ≈ 1.293 kg/m³ at 0°C
T_out = outdoor temp [Kelvin]
T_flue= flue gas temp [Kelvin]
// Simplified Practical Form: ΔP = 0.0342 × h × ( 1/T_out − 1/T_flue ) × 3386.4
// Where: ΔP = draft pressure [Pa or in.H₂O]
C = flue efficiency factor [0.5–1.0]
h = chimney height [m or ft]
g = gravity 9.81 m/s²
ρ_out = outdoor air density ≈ 1.293 kg/m³ at 0°C
T_out = outdoor temp [Kelvin]
T_flue= flue gas temp [Kelvin]
// Simplified Practical Form: ΔP = 0.0342 × h × ( 1/T_out − 1/T_flue ) × 3386.4
Enter values to see live calculation…
Draft Pressure Reference Guide
| Draft (Pa) | Draft (in. H₂O) | Rating | Typical Application | Performance |
|---|---|---|---|---|
| 0 – 5 | 0 – 0.020 | Insufficient | Blocked or very short flue | Smoke backdraft likely; unsafe to operate |
| 5 – 12 | 0.020 – 0.048 | Weak | Marginal residential chimneys | Poor combustion; frequent smoke spillage |
| 12 – 25 | 0.048 – 0.100 | Good | Standard residential wood & pellet stoves | Normal operation; clean combustion |
| 25 – 50 | 0.100 – 0.200 | Strong | Tall chimneys, high-efficiency appliances | Excellent draft; may need damper restriction |
| > 50 | > 0.200 | Excessive | Very tall industrial / masonry stacks | Overcooling; reduce with damper or liner |