Chimney Draft Diagnostic

Calculate your chimney’s actual draft pressure, identify what’s causing poor draw or backdrafting, and get factor-by-factor recommendations to restore strong, safe performance.

Draft Calculation Backdraft Diagnosis Symptom Checker Root Cause Analysis Fix Recommendations

Chimney Parameters

Flue Gas Temp Inside the flue

350°F

Outdoor Temperature Ambient air

38°F

Chimney Height Firebox to cap

18 ft

Flue Area Interior cross-section

90 in²

Altitude Above sea level

400 ft

Flue Condition

Flue Offsets / Bends

Liner / Flue Wall

Cap Type

Symptom Checker

Check all symptoms you are experiencing. These are used to weight the diagnosis.

Draft Measurement

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Pascals (Pa)

Draft (Pa)

—

Pascals

Draft (in H₂O)

—

inches water col.

Flue Velocity

—

ft/min

ΔT Differential

—

°F

Vol. Flow

—

CFM

Efficiency

—

combined factor

Awaiting Input

Adjust the sliders and select your chimney’s conditions to see the draft calculation and diagnosis.

Factor Analysis

Root Cause Diagnosis & Fixes

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Awaiting parameters

Enter your chimney dimensions and conditions to generate a personalised root-cause analysis.

Draft Reference Guide

Draft Range (Pa)	in. H₂O	Status	Typical Causes	Recommended Fix
< 0	< 0	Backdraft	Cold flue, wind pressure, blocked cap, competing exhaust fans	Prime flue with warm air; install anti-downdraft cap; seal house air leaks
0 – 5	0 – 0.02	Insufficient	Short chimney, very low ΔT, severely clogged liner, unlined masonry	Extend chimney height; reline flue; preheat before lighting
5 – 12	0.02 – 0.048	Weak	Moderate blockage, sub-optimal height, wet fuel, too many bends	Annual sweep; use dry fuel; check for debris in cap; add height if possible
12 – 25	0.048 – 0.10	Good	Normal well-maintained residential installation	Maintain with annual inspection; no corrective action needed
25 – 50	0.10 – 0.20	Strong	Tall chimney, large ΔT, cold outdoor temperatures	Excellent draft; use damper to reduce airflow and prevent fuel waste
> 50	> 0.20	Excessive	Very tall stack or extreme ΔT; oversized flue relative to appliance	Install damper; consider smaller liner insert to balance flow