Chimney Liner Size Calculator
Determine the correct chimney flue liner size based on your appliance BTU rating, fuel type, and chimney height.
Covers gas, oil, and wood appliances.
Enter your appliance BTU rating and fuel type to determine the correct chimney liner size.
Understanding Chimney Liner Sizing
A chimney liner must be correctly sized to safely vent combustion gases from your heating appliance. An oversized liner allows gases to cool too quickly, causing condensation and creosote buildup. An undersized liner restricts airflow and can cause dangerous carbon monoxide backflow into the home. The correct size depends on the appliance BTU output, the fuel type, and the height of the chimney.
Liner Size by Appliance BTU Rating (Natural Gas):
| Liner Diameter | Area (sq in) | Maximum BTU Input | Maximum BTU (kW) |
|---|---|---|---|
| 3 in (76 mm) | 7.1 | 74,000 | 21.7 kW |
| 4 in (102 mm) | 12.6 | 119,000 | 34.9 kW |
| 5 in (127 mm) | 19.6 | 186,000 | 54.5 kW |
| 6 in (152 mm) | 28.3 | 268,000 | 78.5 kW |
| 7 in (178 mm) | 38.5 | 370,000 | 108.4 kW |
| 8 in (203 mm) | 50.3 | 484,000 | 141.8 kW |
Fuel Type Considerations:
Different fuels produce different combustion byproducts, which affects the liner material and size needed.
- Natural gas: Produces water vapor and CO2. Requires stainless steel or aluminum liner. Size based on appliance BTU chart above.
- Propane (LP gas): Similar to natural gas but burns slightly hotter. Use stainless steel liner. Sizing follows the same BTU chart.
- Oil: Produces sulfuric acid in condensation. Requires stainless steel liner rated for oil (AL29-4C alloy). Generally needs one size larger than gas for the same BTU rating.
- Wood: Produces creosote and high temperatures (up to 2100 degrees Fahrenheit or 1150 degrees Celsius during a chimney fire). Requires heavy-gauge stainless steel (316L alloy) or clay tile liner. Minimum 6-inch (152 mm) diameter for any wood-burning appliance.
Chimney Height Factor:
Taller chimneys create more draft (suction), which affects liner sizing. The values in the table above assume a chimney height of 15 to 30 feet (4.6 to 9.1 meters). For shorter chimneys under 15 feet, you may need one size larger to compensate for reduced draft. For chimneys over 30 feet tall, the standard size works well, and you may even be able to use one size smaller in some cases.
The Draft Equation:
Draft is created by the temperature difference between hot flue gases and cool outside air. The formula is:
Draft (inches of water) = 0.0255 x H x (1/To - 1/Tf)
Where H is chimney height in feet, To is outside air temperature in Rankine, and Tf is flue gas temperature in Rankine. A properly functioning chimney produces at least 0.04 to 0.05 inches of water column draft per foot of height.
General Rules of Thumb:
The flue area should be approximately 1/10th to 1/12th of the fireplace opening area for an open masonry fireplace. For example, a fireplace opening of 36 inches wide by 28 inches tall (1,008 square inches) needs a flue area of approximately 84 to 101 square inches, which corresponds to a 10 to 12 inch (254 to 305 mm) round liner or equivalent rectangular liner. For a factory-built fireplace or stove, always use the liner size specified by the manufacturer in the installation manual.