Furnace and / or water heater chimney requires proper placement and termination to safely discharge exhaust gases from the appliances combustion (fuel burning) process. Improper chimney placement and termination is often responsible for back-drafts, excessive corrosion of the chimney pipe walls, condensation, and Carbon Monoxide contamination of your house.

The rule explained on the pictures is just one of the several that have to be applied when installing the chimney. Make sure that you have a licensed professional performing gas water heater & furnace flue sizing calculations while replacing or adding any of those appliances in your home.

Chimney located less than 8' to the vertical wall must extend 2' above the roofFurnace and water heater chimney too low, must extend 2'min. above the roof if closer than 8' from vertical wallNatural (gravity) and motor induced draft type gas burning appliances chimney installed closer than 8’ from the vertical wall, must extend a minimum of 2’ above the roof placed over that wall.

Termination (height above the roof surface) of the gas-appliance chimney installed further than 8’ from the vertical wall are listed in the table below (relation to the roof slope). Based on International Residential Code and Uniform Mechanical Code

Termination of B and BW Vents based on [IRC 2426.6.5] and {UMC T8-1}
Roof Slope Height above Roof
up to 6/12 1′
>6/12 to 7/12 1’3″
>7/12 to 8/12 1’6″
>8/12 to 9/12 2′
>9/12 to 10/12 2’6″
>10/12 to 11/12 3’3″
>11/12 to 12/12 4′
>12/12 to 14/12 5′
>14/12 to 16/12 6′
>16/12 to 18/12 7′
>18/12 to 20/12 7’6″
>20/12 to21/12 8′

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