Metal Chimney Inspection
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Single wall metal chimneys are simply unacceptable. Metal chimneys serving appliances with flue gas temperatures at the entrance to the chimney below 350 degrees should be lined with an acid and condensate resistant metal or refractory material. |
Clearances
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Metal chimneys should have enough clearance to avoid heating combustible materials to a temperature in excess of 90 degrees above the ambient temperature.
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Exterior and Interior. Wood frame or other combustible material—18 inches minimum.
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Exterior and Interior. Metal chimney over 18² diameter-Non combustible materials-4 inches minimum.
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Exterior and Interior. Metal chimney 18² diameter or less—Non combustible materials—2 inches minimum.
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Interior. Metal chimneys must be enclosed in a continuous enclosure and maintain the integrity of fire separations—1 hour rating in buildings up to 4 stories.
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Fire stops at floors, walls and roofs must be made of entirely non-combustible materials. 9 inches is the typical clearance, however, it is 18 inches if galvanized steel or approved corrosion-resistant metal is not used.
Specific Metal Chimney Concerns
Metal chimneys will dissipate heat much quicker than masonry chimneys, even insulated metal chimneys, however, to a lesser degree. This causes two issues, and both are related to the draft.
1. The draft in metal chimneys is more difficult to get started. The colder air in the chimney reduces the ability of the smoke to rise, because it is heavier than the smoke. Once the fire is going, the chimney is warmed; there is no problem until the fire starts to go out and the air in the chimney cools.
2. When the fire goes out, and only smoking embers remain, the cooler air starts to accumulate in the top of the chimney. If the outside temperature is low enough, the cold air may cause the last 30 to 45 minutes of smoke to back into the house.
This condition is worse when the outside temperature is below 20 degrees Fahrenheit, and when the chimney is on the outside of the house. When the outside temperature is above 30 degrees Fahrenheit, this is generally not a concern. There is also less concern when the metal chimney goes through the interior of the house.
