Three Wall Chimney Hazards (C) Carson Dunlop Associates How to spot 3-sided Chimneys & Why They are a Hazard


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This article describes visual clues to pick up on unsafe three-sided chimneys. Our sketch of a three-sided chimney is courtesy Carson Dunlop Associates.

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Three-Sided Chimney Fire Hazards

A three-sided or "three walled chimney" is one which does not provide full masonry thickness or fire protection around all sides of the chimney flue.

As Carson Dunlop's sketch shows, a 3-sided chimney may be constructed when a chimney is added to a building as a retrofit project.

Three Wall Chimney Hazards (C) Carson Dunlop Associates

An amateur worker may place the chimney flue tiles right against the building exterior wall, covering only the flue's exposed surfaces with brickwork.

Three sided chimneys can be a very serious fire risk because the chimney has been built close to or even directly against combustible building materials without the necessary fire clearance and masonry fire protection needed.

Chimney Location, Draft & Performance

Chimney location affects performance (C) Carson Dunlop Associates

As Carson Dunlop's sketch shows, a warm chimney works best at developing good draft which in turn helps assure that the appliances or fireplaces being vented by the chimney will perform properly.

Construction of the chimney running through the interior of a home was originally done to get the most heat out of the chimney in cold weather.

Even though it is easier to build the chimney on the outside wall of a building, a central chimney provided heat through its masonry to the building interior on all floors.

Draft: Thermal Performance of Chimneys

A chimney's thermal performance provides the "draft" by maintaining a warm interior lining. The draft is the pressure difference between ambient air and the less dense flue gases within the chimney. The lighter gases are buoyant and rise to be displaced by heavier ambient air.

The chimney must contain the hot gases and protect the surrounding materials against combustion. Residential masonry chimneys must protect the building while under exposure to 1000oF continuous flue gas temperature although most gas appliances operate with a flue gas temperature of about 300oF and oil burners with a flue gas temperature of about 500oF.

See DRAFT REGULATORS - barometric dampers and DRAFT HOOD, GAS HEATER for details about draft, how it works, how it is measured, why it is important for safe appliance operation.

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