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

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Three-sided chimney hazards:

The term "three-sided chimney" refers to masonry chimneys that have three sides exposed to the outdoor elements - a design that may be contrasted with chimneys built into rather than onto buildings. A chimney built into a building has only one side exposed on an exterior wall, while still other chimneys that pass through the building interior may have no sides exposed to the outdoors until the chimney exits through the building roof.

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.

Interior-Routed Chimneys Help Heat the Building

Construction of a chimney that passes through the interior of a home was originally done to get the most heat out of the chimney and into the occupied space during 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.

An additional benefit of constructing the masonry chimney on a route passing through the building interior was lower construction cost. When the brick chimney is routed through the building interior it can be built using common brick rather than more costly face brick that should be used where the brick is exposed to the weather.

Watch out: wherever a masonry chimney is constructed, where it must pass by wood framing the chimney should be separated from wood or other combustibles by a two-inch air space. Depending on fire code requirements where you live it may be permitted or required to fill the air space with non-combustible insulation such as mineral wool or to use a fireproof insulating sealant.

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 1000°F continuous flue gas temperature although most gas appliances operate with a flue gas temperature of about 300°F and oil burners with a flue gas temperature of about 500°F.

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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Continue reading at BRACKET CHIMNEY COLLAPSE & FIRE RISKS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see ANGLED CHIMNEYS

Or see MASONRY CHIMNEY GUIDE - home

Or see UNLINED FLUES

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THREE-SIDED CHIMNEYS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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