Anatomy of a Masonry Chimney and Fireplace

Interior of a bad chimney

Masonry chimneys are more complicated than most people think. 

The structure is carefully designed to conform to proportions and standard building practices per the NFPA 211 Standards and International Residential Code.  If a chimney isn't built or maintained correctly, it may not work properly, and may even be a fire and Carbon Monoxide hazard.  

Chimney sweeps check many different parts of a chimney when doing a routine chimney sweeping and annual inspection.  Some of the interior parts the sweep will check are the firebox (where the fire is built), the damper, the smoke chamber, and the flue liner. 

Click the picture below for a larger size.

The interior flue liner is difficult to see without the use of a chimney camera system with a monitor, such as the Chim-Scan camera.  

The smoke chamber is the area just above the damper. It corbels (angles) inward to a narrower passageway and supports the flue liner.  The smoke chamber is usually constructed with bricks and must have a parge (a thin coat of mortar applied to a surface) coating of insulating mortar to allow for smooth passage of smoke and to help protect hidden combustibles from exposure to heat.  

One of the most common areas for fires to start is at the facial wall in front of the smoke chamber, where builders often place headers or studs without proper clearance to the chimney.  This area is not accessible without some demolition of the facial wall, which is usually not practical.  Wood exposed to heat over time pyrolizes (changes chemical composition).  Pyrolized wood can ignite at a lower temperature than non-pyrolized wood, and without exposure to direct flame.  This is why smoke chamber parging is critical.  

Chimney showing spalling.

The damper - A properly working damper is important to control draft, and close the flue when the fireplace is not being used.  

Standard throat dampers have a handle just inside the top portion of the fireplace.  Older dampers may rust shut, or rust so badly that they must be replaced. 

Top-sealing dampers are a good replacement, and a less expensive alternative since demolition is required to replace an entire throat damper assembly.  Top dampers offer more protection from cold downdrafts in the winter and heat in the summer by making an almost complete seal at the top of the flue.  

There are top-sealing combination damper/caps available, which offer even more protection from the elements and animals.  Top-dampers can keep insects and tiny bats out of flues as well, which is something that a standard chimney cover cannot do.  

The flue - If the chimney has an accumulation of creosote, the chimney sweep will remove it.  There are three types of creosote:

·         Stage I is very dry, light, and removes easily. 

·         Stage II creosote is heavier and takes a lot of elbow grease to brush off. 

o   For these types of creosote in a clay tile flue liner, the chimney sweep will use flat wire or round wire brushes to remove it.  If the flue has a stainless steel liner installed the sweep will use a polyester brush, which does not harm the steel. 

·         Stage III Creosote is baked-on glazed creosote. 

o   While all creosote is flammable and must be removed to avoid chimney fires, glazed creosote is the most flammable and dangerous.  This type of creosote can only be removed using a specialty power tool system, and in some cases, chemicals. 

o   Stage III creosote looks like shiny tar, and is usually created when green wood is burned, if wood is allowed to smolder, or if the flue size is too large for the fireplace. 

o   If a homeowner has glazed creosote in his chimney, he should consult with a professional chimney sweep to see what he can do differently regarding his wood-burning habits. 

After sweeping the flue, the chimney sweep will inspect the interior for missing mortar joints, cracks in flue tiles, open gaps, or deteriorating liner surface.  Gaps in the liner can allow toxic heated flue gases to escape the confines of the flue to the interior chimney or combustible wood framing. If damages are found during inspection the sweep will recommend a proper repair method. 

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