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CHIMNEY INSPECTION DIAGNOSIS REPAIR
BACKDRAFTING HEATING EQUIPMENT
CARBON MONOXIDE - CO
CHIMNEY COMPONENT DEFINITIONS
CHIMNEY FIRE ACTION / PREVENTION
COMBUSTION GASES & PARTICLE HAZARDS
COMBUSTION PRODUCTS & IAQ
FLAME COLOR, BLUE vs YELLOW COMBUSTION
HEATING SYSTEM INSPECTION
HOME HEATING SAFETY
ODORS & SMELLS DIAGNOSIS & CURE
SAFETY RECALLS CHIMNEYS VENTS HEATERS
STAIN DIAGNOSIS on BUILDING EXTERIORS
WOOD, COAL STOVES & FIREPLACES
WOOD STOVE SAFETY
Fireplaces & hearths, construction, inspection and repair: this article provides information about masonry fireplaces, including inspection for damage/hazards (cracks and gaps that appear at masonry fireplaces due to chimney or fireplace settlement or movement), fireplace chimney sizing requirements, draft problems, chimney safety, creosote problems, inserts, and other topics. Fireplace damage from chimney or fireplace settlement or movement may be a fire or gas hazard in a building Fireplace hearth size specifications; How to add support below a settling fireplace hearth Fireplace damper inspection, diagnosis, repair or replacement Photo examples of cracks in on and around masonry fireplaces and a guide to their cause and remedy Masonry fireplace chimney & flue size requirements
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Readers of this article should also see FIREPLACE INSPECTIONS for a professional chimney sweep's fireplace and chimney hazard checklist. At FIREPLACE INSERTS we discuss fireplace inserts and zero-clearance fireplaces, both antique and modern, and their hazards and inspection limitations. Readers should also see Inaccessible Connections Fireplace or Woodstove. The inspection of chimney interiors is discussed at Chimney Inspection: Flue Interiors.
According to the Masonry Institute of America, the required flue size for masonry fireplaces (burning wood) is basically a function of the area of the fireplace opening. Typical standards require a cross-section area of the fireplace flue or chimney/vent to be equal to 1/10 of the area of the fireplace opening itself, for a conventional wood-burning installation and without considering the effects of a glass fire-door. The FHA requires using a 1/8 ratio instead of 1/10 for chimneys that are less than 15 feet high and the 1/10 ratio for chimneys that are 15 feet or more tall. 
How do we measure fireplace chimney height?
Note that "chimney height" for the purpose of determining fireplace requirements, is measured not from the ground outside nor from the "floor" of the fireplace hearth. Rather you should use the distance from the fireplace throat to the top of the chimney. Don't include the chimney cap in height measurements - that added distance does not develop draft in the flue.
Chimney Cross-Sectional Flue Shape Effects on Draft
At FLUE SIZE SPECIFICATIONS where we discuss chimney flue sizing for venting heating appliances such as boilers, furnaces and water heaters, we explain that in comparing two flues of exactly the same square inches of cross-sectional area, a round flue will have better draft than a rectangular one.
For this reason, fireplace and chimney guides offer a table of effective chimney vent area (measured as a cross-section of the flue opening) and flue sizes. You can find effective area and flue size tables in the Uniform Mechanical Code and in the MIA's Masonry Fireplace & Chimney Handbook and also in NFPA 211 - Standards for Chimneys & Fireplaces. 
Un-lined chimney flues are also specified as larger than lined installations and use the 1/8 ratio we explained above.
To use the fireplace chimney flue sizing table below, calculate the area in square inches of the cross section of the inside of the fireplace opening. For a rectangular fireplace opening just multiply its width by its height in inches to calculate the value to look for in column A of the table. The dimensions given in columns B & C of the chimney sizing table present standard clay chimney flue tile dimensions and shapes.
Adapted from Masonry Fireplace and Chimney Handbook, 2nd Ed., James E. Armhein, S.E., M.I.A. Masonry Institute of America, Los Angeles, CA 213-388-0472 prepared to include requirements of the 1994 UBC and other codes. - MIA.
Why are gaps at fireplace fireboxes, hearths, or other components a dangerous fire hazard? What should you do about them?
First at above left we see a gap that has opened up between the fireplace floor and the hearth (above-left). Sparks may fall into this space, causing a building fire.
Second (above right) our photo shows a crack between the face of the fireplace and the fireplace box itself. We don't know without more analysis whether the brick facing has fallen away from a sound and safe fireplace or whether the fireplace has moved away from the facing.
Stuffing a pillow into the chimney throat of a fireplace (above left) might slow the loss of warm air from a home, but it's a dangerous substitute for a missing or broken fireplace damper. What if someone lights a fire without noticing this stuffing?
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