Understand the components of a masonry chimney
8 Aug 2006
Masonry Chimney Diagram
This chimney shows a fireplace flue on the left and a freestanding flue on the right side. The freestanding flue may be used to vent freestanding stove or a furnace.
1. Flue (shown with clay tile liners)
2. Smoke Chamber
3. Smoke Shelf
4. Fireplace, or Firebox
5. Ash Pit
7. Hearth Extension
8. Face Wall
10. Fireplace Chimney
11. Crown, or Wash
12. Chimney Cap
14. Thimble, or Breech
15. Cleanout Door
The Flue (1) is the passageway for smoke & fumes. Clay flue tiles, used in most newer masonry chimneys, provide a barrier that helps contain smoke, fumes and soot so they are not absorbed into the masonry chimney.
Smoke Chamber (2) is the area above the fireplace and below the flue, used to allow smoke to mix and rise into the flue. Because smoke tends to linger here, large deposits of creosote often accumulate here.
Smoke Shelf (3) helps collect small amounts of rain that may enter the chimney. Considerable creosote deposits may collect here. The hinged plate shown is the Damper which is used to close off the chimney when the fireplace is not in used. Helps to prevent loss of heated air up the chimney.
Fireplace (4) is where the fire is built and viewed.
Ash Pit (5) is a passageway that ashes may be emptied into.
Lintel (6) is a piece of metal that supports the face wall (8).
Mantel (9) is a decorative shelf above the fireplace. The one shown is made of brick but others may be made of wood, plaster, stone, cast iron or other decorative materials.
Crown (11), also called a Wash, is a sloped bed of mortar that helps shed rain water. When this crown is cracked or deteriorated then rain can be absorbed into the chimney and cause rapid deterioration.
Chimney Cap (12) helps keep rain from entering the flue. An open flue is like a huge bucket that can allow large amounts of rain water to enter.
Thimble, or breech (14) is the passageway for a freestanding stove or furnace to connect to the chimney.
Cleanout door (15) is used to remove creosote and other foreign matter from the chimney. The cleanout door should be tightly sealed to prevent cool air from entering the chimney as this reduces proper drafting efficiency.
Other terms commonly used in reference to chimneys and fireplaces: Chimney Liner is a separation between the fumes in the flue and the walls of the chimney. Newer chimneys are most commonly built with clay tile liners which are mortared together in sections as the chimney is built. Other chimney lining methods, most often seen when an old chimney is retrofitted with a new liner or where a chimney is re-lined after suffering damage, includes stainless or aluminum pipe and a multitude of cast-in-place liners which may be poured and formed. Different lining methods are recommended depending upon the type of appliance used in the chimney and the chimney's overall condition.
Looking up a chimney from the bottom. The white spot is the sky. Note the creosote deposits in this chimney and the cracked flue tiles. Vertical cracks like the one shown indicate chimney fire damage. Creosote is a flammable black deposit in chimneys that results from burning wood. Creosote removal is the most common reason for sweeping the chimney.
Deteriorated chimney mortar crown, which is allowing water to be absorbed into the brick chimney from the top.
A newly laid mortar crown with a stainless steel chimney cap installed
Spalling is a condition wherein clay tile flue liners chip or peel. This is most often due to moisture mixing with corrosive deposits. Spalling compromises the integrity of the chimney liner and its ability to contain chimney fumes.
Tuckpointing is a masonry term that refers to repair of the mortar joints between bricks. When this mortar has weakened or started to fall out , as in the photo at right, then bricks may become loose. Tuckpointing refers to installing new mortar between the bricks.
Parging is a layer of masonry that covers masonry surfaces (bricks, firebricks, cement block). Sometimes tuckpointing is not a sufficient means to repair bricks, especially in the smoke chamber above the fireplace.
Efflorescence is a white stain that appears on bricks. It is due to moisture problems mixing with chlorides. It is an unsightly stain that indicates a problem with the chimney not venting properly. Most often seen on chimneys venting gas appliances and is an early sign of deterioration.
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