Anatomy of a chimney

A chimney, simply put, is a vertical tube designed to draw combustion products (smoke and gasses) from an appliance like a wood stove or fireplace to the atmosphere outside the house. Here are the basic parts:

Flues

Inside a chimney you'll find one or more vertical passageways called flues. Ideally, each appliance connected to the chimney (such as each fireplace, each furnace, each wood stove) has its own, separate flue. More than one flue might be contained in one masonry chimney. So if you have a furnace and a fireplace connected to the same chimney, there should be at least two vertical passageways up the inside of the chimney.

Metal factory-built chimneys, of course, contain only one passageway for venting combustion products, the inside of the pipe.

Flue liners

In a modern masonry chimney, the inner wall of the flue is lined with some type of material for safety, ease of cleaning, and improved performance. Among the most common types of liners are:

  • Terra-cotta: Baked clay liners, also called terra-cotta or tile liners, are generally about 5/8" thick, and look like two-foot long square, rectangular, or round tubes. They are cemented end-to-end up the inside of the flue to form a continuous, smooth lining.
  • Other modular liners: Terra cotta is not the only material used to make modular liner tubes. Some are composed of refractory cement, volcanic pumice, or a combination of fireproof materials.
  • Stainless Steel: Especially useful in re-lining existing chimneys (But used in new construction, as well), stainless steel liner systems incorporate a metal tube, rigid or flexible, with some type of insulation around it. The metal tube provides a continuous, even lining, and the insulation forms an additional layer of protection and helps keep the flue warm.
  • Aluminum: Some installations of gas-fired equipment allow for the use of lower-cost aluminum liner systems.
  • Cast-in-place: Cast-in-place liners are, in essence, a thick layer of a highly durable, insulative, cement-type material applied to the walls of the flue. One method (but not the only one) for installing a cast-in-place lining involves inserting a removable rubber tube the full length of the flue and pumping the liner material in. The tube is later removed, leaving a smooth-walled, cast-in-place flue liner.

Note: For a metal factory-built chimney, the inner wall of the chimney serves the purpose of a chimney liner.

Chimney Crown

The top of a masonry chimney is called the crown. It should be gently sloped toward the edge, causing rainwater to run off. The flue liners should extend above the crown at least two inches (maybe more, depending on the local building code), so you might be able to see the tops of the liners from the ground.

Cleanout Door

At the base of each flue you should find a small, metal cleanout door. When your chimney professional cleans the flue, the soot and debris will be removed through this cleanout access. The exception is a fireplace, which needs no door, since the soot is cleaned out right at the fireplace opening. If you find a door in the cellar centered below the fireplace, it is probably an ash pit door. We will talk about that in the section on fireplaces.

Those are the basics. We will discuss other parts of chimneys as we cover relevant topics.

 

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