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What do the throat, smoke shelf, and smoke chamber do?

What do the throat, smoke shelf, and smoke chamber do?

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Many people believe that this is the most critical part of a fireplace. The function is to take the exhaust gases from the large firebox and direct them into the relatively small chimney flue. The goal is to gently compress the exhaust gases without creating backdraft. If it works well, we accelerate the exhaust gases up into the chimney and help to promote good draft. If we don't do it well, we end up with smoke in the house.

The

throat

is the top of the firebox below and at the damper.

The

smoke shelf

is the area behind the damper that is nearly level. It is the bottom of the chimney.

The

smoke chamber

is the sloping section of the fireplace above the damper and throat, but below the chimney flue. Typically, the sides and front of the smoke chamber slope. The back of the smoke chamber is usually vertical. This varies.

In some areas, the smoke chamber is referred to as the throat, and there is no particular name for the top of the firebox and the area of the damper. This terminology is somewhat loose and is not critical. The throat, smoke shelf and smoke chamber are typically masonry, although the smoke chamber is not usually firebrick.

Factory-built fireplaces do not usually have a smoke shelf or smoke chamber above the damper. Since most of these fireplaces draw quite nicely, the need for a smoke shelf is questionable. It is, however, a standard part of most masonry fireplaces.

The walls of the smoke chamber may have a minimum required thickness of 51/2 or 6 inches. Some jurisdictions do not allow any hollow units. In some cases, the wall thickness has to be 8 inches. A maximum slope for the sides and front is 45 off vertical. Many say that 30 is a much better arrangement. Good design typically involves the smoke chamber front and both side walls having equal slopes. When the flue is offset to one side of the fireplace or the other, the smoke chamber wall slope has to be asymmetric. This contributes to poor draft, especially if one side of the smoke chamber is vertical and the other has a dramatic slope.

The walls of the smoke chamber should be as smooth as possible. Corbelled brick is not smooth and may contribute to draft problems.

When the back wall of the chamber has to be sloped, again, draft problems may be in the wings. Most designers avoid sloping the back wall of the smoke chamber, although this is common on basement fireplaces, especially those added to an existing home.

The smoke shelf is sometimes flat or almost flat. In other cases it is concave at the bottom, shaped like a cup. Most have a slight slope toward the damper. Part of the thinking is that downdrafts can be stopped and redirected so that they will actually become updrafts and help to improve the overall draft. The absence of a smoke shelf in zero clearance fireplaces may suggest that this theory is flawed.
by Manage My Life
April 26th, 2007
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