Fireplace walls should be a minimum of 8² thick if fireclay brick is used to line the firebox. If there is no fireclay lining, the walls must be a minimum of 12² thick. A lined smoke chamber should be a minimum of 6² thick; an unlined chamber should be at least 8² thick.

Fireplace fireboxes that are larger than 6 sq. ft. should have hearth extensions that extend at least 20² in front of the firebox and at least 12² beyond each side of the opening. Fireboxes that are less than 6 sq. ft. should be at least 16² out and 8² on each side.

In older fireplaces, fireboxes are constructed with standard or common brick. Common brick deteriorates easier than fireclay brick. Firebrick should be used in all new fireplaces, and if the brick cracks or the mortar deteriorates, it should be repaired with refractory or reinforced Portland cement.

The opening for a fireplace should be a minimum of ten times greater than a rectangular flue and twelve times greater than a round flue to ensure adequate drafting.

The hearth, during construction, is normally supported with wood forms. In most cases, forms must be removed after construction, since they pose a fire hazard. Cracks in the hearth should be repaired.

A fireplace should have a properly operating damper. Younger fireplaces are usually built with dampers, but fireplaces that are 70 years old and older are not likely to have dampers. Conventional dampers can be installed, however, it is more common to see external dampers installed on top of the chimney flue of older chimneys.

Any indications of smoking around a fireplace should be noted. Causes for smoking may include: failure to open the damper; improper draft; poor design; setting the wood too close to the front; or inadequate combustion air.

Newer houses are very tightly built and air infiltration as well as air changes are reduced significantly. If there are signs that the fireplace is smoking, and that there may be a problem with the draw, there are a number of possible causes, as outlined above. There are also some relatively easy solutions.

a. Open a window or door 2² to 4², close to the fireplace and prior to lighting and until the fire goes out. This will assist in providing adequate combustion air.

b. Prior to lighting the wood, hold a lighted newspaper up into the flue to warm or prime the chimney shaft. This may be necessary to evacuate to heavier, cold air from the chimney.

c. Opening dampers in other fireplaces will also help, although this is not as good as opening a window located near the fireplace.

d. Reduce the size of the firebox opening. Installing glass doors, and closing the doors on one or both sides easily does this.

e. If the items outlined here do not assist in providing a satisfactory draw, there may be a serious design problem. Consultation may be necessary.

Newer fireplaces have a provision to increase the effectiveness of a fireplace by allowing outside air to come directly into the firebox for combustion. Their biggest advantage, other than providing a reliable source of combustion air, is that they do not allow the fire to consume interior air that was heated by the central heating system.

In 1982, wood-burning appliances accounted for more fires, more fire deaths, and greater property damage than any other kind of heating fuel -- about 140,000 fires, 250 deaths and $257 million in property damage. These losses represented 20 percent of all residential fires in the U.S., 5 percent of all fire deaths, and 8 percent of estimated property damage.

CPSC research indicates that most wood heating fires involve the chimney and not the appliance itself. The majority of these fires are contained within the chimney and cause no damage to the house. The Commission is concerned, however, not only about the chimney fires that did ignite other parts of the house, but also about the potential future hazard from the continued use of chimneys whose structural integrity has been compromised by a chimney fire. This is especially true in light of the fact that many contained chimney fires are not reported to the fire services; in fact, consumers may not even be aware that a chimney fire has occurred.

Therefore, the Consumer Product Safety Commission is issuing a special safety alert concerning chimneys used with woodburning stoves, fireplaces, and fireplace inserts. The Commission urgently warns consumers to be aware of the potential fire hazards associated with these chimneys.

Now that the nation has entered the heating season, the Commission strongly urges you, if you have a stove or fireplace, to check the chimney for any damage that may have occurred in the past heating season. If it is difficult to examine the chimney, a local chimney repairman, chimney "sweep," or dealer can help. Have any damage repaired NOW.

Most fires involving either masonry or prefabricated metal chimneys occur because of improper installation, use or maintenance. The Commission staff has identified the following common causes of fires:

1.)Improper chimney installation too close to wood framing.
2.)Installation of thermal insulation too close to the chimney.
3.)Improperly passing the stovepipe or chimney through a ceiling or wall, causing ignition of wood framing.
4.)Structural damage to the chimney caused by the ignition of creosote (a black tar-like substance that builds up inside the chimney in normal use).

5.)Structural damage to metal prefabricated chimneys that results in wood framing being exposed to excessive temperatures or leakage of potentially toxic gases to the interior of the home can take the following forms:

6.)Corrosion or rusting of the inner liners of metal chimneys.
7.)Buckling, separation of the seam, or collapsing of the inner liner of metal chimneys. (This can result from too hot a fire, especially in high-efficiency stoves and in fireplace inserts, or from a creosote fire.)

Structural damage also occurs in masonry chimneys, often associated with deterioration or improper installation of the chimney. The tile inner liner and the surrounding brick or block structure may crack and separate, perhaps as a result of the ignition of creosote that has built up in the chimney. Many old chimneys do not have a tile liner. If your chimney does not have a liner, the addition of a properly installed liner is advisable. Also, a clay liner should be sealed with refractory cement.

Even when the heating appliance is properly installed, people with either metal or masonry chimney systems should frequently check the chimney for creosote deposits, soot build-up, or physical damage. This involves only a simple visual examination, but it should be done as often as twice a month during heavy use. If you see heavy creosote buildup, suspect a problem, or have had a chimney fire, a qualified chimney repairman or chimney "sweep" should perform a complete safety inspection. They can arrange for any necessary repairs or creosote removal, which must be done before the heating appliance is used again.