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These are typically manufactured metal units. Some have masonry firebrick and some have ceramic coated panels. Installation should be in accordance with the manufacturer’s specifications.
Fireplace walls should be a minimum of 8" thick if fireclay brick is used to line the firebox. If there is no fireclay lining, the walls must be a minimum of 12" thick. A lined smoke chamber should be a minimum of 6" thick; an unlined chamber should be at least 8" thick.
Fireplace fireboxes that are larger than 6 sq. ft. (SF) should have hearth extensions that extend at least 20" in front of the firebox and at least 12" beyond each side of the opening. Fireboxes that are less than 6 SF should be at least 16" out and 8" 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 (10 to 1 rule) and twelve times greater than a round flue (12 to 1 rule) 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 built prior to the early 1900's 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.
Many homes in the United States use wood stoves as primary and secondary source of heat. It is important that such components and associated flues or chimneys operate properly and are well maintained, as they may pose a fire hazard.
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Wood stoves are very popular in rural areas. In some regions of the United States, such as Colorado, the use of wood stoves is regulated based on the air quality.
The following information touches on the issues surrounding wood stoves, and provides some helpful tips that you may wish to consider and apply in your own home.
Clearance to combustibles
The single largest concern with a wood stove is its clearance to combustibles. Many stoves are often installed by the homeowner or unqualified people who violate the required clearances. Manufacturer’s specifications should be obtained in order to verify that the clearances are indeed accurate and comply with regulations.
A common mistake that installers make is they assume that if a fireproof material is installed between the wood stove and combustible materials, they are safe. What they fail to realize is that heat will conduct through ceramic tile, asbestos board, metal or other materials that are rated as fireproof.
Wood stoves are very hazardous to young children for obvious reasons. US Inspect recommends a screening or children’s fence at or around the stove to protect small children.
Hearths for wood stoves must be the same as they are on fireplaces.
In order to have a fire, you must have fuel (wood) and air. Fireplaces that do not have the means to restrict air, such as glass doors, are not as likely to have creosote buildup. Creosote develops when there is insufficient draft, not allowing a fire to reach it's full heat potential. Air reduction will cause the fire to burn at a lower temperature. Fires burning at approximately 1100 degrees Fahrenheit or less will not burn hot enough to eliminate a buildup of creosote. As smoke and rising creosote cools, it condenses, separates itself from the smoke, and attaches itself to the chimney walls.
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.
Improving Chimney Draw
- Open a window or door 2" to 4", close to the fireplace and prior to lighting. Leave open until the fire goes out. This will assist in providing adequate combustion air.
- Prior to lighting the wood, hold a lighted newspaper at the entrance to the flue to warm or prime the chimney shaft. This may be necessary to evacuate to heavier, cold air from the chimney.
- Opening dampers in other fireplaces will also help, although this is not as efficient as opening a window located near the fireplace.
- Reduce the size of the firebox opening. Installing glass doors, and closing the doors on one or both sides easily does this.
- If the items outlined here do not assist in providing a satisfactory draw, there may be a serious design problem. Consultation may be necessary.
Outside Combustion Air
Newer fireplaces have a provision to increase the effectiveness of a fireplace by allowing outside air to come directly into the firebox for combustion. A secondary advantage, in addition to providing a reliable source of combustion air, is they do not allow the fire to consume interior conditioned air that has been previously heated by the central heating system.
See-Through Fireplaces (open on 2 sides)
Fireplaces that are open on two sides may violate the 12 to 1, or 10 to 1, square or rectangular flue size to firebox opening ratio requirement. Check the ratio size of the flue to the total open area of the firebox. It is not unusual for these fireplaces to have draw problems.
Masonry kitchen barbecues that use charcoal were popular around World War II. Most of these devices have been removed. Those that remain are normally of poor design and are not used. It is crucial that they have a proper venting system.
The primary defect with prefabricated fireplaces is clearance to combustibles. Prefabricated fireplaces are framed in wood and proper standoff distances may not be maintained. The other common defect is warped or cracked metal in the firebox, or warped or cracked firebrick. This is often due to a very hot fire or utilizing coal as a fuel.
Common Woodstove Defects
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- Sharing a fireplace flue with another device
- Improper clearance to combustibles
- Routing a stovepipe through an interior wall without proper clearance to combustibles
- Unsupported or sagging stovepipes
- Dampers difficult to operate or inoperative