Many homes in the United States use fireplaces as a primary and secondary source of heat. It is important that such items and associated flues and chimneys operate properly and are well maintained, as they may pose a fire hazard. 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 details the issues surrounding fireplaces, and provides some helpful tips that you may wish to consider and apply in your own home.
Types of Fireplaces
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 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.
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 likely to have creosote buildup. Creosote only develops when the fire cannot get enough air to properly burn the wood. Reducing the amount of air will cause the fire to burn at a lower temperature. Fires that burn at approximately 1100 degrees Fahrenheit or less will not burn hot enough to eliminate the creosote. Cooler burning fires allow the creosote to rise up the chimney with the smoke. As the creosote cools, it condenses 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 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.
- 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.
- Opening dampers in other fireplaces will also help, although this is not as good 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. 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.
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.