If you have a prefabricated unit installed in your home now, or you are considering purchasing one, this information is extremely important. One fact that many people do not realize is how young factory built, or “prefab” units really are. They have only been readily available for purchase for the past 25 years, approximately. However, as a chimney sweep, I deal with them all the time.
Nearly one half of the prefabricated units installed throughout the United States are installed improperly. It is absolutely imperative that installation instructions are followed to the letter, especially specified clearances. Clearances from the firebox and chimney to any combustible material must generally be at least 2″. Most manufacturers require a 2″ air space with no insulation. These specifications are of the utmost importance because they prevent a process known as pyrolisis, in which the wood too close to the chimney dries out and will eventually catch fire, at temperatures as low as 200 degrees!
It is absolutely imperative to install a chimney cap to a prefabricated unit. It is not unusual for chimney sweeps to literally carry out buckets of nesting material from the flue systems of these types of fireplaces. Especially older versions of these units have imitation brick housing above the roof, which birds love to nest in. A cap will prevent these nesting materials from catching your house on fire and blocking air flow, which causes the chimney to overheat.
Although it is difficult to inspect firebox clearances, a trip to the attic to check out clearances of the chimney is definitely worth your while. If you find issues in the attic, it is crucial to install access ports to check the clearances in the firebox and monitor conditions as time goes by.
Another thing to consider, especially if you are in the market for prefabs is the fact that each unit is specifically tested and listed specifically for use with one another. The fireplace and the chimney are tested together and approved, meaning that you must go through that same company to obtain a replacement part years down the road.
Often times, these companies will go out of business leaving you stuck. Your prefab unit will reach the end of its useful life when repair of the unit is no longer possible. Be wary that necessary components of the listings may become unavailable.