What Is a Wood Insert and Why Do I Need A Liner?

A wood insert is basically a high efficient wood stove reconfigured to fit into an existing wood burning fireplace. Most wood burning fireplaces are either true masonry with brick chimneys with or without clay flue tile lining or factory built metal fireboxes with air-cooled metal chimney pipe. Both types are inefficient and drafty. Many take more heat out of the home then they add.

Fireplace inserts were first conceived in the late 1970’s. They were originally intended to simply set into the fireplace and convert the open fireplace into an efficient, solid fuel, heating appliance. However, inserts were soon associated with installation and safety problems. The principle safety issue was accumulation of creosote primarily on the interior walls of the chimney and the fireplace smoke chamber. These conditions often led to dangerous fires in the fireplace (behind the insert) and in the chimney itself. In 1984, in response to numerous chimney fires, National codes and standards (NFPA211) mandated that all inserts be installed with a stainless steel connector from the insert to the first flue liner. The following minimum conditions are also required: a) the cross section or area of the flue can be no greater than three times that of the flue collar or smoke outlet on the insert. b) inspection and cleaning must be possible c) no dilution air is allowed to enter the chimney. However, lining only to the first flue liner is very difficult to seal and cleaning is very difficult and often impossible without removing the insert and stainless steel connector. A masonry chimney is not the ideal chimney anyway for an insert installed without a flue liner. The flue gases in an efficient insert installed without a liner will cool before exiting the cap. By keeping the gases hot with a properly installed liner, they remain in the form of a gas until they have escaped from the chimney cap. The benefits of a full reline are becoming more obvious and will be even more so as you read on. The above issues apply to a masonry fireplace so what about a factory built unit that already has a stainless steel chimney? In this case, National Code (NFPA211) revolves around UL chimney listings and requires that closed combustion wood burning appliances, such as an insert, use UL103 Type HT chimney (2100 degree F tested). The chimneys used in factory built fireplaces only have to meet UL127 requirements (1700 degree F). Therefore, to be in compliance with Code, factory built fireplaces must also be relined. Furthermore, inserts typically have a 6″ flue collar as opposed to most factory built fireplaces have an 8″ or 10″ chimney. In short, to be safe, you must follow Code and use a liner when installing an insert.