Flues and Flue Liners
The chimney flue is the part of the chimney where the exhaust gases actually travel. (The flue is contained inside the chimney chase, which is just a fancy word for “the outside parts of the chimney”.)
There are three common kinds of residential chimney — the kind made entirely of chimney pipe, usually venting free standing wood stoves; the kind made of metal pipe inside a wooden or masonry chase, usually venting prefabricated fireplaces; and the kind made entirely of masonry — this is the “traditional” kind most people envision when they think about fireplaces.
An unlined masonry chimney flue.
Originally, masonry chimneys were made with no special innards — imagine four plain brick walls meeting in a square to form a long tube up which smoke could travel. The “flue” was just this space between the four chimney walls. This arrangement looks great from the outside, but it isn’t safe to use. Over time, the mortar between the bricks begins to deteriorate, leaving holes in the chimney. Take a look at the photo to the right, of an unlined brick chimney — see all the holes where smoke or heat can just pass right through the wall of the chimney?
Terra cotta flue tiles extending from the crown of a masonry chimney.
A modern masonry chimney has a flue liner, which insulates the flue from the chimney chase and helps to contain heat and exhaust gases inside the flue. Most masonry flue liners are made of cylindrical terra cotta flue tiles, stacked on top of one another like building blocks. These tiles are good insulators, because they are made of masonry, but they are fairly fragile and, over time, will move, shift, and crack, forming gaps.
A gap between flue tiles. The raw masonry of the chase is visible through the gap.
Gaps or cracks in a flue liner are potentially hazardous. Creosote and soot, which are flammable, can build up in these spaces or even outside the flue, where they cannot be swept and removed. The gaps can also allow exhaust gases and/or heat out of the sides of the flue and into the chimney or the home. For this reason, we do not recommend using a chimney which has a damaged liner.
Chimneys which have no liner, or which have damaged terra cotta flue liners, can be relined in various ways, including with spray-on ceramic/refractory cement liners and with flexible stainless steel liners. Other alternatives include installation of a wood or gas fireplace insert and accompanying stainless steel liner, or even the installation of vent-free gas logs. Click here to read more about these options.
The chimneys on prefabricated chimney systems (installed in many modern houses), and chimneys made of stove pipe (such as the ones that vent free standing wood stoves), are made of rigid, cylindrical metal pieces which fit into each other like building blocks. These chimneys either do not need a liner (single-wall stove pipe) or come with their own liner (double- or triple-wall stove pipe; Class A pipe; B-vent pipe). If the “liner”, or wall, on these chimneys fails, the only recourse is often to install a whole new chimney. (The pieces are not designed to be pulled apart and rejoined, although in some cases they can be.)
