Chimneys come in many shapes in cross section. Some of the more common ones are the square and rectangular. While the square ones can be easier to manage in terms of installation of the chimney lining (you can use a circular liner offering a snug fit), the rectangular ones are a little bit trickier to work with. This is due to the oblong shape which makes finding a liner that can fit in them a bit harder. Using a circular liner in a rectangular flue is hardly ideal as it means a lot of space is left in the periphery.

One solution to this is getting custom made chimney liners, but this may be a bit expensive. A cheaper and simpler option is to get the oval chimney liners. Due to the geometrical configuration of these liners, they can snugly fit in a rectangular flue, offering a bigger cross section than any other kind of liner. Oval chimney liners can also be used in situations where a chimney has an oval flue, in which case no other type of liner would work. As with all other types of liners, one should consult a professional before installing the liners, as in some jurisdictions there are existing laws that govern how to install liners.