Throats and Voids

Fireplaces should be well throated. That is to say that the void of a fireplace (that transitional volume between the fireplace opening and the flue) should taper gently and smoothly so as to maintain a smooth, laminar flow of smoke into the flue. Anything that interrupts this creates turbulent, chaotic flow, and that will slow the flow and can do so badly enough to create backpressure problems.

Where the void is boxlike and without taper, especially where the flue pipe actually projects down into the box, these problems can be severe. They can lead to preferential deposits around the base of the liner, causing the build-up of soot which will interfere with the proper function of the flue. A similar problem can happen at the top of the flue where the pot is markedly smaller than the flue, and lacks a smooth taper.