Fitting the flue
Every chimney or flue needs to be a regular shaped (rectangular or circular are the most common), and as straight as can be possibly achieved. It is also important to ensure that the internal walls of the chimney are smooth. This ensures that all of the gases being vented from the stove are removed as efficiently as possible. By efficiently venting the exhaust gases, fresh clean air is drawn into the stove through the vents.
Faster moving gases leave less deposits on the walls of the flue, remember that the gases from the stove will contain all sorts of chemicals from the burn fuel. These are generally naturally occurring and harmless when cooled. All modern stoves burn these gases to very high temperatures, ensuring that they do not cause undue pollution and allows the flue to release them efficiently.
However, in a poorly fitted flue these gases are not properly handled and they can and do cause soot and creosote to build up. These can accumulate on the walls the flue, especially at areas which are not smooth. This build up can be seriously detrimental to the efficiency of the stove, as well as attracting more build up in the flue. This spiral can stop your stove from working completely, and in some cases lead to carbon monoxide being released into the house. Carbon monoxide (CO) is potentially fatal to humans at concentrations of only 400 parts per million.
A stove's flue differs from a conventional chimney, because the stove flue is sealed from the room. This arrangement doesn't let draughts into your house. The flue itself should retain a lot of heat because hot gases move faster. Flue liners are available for your traditional chimney. These liners protect the brickwork of chimneys, and provide a perfect smooth sided flue. The chimney liner will also reduce the available space in the chimney, something which will benefit the efficiency of your stove. By compressing the passage the gases can travel through, you will speed them up and encourage condensation once they have been released.