Twin wall flue system design, length of single skin stove flue pipe

Because the single skin flue pipe is not insulated the flue gases will cool faster in this section of pipe and therefore you will get more tar deposits in your chimney. This tar is the cause of chimney fires. Stove manufacturers generally recommend that you do not run more than 1000mm in single skin flue. By building regulations the single skin flue must terminate at least 450mm below combustible ceilings and the flue must be three times it's diameter away from combustible materials - ie for a 150mm single skin flue pipe you must have a 1050mm circle around the centre line of the flue in which there are no combustible materials.

The advantages of single skin flue pipe is that it has a slimmer external diameter than twin wall flue. It is also cheaper than twin wall flue, although the amount saved over a 1000mm length is not so great.
The disadvantages (other than increased tar desposits as mentioned above) are that you have to have two types of flue in the room, two types of flue with different external diamaters - the twin wall flue having a diamter 50mm larger than the single skin flue. This changeover will be visible and may not look as neat as a system all in one type of flue.