There is currently no maximum or minimum length specified for single skin flue pipe which connects a stove to a chimney. Single skin flue pipe may not pass through any roof space, floor or internal wall.
We asked Barrie Meridew, independant chimney consultant and he said:
There is currently no mention of minimum or maximum lengths of flue pipe used to connect a solid fuel appliance to a chimney, other than the flue pipe not being permitted to extend beyond the room in which the appliance is located.
At Stovesonline we recommend that, if you want to use single skin flue pipe, that the most you should have is 1000mm, with a recommendation of a maximum of 1500mm. The reason we make these recommendations is that a lot of heat will transfer to the room from the uninsulated flue pipe. This leaves the flue gases colder and therefore more soot, tar, and other deposits will condense in the chimney. This increases the risk of chimney fire. Please be aware that modern stoves that have very efficient combustion produce relatively cold flue gases. Undoubtedly your chimney will work better if it is entirely insulated so as to keep the flue gases as warm as possible - this will promote a better draw from the chimney as well as reducing deposits.
Some people seem to be under the impression that you have to have a length of single skin flue pipe to start with before connecting to a liner or twin wall flue system. This is not the case, although with low grade or cheap liners or twin wall flue pipe, using a section of single skin flue pipe can be a good idea to reduce heat damage from flames tracking up the chimney.
We asked Barrie Meridew, independant chimney consultant about the heat loss from single skin flue pipe and he said:
As this heat would otherwise be used to ensure good venting characteristics and reduce the condensation potential within the flue, the flue pipe should be kept as short as possible - if used at all. Making a direct contact to the appliance spigot with an insulated flue or chimney section will always provide the better chimney performance.