Existing chimneys, flexible chimney liner, UK stove building regs J
In Document J, page 23 & 24
Lining existing chimneys
A flexible chimney liner may only be used to reline existing chimneys, not new chimneys.
Existing masonry chimneys can suffer from degradation over time. The mortar between courses can fail, the partition between flue runs can fail and holes may have been knocked through the chimney and not repaired. A flexible chimney liner is a way to ensure that the flue gases cannot then leak into the house when you install your stove.
It is important to note that if your chimney has failed like this then you shouldn't just use a liner and think everything is fine. Before you use a flexible liner you should first fix your chimney to make sure it is structurally sound.
A new chimney should be fit for purpose, although if we get involved in your project too late we do sometimes find that building contractors can be fond on using clay liners, which are not really suitable for a stove and will crack if there is a chimney fire. It's for that reason that when you work with us we will specify a pumice chimney system for any new masonry chimneys.
A chimney liner must be installed in one continuous length with no joints within the chimney. This makes good sense as once its installed its hard to check to see if any joins are still sound.
If you have a flexible chimney liner then your sweep should not use coring balls and they should use softer chimney brushes so as not to damage the liner. They should be very familiar with this.
Insulating the liner is not specifically mentioned in Doc J but is a very good idea. This keeps the flue gases warmer which helps the chimney to draw and reduces risk of chimney fire. If something like a floor joist running into the chimney has been overlooked (and that's quite easily done as the end will be blackened like the rest of the chimney) then the insulation will also protect the joist from the heat in the chimney.
