As part of our HETAS Registered Installation service, we carry out smoke tests which can be used to verify the performance and gas-tightness of a clay lined chimney.
Most properties built since 1965, when the first Building Regulations came into force, have a clay or pumice liner system built into the chimney stack.
This is made up of rigid sections of circular pipe, with the joints sealed and pointed with fireproof mortar. The gap between the liner and the brickwork surrounding it should be backfilled for insulation purposes.
Until quite recently, most chimneys have been constructed with open solid fuel fires or decorative fuel effect gas fires in mind, so clay liners in the vast majority of post-1965 housing stock will be 7", 8" or 9" diameter.
In accordance with Document J of the Building Regulations, it is permissible to connect a wood burning or multi-fuel stove up to an existing clay lined chimney, providing the liners are installed correctly and the joints remain sound.
Our test involves lighting smoke pellets in the fireplace or appliance, then sealing the flue at the bottom and top. The pressure created forces the smoke through any weak or failed joints between flue liners, which potentially could pass through the fabric of the building, making the flue unsafe to use.
If the flue passes this test and the clay liners are proven to still be in sound condition, then a new appliance can be connected to the clay lined chimney. This is normally achieved by using components which increase in diameter from the stove's flue outlet, to provide a smooth transition up to the larger clay liner size.
The advantage of using the existing clay liners as your flue is that you can save a significant amount of money by not having to have your chimney relined.
It should be pointed out, though, that while connecting a stove to a clay lined chimney of a larger diameter is perfectly safe and permitted by Building Regulations, the stove will always perform better if the flue is relined in a diameter to match the appliance's flue outlet.
If the flue fails the smoke test it is deemed unsafe for a solid fuel installation, and it must be relined using a twin wall flexible stainless steel liner.