Flue liners are made from clay that has been fired in large industrial kilns. They are mortared into place as chimneys are being bricked up around them. The liners help vent out smoke and combustible gases from burnt fuel. After a clay flue has been fired, the clay is very hard. To cut a vitreous chimney flue liner, you need a diamond blade mounted in an electric- or gas-powered saw. The surface of a diamond blade is impregnated with bits of actual diamonds, the only thing strong enough to cut through kiln-fired, ceramic clay. You'll need to rent or buy the saw and blade. It's an arduous task to cut a flue liner but with the right equipment and focus it can be done.
Things You'll Need
Power saw with diamond blade
Measure and mark on the flue liner where the cut is to be made. Line up the ruler along the marks and draw a line across all four faces of the flue liner.
Put on safety glasses and place the teeth of the diamond saw blade on the pencil line slightly in from the closest corner of the flue. Lift the blade slightly away from the flue, start the saw, slowly bring the blade back into contact with the flue liner and begin cutting.
Cut slowly and allow the saw and blade to dictate how fast to go. Cut all the way across the first of four faces of the flue liner to the opposite corner. Turn off the saw before rotating the flue liner to reveal the next adjoining face.
Repeat Steps 2 and 3 until the flue liner has be cut all the way around.
Sometimes chimneys and flues need to be repaired as a result of damage during a wind storm, tornado or hurricane. The bricks and flue will be torn back to the roof line. A new flue section may have to be cut and mortared in place with new chimney bricks set around it.
Cut a flue liner outdoors so the wind can blow away any dust that is blown up by the saw during cutting.