Clay flue liners or pipes are a requirement in most chimney and wood stove venting projects. They are easy to use and place; however, when it comes time to cut a round hole in a clay flue pipe so you can fit additional venting, you can quickly become frustrated by how easily the pipe will shatter. There is a way to cut a hole in a clay flue pipe without breaking it. You will need enough time to set up the pipe to cut it, but it is worth it considering you won't have to keep buying new sections to replace those that have been broken.
Mark the centre of the hole you want to drill using your tape measure and a pencil. Hold the end of your tape on the centre mark and your pencil on the measurement for the radius for your hole. For example, 4 inches for a pipe 8 inches in diameter, the radius will equal half the diameter or distance across your pipe at the centre. Clay flue liners are rectangular, so marking the hole will be as easy as drawing a circle on a piece of paper.
Fill the clay flue pipe with sand.
Drill two holes in the pipe just inside the circle near the line you draw. You want the holes to overlap so they form an opening large enough to insert the tip of the blade of your reciprocating saw. Make sure you are using a masonry bit in your drill, and drill at a very slow speed. This will prevent the clay flue liner from cracking when you drill.
Insert the tip of the blade of your reciprocating saw into the double hole you drill and, running the saw at a low speed, begin to cut along the line you drew. You don't have to try and use the entire saw blade, just insert the first inch of the blade into the cut. The blade will run into the sand inside, which is fine; you need the sand to absorb the vibrations of the drill and saw so they do not cause the clay pipe to break.
When you have cut all the way around the circle you have drawn, tap the centre of the circle piece (which should be completely separate from the rest of the flue) with a hammer. Tap it several times until it breaks rather then using one mighty swing. When the piece breaks, pull the shards out, empty the sand from the flue and you are finished.
If you don't have sand and won't have a use for a leftover bag of sand from this project, substitute potting soil for sand. Pack it tightly into the clay flue liner and, when finished, empty it back into a bag and reuse in your garden.
Always appropriate personal protective equipment when cutting any kind of fired clay. Safety glasses, safety shoes and gloves will protect you in the event that the clay shatters. Shards of clay can be just as sharp as glass and cause significant damage.