Things You'll Need
The thimble is the round opening that can be found in a brick or stone chimney about five feet above floor level. The thimble enters the flue inside the chimney at a 90 degree angle and is designed to accommodate the entry of a stovepipe from a wood stove. Installing a thimble in a chimney isn't difficult, but the job needs to be approached carefully and systematically to avoid damage to the interior chimney flue.
Lay out the entry area for the thimble by drawing a circle on the face of the brick with a carpenter's pencil.
Chip out the brick within the circle using a chisel and small sledgehammer. Alternatively, you can drill through the brick with a masonry drill, although this is a specialty tool and requires some skill to run. If you choose to pursue this option, you are advised to hire a professional to do it for you, because a masonry drill in the hands of an amateur is a dangerous thing, particularly when drilling such a large hole.
Cut through the side of the flue inside the chimney carefully, making the dimensions of the hole the same as the hole that you put through the brick.
Apply mortar to the inside face of the hole that you made, being sure to cover all areas of both the brick and the interior clay flue.
Place the thimble into the hole. Reach through the thimble so that you can feel the joint between the end of the thimble and the inside of the clay flue. Stabilizing the thimble with one hand, press the mortar around the thimble on all sides with a trowel using the other hand.
Fill any gaps that you feel on the inside of the clay flue liner between it and the inside edge of the thimble. The inside edge of the thimble should be flush with the inside face of the clay flue liner, with all gaps filled with mortar.
Fill any gaps that surround the thimble on the outside face of the chimney, and smooth the face of the mortar between the thimble and the bricks. Be sure that the thimble is level; the part that protrudes from the chimney should be perpendicular to the face of the brick.