Stove Chimney Clearances

Thimbles and Wall Penetrations

A thimble is a lining for the opening from the room into the chimney, into which the chimney connector is inserted (this opening is also called a "breaching"). Assuming that the chimney is exposed to the room (i.e., not concealed by combustible materials), thimbles should be constructed of round fireclay liners not less than 5/8 inch thick, or made of a preformed refractory material. Fireclay thimbles should be cemented into the masonry wall of the chimney with refractory cement. The thimble should extend to, but not past, the flue wall.

Penetrating a combustible wall with a solid fuel venting system must be undertaken with great caution because of the many potential dangers inherent in this type of installation. The first of the dangers is the fact that the combustible materials in the wall will be concealed from inspections. Second, the horizontal run may be a source of creosote productions and the collection point of falling creosote. Finally, this section of the vent may be close enough to the appliance to attain creosote ignition temperatures as well as sustain high temperatures in normal operation.

Sometimes it is necessary for chimney connector to pass through a combustible wall before connecting to a masonry chimney. In order for the chimney connector to pass through a listed wall pass-through that has been installed in accordance with the conditions of the listing, or it must utilize one of the home-constructed systems recognized in the National Fireplace Institute 211. Never insert a chimney connector directly through a combustible wall without an approved pass-through method.