Chimneys are classified by maximum allowable flue gas temperature and suitability for different types of fuel. There are three types of chimney commonly used in Canada:
A masonry chimney is constructed of brick, stone or concrete block, and encases a liner made of material selected to withstand the effects of high temperatures and corrosive acids. Liners of clay pipe or tile are usually used in new construction. Where a masonry chimney is to be renovated without being dismantled, a steel liner can be inserted. A masonry chimney can be used with furnaces, stoves, fireplaces or any other appliances that burn gas, oil or solid fuels.
This type of chimney is commonly referred to as a “Type A” chimney. It is constructed of factory-made metal sections, or lengths that can be joined together on site. It is intended for installation on the inside or outside of a building. The liner is usually stainless steel, but the outer wall may be of a different metal since it is not subject to corrosion or to the same high temperatures. The Type A chimney has a mineral based thermal insulation between its walls to reduce the temperature of its outside surface. It is suitable for use with an appliance that burns gas, oil or solid fuel; provided that the temperature of the flue gas does not exceed 1000 F.
Factory-built Gas Vent Chimney
This chimney consists of factory-made metal sections that can be joined together on site. It may appear to be generally similar to the Type A chimney, but it is not intended to withstand flue gas temperatures higher than 470 F. Hence, it is not suitable for use with appliances that burn oil or solid fuels. It is normally designed with only an air gap between the inner and outer walls to provide insulation.
The chimney cap surrounds the flue lining at the top of the chimney. Its purpose is to protect the brick work from rain and moisture. The cap of the chimney should extend 1 inch beyond the brick work of the chimney stack. The cap in older chimneys can be made of slabs of stone or a thick coat of mortar. Poured concrete caps and cast concrete caps are a more effective choice. In the event that the chimney cap is cracked or damaged, temporary repairs can be made with silicone caulking, however, replacement is the ideal solution.
The brick work and liner are installed in sections from the base of the chimney to the top of the stack. The brickwork is exposed not only to the temperatures within the chimney, but also the elements, therefore, damage is inevitable. Flaking or chipping can be caused during freezing, due to the moisture in the bricks. As moisture or temperature changes expand and contract the mortar, it can loosen and crumble, causing the brick to become unstable. Damage can often be repaired by repointing the mortar, but if it is severe, the entire chimney may need replacement from the roof line up.
This is a valve which regulates the draft level by allowing room temperature air into the chimney. This lowers the flue gas temperature. Chimney top dampers are designed to reduce heat loss when the fireplace is not in operation and to exclude birds and debris.
Most chimney fires are caused by the build up of creosote. Creosote is a by product of incomplete combustion, it is a tar like substance created from condensed gases. As wood is burned, the sticky creosote residue (which is as fine as dust) travels up the chimney in the smoke. As it cools, it falls, and collects on the inside wall of the chimney. Temperatures released by most fireplaces or stoves range from 300F to 900 F. Creosote ignites at 451 F, the same low burning point as paper. Once the creosote is ignited, the temperature in a chimney can rise to 2,000 F in seconds, causing an extremely dangerous situation.
Most chimney fires can be avoided. The chimney is one of the most neglected components of the house. In some cases minor repairs can be done by the homeowner, but it is recommended that serious repairs be left to a professional. The chimney should be inspected annually, but ideally, once in the fall and again in the spring. A chimney cleaning and inspection costs approximately $75.00. The cost of replacing a chimney from the roof line up can cost up to $1,000.00. Proper maintenance is all that a chimney requires to provide safe, comforting heat.