double wall chimney pipe – Ask the Chimney Sweep

When it comes to the inside of your chimney, things can get a little confusing. Terms that seem to mean the same thing or that look very similar have caused people to ask me many questions over my years in the chimney business. The term "chimney liner" is extremely vague. Loosely defined, a chimney liner is anything that is used to vent exhause from an appliance or fireplace through a chimney. A chimney liner can be clay tiles, stainless steel pipe (for wood burning appliances) or aluminum pipe (for gas or oil). Clay pipes have no warranty and they are usually extruded in a factory using ceramic materials. Clay flue liners are meant to hold the product of combustion (fire) but not fire itself. Fire in the actual clay flue system will cause cracks to occur, and they are intended for use in residential chimneys that burn less than 1000 degrees Fahrenheit. This particular type of flue liner is uniquely orange but baking can turn it more brown. In Europe, most liners whether they are clay or metal are round. Here in the United States we use modular construction which is square in size because square clay flue liners are easier to store, manufacture, and ship. Round flue pipes vent a lot better than square flue pipes because the products of combustion rifle or spiral up the round pipe. Often, a square edge clay chimney liner is not even fully used because the gases will still spiral up the pipe as much as they possibly can. This type of chimney liner is used when clay tiles fail. Generally used to vent solid fuel burning appliances and fireplaces Stainless steel chimney liners are primarily used as a relying system for damaged clay flue tiles. Generally they are used to vent solid fuel burning appliances and fireplaces. In an open fireplace (no wood stove or insert) NFPA 11 states that the exhausting flue system must not be any less than 10% of the opening of the mouth of the fireplace. Keep this in mind when you are converting from a clay flue liner and installing a stainless steel chimney liner. You must be sure not to reduce the square surface inches when you are replacing a clay chimney liner with a stainless steel chimney pipe. A wood stove pipe is generally black and comes out of the top of a wood stove and goes into the wall. It is necessary for the wood stove pipe to be the same size as the collar of the appliance. You cannot downsize the pipe. Black pipe like this can be either single or double walled. Any single wall pipe needs 18" of clearance from combustibles no matter what type of appliance or fireplace it is venting. A double wall chimney pipe can reduce clearance necessities by about 66%. It is IMPERATIVE to follow manufacturers instructions however because not all double wall chimney pipes are qualified for this. Double wall pipes are sometimes known as "factory built chimney systems." Sometimes pipes that pass through a wall, such as a wood stove pipe that passes from the top of the stove through the wall, require an insulated wall thimble or a pass through. These fit around the pipe at the site where the pipe passes through the wall and dissipates heat so as to not catch the wall on fire. Finally, there are chimney pipes that are called Class A chimney pipes. They are installed on the outside of a home. In most cases, these chimney pipes are spaced about 2" from combustibles. Class A chimney pipes must meet the code requirement of being two feet higher than anything within a ten foot radius. This helps ensure proper draw. All chimney pipe and chimney liners must be installed according to the manufacturers listing standards which may require braces, spacers, particular clearances, and caps.