Fireplaces Versus Wood Stoves
Fireplaces aren't designed to vent or carry away the combustion by-products from wood stoves or wood burning inserts; they are a uniquely engineered, solid-fuel burning system. The fireplace system consists of the firebox, a damper (the mechanism that regulates air flowing up the chimney), a smoke chamber (the area between the damper and the flue) and a flue (a passageway inside the chimney through which the smoke rises). Together, they vent the smoke and gasses produced by burning wood. How precise the alignment of the firebox, smoke chamber and flue are to each other, determines the system's performance.
The accepted rule of thumb, included in many local codes, dictates that the area of the firebox opening should be approximately 10 times larger than the area of the flue's interior volume - a 10:1 ratio. A 10:1 ratio should be used for chimneys less than 25 feet tall and 12:1 ratio for chimneys greater than 25 feet.
Whenever a wood stove or insert is vented through a masonry fireplace system, the initial ratios on which that masonry system was based may need to be changed. For example, if the size of an inserts firebox is smaller than that of the masonry firebox, the existing masonry flue may now be proportionately too large. An over-sized flue causes a decrease in the speed at which air moves out of the chimney. This lets the smoke exiting the wood stove linger in the chimney, cool down and deposit condensed creosote on the chimney’s interior. Creosote is a brown or black combustible deposit, occurring when smoke condenses, and it must be monitored and swept out to keep your system safe.
Major creosote deposits are created when wood stoves or inserts do not meet the 1984 accepted standard and vent smoke directly into the fireplace or smoke chamber. Smoke condenses inside the firebox and smoke chamber, and may produce a ceramic-hard glaze of condensed creosote which is hazardous, difficult and potentially expensive to remove. Unattended, this eventually damages masonry materials through the corrosive action of acids it contains. It is never advisable to continue the use of an improperly installed stove, even if it is old enough that manufacturer's instructions did not require the connector pipe to extend into the first flue tile. Insist on a safe installation for you and your family.