IMPROPER FLUE SIZING

When your masonry fireplace and chimney was being constructed, the flue sizing
was determined by the size of the fireplace opening below. Masons usually
calculated the size of fireplace flues by using a "rule of thumb" that the CSA
(Cross-Sectional Area) of the masonry flue be at least 1/10 the CSA of the fireplace
opening. For example, an 8x12 (id) flue liner is used in a chimney venting a
fireplace with an opening measuring up to 40" wide x 24" tall.  The larger  the
fireplace, the larger the flue needed to be. If your fireplace smokes because the
flue is too small, try temporarily reducing the size of the fireplace opening with
pieces of sheet metal, this could resolve the issue.  What to do next really is
determined by this simple question: Is your fireplace used for a heat source or
strictly for ambience.  If you are using it for an actual heat source, I recomend
installing an insert.  Inserts today, come with fans, timers, and even remote
controls, to regulate the heat. 

When you have an woodstove, pellet stove, coal stove, either free-standing or an
insert, flue sizing is determined by the stove manufacturer for each model during
the testing process. In order for a woodstove chimney to do the best possible
venting job, the flue opening must have exactly the same cross-sectional area as
the vent opening on the appliance. If the chimney is too small, it may not have
room for the volume of rising air the stove requires. If it is too big, it may draw
too slowly for the appliance, and may never heat up enough to compensate. For
this reason, both flue undersizing and flue oversizing should be avoided. If your
chimney is too small, replace either the chimney or the appliance. If your chimney
is too big, install a stainless steel flue liner that has the same CSA as the vent
opening on the appliance. For a chart showing recommended masonry liner sizes
for woodstoves and inserts, contact me as soon as you can.
Thank You