effect tries to draw air down the
chimney. In a worst case situation,
air and smoke (or gas) will be drawn
down the chimney, spill out of the
appliance and try to rise up through
the middle of the home. In effect,
the house has become a better
chimney than the one built for the
basement fireplace or furnace. This
situation is worsened if the chimney
in question is located on the outside
of the home, outside of the warm
envelope, as is the case with many
basement fireplaces. The chimney
system now not only has to
overcome this "stack effect", it also is
faced with an impassable 30 ft (9m)
plug of cold air. (In our Temp-Cast
Planning Guide, we strongly
recommend against the combination
of outside chimneys and basement
installations for these reasons.)
OTHER FACTORS AFFECTING
DRAFT
There are other factors besides the
"stack effect" which can cause poor
draft and smoke spillage. Some of
these are:
• blocked or partially blocked
chimney or base manifold
• insufficient chimney height
• chimney with too many offsets
• chimney cross-sectional area too
small
• poorly aligned chimney liners
causing partial obstructions
• lack of chimney cap (or roof-top
damper) to keep out wind & rain
• chimney built outside of the warm
envelope of the home
• locating the fireplace in the
basement, where the "stack
effect" is greatest
• operating a poorly balanced
mechanical ventilator
• operating too many or too
powerful exhaust fans which
compete with the fireplace for
combustion air. (This happens
most often in very tight houses,
when a strong fan can actually
de-pressurize the home, and
draw fresh air down the chimney.
"Down-draft" range fans are the
most common culprits.)
• strong winds against poorly
sealed windows & doors
(increasing the stack effect)
• open or leaky windows upstairs in
a 2 story home (increasing stack
effect)
• mild weather, when the
temperature difference between
air in the chimney and outside air
is very little.
CHECKING FOR DRAFT
Smoke spillage caused by poor draft
is most common during the start-up
phase of a fire, and the "tail-out"
phase, at the end of the fire.
Spillage is not likely to occur when a
strong fire and good draft have been
established.
Since poor draft and negative
pressure are difficult to detect before
lighting the fire, a simple pre-fire
routine will quickly tell you if a draft
problem exists.
Load your firebox with the
appropriate amount of wood, but do
not light it yet. Open the chimney
damper and fresh air supply. Close
the firebox doors over, but do not
latch the door, so that a slight space
of about 1/4" (6.5mm) is left between
door gasket and door frame. Light a
match and hold it near this gap. .
If the flame is drawn strongly into
the firebox, you can be confident that
a strong draft is present. You can
proceed to light the fire and close the
door(s).
If the flame of the match is blown
towards you, away from the firebox,
or is blown out, then you probably
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