Spec Sheet

2
outside chase tends to cool grad-
ually to below room tempera-
ture, even if the chase is sealed
and insulated. As the chimney
temperature falls, draft declines
along with it. The loss of standby
draft is worsened by the fact that
negative pressure inside the house
can work against upward flow
in the chimney. The three most
common causes of negative pres-
sure in houses are stack effect,
exhaust fan effects and air cir-
culation system imbalances. One
or more of these sources are
found in all houses to varying
degrees.
Stack Effect
Just as warm air in a chimney
produces draft, the air in the
house is warm relative to the
outdoor air so it tends to rise, produc-
ing a pressure difference called stack
effect. Stack effect creates negative pres-
sure in the lower levels of the house,
where fireplaces are normally installed.
The negative pressure due to stack effect
is enough to force a cold backdraft in
a fireplace chimney that is cooler than
room temperature. If one attempts to
light a fire in a backdrafting fireplace,
smoke will flow into the room instead
of up the chimney. The negative pres-
sure due to stack effect experienced by
the fireplace is strengthened if the house
has excessive leakage at high levels
(see below). The cure for this form of
venting failure is straightforward: Install
chimneys through the warm space
enclosed by the building envelope.
A chimney installed inside the house
will produce more standby draft than
the house produces in stack effect. This
is because the air inside the chimney
is at room temperature, but the chim-
ney is taller than the warm space of
the house. The negative pressure due
to stack effect in the house is never
strong enough to overcome standby draft
in an interior chimney (subject to the
effect discussed below).
Penetrate the Top of the
Building Envelope
Keeping a chimney at room tempera-
ture during standby conditions deals
with one of the main influences on
chimney draft. The other factor is height
and it is just as important. To produce
enough standby draft, a chimney must
not only be as warm as the house, but
it must also be as tall as the highest
part of the building envelope in order
to overcome the negative pressure due
to stack effect.
A chimney installed in a single-story
section of a two-story house can suf-
fer cold backdrafting at standby (Fig.
1). This is because the chimney may
be shorter than the warm part of the
building and in addition, cooling of the
chimney’s exposed section reduces its
average temperature. A chimney
installed low on the eaves of a house
with a vaulted ceiling can backdraft for
the same reasons.
The problems resulting from a chim-
ney being installed in an outside chase
or penetrating the building envelope at
or below its top are made far worse if
the majority of leaks in the building
envelope are concentrated at high lev-
els. High level leakage can be created
by recessed light fixtures, ceiling fan
mounts, poorly fitted attic hatches, fold-
up stairs and skylights. High level leak-
age sites should be minimized if pos-
sible and, where unavoidable, they
should be carefully sealed to the build-
ing air barrier.
To avoid these problems, a chim-
ney should penetrate the highest part
of the building envelope. For most com-
mon house designs, this means
that the fireplace and chimney
should be located on an interior
wall rather than a perimeter wall.
Fireplaces and chimneys that
are located on perimeter walls
(or outside perimeter walls in
chases) are more vulnerable to
the adverse effects of wind
because their chimneys tend to
be low relative to roof lines. The
general strategy for preventing
cold backdrafts under standby
conditions is to locate fireplaces
and their chimneys centrally in
buildings rather than against
perimeter walls. The same
approach is effective in reduc-
ing the potentially adverse effects
of wind by placing the top of
the chimney above areas of wind
turbulence.
Even in cases where venting
problems are assumed to be
wind-related, the replacement of
the standard chimney cap with
one of specialized design should only
be done with the approval of the fire-
place manufacturer. The standard chim-
ney caps supplied by manufacturers of
factory-built fireplaces and chimneys
are usually of good design, so replac-
ing standard caps supplied with these
fireplaces is not necessary. In fact, the
supplied cap should never be replaced
or left off for aesthetic or other reasons
5
Chimney offsets
can be impli-
cated in smoke
spillage.
To produce enough
standby draft, a chim-
ney must not only be
as warm as the
house, but it must
also be as tall as the
highest part of the
building envelope in
order to overcome the
negative pressure due
to stack ef fect.
Location
Not
Recommended
Marginal
Location
Recommended
Location
Chimney should penetrate the envelope
at or near its highest point.