Spec Sheet

1
practice cannot be met, fireplace per-
formance might not suffer noticeably.
However, installations with several char-
acteristics known to cause problems are
much more likely to create user dis-
satisfaction because of smoking and
backdrafting. Therefore, while com-
plying with all these best practices is
a good target to aim for in all instal-
lations, some compromise will not nec-
essarily result in poor performance.
These best practices can be expected
to produce good performance in most
fireplaces. However, the installation
instructions provided by the fireplace
manufacturer take precedence, so they
should be followed if they differ from
the recommendations made here.
Venting failure is usually caused by
the location of the fireplace and its
chimney in the house and the way
other equipment affects fireplace oper-
ation. For this reason the best time to
prevent smoke spillage from fireplaces
is during the planning stage as the fire-
place system is integrated with the
house design. Following, and in Fig-
ures 1 and 2, is a summary of the ele-
ments of best practice that are discussed
in this paper.
1. Install fireplaces and
chimneys inside the building
envelope.
2. Penetrate the building
envelope at or near its
highest level.
3. Avoid large, uncompensated
exhausts.
4. Avoid very short chimney
systems.
5. Use straight chimney systems.
6. Provide glass doors.
Locate Fireplaces and
Chimneys Inside Houses
A common installation practice involves
locating the fireplace and its chimney
in a chase projecting outside the build-
ing envelope to conserve interior floor
space. Despite its popularity, the prac-
tice of locating fireplaces in outside
chases is one of the main causes of
complaints about fireplaces that spill
odors and cold air when not in use and
smoke when a fire is lit. A fireplace
depends on the buoyancy of its hot
exhaust to draw in combustion air and
create flow up the chimney. But when
the chimney runs up the outside of the
house, its ability to resist the negative
pressure in the house due to stack effect
is weakened (see discussion of stack
effect below).
Draft, which is the pressure differ-
ence needed to vent fireplaces success-
fully, is influenced by the temperature
of the exhaust and the height of the
chimney. That is, the hotter the exhaust
gases, the stronger the draft. And the
taller the chimney (at a given temper-
ature), the stronger the draft. In well-
designed systems there tends to be a
low level of draft present during the
heating season even when the fireplace
is not in use. This “standby draft” is
produced because the room tempera-
ture air in the chimney is warmer than
the outdoor air, so it rises. Without
standby draft, chimney flow can reverse,
spilling foul odors and cold air from
an unused fireplace.
Under standby conditions the air in
a fireplace and chimney installed in an
FIG. 2
3
2
6
1
4
Fig. 2 This system will function reliably because it has the characteristics that promote successful venting.