Spec Sheet

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because the result could be venting fail-
ure due to adverse winds. Decorative
shrouds for chimney tops should never
be used unless listed by the fireplace
manufacturer.
Avoid Large,
Uncompensated
Exhausts
When a large exhaust, such as
a downdraft kitchen barbecue
fan, is turned on in a modern
tightly constructed house, the
pressure inside can drop below
atmospheric pressure outside.
This negative pressure competes
with the upward flow in a chim-
ney produced by draft and can
result in the spillage of smoke
into the house. Once they are
aware of it, most homeowners
can manage this potential prob-
lem, but it can be alarming
when first experienced.
One way to prevent this is to avoid
the use of large exhaust fans in houses
with wood-burning equipment. Another
way is to compensate for the volume
of air exhausted with a roughly equal
amount of make-up air brought in from
outdoors. A make-up air fan can be elec-
trically interlocked with the exhaust fan
switch so that house depressurization is
prevented. Make-up air systems of this
type can function without the home-
owner having to take action or even
being aware that it is operating.
Pressure imbalances in houses can
also be created by design problems with
forced-air heating and cooling systems.
Leaking ducts and air handlers located
outside the building envelope in attics
or crawl spaces are common causes of
negative pressure problems. Unbalanced
supply and return airflows can also
depressurize the space where a fireplace
is located, leading to spillage of smoke
and cold air.
Avoid Very Short
Chimney Systems
At a given temperature difference, a
taller chimney will produce more draft
than a shorter chimney. While factory-
built fireplaces are normally approved
for minimum system heights in the 14-
to 15-foot range, taller systems than
this are preferable for good performance,
particularly for fireplaces with gener-
ous hearth openings relative to chim-
ney diameter. The larger the fireplace
opening, the more air must pass through
it to prevent spillage. For fireplaces
with large openings or with more than
one open side, such as see-through or
corner fireplaces, a system height of
at least 20 feet is preferable. Note that
the installation instructions for listed
factory-built fireplaces provide chim-
ney height limitations and these should
be followed precisely.
Use Straight Chimney Systems
Chimney offsets should be avoided where
possible. Each elbow or offset in a vent-
ing system creates turbulence in the flue
gas stream and therefore presents resis-
tance to flow. Offsets in chimneys are
not uncommon but their use demands
special attention to the other aspects
of system design. Manufacturer’s instruc-
tions for factory-built fireplaces should
be reviewed carefully for criteria asso-
ciated with the use of offsets.
Provide Glass Doors
Open fireplaces are particularly
vulnerable to smoke spillage for
two reasons. First, the main
restriction to flow – the chim-
ney – is downstream of the fire,
and there is little resistance to
spillage flow into the room. Sec-
ond, most new homes do not
provide enough natural leakage
to supply the large amount of
air that an open fireplace con-
sumes. Spillage resistance is
increased significantly if the
main flow restriction is on the
room side of the fire in the form
of glass doors, which also reduce
the amount of air that the fireplace needs
for satisfactory operation.
Many homeowners enjoy the crackle
of an open fire and some fireplaces
in some houses can operate success-
fully this way. But changing condi-
tions, like adverse winds, the operation
of an exhaust fan, unbalanced airflows
from one room to another, or even
people walking in front of the fire-
place, can induce spillage from the
open hearth. See-through or multi-
sided open fireplaces are particularly
vulnerable to these pressure differ-
ences and airflows.
If smoke spillage occurs, the quick-
est and most effective way to stop the
spillage is to close the glass doors. Glass
doors can also help to reduce air leak-
age up the chimney and hearth odors
from spilling into the room when the
fireplace is not in use. For these rea-
sons all fireplaces should be provided
with glass doors, even if the purchaser
expects to use the fireplace as an open
hearth most of the time.
For more information about the Best
Practices recommended in this docu-
ment, including technical references and
alternate design strategies, please visit
the HPBA Web site at www.hpba.org
This article is a joint effort of Hearth
& Home Magazine, and the Technical
Committee of the Hearth, Patio and
Barbecue Association. Text created by
John Gulland.
Spillage r esistance
is incr eased signifi -
cantly if the main flow
restriction is on the
room side of the fire
in the for m of glass
doors
Why ducted outdoor
combustion air is not in
this list of best practices.
A ducted supply of combustion
air from outdoors has been
promoted – and even made
mandatory in some building
codes – as a solution to spillage
problems from fireplaces.
Although there is anecdotal
evidence of effectiveness in
some situations, the available
research shows that outdoor
air supplies do not reliably
prevent smoke spillage. Where
required by regulation, these
outdoor air supplies must be
installed, but they should not be
relied upon as a primary mea-
sure to ensure good fireplace
performance.