1. Best Practices for
Woodburning Fireplace
Installation
Prevent smoking, drafts and odors
from fireplaces installed in new homes
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4
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FIG. 1 See legend next page.
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ntroduction not only in lost time but in the repu- lems are difficult and expensive to cor-
The ideal wood-burning fireplace is tation of everyone concerned. rect after the fireplace is installed, so
a pleasure to use. It doesn’t smoke Over the years the fireplace indus- the installation design stage is critical
when lit or spill cold air and odors try has spent a lot of time and money to success.
when not in use; it doesn’t backdraft investigating problems and working to Although installations that meet all
when the kitchen fan is on, and it works improve fireplace performance. We now of these best practices are ideal and are
well regardless of wind speed or direc- know how to prevent problems through most likely to give trouble-free perfor-
tion. For obvious reasons, everyone effective installation design. This paper mance, sometimes house designs or
involved in putting fireplaces into provides a concise overview of the char- client objectives make compromises
houses, including manufacturers, archi- acteristics of good design. But before necessary. Where one aspect of best
tects, builders and installation contrac- getting to the details of best practices
tors, want every fireplace to give for integrating fireplaces into today’s Fig. 1 The hearth system illustrated
pleasure and never frustrate the home- houses, there is one essential fact you above would not operate reliably
owner. But sometimes fireplaces don’t need to know: because it has many of the features that
work well and the results are costly, The most common fireplace prob- lead to problems.

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

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

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