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Fig.1 The hearth system illustrated above would not operate reliably because it has many
of the features that lead to problems.
Introduction
The ideal wood-burning fireplace is a pleasure to use. It doesn't smoke
when lit or spill cold air and odors when not in use; it doesn't back
draft when the kitchen fan is on, and it works well regardless of wind
speed or direction. For obvious reasons, everyone involved in putting
fireplaces into houses, including manufacturers, architects, builders
and installation contractors, want every fireplace to give pleasure and
never frustrate the home owner. But sometimes fireplaces don't work well
and the results are costly, not only in lost time but in the reputation
of everyone concerned.
Over the years the fireplace industry has spent a lot of time and money
investigating problems and working to improve fireplace performance. We
now know how to prevent problems through effective installation design.
This paper provides a concise overview of the characteristics of good
design. But before getting to he details of best practices for integrating
fireplaces into today's houses, there is one essential fact you need to
know. The most common fireplace problems are difficult and expensive to
correct after the fireplace is installed, so the installation design stage
is critical to success.
Although installations that meet all of these best practices are ideal
and are most likely to give trouble-free performance, sometimes house
designs or client objectives make compromises necessary. Where one aspect
of best practice cannot be met, fireplace performance might not suffer
noticeably. However, installations with several characteristics known
to cause problems are much more likely to create user dissatisfaction
because of smoking and backdrafting. Therefore, while complying with all
the best practices is a good target to aim for in all installation, some
compromise will not necessarily result in poor performance.
Fig. 2 This system will function reliably becuase
it has the characteristics that promote successful venting.
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
operation. For this reason the best time to prevent smoke spillage from
fireplaces is during the planning stage as the fireplace system is integrated
with the house design. Following, and in Figures 1 and 2, is a summary
of the elements of best practice that are discussed in this paper.
1. Install fireplaces and chimneys inside the building.
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.
1. Locate Fireplaces and Chimneys Inside Houses
A common installation practice involves locating the fireplace and its
chimney in a chase projecting outside the building envelope to conserve
interior floor space. Despite is popularity, the practice 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 difference needed to vent fireplaces successfully,
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 temperature), 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 temperature
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 outside chase tends to cool gradually to below room temperature,
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 pressure
in houses are stack effect, exhaust fan effects and air circulation 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, producing a pressure
difference called stack effect. Stack effect creates negative pressure
in 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 pressure due to stack effect
experienced by the fireplace is strengthened if the house had 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 chimney is taller than the warm
space of the house. The negative pressure due to the stack effect in the
house is never strong enough to overcome standby draft in an interior
chimney (subject to the effect discussed below).
2. Penetrate the Top of the Building Envelope
Keeping a chimney at room temperature 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
suffer cold backdrafting at standby (Fig. 1). This is because the chimney
may be shorter than the warm part to of the building and in addition,
cooling of the chimney's exposed section reduces its average temperature.
A chimney installed low to the caves of a house with a vaulted ceiling
can backdraft for the same reasons. (See Fig. 3)
Fig. 3 The chimney should penetrate a vaulted ceiling at or near its
highest point
The problems resulting from a chimney 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 concentrate
at high levels. High level leakage can be created by recessed light fixtures,
ceiling fan mounts, poorly fitted attic hatches, fold-up stairs and skylights.
High level leakage sites should be minimized if possible and, where unavoidable,
they should be carefully sealed to the building air barrier.
To avoid these problems, a chimney should penetrate the highest part of
the building envelope. For most common house designs, this means that
the fireplace and chimney should be located on an interior wall rather
than a perimeter wall.
Wind Effects
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 reducing
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 on of specialized design
should only be done with the approval of the fireplace manufacturer. The
standard chimney caps supplied by manufactures of factory-built fireplaces
and chimneys are usually of good design, so replacing 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 because the
result could be venting failure due to adverse winds. Decorative shrouds
for chimney tops should never be used unless listed by the fireplace manufacturer.
3. Avoid Large, Uncompensated Exhausts
When a large exhaust, such as a down-draft 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
upward flow in a chimney produced by draft and can result in spillage
of smoke into the house. Once they are aware of it, most homeowners can
manage this potential problem, 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 electrically interlocked with the exhaust fan switch so that house
depressurization is prevented. Make-up air systems of this type can function
without the homeowner 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 handler
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.
4. 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 generous heath openings relative to chimney 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 chimney height limitations
and these should be followed precisely.
5. Use Straight Chimney Systems
Chimney offsets should be avoided where possible. Each elbow or offset
in venting system creates turbulence in the flue gas steam and therefore
presents resistance to flow. Offsets in chimneys are not uncommon but
their use demands special attention to the other aspects of system design.
Manufacturer’s instructions for factory-built fireplaces should
be reviewed carefully for criteria associated with the use of offsets.
Fig. 4 Chimney offsets can be implicated in
smoke spillage
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 form 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 measure to ensure good fireplace performance.
6. Provide Glass Doors
Open fireplaces are particularly vulnerable to smoke spillage for two
reasons. First, the main restriction to flow - the chimney - is downstream
of the fire, and there is little resistance to spillage flow into the
room. Second, most new homes do not provide enough natural leakage to
supply the large amount of air that an open fireplace consumes. 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 he fireplace needs for satisfactory operation.
Many homeowners enjoy the crackle of an open fire and some fireplaces
in some houses can operate successfully this way. But changing conditions,
like adverse winds, the operation of an exhaust fan, unbalanced airflows
from one room to another, or even people walking in front of the fireplace,
can induce spillage from the open heath. See-through or multi-sided open
fireplaces are particularly vulnerable to these pressure differences and
airflows. If smoke spillage occurs, the quickest and most effective way
to stop the spillage is to close the glass doors. Glass doors can also
help to reduce air leakage up the chimney and hearth odors from spilling
into the room when the fireplace is not in use. For these reasons all
fireplaces should be provided with glass doors, even if the purchaser
expects to use the fireplace as an open heath most of the time.
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