SIZE OF THE FIREPLACE
Once the design option has been
determined for your fireplace, careful
consideration should be given to the
size best suited to the room in which
it is located. This is important not
only from an appearance standpoint,
but also for it to operate efficiently.
If it is too small for the room, for
example, it may not produce a
sufficient amount of heat – even if it
functions properly. (Follow Chart I
as a guide to the selection of the size
best suited to your application.) Once
the proper size has been determined,
careful consideration should be given
to other important areas, such as
construction of the foundation, as
well as the hearth, firebox, and
smoke chamber.
FOUNDATION
The foundation of the fireplace
consists of concrete footings and
masonry walls. Minimum code
requirements must be met with
respect to foundation and wall
design. Unless specifically designed
for additional loads, no other part of
the structure or adjoining structure
should be supported by the chimney.
Immediately above the foundation
wall, support for the combustion
chamber and the extended hearth
should be provided by a cast-in-place
or precast concrete slab. The slab
should include openings for outside
combustion air vents and ash dumps.
If a permanent form is used it must
be non-combustible (i.e., steel, slate,
SMOKE CHAMBER
The smoke chamber located
directly above the firebox should be
constructed using a choice of firebrick
laid in with refractory mortar (ASTM
C-199 medium duty) at a minimum
thickness of 2 inches, a manufactured
vitrified clay smoke chamber, or 4
inches of solid masonry. As required
by code, an insulating refractory
parging product (also ASTM C-199
medium duty) must be applied 1/2
to 3/4 inch thick over the corbeled
firebrick to form a smooth surface,
allowing for an easy transfer of visible
smoke and other gases through the
chamber. (Refer to drawings for the
minimum total wall thickness.)
The smoke chamber in the standard
fireplace design should be constructed
so that the sidewalls and front wall
taper inward to form the support of
the fireplace chimney. In this design,
the chimney should be positioned so
that it is centered on the width of the
fireplace, and the back of the flue
liner is aligned flush with the vertical
rear surface of the smoke chamber.
This configuration provides for
smooth uninterrupted exiting of
smoke and gases from the fireplace
into the chimney. NOTE: The
Rumford throat and smoke chamber
are usually made of preformed parts.
Follow the manufacturer’s directions
when building a Rumford fireplace.
CHIMNEY
The chimney is constructed directly
on the smoke chamber. It consists of
a flue liner and chimney wall. The
chimney wall should have at least
a 4 inch thickness of solid masonry
between the flue liner and any
exterior surface. Solid masonry
products used in the chimney wall
construction are those which are
either 100 percent solid (brick, block
or stone) or those which have cores
or holes which do not exceed 25
percent of the gross cross sectional
area of any load bearing surface. The
chimney should be separated from the
flue liner by an air space that is not
greater than the thickness of the clay
our national building codes (ASTM
C-199 medium duty). The back side
of the firebox should be parged with
mortar to provide added strength and
insulation. Regular masonry mortar
may be used here, but you may want
to consider the specialty refractory
products available which are designed
for parging. The fill behind the
firebox should be non-combustible
rubble or solid masonry.
PLACEMENT OF THE
DAMPER AND LINTELS
Metal parts have a greater coefficient
of expansion than masonry units.
Since damper and lintels are metal,
room for expansion or movement
must therefore be provided. The
damper should be laid on top of
the firebox in a bed of refractory
mortar only thick enough to ensure
a level installation. A 1/8 inch to 1/4
inch space should be left at the ends
of the damper flanges. The damper
should not support any masonry. Any
fireplace lintel should be installed in
a similar manner on a bed of mortar
(for leveling purposes) with a 1/8 inch
to 1/4 inch space on the sides and
ends. This space may be filed with
non-combustible soft material such
as fiberglass insulation or a ceramic
blanket. Any masonry corbeled from
the firebox should not bear weight
on the damper assembly. To prevent
this, a second lintel may be installed
with the same provisions for thermal
expansion as the lintel installed above
the fireplace opening.
(e.g., by using a corbel header) to
ensure structural integrity of the
clay flue lining. The flue lining and
surrounding masonry should touch
each other only in spot locations
(not to exceed 2 inches square each)
to ensure proper alignment and
support. In no circumstances should
the clay flue liner be encased com-
pletely around its circumference in
one location. Clay flue liners installed
as such will have sufficient room for
expansion and contraction during
normal operating temperatures of
residential fireplaces and chimneys.
When more than one flue is contained
within the chimney, a solid masonry
wall (wythe) separation of at least
4 inches should be maintained
between adjacent flues. The walls
should be bonded to the chimney wall
with either ties or masonry mortar.
or corrugated metal). The concrete
slab must not support the floor system.
FIREBOX AND HEARTH
All materials used to construct the
firebox, hearth and extension must
be made of non-combustible masonry
materials. Brick or stone are the
most popular. In no instance should
combustible elements be used to
provide support to the hearth of
the fireplace.
For fireplaces up to 6 sq. ft. (or an
opening approximately 3 foot x
2 foot) the hearth extension must
project 16 inches in front of the
fireplace and 8 inches beyond each
side of the fireplace opening. For
larger sized openings these dimensions
increase to 20 inches in front and
12 inches on the sides. The fireplace
hearth and sidewalls should be
constructed of a minimum of 2 inches
of firebricks laid in refractory mortar.
The firebox should be surrounded on
three walls (front and two sides) by
4 inch thick solid block for structural
stability and thermal heat storage
(refer to the drawings for minimum
total wall thickness). Firebrick made
to these specifications will provide
minimal expansion and contraction
throughout the operating temperature
of residential fireplaces and chimneys.
Therefore, no allowance to accommo-
date movement of firebrick is included.
All firebrick must be installed in
refractory mortar conforming to
flue liner. This will enable the flue
liner to stay in alignment and permit
expansion and contraction.
All spaces between the chimney
and the floor and ceilings should
be insulated with non-combustible,
fire-proof material. The fire proofing
material of choice for use between
the chimney and nearby wood joists,
beams or headers should be galva-
nized steel that is at least 26 gauge
and no thinner than 1/2 inch. Clay
flue liners should be installed with
non-water soluble refractory mortar
bedding with close fitting joints left
smooth on the inside and outside
facing of the flue liner. Depending on
the size and design of the chimney,
the mason may choose to install an
additional vertical section of masonry
one unit wide (wythe) or support the
structure by other accepted methods
CHART I
Fireplace Opening Size of Room
Width in Inches
in Feet
24 to 32
10 x 14
32 to 36
12 x 16
36 to 40
12 x 20
40 to 48
14 x 28
48 to 60
16 x 30
60 to 72
20 x 26
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