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Clearance To Combustibles Background
In the most comprehensive re-writing of the code since the 1940s, Chapter 10 of the IRC and Chapter 21 of the IBC, dealing with fireplaces and chimneys, takes an
encompassing view of building styles and construction methods practiced
all over the country, and also addresses some of the ways fireplaces and chimneys used to be built before the post World War II building boom and continue to be built in some custom homes and restorations.
The old (current) codes were promulgated during the unprecedented
building boom after WWII and focused narrowly on the building methods
commonly used then - mostly wood-frame construction. It was
assumed that masonry chimneys would be built in houses built of wood,
and the codes generally required a 2-in. clearance, or air space, to
combustibles.
Tests showed that, while masonry takes a long time to heat up, it isn't a very good insulator and with steady operating temperatures of 1,000 degrees F or more the temperature on the outside surface of the chimney would never reach an acceptable equilibrium and would keep on getting hotter. The idea of the air space was to have some air circulating around the masonry to cool it. You wouldn't want to enclose the chimney within combustible materials or have combustible materials in contact with large areas of the surface. And you wouldn't want to seal up and insulate the air space, which would interfere with the cooling air circulation, but the edges (not the flat sides) of "sheet materials", like siding, flooring and drywall, could safely touch the masonry, some said.
None of this was very clearly spelled out in any of the model codes but in practice code officials allowed builders to close the gap between a frame wall and the masonry chimney with sheet materials such as drywall and siding. Most allowed the "air space" within the wall to be insulated but this practice was deplored by other officials.
In recent years these old arguments and the "intent of the code" have all but been forgotten. In some areas of the country building officials who have worked their profession for ten years may never have inspected a masonry chimney and when asked to do so (guess what?) they read the code.
Unfortunately the codes are poorly written, self contradictory and unclear. No combustible materials within 2" of the chimney is what the codes say. So how do you close the gap between the wall and the chimney or around the hearth? With sheet metal? How many living rooms have you seen with sheet metal trim around the mantel? And in the next paragraph the mantel, it says, must be 6" away from the fireplace opening - but presumably 6" away it is in contact with the masonry. What about the clearance to combustibles or are wooden mantels not "combustible"?
And how does that distance-to-combustibles rule apply to a chimney in a masonry wall? A few companies are promoting brick or block residential wall systems. We've heard of more than one official who wouldn't allow a combustible window or door frame several feet away from the chimney unless there was a 2" air space somewhere between the chimney and the combustible framing. The rule may be silly but usually the builder just puts a metal fireplace and metal chimney within a wooden chase on his masonry house rather than deal with the patched and ambiguous language governing masonry chimneys and fireplaces. (Wooden chimneys on masonry houses! It should be illegal.)
The new International Residential Code (IRC) clearly restates the basic clearance to combustibles rules but would permit masonry fireplaces and chimneys to be in contact with combustibles in specific situations expressed as exceptions. See the new code language above.
While the new clearance to combustible language is much better than the current linguistic ambiguity and the resultant policy of letting builders and inspectors figure it out in the field, most of us would like to see some testing or at least heat transfer calculations. Many tests have been performed on code minimum masonry chimneys (a clay flue lining surrounded by 4" on masonry) completely wrapped in a plywood enclosure as specified by UL 1777. The masonry chimneys don't pass, meaning the combustible plywood gets hotter than the allowable 90 degrees F above ambient temperature, unless the flue lining is insulated. But what if only the edge of the plywood sheeting touches the chimney? Or maybe 6" of combustible trim only touching on the sides of the chimney 8" or 12" away from the flue? It would stand to reason that a masonry chimney with its surface mostly exposed would dissipate more heat and not get as hot as a totally enclosed chimney. But would it be too hot? How about a bigger chimney with more mass than the minimal chimney - one that is five feet wide with three flues or one that is part of a masonry wall?
These are not new issues. In a larger historical context, modern codes have narrowly focused on the kind of production frame houses built after WWII. Older and historic masonry chimneys, especially the ones in better houses, often contained three or four flues or were built into thick solid masonry walls. For safety many old chimneys were 8" thick or corbeled out so the chimney walls were increased in thickness to 8" where they passed through combustible floors and roofs. These were long standing practices based on experience. It would be nice to re-examine some of these methods of building safe masonry chimneys, not only to help us maintain and restore these older chimneys but also because some new houses are being well built using masonry and traditional building techniques.
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Rumford Fireplaces and Masonry Heaters
The IBC and the IRC now both permit Rumford fireplaces and masonry heaters
and provide guidelines about their construction and safety.
Rumford fireplaces were so common in the 19th and early 20th century that
"Rumford" was synonymous with "fireplace" and Thoreau listed Rumfords along with plaster walls and venetian blinds as luxuries a civilized man could take for granted. Masonry heaters, a tradition even older than Rumford fireplaces, are culturally important features in the homes built by Scandinavians, Germans, and Russians. Both Rumfords and
masonry heaters were not clearly permitted by the codes developed with the decorative but ineffective fireplace of the 1950s in mind. The IBC and the IRC now specifically permit the construction and restoration of these efficient and clean-burning Rumford fireplaces and masonry heaters. Rumford fireplaces have been specifically permitted in the BOCA and UBC codes for several years but the code issues involved in building masonry heaters have never before been addressed in any of the model codes. Here is the language:
RUMFORD FIREPLACES
R1001.11 Firebox dimensions. The firebox of a concrete or masonry fireplace shall have a minimum depth of 20 inches (508 mm). The throat shall not be less than 8 inches (203 mm) above the fireplace opening. The throat opening shall not be less than 4 inches (102 mm) in depth. The cross-sectional area of the passageway above the firebox, including the throat, damper and smoke chamber, shall not be less than the cross-sectional area of the flue.
Exception: Rumford fireplaces shall be permitted provided that the depth of the fireplace be at least 12 inches (305 mm) and at least one-third of the width of the fireplace opening, and that the throat be at least 12 inches (305 mm) above the lintel, and be at least 1/20 the cross-sectional area of the fireplace opening.
SECTION R1005
MASONRY HEATERS
R1005.1 Definition. A masonry heater is a heating appliance constructed of
concrete or solid masonry, hereinafter referred to as masonry, having a mass of at least 800 kg (1760 lbs.), excluding the chimney and foundation, which is designed to absorb and store heat from a solid fuel fire built in the firebox by routing the exhaust gases through internal heat exchange channels in which the flow path downstream of the firebox includes at least one 180 degree change in flow direction before entering the chimney and which delivers heat by radiation from the masonry surface of the heater which shall not exceed 230 degrees F (110 degrees C) except within 8 inches (203 mm) surrounding the fuel loading door(s).
R1005.2 Installation. Masonry Heaters shall be listed or installed in accordance to ASTM E-1602
R1005.3 Seismic reinforcing. Seismic reinforcing shall not be required within the body of a masonry heater whose height is equal to or less than 2.5 times it's body width and where the masonry chimney serving the heater is not supported by the body of the heater. Where the masonry chimney shares a common wall with the facing of the masonry heater, the chimney portion of the structure shall be reinforced in accordance with Section R1006.
R1005.4 Masonry heater clearance. Wood or other combustible framing shall not be placed within 4 inches (102 mm) of the outside surface of a masonry heater, provided the wall thickness of the firebox is not less than 8 inches (203 mm) and the wall
thickness of the heat exchange channels is not less than 5 inches (127 mm). A clearance of at least 8 inches (203 mm) shall be provided between the gas tight capping slab of the heater and a combustible ceiling. The required space between the heater and combustible material shall be fully vented to permit the free flow of air around all heater surfaces.
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Fire Safety Lessons
The late 1970s and early 1980s saw many chimney fires caused by air-tight wood stoves and inserts inappropriately installed or improperly used by many of us following the energy crisis a few years earlier. As one might expect, regulations followed the crisis and in the next few years various standards were developed to specify how these appliances should be installed and used. Now, after the dust has settled, the best of these standards - a positive connection to the flue and UL listed relining systems - have been incorporated, either directly in the IBC and IRC or by reference. We have learned from the rash of fires in the '70s and '80's and the new code will result in safer building practices.
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Seismic and Wind Load Design Issues
The new model codes also make more sense out of seismic and wind load design issues that were often unclear, self-contradictory or at least varied widely from one model
code to another.
In most of the country, for example, clay flues were installed with an
air space around them for expansion, but in the earthquake-prone areas
of the west that space was often filled with reinforcing bars and
grouted solid. Then again, maybe not. In California, builders and inspectors
worried about thermal expansion and the code didn't specifically say the
steel had to be grouted. Lots of the chimneys that fell over in the
recent California quakes were not grouted nor attached to the house as required by code - sort of.
The new IRC will fix all that. The new code preserves the air space for
expansion and, at the same time, is clear about proper reinforcing and
grout placement where required.
R1006.3 Seismic reinforcing. Masonry or concrete chimneys shall be constructed, anchored, supported and reinforced as required in this chapter. In Seismic Design Category D-1 and D-2, masonry and concrete chimneys shall be reinforced and anchored as detailed in Section R1006.3.1 and R1006.3.2. In Seismic Design Categories A, B or C, reinforcement and seismic anchorage is not required. In Seismic Design Categories E and F, masonry and concrete chimneys shall be reinforced in accordance with the requirements of Section 2101 through 2109.
R1006.3.1 Vertical reinforcing. For chimneys up to 40 inches (1016 mm) wide, four No. 4 continuous vertical bars, anchored in the foundation, shall be placed in the concrete, or between wythes of solid masonry, or within the cells of hollow unit masonry, and grouted in accordance with Section R609. Grout shall be prevented from bonding with the flue liner so that the flue liner is free to move with thermal expansion. For chimneys greater than 40 inches (1016 mm) wide, two additional No. 4 vertical bars shall be provided for each additional 40 inches (1016 mm) in width or fraction thereof.
R1006.3.2 Horizontal reinforcing. Vertical reinforcement shall be placed
enclosed within 1/4-inch ties, or other reinforcing of equivalent net
cross-sectional area, spaced not to exceed 18-inches on center in concrete,
or placed in the bed joints of unit masonry, at a minimum of every 18 inches (457 mm) of vertical height. Two such ties shall be provided at each bend in the vertical bars.
R1006.4 Seismic anchorage. Masonry and concrete chimneys and foundations in Seismic Design Category D shall be anchored at each floor, ceiling or roof line more than 6 feet (1829 mm) above grade, except where constructed completely within the exterior walls. Anchorage shall conform to the following requirements:
R1006.4.1 Anchorage. Two 3/16-inch by 1-inch (4.8 mm by 25 mm) straps shall be embedded a minimum of 12 inches (305 mm) into the chimney. Straps shall be hooked around the outer bars and extend 6 inches (153 mm) beyond the bend. Each strap shall be fastened to a minimum of four floor joists with two 1/2-inch (12.7 mm) bolts.
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Background Clearance Code Issues
A Proposal to Rationalize and Simplify the Code language on Clearance-to-Combustibles Relating to Masonry Fireplaces and Chimneys
by Jim Buckley, 5/9/00
Notes
Jerry Frisch - Work masonry heaters into other sections in code: clearances, flue offsets, seismic reinforcement
John Guland
5/7/00
john@gulland.ca
Jim,
Here is the reference from the 1995 (current) National Building Code of Canada:
Subsection 9.21.5. Clearance from Combustible Construction
9.21.5.3. Support of Joists or Beams
1) Joists or beams may be supported on masonry walls which enclose chimney flues provided the combustible members are separated from the flue by not less than 290 mm of solid masonry.
This requirement has been in the code since I got involved in 1980.
Cheers,
John
Code:
Crown Publications
546 Yates Street
Victoria, BC V8W 1K8
(250) 386-4636
Masonry Heater Association April 19 - 20, 2000
http://mha-net.org/msb/html/lopezi.htm
Discussion:
UL-127 Temperature limits on combustible surfaces are:
90F over ambient room temperature for unexposed (ie., in contact and covered)
surfaces and 117F over ambient room temperatures for surface exposed to ambient air.
Testing done at Lopez in January 2000, indicates that in order to obtain 117F above
ambient with a 4" ventilated air space, a heater surface temperature of approximately
350F is required.
Below are the results with a horizontal line indicating the
350F surface temperature limit.
[chart]
Note that the firebox wall built in accordance with current fireplace building code, ie., with a firebrick liner and an 8" wall thickness, clearly meets the UL surface temperature requirement at 4" clearance to combustibles when overfired by 400%.
February 20, 2000
Paul Tiegs
paultiegs@omni-test.com
http://omni-test.com
Phone: 503-643-3788
Fax: 503-643-3799
Hello Paul:
Thanks for forwarding your proposed protocol for comment. It
looks good, in terms of obtaining a U.L. equivalent clearance.
As you know, some manufacturers would like to get away from U.L.
labelling, and instead extend the building code to specify
clearances for appliances that now fall outside of the wall
thickness specifications for masonry fireplaces. This is an issue
with masonry heater manufacturers, in particular.
One view is that the existing U.L. protocol was designed for low
mass metal stoves, and is inappropriate for testing high mass
appliances, since the temperature stabilization requirement
imposes an unfair penalty on certain appliances, and certain
materials by requiring unrealistic quantities of wood to be
consumed. There are anecdotal stories of laboratory personnel
having to approach the heater in fireproof suits in order to be
able to get near enough to the loading doors to add another
charge of brands. Everyone would probably agree that this would
never be a real world scenario of fireplace operation.
On the other hand, two years ago during the Great Ice Storm here
in the Northeast, several fires and deaths resulted from people
using their masonry fireplaces as a sole source of emergency heat
for extended periods (days). So, one could also question the burn
rate assumptions of the UL test. In the Ice Storm scenario,
temperature stabilization data would be useful, but at realistic
burn rates, and with realistic fuel.
MHA has discussed this recently, also with HPA, and there is a
consensus that it would be useful to have a new fueling protocol
for safety testing of masonry appliances, perhaps as an ASTM
standard, in order to determine prescribed clearances for a wider
range of masonry appliances and wall sections.
I have put forward one proposal for masonry heaters, as follows:
- define a fuel load, in kg., based on firebox floor area or
volume
- define an overfiring percentage, say 500%
- using 20% moisture cordwood, have the specified number of
back-to-back burns
- using an infrared thermometer, record surface temperatures on
the heater, using a standardized grid, to be defined
- define a methodology for doing the surface temperature scan,
including calibration and quality control issues, and administer
it, for example, through the MHA
Certified Heater Mason program. It could be taught as a
continuing education module, for instance. This could provide a
large and robust data pool on which to base code requirements.
- obtain laboratory data to correlate masonry surface
temperatures with adjacent combustibles surface temperatures
- evaluate existing thermal simulation software to define a
software model for the above process, using laboratory data to
calibrate the software model.
Since heat transfer physics is simpler than combustion chemistry,
it makes sense to use a low cost method where appropriate, as in
safety testing. This would allow us to affordably build a
database of heater types and wall sections, and confidently
extend the clearance language in the code to more types of
construction.
Laboratory data from an accredited lab would, of course, still be
needed. It would be necessary to calibrate the proposed field
method, and to define some reference cases. In addition, one
could perhaps devise a conversion factor to the UL protocol. The
use of cordwood instead of brands would lower the cost of testing
significantly.
Having said that, the following data would be very useful from
your upcoming tests:
- calibration data for Jerry Frisch's infrared thermometer. For
example, obtain a temperature reading every 30 minutes for a
heating-cooling cycle on a surface point of the masonry for which
there is also calibrated thermocouple data. Ditto for a point on
the plywood surround. For an open fireplace, the hottest point,
depending on wall thickness, is probably about 8 - 12" from
the floor, at the centre of the fireback. Jerry or Jim Buckley
could advise on this. It may be necessary to take this into
account when designing the surround. For example, one may need to
access the plywood from the back and remove a plug in order to
get an IR reading off the back of the firebox.
- characterization of the wall section at this point. A sample of
the actual face brick and firebrick should be labelled and
retained. The density of both materials should be known. This
could be obtained with a ruler and a weight scale. These data are
needed to calibrate potential software models. We have done some
preliminary testing, and show an example of simulation software,
at
http://mha-net.org/msb/html/lopezd.htm
- complete fueling description. For brands, it would probably be
most useful to have photographs showing exact fuel location, and
appearance of the firebox and coal bed at different stages of the
test.
I'll post your testing protocol, along with these and any other
comments, in the MHA Members' Lounge.
Best Regards ................. Norbert
----------------------------------------
Norbert Senf---------- mheat@mha-net.org-nospam
Masonry Stove Builders
RR 5, Shawville------- www.mha-net.org/msb/
Quebec J0X 2Y0-----fax:-----819.647.6082
---------------------- voice:---819.647.509
----------------message
separator-----------------------------------------------
February 18, 2000
Dear Mr. Senf,
Attached please find a description of the test protocol we
propose to use on the BIA standardized fireplace. Testing is
scheduled to begin Wednesday March 1, 2000. It is anticipated
that Jerry Frisch and Jim Buckley will be here to observe test
progress. Please let us know if you have additional suggestions
on procedures or specifications or even other data that can be
collected.
The attached file is MS Word. If anyone needs something else,
let me know.
Paul Tiegs
paultiegs@omni-test.com
http://omni-test.com
Phone: 503-643-3788
Fax: 503-643-3799
Exterior Air
From ???@??? Sun Apr 16 16:27:52 2000
To: Jim Brewer
From: Jim Buckley
Subject: Exterior air
Cc:
Bcc:
X-Attachments:
Message-Id:
Exterior air
Jim,
Do you want to work on a new version of exterior air for next
year while we're here and can work on Chip?
If so, here is where I want to start. What it does that
your RB 114 doesn't do is allow unlisted but emissions tested and
certified fireplaces to have air systems designed by their
manufacturers. It also permits, but does not require, the
outlet to be within the firebox which will go a long way toward
appeasing Chip and Vestal and those with certified closed combustion
chamber fireplaces. It's optional, exempts small fireplaces and
probably all closed combustion chamber fireplaces that exhaust less
then 150 cfm.
I have not required back draft dampers for systems entirely
enclosed within the masonry. Such systems are common and don't
present a fire hazard even if they do back draft.
Finally, to answer Chip's concern, I would be open to requiring
air tight doors on fireplaces to be listed or to be installed only on
fireplaces with exterior air in the firebox. Tie the
requirement to the doors rather than to all fireplaces.
Best,
Jim Buckley
2116.1 Combustion air. Factory-built or masonry
fireplaces covered in this chapter which have a net exhaust flow
exceeding 150 cfm shall be provided with a carbon monoxide detector
with alarm installed at or near ceiling of the room containing the
fireplace, or shall be provided with combustion air supply to assure
proper fuel combustion and exhaust of flue gasses by:
(a) installing a powered make up air supply to the room
containing the fireplace of sufficient capacity to reduce the net
exhaust flow caused by the fireplace to less than 150 cfm,
or
(b) providing a supply of combustion air from the
exterior of the dwelling directly to the fireplace of sufficient
capacity to reduce the net exhaust flow caused by the fireplace to
less than 150 cfm.
2116.2 Exterior air. Where a supply of combustion air is
provided from the exterior of the dwelling directly to the appliance,
the following requirements shall be met:
2116.2.1 Factory-built fireplaces. Exterior
combustion air ducts for factory-built fireplaces shall be listed as
a component of the fireplace and shall be installed according to the
fireplace manufacturer's instructions.
2116.2.2 Masonry fireplaces. Exterior combustion
air ducts for masonry fireplaces shall be a minimum of 6 square
inches (3870 mm2) and not more than 55 square inches (0.035 m2)
except that combustion air systems for listed or certified fireplaces
shall be constructed according to the fireplace manufacturer's
instructions.
2116.2.3 Exterior air intake. The exterior air
intake shall be located on the exterior of the dwelling or from
spaces within the dwelling ventilated with outside air such as crawl
or attic spaces. The exterior air intake shall not be located within
the garage, or basement of the dwelling. The air intake shall not be
located at an elevation higher than the firebox. The exterior air
intake shall be covered with a corrosion-resistant screen of 1/4-inch
(6.4 mm) mesh.
2116.2.4 Clearance. Unlisted combustion air ducts
shall be installed with a minimum 1-inch (25 mm) clearance to
combustibles for all parts of the duct within 5 feet (1524 mm) of the
duct outlet at the appliance. Where any part of the exterior
air system is not entirely within the masonry walls of the fireplace
or the fireplace foundation the system shall be fitted with a back
draft damper to prevent back drafting.
2116.2.5 Outlet. The exterior air outlet is
permitted to be located in the hearth, back or sides of the firebox
chamber, or within 24 inches (610 mm) of the firebox opening on or
near the floor. The outlet shall be closable and designed to prevent
burning material from dropping into concealed combustible
spaces.
________
Reason: The whole concept of requiring an exterior source
of combustion air for fireplaces is flawed and has led to some unsafe
practices. Rather than try to fix the most grievous problems, this
proposal takes a fresh look at the issue. The proposed language would
allow for other ways to insure that fireplaces are provided with
adequate combustion air and are capable of exhausting flue gasses.
The proposal makes the provision of exterior combustion air optional
and only permitted if certain safety rules are followed.
From ???@??? Mon Apr 17 10:25:08 2000
To: Jim Brewer
From: Jim Buckley
Subject: Exterior air
Cc:
Bcc:
X-Attachments:
Message-Id:
Exterior air
Jim,
OK. Let's play it again .... Sam.
Best,
Jim Buckley
2116.1 Combustion air. Factory-built or masonry
fireplaces covered in this chapter which have a net opening exceeding
6 square feet (0.557 m2) shall be provided with combustion air supply
to assure proper fuel combustion and exhaust of flue gasses by:
(a) providing to the room containing the fireplace at
least one cubic foot per minute (cfm) of make up air for each square
inch of flue size as determined by TABLE 2116.12b, or
(b) providing a supply of combustion air from the
exterior of the dwelling directly to the fireplace in accordance with
2116.2
2116.2 Direct exterior combustion air. Where a supply of
combustion air is provided from the exterior of the dwelling directly
to the appliance, the following requirements shall be met:
2116.2.1 Factory-built fireplaces. Exterior
combustion air ducts for factory-built fireplaces shall be listed as
a component of the fireplace and shall be installed according to the
fireplace manufacturer's instructions.
2116.2.2 Masonry fireplaces. Exterior combustion
air ducts for masonry fireplaces shall be a minimum of 6 square
inches (3870 mm2) and not more than 55 square inches (0.035 m2)
except that combustion air systems for listed or certified fireplaces
shall be constructed according to the fireplace manufacturer's
instructions.
2116.2.3 Exterior air intake. The exterior air
intake shall be located on the exterior of the dwelling or from
spaces within the dwelling ventilated with outside air such as crawl
or attic spaces. The exterior air intake shall not be located within
the garage, or basement of the dwelling. The air intake shall not be
located at an elevation higher than the firebox. The exterior air
intake shall be covered with a corrosion-resistant screen of 1/4-inch
(6.4 mm) mesh.
2116.2.4 Clearance. Unlisted combustion air ducts
shall be installed with a minimum 1-inch (25 mm) clearance to
combustibles for all parts of the duct within 5 feet (1524 mm) of the
duct outlet at the appliance. Where any part of the exterior
air system is not entirely within the masonry walls of the fireplace
or the fireplace foundation, the system shall be fitted with a back
draft damper to prevent back drafting.
2116.2.5 Outlet. The exterior air outlet is
permitted to be located in the hearth, back or sides of the firebox
chamber, or within 24 inches (610 mm) of the firebox opening on or
near the floor. The outlet shall be closable and designed to prevent
burning material from dropping into concealed combustible
spaces.
________
Reason: This proposal takes a fresh look at the issue of
requiring an exterior source of combustion air for fireplaces. The
proposed language would allow for flexibility in insuring that
fireplaces are provided with adequate combustion air and are capable
of exhausting flue gasses. It provides guidelines about the
capacity of combustion air needed based on established ASHRAE
provisions and the existing flue sizing table in the IBC code.*
The proposal also prescribes ways to prevent backdrafting and improve
fire safety.
* The 1979 ASHRAE Handbook reports on empirical studies which
show that modern masonry fireplaces require a minimum average face
velocity or 0.2 feet/second in order to exhaust all the products of
combustion. According to this calculation, a fireplace with an
opening of 6 square feet would need a minimum of only 72 cfm so we
exempted fireplaces smaller than 6 square feet in opening area.
This same fireplace, with a 12"x12" flue at one cfm per
square inch of cross-sectional flue area would, from TABLE 2116.12b
require 91 cfm and a four foot wide fireplace with a
16"x20" flue would require a minimum of 214 cfm makeup
air. While these fireplaces might actually require more air
with a brisk fire, the greatest potential for spillage is when the
fire is dying down and the draft and required makeup air volumes are
reduced, so we think sizing the minimum makeup air requirement to the
flue size and based on the ASHRAE formula is about right and easy to
regulate without having to go beyond the IBC code.
**************
By 2000 (next year or shortly thereafter) the 50 states and all local city and county building authorities will adopt the new International Building Code (IBC) and its companion the International Residential Code (IRC). It's good news for masonry fireplaces and chimneys in several ways:
Results of the March Code Change Meetings in Cosa Mesa
Masonry fireplace and chimney issues are in pretty good shape in both the new IBC and the IRC as a result of the code change committee meetings held in Costa Mesa, CA March 15 through March 26, 1999. Code officials and industry associations had been working on the new codes for several years but, as the code change meetings began, there were still several unresolved issues.
IBC Chapter 21 Rewritten
On March 21 IBC Committee approved a complete re-organization of the sections of Chapter 21 dealing with masonry fireplaces and chimneys, after tabling the issue twice to allow all the proponents to rework the language. Although we did eliminate the controversial clearance to combustibles exceptions to get consensus, we did re-organize the IBC to reflect the IRC in structure, got masonry heaters in the IBC code for the first time, as well as all the ASTM references, seismic and flue sizing language we've been working on for the last few years in the IRC arena. It was an important victory crafted within the Masonry Alliance for Codes and Standards (MACS) with support from the Brick Industry Association (BIA), the National Concrete Masonry Association (NCMA), the Portland Cement Association (PCA), the Clay Flue Lining Institute (CFLI), the Masonry Heater Association (MHA), the National Fire Protection Association (NFPA), the Hearth Products Association (HPA) and the National Chimney Sweep Guild (NCSG) among others.
Clearance to Combustibles
Later in the week, while other code changes were being addressed, we hammered out an agreement on the clearance to combustibles issues. All the codes - the BOCA, UBC, One and Two Family, even the NFPA 211 - have long been ambiguous and even self contradictory on the distance to combustibles issue (see background below).
Two years ago in Nashville, in the effort to combine all these codes into one, some of this ambiguity was resolved but in such a way that the home builders felt they couldn't continue to abut drywall, exterior sheathing or wood trim to masonry chimneys ("combustible framing" was changed to "combustible materials"). The National Home Builders Association (NHBA) said the code was "un-buildable" if the only way to close off the air space around masonry fireplaces and chimneys is with sheet metal. So last year the NHBA changed the wording in the second draft of the IRC to:
"Sheathing materials shall be permitted to abut the masonry chimney walls. The juncture of the sheathing materials and the chimney walls shall be sealed by methods approved by the code official."
While this language essentially re-stated the way builders have been closing off masonry chimneys all along, the vague and permissive language elicited objections in the form of eight code change proposals from a wide range of safety minded organizations - all of which (except the MACS proposal) would have eliminated all combustible material in contact with masonry fireplace and chimneys. As the NHBA spokesman pointed out, these proposals left no acceptable way to trim around the masonry. "How are we going to keep out the bats and the rats?" he complained.
For three days and nights Jim Brewer (NCSG), Chip Clark (BIA), Jerry Frisch (MHA and I worked on language. We might not have come to an agreement except that, with three homebuilders on the IRC Committee, not agreeing would likely mean the permissive NAHB language would stay in the code. In the end we did all agree and we secured the support of all the other code change proponents on this issue and worked with the NHBA so that their opposition was only half hearted.
Here then is what the new International Residential Code (IRC) will say about clearance to combustibles around masonry fireplaces and chimneys:
R1001.15 Chimney clearances. Any portion of a masonry chimney located in the interior of the building or within the exterior wall of the building shall have a minimum air space clearance to combustibles of 2 inches (51 mm). Chimneys located entirely outside the exterior walls of the building, including chimneys that pass through the soffit or cornice, shall have a minimum air space clearance of 1 inch (25 mm). The air space shall not be filled, except to provide fire blocking in accordance with Section R1001.16.
Exceptions:
1. Masonry chimneys equipped with a chimney lining system listed and labeled for use in chimneys in contact with combustibles in accordance with UL 1777, and installed in accordance with the manufacturer's installation instructions, are permitted to have combustible material in contact with their exterior surfaces.
2. When masonry chimneys are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete wall less than 12 inches (306 mm) from the inside surface of the nearest flue lining.
3. Exposed combustible trim and the edges of sheathing materials, such as wood siding, shall be permitted to abut the masonry chimney side walls, in accordance with FIGURE R1001.15, provided such combustible trim or sheathing is a minimum of 12 inches (306 mm) from the inside surface of the nearest flue lining. Combustible material and trim shall not overlap the corners of the chimney by more than 1 inch (25 mm).
And in the Fireplace Section:
R1003.12 Fireplace clearance. All wood beams, joists, studs, and other combustible material shall have a clearance of not less than 2 inches (51 mm) from the front faces and sides of masonry fireplaces and not less than 4 inches (102 mm) from the back faces of masonry fireplaces. The air space shall not be filled, except to provide fire blocking in accordance with Section R1003.14.
Exceptions:
1. Masonry fireplaces listed and labeled for use in contact with combustibles in accordance with UL 127, and installed in accordance with the manufacturer's installation instructions, are permitted to have combustible material in contact with their exterior surfaces.
2. When masonry fireplaces are constructed as part of masonry or concrete walls, combustible materials shall not be in contact with the masonry or concrete walls less than 12 inches (306 mm) from the inside surface of the nearest firebox lining.
3. Exposed combustible trim and the edges of sheathing materials, such as wood siding, flooring and drywall, shall be permitted to abut the masonry fireplace side walls and hearth extension, in accordance with FIGURE R1003.12, provided such combustible trim or sheathing is a minimum of 12 inches (306 mm) from the inside surface of the nearest firebox lining.
4. Exposed combustible mantels or trim may be placed directly on the masonry fireplace front surrounding the fireplace opening provided such combustible materials shall not be placed within 6 inches (153 mm) of a fireplace opening. Combustible material within 12 inches (305 mm) of the fireplace opening shall not project more than 1/8 inch (3.2 mm) for each 1-inch (25 mm) distance from such opening.
ASTM Standards Referenced
Of particular interest to flue liner manufactures the new codes will reference ASTM C315, the flue lining standard and ASTM C 1283, the clay flue installation standard and chimney construction in seismic areas was also rationalized.
The Uniform Building code (UBC) adopted by most western states has not referenced ASTM C315 and has allowed reinforcing bars to be grouted in solid between the flues and the masonry chimney. "Seismic zones" have been replaced by "seismic design categories" that depend on new detailed maps and soil conditions. Seismic requirements are definitely moving east. The new language in both Chapter Ten of the IRC and Chapter 21 of the IBC will require an air space around the flues in accordance with ASTM C315 even when seismic reinforcing is required. (See seismic requirements below.)
The code also references ASTM C 27 or C 1261 for firebrick and requires that firebrick be laid with medium-duty refractory mortar conforming to ASTM C 199.
Masonry heaters were included in the code for the first time with a reference to ASTM E 1602. For the actual code language check below.
Exterior Air Supply
We failed to come to an agreement at the Cosa Mesa code meeting on the exterior air supply issue. The NFPA 211 representative as well as those representing the HPA and the NCSG wanted to eliminate or make the exterior air supply optional while the BIA and the NAHB representatives wanted more research. So the IRC Committee suggested we get it together for next year and disapproved all four proposals dealing with the exterior air supply for fireplaces issue.
The whole concept of requiring an exterior source of combustion air for fireplaces is flawed and has led to some unsafe practices. Three of the four proposed changes to the IRC would
have made the provision of exterior combustion air optional and only permitted if certain safety rules to prevent back drafting were followed.
The value of exterior air supplied directly to the firebox of a fireplace has never been tested. Many say it simply doesn't work. At best the six square inches required is insufficient for an open fireplace. And it can lead to bad smells, blowing ashes, enough turbulence to make the fireplace smoke and, if it can back draft as in a positively pressured space such as an upstairs bedroom, it can be a fire hazard.
There are those who feel that the exterior air supply is needed to keep the fireplace cool in the event glass doors are added to the fireplace. In any event, since we couldn't agree among ourselves, the IRC Committee left the code language as it was, requiring exterior air supply.
Codes Still Being Finalized
The code changes approved in Cosa Mesa were published May 17, 1999 and any that were challenged on the floor at Cosa Mesa could have been challenged by June 18, 1999. The ones that are challenged will be published and on the agenda for ICC "membership action" at the annual meetings scheduled for September 12 - 17, 1999 in St. Louis.
We have not challenged any of the code changes ourselves even though we'd still like to make some minor changes. The September ICC meeting with hundreds of code officials voting on hundreds of code issues is just too complicated. Rather, we will make some code change proposals through the Masonry Alliance for Codes and Standards (MACS) next year in the 2000 code change cycle.
The International Building Code (IBC) and the International Residential Code (IRC) will be published in April, 2000 and will be ready for adoption by the various states and local governments that are charged with code enforcement. The code change process goes on, of course, and there will be opportunities to propose changes in future years, but the Code 2000, and all the work it represents, will be seminal.
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Possible Side Bars
Clearance To Combustibles Background
In the most comprehensive re-writing of the code since the 1940s, Chapter 10 of the IRC and Chapter 21 of the IBC, dealing with fireplaces and chimneys, takes an
encompassing view of building styles and construction methods practiced
all over the country, and also addresses some of the ways fireplaces and chimneys used to be built before the post World War II building boom and continue to be built in some custom homes and restorations.
The old (current) codes were promulgated during the unprecedented
building boom after WWII and focused narrowly on the building methods
commonly used then - mostly wood-frame construction. It was
assumed that masonry chimneys would be built in houses built of wood,
and the codes generally required a 2-in. clearance, or air space, to
combustibles.
Tests showed that, while masonry takes a long time to heat up, it isn't a very good insulator and with steady operating temperatures of 1,000 degrees F or more the temperature on the outside surface of the chimney would never reach an acceptable equilibrium and would keep on getting hotter. The idea of the air space was to have some air circulating around the masonry to cool it. You wouldn't want to enclose the chimney within combustible materials or have combustible materials in contact with large areas of the surface. And you wouldn't want to seal up and insulate the air space, which would interfere with the cooling air circulation, but the edges (not the flat sides) of "sheet materials", like siding, flooring and drywall, could safely touch the masonry, some said.
None of this was very clearly spelled out in any of the model codes but in practice code officials allowed builders to close the gap between a frame wall and the masonry chimney with sheet materials such as drywall and siding. Most allowed the "air space" within the wall to be insulated but this practice was deplored by other officials.
In recent years these old arguments and the "intent of the code" have all but been forgotten. In some areas of the country building officials who have worked their profession for ten years may never have inspected a masonry chimney and when asked to do so (guess what?) they read the code.
Unfortunately the codes are poorly written, self contradictory and unclear. No combustible materials within 2" of the chimney is what the codes say. So how do you close the gap between the wall and the chimney or around the hearth? With sheet metal? How many living rooms have you seen with sheet metal trim around the mantel? And in the next paragraph the mantel, it says, must be 6" away from the fireplace opening - but presumably 6" away it is in contact with the masonry. What about the clearance to combustibles or are wooden mantels not "combustible"?
And how does that distance-to-combustibles rule apply to a chimney in a masonry wall? A few companies are promoting brick or block residential wall systems. We've heard of more than one official who wouldn't allow a combustible window or door frame several feet away from the chimney unless there was a 2" air space somewhere between the chimney and the combustible framing. The rule may be silly but usually the builder just puts a metal fireplace and metal chimney within a wooden chase on his masonry house rather than deal with the patched and ambiguous language governing masonry chimneys and fireplaces. (Wooden chimneys on masonry houses! It should be illegal.)
The new International Residential Code (IRC) clearly restates the basic clearance to combustibles rules but would permit masonry fireplaces and chimneys to be in contact with combustibles in specific situations expressed as exceptions. See the new code language above.
While the new clearance to combustible language is much better than the current linguistic ambiguity and the resultant policy of letting builders and inspectors figure it out in the field, most of us would like to see some testing or at least heat transfer calculations. Many tests have been performed on code minimum masonry chimneys (a clay flue lining surrounded by 4" on masonry) completely wrapped in a plywood enclosure as specified by UL 1777. The masonry chimneys don't pass, meaning the combustible plywood gets hotter than the allowable 90 degrees F above ambient temperature, unless the flue lining is insulated. But what if only the edge of the plywood sheeting touches the chimney? Or maybe 6" of combustible trim only touching on the sides of the chimney 8" or 12" away from the flue? It would stand to reason that a masonry chimney with its surface mostly exposed would dissipate more heat and not get as hot as a totally enclosed chimney. But would it be too hot? How about a bigger chimney with more mass than the minimal chimney - one that is five feet wide with three flues or one that is part of a masonry wall?
These are not new issues. In a larger historical context, modern codes have narrowly focused on the kind of production frame houses built after WWII. Older and historic masonry chimneys, especially the ones in better houses, often contained three or four flues or were built into thick solid masonry walls. For safety many old chimneys were 8" thick or corbeled out so the chimney walls were increased in thickness to 8" where they passed through combustible floors and roofs. These were long standing practices based on experience. It would be nice to re-examine some of these methods of building safe masonry chimneys, not only to help us maintain and restore these older chimneys but also because some new houses are being well built using masonry and traditional building techniques.
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Rumford Fireplaces and Masonry Heaters
The IBC and the IRC now both permit Rumford fireplaces and masonry heaters
and provide guidelines about their construction and safety.
Rumford fireplaces were so common in the 19th and early 20th century that
"Rumford" was synonymous with "fireplace" and Thoreau listed Rumfords along with plaster walls and venetian blinds as luxuries a civilized man could take for granted. Masonry heaters, a tradition even older than Rumford fireplaces, are culturally important features in the homes built by Scandinavians, Germans, and Russians. Both Rumfords and
masonry heaters were not clearly permitted by the codes developed with the decorative but ineffective fireplace of the 1950s in mind. The IBC and the IRC now specifically permit the construction and restoration of these efficient and clean-burning Rumford fireplaces and masonry heaters. Rumford fireplaces have been specifically permitted in the BOCA and UBC codes for several years but the code issues involved in building masonry heaters have never before been addressed in any of the model codes. Here is the language:
RUMFORD FIREPLACES
R1001.11 Firebox dimensions. The firebox of a concrete or masonry fireplace shall have a minimum depth of 20 inches (508 mm). The throat shall not be less than 8 inches (203 mm) above the fireplace opening. The throat opening shall not be less than 4 inches (102 mm) in depth. The cross-sectional area of the passageway above the firebox, including the throat, damper and smoke chamber, shall not be less than the cross-sectional area of the flue.
Exception: Rumford fireplaces shall be permitted provided that the depth of the fireplace be at least 12 inches (305 mm) and at least one-third of the width of the fireplace opening, and that the throat be at least 12 inches (305 mm) above the lintel, and be at least 1/20 the cross-sectional area of the fireplace opening.
SECTION R1005
MASONRY HEATERS
R1005.1 Definition. A masonry heater is a heating appliance constructed of
concrete or solid masonry, hereinafter referred to as masonry, having a mass of at least 800 kg (1760 lbs.), excluding the chimney and foundation, which is designed to absorb and store heat from a solid fuel fire built in the firebox by routing the exhaust gases through internal heat exchange channels in which the flow path downstream of the firebox includes at least one 180 degree change in flow direction before entering the chimney and which delivers heat by radiation from the masonry surface of the heater which shall not exceed 230 degrees F (110 degrees C) except within 8 inches (203 mm) surrounding the fuel loading door(s).
R1005.2 Installation. Masonry Heaters shall be listed or installed in accordance to ASTM E-1602
R1005.3 Seismic reinforcing. Seismic reinforcing shall not be required within the body of a masonry heater whose height is equal to or less than 2.5 times it's body width and where the masonry chimney serving the heater is not supported by the body of the heater. Where the masonry chimney shares a common wall with the facing of the masonry heater, the chimney portion of the structure shall be reinforced in accordance with Section R1006.
R1005.4 Masonry heater clearance. Wood or other combustible framing shall not be placed within 4 inches (102 mm) of the outside surface of a masonry heater, provided the wall thickness of the firebox is not less than 8 inches (203 mm) and the wall
thickness of the heat exchange channels is not less than 5 inches (127 mm). A clearance of at least 8 inches (203 mm) shall be provided between the gas tight capping slab of the heater and a combustible ceiling. The required space between the heater and combustible material shall be fully vented to permit the free flow of air around all heater surfaces.
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Fire Safety Lessons
The late 1970s and early 1980s saw many chimney fires caused by air-tight wood stoves and inserts inappropriately installed or improperly used by many of us following the energy crisis a few years earlier. As one might expect, regulations followed the crisis and in the next few years various standards were developed to specify how these appliances should be installed and used. Now, after the dust has settled, the best of these standards - a positive connection to the flue and UL listed relining systems - have been incorporated, either directly in the IBC and IRC or by reference. We have learned from the rash of fires in the '70s and '80's and the new code will result in safer building practices.
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Seismic and Wind Load Design Issues
The new model codes also make more sense out of seismic and wind load design issues that were often unclear, self-contradictory or at least varied widely from one model
code to another.
In most of the country, for example, clay flues were installed with an
air space around them for expansion, but in the earthquake-prone areas
of the west that space was often filled with reinforcing bars and
grouted solid. Then again, maybe not. In California, builders and inspectors
worried about thermal expansion and the code didn't specifically say the
steel had to be grouted. Lots of the chimneys that fell over in the
recent California quakes were not grouted nor attached to the house as required by code - sort of.
The new IRC will fix all that. The new code preserves the air space for
expansion and, at the same time, is clear about proper reinforcing and
grout placement where required.
R1006.3 Seismic reinforcing. Masonry or concrete chimneys shall be constructed, anchored, supported and reinforced as required in this chapter. In Seismic Design Category D-1 and D-2, masonry and concrete chimneys shall be reinforced and anchored as detailed in Section R1006.3.1 and R1006.3.2. In Seismic Design Categories A, B or C, reinforcement and seismic anchorage is not required. In Seismic Design Categories E and F, masonry and concrete chimneys shall be reinforced in accordance with the requirements of Section 2101 through 2109.
R1006.3.1 Vertical reinforcing. For chimneys up to 40 inches (1016 mm) wide, four No. 4 continuous vertical bars, anchored in the foundation, shall be placed in the concrete, or between wythes of solid masonry, or within the cells of hollow unit masonry, and grouted in accordance with Section R609. Grout shall be prevented from bonding with the flue liner so that the flue liner is free to move with thermal expansion. For chimneys greater than 40 inches (1016 mm) wide, two additional No. 4 vertical bars shall be provided for each additional 40 inches (1016 mm) in width or fraction thereof.
R1006.3.2 Horizontal reinforcing. Vertical reinforcement shall be placed
enclosed within 1/4-inch ties, or other reinforcing of equivalent net
cross-sectional area, spaced not to exceed 18-inches on center in concrete,
or placed in the bed joints of unit masonry, at a minimum of every 18 inches (457 mm) of vertical height. Two such ties shall be provided at each bend in the vertical bars.
R1006.4 Seismic anchorage. Masonry and concrete chimneys and foundations in Seismic Design Category D shall be anchored at each floor, ceiling or roof line more than 6 feet (1829 mm) above grade, except where constructed completely within the exterior walls. Anchorage shall conform to the following requirements:
R1006.4.1 Anchorage. Two 3/16-inch by 1-inch (4.8 mm by 25 mm) straps shall be embedded a minimum of 12 inches (305 mm) into the chimney. Straps shall be hooked around the outer bars and extend 6 inches (153 mm) beyond the bend. Each strap shall be fastened to a minimum of four floor joists with two 1/2-inch (12.7 mm) bolts.
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