01-000 COMBUSTION AIR
SECTION 708 SECTION 710
COMBUSTION AIR DUCTS OPENING LOCATION AND PROTECTION !,
708.1 General. Combustion air ducts shall: 710.1 General. Combustion air openings to the outdoors shall I
1. Be of galvanized steel complying with Chapter 6 or of comply with the location and protection provisions of Sections
equivalent corrosion resistant material approved for this 401.4 and 401.5 applicable to outdoor air intake openings. '
application.
Exception: Within dwelling units, unobstructed stud
and joist spaces shall not be prohibited from convey-
ing combustion air, provided that not more than one
required fireblock is removed.
2. Have a minimum cross - sectional dimension of 3 inches
(76 mm).
3. Terminate in an unobstructed space allowing free move-
ment of combustion air to the appliances. ,, "'
4. Have the same cross - sectional areas as the free area of
the openings to which they connect. i i'u
5. Serve a single appliance enclosure
6. Not serve both upper and lower combustion air openings
where both such openings are used The separation I l
between ducts serving upper and lower combustion air I ' ,
openings shall be maintained to the source of combus-
tion air.
7. Not be screened where terminating in an attic space. =rles
8. Not slope downward toward the source of combustion
air, where serving the upper required combustion air 1
opening.
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SECTION 709
OPENING OBSTRUCTIONS
709.1 General. The required size of openings for combustion Ia
and dilution air shall be based on the net free area of each open
ing. The net free area of an opening shall be that specified by I � i Iti
the manufacturer of the opening covering. In the absence of
such information, openings covered with metal louvers shall beA-
deemed to have a net free area of 75 percent of the area of the rt
opening, and openings covered with wood louvers shall be '[u s
deemed to have a net free area of 25 percent of the area of the
opening. Louvers and grills shall be fixed in the open position. 1 1 [
Exception: Louvers in with the appliance so that Ck
they are proven to be in the full open position prior to main
burner ignition and during main burner operation. Means �i
shall be provided to prevent the main burner from igniting if € '.
the louvers fail to open during burner startup and to shut s j
down the main burner if the louvers close during operation.
709.2 Dampered openings. Where the combustion air open { u
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ings are provided with volume, smoke or fire dampers, the
dampers shall be electrically interlocked with the firing cycle 1 11 i
of the appliances served, so as to prevent operation of any EE
appliance that draws combustion and dilution air from the (R II
room when any of the dampers are closed. Manually operated
dampers shall not be installed in combustion air openings. ''
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COMBUSTION AIR
703.1.1 Number and location of openings. Two openings both such openings as determined in accordance with Sec -
shall be provided, one within 1 foot (305 mm) of the ceiling tion 703.1.3.
of the room and one within 1 foot (305 mm) of the floor. 704.1.4 Ratio of vertical openings. Where openings con -
703.1.2 Size of direct openings. The net free area of each nected to the outdoors through vertical ducts are provided in
direct opening to the outdoors, calculated in accordance accordance with Section 703.1, the ratio of vertical open -
with Section 709, shall be a minimum of 1 square inch per ings shall be the sum of the net free areas of both such open -
4,000 Btu /h (550 mm /kW) of combined input rating of the ings, divided by the sum of the required areas for both such
fuel - burning appliances drawing combustion and dilution openings as determined in accordance with Section 703.1.4.
air from the room. 704.1.5 Ratio of interior spaces. The ratio of interior
703.1.3 Size of horizontal openings. The net free area of spaces shall be the available volume of all communicating
each opening, calculated in accordance with Section 709 spaces, divided by the required volume as determined in
and connected to the outdoors through a horizontal duct, accordance with Sections 702.2 and 702.3.
shall be a minimum of 1 square inch per 2,000 Btu/h (1100
mm of combined input rating of the fuel- burning 704.1.6 Prorating of indoor and outdoor air. In spaces
appliances drawing combustion and dilution air from the that utilize a combination of indoor and outdoor air, the sum
room. The cross - sectional area of the duct shall be equal to of the ratios of all direct openings, horizontal openings, ver
tical openings and interior spaces shall equal or exceed 1.
or greater than the required size of the opening.
703.1.4 Si of vertical openings. The net free area of each
opening, calculated in accordance with Section 709 and SECTION 705
connected to the outdoors through a vertical duct, shall be a COMBINED USE OF INDOOR AND OUTDOOR AIR
minimum of 1 square inch per 4,000 Btu /h (550 mm
(CONDITION 2)
of combined input rating of the fuel- burning appliances
drawing combustion and dilution air from the room. The 705.1 General. This section shall apply only to appliances
cross - sectional area of the duct shall be equal to or greater located in unconfined spaces in buildings of unusually tight
than the required size of the opening. construction. Combustion air supplied by a combined use of
indoor and outdoor air shall be supplied through openings and
ducts extending to the appliance room or to the vicinity of the
SECTION 704 appliance.
COMBINED USE OF INDOOR AND OUTDOOR AIR 705.1.1 Openings and supply ducts. Openings shall be
(CONDITION 1)
provided, located and sized in accordance with Sections
704.1 Combination of air from indoors and outdoors. This 702.3.1 and 702.3.2; additionally, there shall be one open -
section shall apply only to appliances located in confined ing to the outdoors having a free area of at least 1 square inch
spaces in buildings not of unusually tight construction. Where per 5,000 Btu /h (440 mm /kW) of total input of all appli-
the volumes of rooms and spaces are combined for the purpose ances in the space.
of providing indoor combustion air, such rooms and spaces
shall communicate through permanent openings in compliance
with Sections 702.3.1 and 702.3.2. The required combustion SECTION 706
and dilution air shall be obtained by opening the room to the FORCED COMBUSTION AIR SUPPLY
outdoors using a combination of indoor and outdoor air, pro- '.
rated in accordance with Section 704.1.6. The ratio of interior 706.1 General. Where all combustion air and dilution air is
spaces shall comply with Section 704.1.5. The number, loca provided by a mechanical forced air system, the combustion 706.1
supplied Y
air and dilution air shall b lied at the minimum rate of 1 ;.
tion and ratios of openings connecting the space with the out- pp
door air shall comply with Sections 704.1.1 through 704.1.4.
cfm per 2,400 Btu/h [0.00067 m /(s • kW)] of combined input
rating of all the fuel- burning appliances served. Each of the
704.1.1 Number and location of openings. At least two appliances served shall be electrically interlocked to the
openings shall be provided, one within 1 foot (305 mm) of mechanical forced -air system so as to prevent operation of the
the ceiling of the room and one within 1 foot (305 mm) of appliances when the mechanical system is not in operation.
the floor. Where combustion air and dilution air is provided by the build -
704.1.2 Ratio of direct openings. Where direct openings to ing's mechanical ventilation system, the system shall provide
the outdoors are provided in accordance with Section 703.1, the specified combustion/dilution air rate in addition to the
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the ratio of direct openings shall be the sum of the net free required ventilation air.
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areas of both direct openings to the outdoors, divided by the
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sum of the required areas for both such openings as deter-
mined in accordance with Section 703.1.2. SECTION 707
704.1.3 Ratio of horizontal openings. Where openings DIRECT CONNECTION
connected to the outdoors through horizontal ducts are pro- 707.1 General. Fuel- burning appliances that are listed and
vided in accordance with Section 703.1, the ratio of hori- labeled for direct combustion air connection to the outdoors
zontal openings shall be the sum of the net free areas of both shall be installed in accordance with the manufacturer's instal -
such openings, divided by the sum of the required areas for lation instructions.
66 2006 INTERNATIONAL MECHANICAL CODE®
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CHAPTER 7
COMBUSTION AIR
SECTION 701 701.5 Prohibited sources. Openings and ducts shall not con -
GENERAL nect appliance enclosures with a space in which the operation
701.1 Scope. The provisions of this chapter shall govern the of a fan will adversely affect the flow of the combustion air.
requirements for combustion and dilution air for fuel- burning Combustion air shall not be obtained from a hazardous loca-
appliances other than gas-fired appliances. The requirements tion, except where the fuel -fired appliances are located within
for combustion and dilution air for gas -fired appliances shall be the hazardous location and are installed in accordance with this
in accordance with the International Fuel Gas Code. code. Combustion air shall not be taken from a refrigeration
machinery room, except where a refrigerant vapor detector
701.2 Combustion and dilution air required. Every room or system is installed to automatically shut off the combustion
space containing fuel- burning appliances shall be provided process in the event of refrigerant leakage. Combustion air
with combustion and dilution air as required by this code. Com- shall not be obtained from any location below the design flood
bustion and dilution air shall be provided in accordance with elevation.
Section 702, 703, 704, 705, 706 or 707 or shall be provided by
an approved engineered system. Direct vent appliances or
equipment that do not draw combustion air from inside of the SECTION 702
building re not required to be considered in the determination
g q INDOOR AIR
of the combustion and dilution air requirements. Combustion 702.1 All air from indoors. Combustion and dilution air shall
air requirements shall be determined based on the simultaneous be permitted to be obtained entirely from the indoors in build -
operation of all fuel - burning appliances drawing combustion ings that are not of unusually tight construction. In buildings of
and dilution air from the room or space. unusually tight construction, combustion air shall be obtained
701.3 Circulation of air. The equipment and appliances within from the outdoors in accordance with Section 703, 705, 706 or
every room containing fuel- burning appliances shall be 707.
installed so as to allow free circulation of air. Provisions shall 702.2 Air from the same room or space. The room or space
be made to allow for the simultaneous operation of mechanical containing fuel burning appliances shall be an unconfined
exhaust systems, fireplaces or other equipment and appliances space as defined in Section 202. `"
operating in the same room or space from which combustion p
and dilution air is being drawn. Such provisions shall prevent 702.3 Air from adjacent spaces. Where the volume of the
the operation of such appliances, equipment and systems from room in which the fuel- burning appliances are located does not
affecting the supply of combustion and dilution air. comply with Section 702.2, additional indoor combustion and 1
701.4 Crawl space and attic space. For the purposes of this dilution air shall be obtained by opening the room to adjacent III
chapter, an opening to a naturally ventilated crawls ace or attic spaces, so that the combined volume of all communicating p
a e shall be considered equivalent to an opening p to the out- spaces meets the volumetric requirement of Section 702.2. i
space q Openings connecting the spaces shall comply with Section
doors. 702.3.1 and 702.3.2. '� 1'
701.4.1 Crawl space. Where lower combustion air open-
ings connect with crawl spaces, such spaces shall have 702.3.1 Number and location of openings. Two openings
unobstructed openings to the outdoors at least twice that shall be provided, one within 1 foot (305 mm) of the ceiling
required for the combustion air openings. The height of the
of the room and one within 1 foot (305 mm) of the floor. ' I
crawl space shall comply with the requirements of the Inter- 7023.2 Size of openings. The net free area of each opening, -,
national Building Code and shall be without obstruction to calculated in accordance with Section 708, shall be a mini- 1
the free flow of air. mum of 1 square inch per 1,000 Btu/h (2201 mm of
701.4.2 Attic space. Where combustion air is obtained from input rating of the fuel burning appliances drawing com
an attic area the attic ventilating openings shall not be sub bustion and dilution air from the communicating spaces and
ject to ice or snow blockage, and the attic shall have not less shall be not less than 100 square inches (64 516 mm
than 30 inches (762 mm) vertical clear height at its maxi-
mum point. Attic ventilation openings shall be sufficient to SECTION 703
provide the required volume of combustion air and the attic OUTDOOR AIR
ventilation required by the International Building Code. ■,
The combustion air openings shall be provided with a sleeve 703.1 All air from the outdoors. Where all combustion and 1 ri
of not less than 0.019 -inch (0.5 mm) (No. 26 Gage) galva- dilution air is to be provided by outdoor air, the required com- l i
nized steel or other approved material extending from the bustion and dilution air shall be obtained by opening the room
appliance enclosure to at least 6 inches (152 mm) above the to the outdoors. Openings connecting the room to the outdoor , T'
top of the ceiling joists and insulation. air shall comply with Sections 703.1.1 through 703.1.4.
2006 INTERNATIONAL MECHANICAL CODE® 65 ,
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