SYSTEM OPERATION
13
(c) The minimum size of outdoor opening(s) shall be the full
size of outdoor opening(s) calculated in accordance with
9.3.3, multiplied by the reduction factor. The minimum
dimension of air openings shall not be less than 3 in. (80
mm).
9.3.5 Engineered Installations.
Engineered combustion air instal-
lations shall provide an adequate supply of combustion, ventilation,
and dilution air and shall be approved by the authority having
jurisdiction.
9.3.6 Mechanical Combustion Air Supply.
Where all combustion
air is provided by a mechanical air supply system, the combustion
air shall be supplied form outdoors at the minimum rate of 0.35 ft
3
/
min per 1000 Btu/hr (0.034 m
3
/min per kW) for all appliances located
within the space.
9.3.6.1
Where exhaust fans are installed, additional air shall be
provided to replace the exhausted air.
9.3.6.2
Each of the appliances served shall be interlocked to the
mechanical air supply system to prevent main burner operation
where the mechanical air supply system is not in operation.
9.3.6.3
Where combustion air is provided by the building’s me-
chanical ventilation system, the system shall provide the specified
combustion air rate in addition to the required ventilation air.
9.3.7 Louvers, Grilles, and Screens.
9.3.7.1 Louvers and Grilles.
The required size of openings for
combustion, ventilation, and dilution air shall be based on the net
free area of each opening. Where the free area through a design
of louver or grille or screen is known, it shall be used in calculating
the size opening required to provide the free area specified. Where
the louver and grille design and free area are not known, it shall be
assumed that wood louvers will have 25 percent free area, and metal
louvers and grilles will have 75 percent free area. Nonmotorized
louvers and grilles shall be fixed in the open position.
9.3.7.2 Minimum Scree Mesh Size.
Screens shall not be smaller
than 1/4 in. mesh.
9.3.7.3 Motorized Louvers.
Motorized louvers shall be interlocked
with the appliance so they are proven in the full open position prior
to main burner ignition and during main burner operation. Means
shall be provided to prevent the main burner form igniting should the
louver fail to open during burner startup and to shut down the main
burner if the louvers close during burner operation.
9.3.8 Combustion Air Ducts.
Combustion air ducts shall comply
with 9.3.8.1 through 9.3.8.8.
9.3.8.1
Ducts shall be constructed of galvanized steel or a material
having equivalent corrosion resistance, strength, and rigidity.
Exception: Within dwellings units, unobstructed stud and joist spaces
shall not be prohibited from conveying combustion air, provided that
not more than one fireblock is removed.
9.3.8.2
Ducts shall terminate in an unobstructed space, allowing
free movement of combustion air to the appliances.
9.3.8.3
Ducts shall serve a single space.
9.3.8.4
Ducts shall not serve both upper and lower combustion
air openings where both such openings are used. The separation
between ducts servicing upper and lower combustion air openings
shall be maintained to the source of combustion air.
9.3.8.5
Ducts shall not be screened where terminating in an attic
space.
9.3.8.6
Horizontal upper combustion air ducts shall not slope down-
ward toward the source of combustion air.
9.3.8.7
The remaining space surrounding a chimney liner, gas vent,
special gas vent, or plastic piping installed within a masonry, metal,
or factory built chimney shall not be used to supply combustion air.
Exception: Direct vent appliances designed for installation in a
solid fuel-burning fireplace where installed in accordance with the
manufacture’s installation instructions.
9.3.8.8
Combustion air intake openings located on the exterior of
the building shall have the lowest side of the combustion air intake
openings located at least 12 in. (300 mm) vertically from the ad-
joining grade level.
Category I Venting (Vertical Venting)
(80% Furnaces Only)
WARNING
T
O
P
R
EVENT
PO
SS
IBLE
PE
RS
ONAL
IN
J
U
R
Y
O
R
DEATH
DUE
TO
A
S
PHY
X
IATION,
NON
-
CONDEN
S
ING
GA
S
FI
R
ED
WA
R
M
AI
R
FU
R
NACE
S
MU
S
T
BE
C
ATEGO
R
Y
I
VENTED.
D
O
NOT
VENT
ANY
OF
THE
S
E
FU
R
NACE
S
U
S
ING
C
ATEGO
R
Y
III
VENTING.
Category I Venting is venting at a non-positive pressure. A
furnace vented as Category I is considered a fan-assisted
appliance and does not have to be “gas tight.”
NOTE:
Single-
Stage and Two-Stage gas furnaces with induced draft blowers
draw products of combustion through a heat exchanger
allowing in some instances common venting with natural draft
appliances (i.e. water heaters).
All installations must be vented in accordance with National
Fuel Gas Code NFPA 54/ANSI Z223.1 - latest edition. In
Canada, the furnaces must be vented in accordance with the
National Standard of Canada, CAN/CGA B149.1 and CAN/
CGA B149.2 - latest editions and amendments.
NOTE:
The vertical height of the Category I venting system
must be at least as great as the horizontal length of the
venting system.
WARNING
T
O
P
R
EVENT
PO
SS
IBLE
DEATH
O
R
PE
RS
ONAL
IN
J
U
R
Y
DUE
TO
A
S
PHY
X
IATION,
COMMON
VENTING
WITH
OTHE
R
MANUFACTU
R
E
R'S
INDUCED
D
R
AFT
APPLIANCE
S
I
S
NOT
ALLOWED.
When an existing furnace is removed from a venting system
serving other appliances
, the venting system may be too
large to properly vent the remaining attached appliances. For
complete details refer to
Exisiting Furnace Removal
section
of this manual.
