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5. Combination Indoor and Outdoor Combustion Air: If
the required volume of indoor air exceeds the
available indoor air volume, outdoor air openings or
ducts may be used to supplement the available
indoor air provided:
a. The size and location of the indoor openings
comply with Subsection 3.
b. The outdoor openings are to be located in
accordance with Subsection 4.
c. The size of the outdoor openings are to be sized
as follows:
where:
A
req
= minimum area of outdoor openings.
A
full
= full size of outdoor openings calculated
in accordance with Subsection 4.
V
avail
=available indoor air volume
V
req
= required indoor air volume
6. Engineer
ed Installations: Engineered combustion air
installations shall provide an adequate supply of
combustion, ventilation, and dilution air and shall be
approved by the authority having jurisdiction.
7. Mechanical Combustion Air Supply
:
a. In installations where all combustion air is
provided by a mechanical air supply system, the
combustion air shall be supplied from the
outdoors at the minimum rate of 0.35 ft
3
/min per
1000 Btu/hr (0.034 m
3
/min per 1000 W) of the
total rated input of all appliances in the space.
b. In installations where exhaust fans are installed,
additional air shall be provided to replace the
exhaust air.
c. Each of the appliances served shall be
interlocked to the mechanical air supply to
prevent main burner operation when the
mechanical air supply system is not in operation.
d. In buildings where the combustion air is provided
by the mechanical ventilation system, the system
shall provide the specified combustion air rate in
addition to the required ventilation air.
8. Louvers & Grills:
a. The required size of openings for combustion,
ventilation, and dilution air shall be based on the
net free area of each opening.
i. Where the free area through a louver or grille
is known, it shall be used in calculating the
opening size required to provide the free area
specified.
ii. Where the free area through a louver or grille
is not known, it shall be assumed that wooden
louvers will have 25% free area and metal
louvers and grilles will have 75% free area.
iii. Nonmotorized dampers shall be fixed in the
open position.
b. Motorized dampers shall be interlocked with the
equipment so that they are proven in the full
open position prior to ignition and during
operation of the main burner.
i. The interlock shall prevent the main burner
from igniting if the damper fails to open
during burner startup.
ii. The interlock shall shut down the burner if
the damper closes during burner operation.
9. Combustion Air Ducts
a. Ducts shall be constructed of galvanized steel or
an equivalent corrosion- resistant material.
b. Ducts shall terminate in an unobstructed space,
allowing free movement of combustion air to the
appliances.
c. Ducts shall serve a single space.
d. Ducts shall not serve both upper and lower
combustion air openings where both such
openings are used. The separation between ducts
serving upper and lower combustion air
openings shall be maintained to the source of
combustion air.
e. Ducts shall not be screened where terminating in
an attic space.
f. Horizontal upper combustion air ducts shall not
slope downward toward the source of the
combustion air.
g. 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.
h. Combustion air intake openings located on the
exterior of buildings shall have the lowest side of
the combustion air intake opening at least 12
inches (305 mm) above grade.
D. CHIMNEY OR VENT
1. Inspect the existing chimney or vent system. Make
sure it is in good condition. Inspect chimney liner
and repair or replace if necessary.
2. The vent system and installation must be in
accordance with the current edition of the National
Fuel Gas Code, ANSI Z223.1/NFPA 54, under
“Venting of Equipment”, or CAN/CGA B149,
Installation codes, under “Venting Systems and Air
Supply for Appliances”, or applicable provisions of
the local building codes.
5
PREINSTALLATION
V
avail
1 –
V
req
A
req
= A
full
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⎜
⎝
⎛
⎜
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