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method used to provide fresh air to the con
fi
ned space
and the total Btu/hr input rating of all appliances installed
in the space.
DIRECT VENT APPLIANCES
Other appliances installed in a Direct Vent con
fi
guration
that derive all air for combustion from the outdoor
atmosphere through sealed intake air piping are not
factored in the total appliance input Btu/hr calculations
used to determine the size of openings providing fresh
air into con
fi
ned spaces.
EXHAUST FANS
Where exhaust fans are installed, additional air should be
provided to replace the exhausted air. When an exhaust
fan is installed in the same space with a water heater,
suf
fi
cient openings to provide fresh air must be provided
that accommodate the requirements for all appliances
in the room and the exhaust fan. Undersized openings
will cause air to be drawn into the room through the
water heater’s vent system causing poor combustion.
Sooting, serious damage to the water heater and the risk
of
fi
re or explosion may result. It can also create a risk of
asphyxiation.
FRESH AIR OPENINGS FOR CONFINED SPACES
The following instructions should be used to calculate the
size, number and placement of openings providing fresh air
for combustion, ventilation and dilution in con
fi
ned spaces.
The illustrations shown in this section of the manual are
a reference for the openings that provide fresh air into
con
fi
ned spaces only. Do not refer to these illustrations
for the purpose of vent installation. See “Installation of
Vent System” section for complete venting installation
instructions.
Chemical vapor corrosion of the
fl
ue, blower assembly and
vent system may occur if the air supply contains certain
chemical vapors. Spray can propellants, cleaning solvents,
refrigerator and air conditioner refrigerants, swimming pool
chemicals, calcium and sodium chloride (water softener
salt), waxes, bleach and process chemicals are typical
compounds which are potentially corrosive.
A. ALL AIR FROM INSIDE BUILDINGS: (See Figure 17)
The confined space shall be provided with two
permanent openings communicating directly with an
additional room(s) of suf
fi
cient volume so that the
combined volume of all spaces meets the criteria for an
uncon
fi
ned space. The total input of all gas utilization
equipment installed in the combined space shall be
considered in making this determination. Each opening
shall have a minimum free area of one square inch per
1,000 Btu/hr (22 cm
2
/kW) of the total input rating of all
gas utilization equipment in the con
fi
ned space, but not
less than 100 square inches (645cm
2
). One opening
shall commence within 12” (30cm) of the top and one
commencing within 12” (30cm) of the bottom of the
enclosures.
Figure 17
B. ALL AIR FROM OUTDOORS: (See Figures 18-20)
The confined space shall be provided with two
permanent openings, one commencing within 12”
(30cm) of the top and one commencing within
12“ (30cm) from the bottom of the enclosure. The
openings shall communicate directly, or by ducts,
with the outdoors or spaces (crawl or attic) that freely
communicate with the outdoors.
1. When directly communicating with the outdoors,
each opening shall have a minimum free area of 1
square inch per 4,000 Btu/hr (5.5 cm
2
/kW) of total
input rating of all equipment in the enclosure (see
Figure 18).
Figure 18
2. When communicating with the outdoors through
vertical ducts, each opening shall have a minimum
free area of 1 square inch per 4,000 Btu/hr (5.5
cm
2
/kW) of total input rating of all equipment in the
enclosure (see Figure 19).
3. When communicating with the outdoors through
horizontal ducts, each opening shall have a
minimum free area of 1 square inch per 2,000 Btu/hr
(11 cm
2
/kW)) of total input rating of all equipment in
the enclosure (see Figure 20).
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