Fresh Air Openings For Confined Spaces; Outdoor Air Through Two Openings; Outdoor Air Through One Opening; Outdoor Air Through Two Horizontal Ducts - A.O. Smith VB/VW 1500, 2000 Instruction Manual

air to the confined space and the total Btu/hr input rating of all
appliances installed in the space.
DIRECT VENT APPLIANCES
Appliances installed in a Direct Vent configuration 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 confined 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,
sufficient 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 appliance vent system causing
poor combustion. Sooting, serious damage to the appliance
and the risk of fire or explosion may result. It can also create
a risk of asphyxiation.
LOUVERS AND GRILLES
The free areas of the fresh air openings in the instructions
that follow do not take in to account the presence of louvers,
grilles or screens in the openings.
The required size of openings for combustion, ventilation and
dilution air should 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 should be used in calculating the size of
opening required to provide the free area specified. Where the
louver and grille design and free area are not known, it should
be assumed that wood louvers will have 25% free area and
metal louvers and grilles will have 75% free area. Non motorized
louvers and grilles should be fixed in the open position.

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 confined spaces. The
illustrations shown in this section of the manual are a reference
for the openings that provide fresh air into confined spaces
only. Do not refer to these illustrations for the purpose of vent
installation. See Venting Installation on page 18 for complete
venting installation instructions.

OUTDOOR AIR THROUGH TWO OPENINGS

FIGURE 9A.
The confined space should be provided with two permanent
openings, one commencing within 12 inches (300 mm) of the
top and one commencing within 12 inches (300 mm) of the
bottom of the enclosure. The openings should communicate
directly with the outdoors. See Figure 9A.
Each opening should have a minimum free area of 1 square
inch per 4,000 Btu/hr (550 mm
input rating of all appliances installed in the enclosure.
Each opening should not be less than 100 square inches
(645 cm2).

OUTDOOR AIR THROUGH ONE OPENING

Alternatively a single permanent opening, commencing within 12
inches (300 mm) of top of enclosure, should be provided. See
Figure 9B. The appliance should have clearances of at least 1
inch (25 mm) from sides and back and 6 inches (150 mm) from
front. The opening should directly communicate with outdoors
or should communicate through a vertical or horizontal duct to
outdoors or spaces that freely communicate with outdoors and
should have a minimum free area of the following:
1. 1 square inch per 3000 Btu/hr (700 mm
input rating of all appliances located in the enclosure, and
2. Not less than the sum of areas of all vent connectors
in the space.

OUTDOOR AIR THROUGH TWO HORIZONTAL DUCTS

13
per kW) of the aggregate
2
FIGURE 9B.
per kW) of the total
2
FIGURE 9C.