John Wood Commercial - 300-400K BTU - Manual (Page 18)

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Alternatively a single permanent opening, commencing within 12
inches (300 mm) of the top of the enclosure, shall be provided.
See Figure 11. The water heater shall have clearances of at
least 1 inch (25 mm) from the sides and back and 6 inches (l50
mm) from the front of the appliance. The opening shall directly
communicate with the outdoors or shall communicate through a
vertical or horizontal duct to the outdoors or spaces that freely
communicate with the outdoors and shall have a minimum free
area of the following:
1. 1 square inch per 3000 btu/hr (700 mm2 per kW) of the total
input rating of all appliances located in the enclosure, and
2. Not less than the sum of the areas of all vent connectors in
the space.
outdoor air through two horiZontal ducts
figure 12
The confined space shall be provided with two permanent
horizontal ducts, 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 horizontal ducts shall communicate
directly with the outdoors. See Figure 12.
Each duct opening shall have a minimum free area of 1 square
inch per 2,000 btu/hr (1100 mm2 per kW) of the aggregate input
rating of all appliances installed in the enclosure.
When ducts are used, they shall be of the same cross sectional
area as the free area of the openings to which they connect.
The minimum dimension of rectangular air ducts shall be not less
than 3 inches.
fresh air openings for confined spaces
The following instructions shall 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 20 for complete
venting installation instructions.
outdoor air through two openings
figure 10
The confined space shall 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 shall communicate directly with the
outdoors. See Figure 10.
Each opening shall have a minimum free area of 1 square inch
per 4,000 btu/hr (550 mm2 per kW) of the aggregate input rating
of all appliances installed in the enclosure. Each opening shall
not be less than 100 square inches (645 cm2).
outdoor air through one opening
figure 11