State GP6 75 YTPDT - Manual (Page 17)

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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 6. The water heater shall have clearances of at
least 1 inch (25 mm) from the sides and back and 6 inches (150
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 (733 mm 2 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: 7
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 7.
Each duct opening shall have a minimum free area of 1 square inch
per 2,000 Btu/hr (1100 mm 2 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.
AIR FROM OTHER INDOOR SPACES
Figure: 8
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. See Figure 8.
Each opening shall communicate directly with an additional room(s)
of sufficient volume so that the combined volume of all spaces
meets the criteria for an Unconfined Space.
Each opening shall have a minimum free area of 1 square inch per
1,000 Btu/hr (2200 mm 2 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 cm 2 ).