American Water Heater GAS-FIRED COMMERCIAL WATER HEATER - Manual (Page 17)

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2. If the openings communicate with the outdoors through horizontal ducts,
each opening shall have a minimum free area of 1 in.
2 /2000 BTU
(2.54cm 2 /0.59kW) per hour of the total rating of all gas appliances in the
enclosure.
3. If the openings communicate directly with the outdoors or through vertical
ducts with the outdoors, each opening shall have a minimum free area of
1
in. 2 /4000 BTU (2.54cm 2 /1.18kW) per hour of the total rating of all gas
appliances in the enclosure.
All Air From Inside the Building: The confined space shall be provided with
two permanent openings communicating directly with an additional room(s) of
sufficient volume so that the combined volume of all spaces meets the criteria
for an unconfined 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 1 in.
2 /1000
BTU (2.54cm 2 /0.29kW) per hour of the total input rating of all gas utilization
equipment in the confined space, but not less than 100 square inches
(254cm
2 ). One opening shall be within 12 inches (30.5 cm) of the top and one
within 12 inches (30.5 cm) of the bottom of the enclosure.
Unconfined Spaces
In unconfined spaces in buildings, infiltration may be adequate to provide air
for combustion, ventilation and dilution of flue gases. However, in buildings of
tight construction (for example, weather stripping, heavily insulated, caulked,
vapor barrier, etc.), additional air may need to be provided using the methods
described above under CONFINED SPACES: All Air From Outdoors or
SPECIALLY ENGINEERED INSTALLATIONS.
Specially Engineered Installations
The requirements noted under CONFINED SPACES above shall not
necessarily govern when special engineering, approved by the authority
having jurisdiction, provides an adequate supply of air for combustion,
ventilation, and dilution of flue gases.
Water Connections
CAUTION
The draft diverter relief opening of the water heater and combustion
air inlet must be in the same atmospheric pressure zone. Large
exhaust fans in kitchens and other locations can lower the air
pressure inside an enclosure and interfere with the proper operation
and venting of the water heater . In these cases, the water heater
should be installed in a separate room with the combustion and
ventilation air supplied directly from outdoors as previously
described.
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