Installation / Operation Instruction Manual
12
1. If the openings communicate directly with an additional room(s) of sufficient volume, each opening must have a
minimum free area opening of 1 in.
2
/1000 BTU (2.54cm
2
/0.29kW) per hour of the total input rating of all gas
appliances in the confined space, but not less than 100 in.
2
(254 cm
2
).
2. If the openings communicate with the outdoors through horizontal ducts, each opening must 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 must
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 must 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 must
be considered in making this determination. Each opening must 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 must 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 must 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.
The draft hood 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.
CAUTION