# 45464K003 Page 5
Figure 4
Equipment in Confined Space
All Air from Outside
(Inlet Air from Crawl Space and
Outlet Air to Ventilated Attic)
Chimney or
Gas Vent
Ventilation Louvers
(Each end of attic)
Water
Heater
Furnace
Outlet
Air
Inlet
Air
Ventilation
Louvers
(For unheated
crawl space)
Note: The inlet and outlet air opening shall each have a free area of
at least one square inch per 4000 BTU per hour of the total
input rating of all equipment in the enclosure.
Figure 6
Equipment in Confined Space
All Air from Outside
(All Air Through Horizontal Ducts)
Chimney or
Gas Vent
Furnace
Water Heater
Outlet Air
Inlet Air
Note: Each air duct opening shall have a free area of at least one
square inch per 2000 BTU per hour of the total input rating of
all equipment in the enclosure. If the equipment room is
located against an outside wall and the air openings communi-
cate directly with the outdoors, each opening shall have a free
area of at least one square inch per 4000 BTU per hour of the
total input rating of all other equipment in the enclosure.
Figure 5
Chimney or
Gas Vent
Ventilation Louvers
(Each end of attic)
Inlet Air
(Ends 12”
above bottom)
Outlet Air
Water
Heater
Furnace
Equipment in Confined Space
All Air from Outside
(All Air Through Ventilated Attic)
Note: The inlet and outlet air opening shall each have a free area of
at least one square inch per 4000 BTU per hour of the total
input rating of all equipment in the enclosure.
Another option is to use horizontal combustion ducts (see
Figure 6). If horizontal combustion ducts are run, 1 square
inch per 2000 BTU/HR is required.
Contaminated Combustion Air
Excessive exposure to contaminated combustion air will
result in safety and performance related problems. The
recommended source of combustion air is outdoor air.
However, the use of indoor air in most applications is
acceptable if the following guidelines are followed:
1. If the furnace is installed in a confined space, it is
recommended that the necessary combustion air
come from the outdoors by way of an attic, crawl
space, air duct, or direct opening.
2. If indoor combustion air is used, there must be no
exposure to the substances listed in item 5.
3. All provisions for indoor combustion air must meet the
requirements for combustion air indicated in the
National Fuel Gas Code, ANSI Z223.1/NFPA 54 (latest
edition), and/or any applicable local codes. In Canada,
see CSA B149.1, Natural Gas and Propane Installa-
tion Codes (latest edition).
4. The following types of installation may require out-
door air for combustion, due to chemical exposures:
• Commercial buildings
• Buildings with indoor pools
• Furnaces installed in laundry rooms
• Furnaces installed in hobby or craft rooms
• Furnaces installed near chemical storage areas
5. Exposure to the following substances in the combustion
air supply may also require outdoor air for combustion:
• Permanent wave solutions
• Chlorinated waxes and cleaners
• Chlorine-based swimming pool chemicals
• Water softening chemicals
• Deicing salts or chemicals
• Carbon tetrachloride
• Halogen-type refrigerants
• Cleaning solvents (such as perchloroethylene)
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