Service Guide
SYSTEM OPERATION
12
Two permanent open-
ings, one commencing within 12 in. (300 mm) of the top and one
commencing within 12 in. (300 mm) of the bottom, of the enclosure
shall be provided. The openings shall communicate directly, or by
ducts, with the outdoors or spaces that freely communicate with the
outdoors, as follows:
(1)* Where directly communicating with the outdoors or where com-
municating to the outdoors through vertical ducts, each opening
shall have a minimum free area of 1 in.
2
/4000 Btu/hr (550 min
2
/
kW) of total input rating of all appliances in the enclosure. [See
Figure A.9.3.3.1(1)(a) and Figure A.9.3.3.1(1)(b).]
Furnace
Water
Heater
Outlet Air
Chimney or Gas Vent
Inlet Air
Ventilation louvers for
unheated crawl space
Alternate
air inlet
Ventilation louvers
(each end of attic)
Furnace
Water
Heater
Outlet Air
Chimney or Gas Vent
Inlet air duct
[ends 1 ft (300 mm)
above floor]
Ventilation louvers
(each end of attic)
(2)* Where communicating with the outdoors through horizontal
ducts, each opening shall have a minimum free area of 1
in.
2
/2000 Btu/hr (1100 min
2
/kW) of total input rating of all appli-
ances in the enclosure. [See Figure A.9.3.3.1(2).]
Furnace
Water
Heater
Chimney or Gas Vent
Outlet air duct
Inlet air duct
One permanent open-
ings, commencing within 12 in. (300 mm) of the top of the enclosure,
shall be provided. The appliance shall have clearances of at least
1 in. (25 mm) from the sides and back and 6 in. (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 (see Figure A.9.3.3.2) and shall have a minimum free area
of the following:
(1) 1 in.
2
/3000 Btu/hr (700 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.
Furnace
Water
Heater
Opening
Chimney or Gas Vent
Alternate
Opening
Location
The use
of a combination of indoor and outdoor combustion air shall be in
accordance with (1) through (3) (see example calculation in Annex J]:
(1) Indoor Openings: Where used, openings connecting the interior
spaces shall comply with 9.3.2.3.
(2) Outdoor Opening(s) Location. Outdoor opening(s) shall be
located in accordance with 9.3.3.
(3) Outdoor Opening(s) Size. The outdoor opening(s) size shall be
calculated in accordance with the following:
(a) The ratio of the interior spaces shall be the available vol-
ume of all communicating spaces divided by the required
volume.
(b) The outdoor size reduction factor shall be 1 minus the ratio
of interior spaces.