10
d. Vertical duct openings or openings directly communic-
ating with the outdoors require 1 square inch of free
area per 4,000 BTUH (5.5 cm
2
/kW) for combined in-
put of all gas appliances in the space
(
see
Table 1).
3. When one permanent outdoor opening is used, the opening
requires:
a. 1 sq. in of free area per 3,000 BTUH (7 cm
2
/kW) for
combined input of all gas appliances in the space (see
Table 1) and
b. not less than the sum of the areas of all vent connect-
ors in the space.
The opening shall commence within 12
″
(305 mm) of the top of
the enclosure. Appliances shall have clearances of at least 1
″
(25
mm) from the sides and back and 6
″
from the front. The opening
shall directly communicate with the outdoors or shall
communicate through a vertical or horizontal duct to the outdoors
or spaces (crawl or attic) that freely communicate with the
outdoors.
4. Combination of Indoor and Outdoor Air shall have:
a. Indoor openings that comply with the
Indoor Com-
bustion Air
Method below and
b. Outdoor openings located as required in the
Outdoor
Combustion Air
Method above and
c. Outdoor openings sized as follows.
(1.) Calculate the
Ratio
of all Indoor Space volume
divided by required volume for
Indoor Combus-
tion Air
Method.
(2.) Outdoor opening size reduction
Factor
is
1
minus
the
Ratio
in 1) above.
(3.) Minimum size of Outdoor openings shall be the
size required in
Outdoor Combustion Air
Meth-
od above multiplied by reduction
Factor
.
Table 1 – Free Area
BTUH
Input
Rating
Minimum Free Area Required for Each Opening or Duct to Outdoors
Two Horizontal Ducts
(sq. in./2,000 BTUH)
Single Opening
(sq. in./3,000 BTUH)
Two Vertical Ducts or Open-
ings
(sq. in./4,000 BTUH)
Round Duct
(sq. in. /4,000
BTUH)
60,000
30 sq. in. (194 cm
2
)
20 sq. in. (129 cm
2
)
15 sq. in. (97 cm
2
)
5
″
(127 mm)
80,000
40 sq. in. (258 cm
2
)
26.7 sq. in. (172 cm
2
)
20.0 sq. in. (129 cm
2
)
5
″
(127 mm)
100,000
50 sq. in. (322 cm
2
)
33.3 sq. in. (215 cm
2
)
25 sq. in. (161 cm
2
)
6
″
(152 mm)
EXAMPLE:
Determining Free Area
Furnace
100,000
Furnace
100,000
+
+
Water Heater
30,000
Water Heater
30,000
=
=
Total Input
(130,000
÷
4,000)
Total Input
(130,000
÷
2,000)
=
32.5 Sq. In. Vertical
=
65 Sq. In. Horizontal
Indoor Combustion Air (Unconfined Space)
CARBON MONOXIDE POISONING HAZARD
Failure to follow this warning could result in personal injury or
death.
Most homes will require additional air from outdoors for
combustion and ventilation. A space with at least 50 cubic
feet per 1,000 Btuh (4.8 M
3
/kW) Input rating or homes with
tight construction may need outdoor air, supplied through
ducts, to supplement air infiltration for proper combustion and
ventilation of flue gasses.
!
WARNING
Standard
and
Known- Air- Infiltration Rate
Methods
©
NFPA & AGA
Indoor air
is permitted for combustion and ventilation, if the
Standard
or
Known--Air--Infiltration Rate
Method is used.
The
Standard
Method may be used, if the space has no less
volume than 50 cubic feet per 1,000 BTUH (4.8 M
3
/kW) input
rating for all gas appliances installed in the space. The
standard
method permits indoor air to be used for combustion and
ventilation air.
The
Known Air Infiltration Rate
Method shall be used if the
infiltration rate is known to be less than 0.40 air changes per hour
(ACH) and equal to or greater than 0.10 ACH. Infiltration rates
greater than 0.60 ACH shall not be used. The minimum required
volume of the space varies with the number of ACH and shall be
determined per
Table 2
or
Equations 1 and 2
. Determine the
minimum required volume for each appliance in the space, and
add the volumes together to get the total minimum required
volume for the space.
CARBON MONOXIDE POISONING HAZARD
Failure to supply additional air by means of ventilation grilles
or ducts could result in personal injury or death.
An unconfined space or homes with tight construction may not
have adequate air infiltration for proper combustion and
ventilation of flue gasses. Most homes will require additional
air.
!
WARNING
359AA
V
