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Table 1
Free Area
BTUH (kW)
Input Rating
Minimum Free Area Required for Each Opening or Duct to Outdoors
Two Horizontal Ducts
BTUH (kW)
sq. in./2,000(1 cm
2
/.09)
Single Opening
BTUH (kW)
sq. in./3,000 (1 cm
2
/.135)
Two Vertical Ducts or
Openings
BTUH (kW)
sq. in./4,000(1 cm
2
/.18)
Round Duct
BTUH (kW)
sq.
in./4,000(6.5cm
2
/.18)
40,000 (11.72) 20 sq. in. (129 cm
2
) 13.34 sq. in. (86 cm
2
) 10 sq. in. (65 cm
2
) 4″ (101.6mm)
50,000 (14.65) 25 sq. in. (161 cm
2
) 16.7 sq. in. (108 cm
2
) 12.5 sq. in. (81 cm
2
) 4″ (101.6mm)
60,000 (17.58) 30 sq. in. (194 cm
2
) 20 sq. in. (129 cm
2
) 15 sq. in. (97 cm
2
) 5″ (127mm)
75,000 (21.98) 37.5 sq. in. (242 cm
2
) 25 sq. in. (161 cm
2
) 18.75 sq. in. (121 cm
2
) 5″ (127mm)
80,000 (23.45) 40 sq. in. (258 cm
2
) 26.7 sq. in. (172 cm
2
) 20sq. in. (129 cm
2
) 5″ (127mm)
100,000 (29.31) 50 sq. in. (322 cm
2
) 33.3 sq. in. (215 cm
2
) 25sq. in. (161 cm
2
) 6″ (152.4mm)
125,000 (36.63) 62.50 sq. in. (403 cm
2
) 41.7 sq. in. (269 cm
2
) 31.25 sq. in. (202 cm
2
) 7″ (177.8mm)
EXAMPLE: Determining Free Area
Furnace
100,000
29.31
Furnace
100,000
29.31
+
+
Water Heater
30,000
8.8
Water Heater
30,000
8.8
=
=
Total Input
(130,000 ÷ 4,000)
(38.11 ÷ .18)
Total Input
(130,000 ÷ 2,000)
(38.11 ÷ .09)
= 32.5 Sq. In. Vertical
= 210 c
m
2
Vertical
= 65 Sq. In. Horizontal
= 423
cm
2
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
(1.4
3
M) per 1,000 (2.33 cm
2
/kW) Btuh 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 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 a ir 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
Table 2
MINIMUM SPACE VOLUME FOR 100% COMBUSTION AND VENTILATION AIR FROM INDOORS
Other Than Fan-Assisted Total Fan-assisted Total
ACH
30,000 BTU
(8,790 kW)
40,000 BTU
(11,720 kW)
50,000 BTU
(14,650 kW)
50,000 BTU
(14,650 kW)
75,000
(21,975 kW)
100,000 BTU
(29,300 kW)
125,000 BTU
(36,625 kW)
ft
3
(m
3
)
0.60 1,050 (29.7) 1,400 (39.2) 1,750 (49) 1,250 (35) 1,875 (52.5) 2,500 (70) 3,125 (87.5)
0.50 1,260 (35.3) 1,680 (47.04) 2,100 (58.8) 1,500 (42) 2,250 (63) 3,000 (84) 3,750 (105)
0.40 1,575 (44.1) 2,100 (58.8) 2,625 (73.5) 1,875 (52.5) 2,813 (78.8) 3,750 (105) 4,688 (131.3)
0.30 2,100 (58.8) 2,800 (78.4) 3,500 (98) 2,500 (70) 3,750 (105) 5,000 (140) 6,250 (175)
0.20 3,150 (88.2) 4,200 (117.6) 5,250 (147) 3,750 (105) 5,625 (157.5) 7,500 (210) 9,375 (262.5)
0.10 6,300 (176.4) 8,400 (235.2) 10,500 (294) 7,500 (210) 11,250 (315) 15,000 (420) 18,750 (525)
0.00 NP NP NP NP NP NP NP
ACH = Air Changes per Hour
NP = Not Permitted
Table 2 Minimum Space Volumes were determined by using the following equations from the National Fuel Gas Code ANS I Z223.1/NFPA 54---2006, 9.3.2.2:
1. For appliances other than fan-- assisted appliances (such as
a draft hood--equipped water heater), calculate using the fol-
lowing equation:
1000 Btu / hr
21 ft
3
(
I
other
)
Required Volume
other
²
ACH
58HDV