3. When one permanent outdoor opening is used, the opening
requires:
a. 1 sq. in of free area per 3,000 BTUH (700 mm2/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 connectors in
the space.
The opening shall commence within 12" of the top of the enclo-
sure. Appliances shall have clearances of at least 1" from the sides
and back and 6" from the front. The opening shall directly commu-
nicate 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.
b.
C.
Indoor openings that comply with the Indoor Combus-
tion Air Method below and
Outdoor openings located as required in the Outdoor
Combustion Air Method above and
Outdoor openings sized as follows.
1) Calculate the Ratio of all Indoor Space volume divid-
ed by required volume for Indoor Combustion Air Meth-
od.
2) Outdoor opening size reduction Factor is I minus the
Ratio in 1) above.
3) Minimum size of Outdoor openings shall be the size
required in Outdoor Combustion Air Method above
multiplied by reduction Factor.
Area
BTUH MinimumFree Area Requiredfor EachOpening or Ductto Outdoors
Input TwoHorizontalDucts SingleOpening TwoVerticalDuctsor Openings RoundDuct
Rating (sq. inJ2,000BTUH) (sq. in.i3,000BTUH) (sq. inJ4,000BTUH) (sq. in./4,000 BTUH)
50,000 25 sq. in. 16.7 sq. in. 12.8sq. in. 4"
75,000 37.5 sq. in. 25 sq. in. 1828 sq. in. 5"
80,000 40 sq. in. 26.7 sq. in. 20.0 sq. in. 5"
100,000 80sq. in. 33.3sq. in. 25 sq. in. 6"
125,000 62.50sq. in. 41.7 sq. in. 31.25 sq. in. 7"
EXAMPLE: Determining Free Area
Furnace Water Heater
100,000 + 30,000
Furnace Water Heater
100,000 + 30,000
Indoor Combustion Air
Total Input
(130,000 + 4,000) 32.5 Sq. In. Vertical
Total Input
(130,000 + 2,000) 65 Sq. In. Horizontal
Standard and Known-Air-Infiltration Rate Methods
© NFPA&AGA
CARBON MONOXIDE POISONING HAZARD.
Failure to supply adequate combustion air could
result in death or personal injury.
Most homes will require additional air from
outdoors for combustion and ventilation. A space
with at least 50 cubic feet per 1,000 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 gases.
Indoor air is permitted for combustion and ventilation, if the Stan-
dard or Known-Air-Infiltration Rate Method is used.
The Standard Method may be used, if the space has no less vol-
ume than 50 cubic feet per 1,000 BTUH input rating for all gas ap-
pliances 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 in-
filtration 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 vol-
ume for the space.
ACH
0.60
0.50
0.40
0.30
0.20
0.10
0.00
NP = Not Permitted
44001 102102
e
MINIMUMSPACEVOLUMEFOR100%COMBUSTIONANDVENTILATIONAIR FROMINDOORS(ft3)
30 50 50 125
1,050 1,750 1,250 3,125
1,260 2,100 1,500 3,750
1,575 2,625 1,875 2,813 4,688
2,100 3,500 2,500 3,750 6250
3,150 5250 3,750 5,625 9,375
6,300 10,500 7,500 11,250 18,750
NP NP NP NP NP
Other Than Fan-AssistedTotal
(1,000's Btuh)
40
1,400
1,680
2,100
2,800
4,200
8,400
NP
Fan-assistedTotal
(1,OOO'sBtuh)
75 100
1,875 2,500
2,250 3,000
3,750
5,000
7,500
15,000
NP
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