Outdoor Combustion Air Method
1. Provide the space with sufficient air for proper combustion,
ventilation, and dilution of flue gases using permanent
horizontal or vertical duct(s) or opening(s) directly commu-
nicating with the outdoors or spaces that freely communi-
cate with the outdoors.
2. Fig. 18 illustrates how to provide TWO OUTDOOR OPEN-
INGS, one inlet and one outlet combustion and ventilation
air openings to the outdoors.
a. One opening MUST commence within 12″ (300 mm) of
the ceiling and the second opening MUST commence
within 12″ (300 mm) of the floor.
b. Size openings and ducts per Fig. 18 and Table 1.
c. TWO HORIZONTAL DUCTS require 1 square inch of
free area per 2,000 Btuh (1,100 mm
2
/kW) of combined
input for all gas appliances in the space per Fig. 18 and
Table 1.
d. TWO OPENINGS OR VERTICAL DUCTS require 1
square inch of free area per 4,000 Btuh (550 mm
2
/kW)
for combined input of all gas appliances in the space per
Fig. 18 and Table 1.
3. ONE OUTDOOR OPENING requires:
a. 1 square inch of free area per 3,000 Btuh (734 mm
2
/kW)
for combined input of all gas appliances in the space per
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″ (300 mm) of
the ceiling. Appliances in the space shall have clearances
of at least 1″ (25 mm) from the sides and back and 6″
(150 mm) 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.
Indoor Combustion Air© NFPA & AGA
Standard and Known-Air-Infiltration Rate Methods
Indoor combustion air is permitted for combustion, ventilation,
and dilution, if the Standard or Known-Air-Infiltration Rate
Method is used.
WARNING: CARBON MONOXIDE POISONING
HAZARD
Failure to supply outdoor air via grilles or ducts could
result in death and/or personal injury.
Many homes require air to be supplied from outdoors for
furnace combustion, ventilation, and dilution of flue
gases.
The furnace combustion air supply must be provided in
accordance with this instruction manual.
The Standard Method:
1. The space has no less volume than 50 cubic feet per 1,000
Btuh of the maximum input ratings for all gas appliances
installed in the space and
2. The air infiltration rate is not known to be less than 0.40 air
changes per hour (ACH).
The Known Air Infiltration Rate Method shall be used, if the
infiltration rate is known to be:
1. Less than 0.40 ACH and
2. Equal to or greater than 0.10 ACH
TABLE 1—MINIMUM FREE AREA REQUIRED FOR EACH COMBUSTION AIR OPENING OR DUCT TO OUTDOORS
FURNACE
INPUT
(BTUH)
TWO HORIZONTAL DUCTS
(1 SQ. IN./2,000 BTUH) (1,100 SQ. MM/KW)
SINGLE DUCT OR OPENING
(1 SQ. IN./3,000 BTUH) (734 SQ. MM/KW)
TWO OPENINGS OR VERTICAL DUCTS
(1 SQ. IN./4,000 BTUH) (550 SQ. MM/KW)
Free Area of
Opening and Duct
(Sq. In.)
Round Duct
(in. Dia)
Free Area of
Opening and Duct
(sq In.)
Round Duct
(in. Dia)
Free Area of
Opening and Duct
(Sq In.)
Round Duct
(In. Dia)
44,000 22 6 14.7 5 11 4
66,000 33 7 22 6 16.5 5
88,000 44 8 29.3 7 22 6
110,000 55 9 36.7 7 27.5 6
132,000 66 10 44 8 33 7
154,000 77 10 51.3 9 38.5 8
EXAMPLES: DETERMINING FREE AREA
FURNACE WATER HEATER TOTAL INPUT
110,000 + 30,000 = (140,000 divided by 4,000) = 35.0 Sq. In. for each two Vertical Ducts or Openings
66,000 + 40,000 = (106,000 divided by 3,000) = 35.3 Sq. In. for a Single Duct or Opening
88,000 + 30,000 = (118,000 divided by 2,000) = 59.0 Sq. In. for each of two Horizontal Ducts
→ TABLE 2—MINIMUM SPACE VOLUMES FOR 100% COMBUSTION, VENTILATION AND DILUTION AIR FROM OUTDOORS
ACH
OTHER THAN FAN-ASSISTED TOTAL
(1,000’S BTUH GAS INPUT RATE)
FAN-ASSISTED TOTAL
(1,000’S BTUH GAS INPUT RATE)
30 40 50 44 66 88 110 132 154
Space Volume (ft
3
)
0.60 1,050 1,400 1,750 1,100 1,650 2,200 2,750 3,300 3,850
0.50 1,260 1,680 2,100 1,320 1,980 2,640 3,300 3,960 4,620
0.40 1,575 2,100 2,625 1,650 2,475 3,300 4,125 4,950 5,775
0.30 2,100 2,800 3,500 2,200 3,300 4,400 5,500 6,600 7,700
0.20 3,150 4,200 5,250 3,300 4,950 6,600 8,250 9,900 11,550
0.10 6,300 8,400 10,500 6,600 9,900 13,200 16,500 19,800 23,100
0.00 NP NP NP NP NP NP NP NP NP
—17—
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