Indoor Combustion Air 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.
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ACH
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.
MINIMUM SPACE VOLUME FOR 100% COMBUSTION AND VENTILATION AIR FROM INDOORS (ft 3)
Other Than Fan-Assisted Total
(1,OOO'sBtuh)
30 40 50 50 125 150
0.60 1,050 1,400 1,750 1250 3,125 3,750
0.50 1,260 1,680 2,100 1,500 3,750 4,500
0.40 1,575 2,100 2,625 1,875 4,688 5,625
0.30 2,100 2,800 3,500 2,500 6250 7,500
0.20 3,150 4200 5,250 3,750 9,375 11,250
0.10 6,300 8,400 10,500 7,500 18,750 22,500
0.00 NP NP NP NP NP NP
NP = Not Permitted
Table 2 Minimum Space Volumes were determined by using the
following equations from the National Fuel Gas Code ANSI
Z223.1/NFPA 54-2002, 8.3.3.2:
1. For other than fan-assisted appliances such as a draft
hood-equipped water heater,
21 ft 3 ( ! other )
Volume other - ACH 1000 Btu / hr
2. For fan-assisted appliances such as this furnace,
15ft3 ( Iran )
Volume fan = ACH 1000Btu ! hr
[ other = combined input of all other than fan-assisted
appliances in Btu/hr
[ ran = combined input of all fan-assisted appliances in Btu/hr
If:
ACH = air changes per hour (ACH shall not exceed 0.60.)
The following requirements apply to the Standard Method and to
the Known Air Infiltration Rate Method.
• Adjoining rooms can be considered part of a space, if there
are no closable doors between rooms.
• Combining spaces on the same floor level. Each opening
shall have a free area of at least 1 square inch/1,000 BTUH
(2,000 mm2/kW) of the total input rating of all gas appliances
in the space, but not less than 100 square inch (0.06 m2).
Once opening shall commence within 12" (300 mm) of the
ceiling and the second opening shall commence within 12"
(300 mm) of the floor. The minimum dimension of air open-
ings shall be at least 3 in. (80 mm).
Fan-assistedTotal
(1,000'sBtuh)
75 100
1,875 2,500
2,250 3,000
2,813 3,750
3,750 5,000
5,625 7,500
11,250 15,000
NP NP
An attic or crawl space may be considered a space that freely
communicates with the outdoors provided there are adequate
ventilation openings directly to outdoors. Openings MUST re-
main open and NOT have any means of being closed off. Ven-
tilation openings to outdoors MUST be at least I square inch
of free area per 4,000 BTUH of total input rating for all gas ap-
pliances in the space.
In spaces that use the Indoor Combustion Air Method, in-
filtration should be adequate to provide air for combustion,
ventilation and dilution of flue gases. However, in buildings
with unusually tight construction, additional air MUST be pro-
vided using the methods described in section titled Outdoor
Combustion Air Method:
• Unusually tight construction is defined as Construction with:
1. Walls and ceilings exposed to the outdoors have a continu-
ous, sealed vapor barrier. Openings are gasketed or
sealed and
2,
3.
Doors and openable windows are weather stripped and
Other openings are caulked or sealed. These include
joints around window and door frames, between sole
plates and floors, between wall-ceiling joints, between
wall panels, at penetrations for plumbing, electrical and
gas lines, etc.
Ventilation Air
Some provincial codes and local municipalities require ventilation
or make-up air be brought into the conditioned space as replace-
ment air. Whichever method is used, the mixed return air tempera-
ture across the heat exchanger MUST not fall below 60 °
continuously, or 55 ° on an intermittent basis so that flue gases will
not condense excessively in the heat exchanger. Excessive con-
densation will shorten the life of the heat exchanger and possibly
void your warranty.
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