9
441 01 5003 01
Specifications are subject to change without notice
Indoor Combustion Air
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.
!
CARBON MONOXIDE POISONING HAZARD.
Failure to supply adequate combustion air 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 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.
The
Standard
Method may be used, if 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. 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.
Table 2
MINIMUM SPACE VOLUME FOR 100% COMBUSTION AND VENTILATION AIR FROM INDOORS (ft
3
)
Other Than Fan- Assisted Total
(1,000’s Btuh)
Fan- assisted Total
(1,000’s Btuh)
ACH
30
40
50
50
75
100
125
150
0.60
1,050
1,400
1,750
1,250
1,875
2,500
3,125
3,750
0.50
1,260
1,680
2,100
1,500
2,250
3,000
3,750
4,500
0.40
1,575
2,100
2,625
1,875
2,813
3,750
4,688
5,625
0.30
2,100
2,800
3,500
2,500
3,750
5,000
6,250
7,500
0.20
3,150
4,200
5,250
3,750
5,625
7,500
9,375
11,250
0.10
6,300
8,400
10,500
7,500
11,250
15,000
18,750
22,500
0.00
NP
NP
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--2006
, 9.3.2.2:
1.
For
other than fan--assisted appliances
such as a draft
hood--equipped water heater,
1000 Btu / hr
21 ft
3
(
I
other
)
Volume
other
=
ACH
2.
For
fan--assisted appliances
such as this furnace,
1000 Btu / hr
15 ft
3
(
I
fan
)
Volume
fan
=
ACH
If:
I
other
= combined input of all
other than fan--assisted
appliances
in Btu/hr
I
fan
= combined input of all
fan--assisted appliances
in Btu/hr
ACH = air changes per hour (ACH shall not exceed 0.60.)
The following requirements apply to the
Standar
d 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.
•
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
remain open and
NOT
have any means of being closed off.
Ventilation openings to outdoors
MUST
be at least 1 square
inch of free area per 4,000 BTUH of total input rating for all gas
appliances in the space.
•
In spaces that use the
Indoor Combustion Air
Method,
infiltration 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 provided
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
continuous, sealed vapor barrier. Openings are gasketed or
sealed and
2. Doors and openable windows are weather stripped and
3. 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
replacement air. Whichever method is used, the mixed return air
temperature across the heat exchanger
MUST
not fall below 60
°
F
so that flue gases will not condense excessively in the heat
exchanger. Excessive condensation will shorten the life of the heat
exchanger and possibly void your warranty.
