10
Alternate Method of Providing Air from
Outside
If acceptable under local Codes, it is permitted to provide
outside air using one opening (See NFGC). Generally,
confined spaces must have two openings in the space for
combustion air. One opening must be within 12 inches of
the ceiling, and the other must be within 12 inches of the
floor. However, an alternative method recently adopted by
the NFGC uses one opening within 12 inches of the top
of the space. This method may be used if it is acceptable
to the local codes.
THE FOLLOWING CONDITIONS MUST BE MET:
1. The opening must start within 12” of the top of the
structure and connect with the out of doors through
vertical or horizontal ducts or be ducted to a crawl or
attic space that connects with the out of doors.
2. The opening must have a minimum free area of 1 sq.
in. per 3,000 Btu per hour of the total input rating of all
equipment located in the enclosure.
3. The free area must not be less than the sum of all the
areas of the vent connectors in the enclosure.
An unconfined space is an area including all rooms not
separated by doors with a volume greater than 50 cubic
feet per 1,000 Btuh of the combined input rates of all
appliances which draw combustion air from that space.
In general, a furnace installed in an unconfined space will
not require outside air for combustion. However, in homes
built for energy efficiency (low air change rates), it may
be necessary to provide outside air to ensure adequate
combustion and venting, even though the furnace is located
in an unconfined space. See Example below.
EXAMPLE
A space with a water heater rated at 45,000 Btuh
input and a furnace rated at 75,000 Btuh requires a
volume of 6,000 cubic feet [50 x (45 + 75) = 6,000] to
be considered unconfined. If the space has an 8 foot
ceiling, the floor area of the space must be 750 sq. ft.
(6,000 / 8 = 750).
WARNING:
This furnace is classified as a “Category IV” appliance,
which requires special venting materials and installation
procedures. This section specifies installation requirements
for Conventional (1-pipe) and Direct Vent (2-pipe) piping.
For 1- pipe installations, install vent piping as described in
this section and provide air for combustion and ventilation
according to page 7, page 8, page 9 & page 10.
The length of vent and combustion air piping for either
type of installation are llisted in Table 1.
FURNACE
MODELS
(BTU)
FURNACE
INSTALLATION
SINGLE VENT PIPE LENGTH (FT.)
WITH 1 LONG RADIUS ELBOW†
DUAL VENT PIPE LENGTH (FT.)
WITH 1 LONG RADIUS ELBOW ON EACH PIPE†
OUTLET
2” DIAMETER
OUTLET
3” DIAMETER
2” DIAMETER
3” DIAMETER
38,000 Upflow 50 70 50 70
54,000
Upflow 70 90 70 90
Downflow 70 90 70 90
72,000
Upflow 50 90 50 90
Downflow 50 90 50 90
90,000
Upflow 60 90 60 90
Downflow 60 90 60 90
108,000 Upflow N/A 90 N/A 90
118,000 Downflow N/A 90 N/A 90
120,000
Upflow N/A 90 N/A 90
Downflow N/A 90 N/A 90
†
NOTES:
1. Subtract 2.5 ft. for each additional 2 inch long radius elbow, 5 ft. for each additional 2 inch short radius elbow, 3.5 ft. for each additional 3 inch long radius elbow,
and 7 ft. for each additional 3 inch short radius elbow. Subtract 5 ft for each 2” tee and 8 ft for each 3” tee.
2. Two 45 degree elbows are equivalent to one 90 degree elbow.
3. This table applies for elevations from sea level to 2,000 ft. For higher elevations, decrease pipe lengths by 8% per 1,000 ft of altitude.
4. A long radius elbow’s centerline radius is equal to or greater than 1.5 times the vent diameter.
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