Installation In Confi Ned Spaces; Air From Inside; Outdoor Air From A Crawl Space Or Vented Attic; Outdoor Air Using Vertical Ducts - Nordyne M4RC Upflow Installation Instructions Manual

Furnace closets, small equipment rooms and garages are
confi ned spaces. Furnaces installed in a confi ned space
which supply heated air to areas outside the space must
draw return air from outside the space and must have the
return air ducts tightly sealed to the furnace.
The required sizing of these openings is determined by
whether inside or outside air is used to support combustion,
the method by which the air is brought to the space, and
by the total input rate of all appliances in the space. In
all cases, the minimum dimension of any combustion air
opening is 3 inches.

Air From Inside

If combustion air is taken from the heated space, the two
openings must each have a free area of at least one square
inch per 1,000 Btuh of total input of all appliances in the
confi ned space, but not less than 100 square inches of
free area. See Example and Figure 1.
Example:
If the combined input rate of all appliances is less than
or equal to 100,000 Btuh, each opening must have a
free area of at least 100 square inches. If the combined
input rate of all appliances is 120,000 Btuh, each opening
must have a free area of at least 120 square inches.
Vent or
Chimney
NOTES:
Each opening must be
at least 100 sq. in. or
1 sq. in. per 1,000 Btuh
of total input rating,
whichever is greater.
Openings must start at
no more than 12 inches
from the top and bottom
of the enclosure.
Total Input Rating Minimum Free Area
(Btuh) (Each Opening)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 1. Combustion Air Drawn from Inside

Outdoor Air from a Crawl Space or Vented Attic

When the openings can freely exchange air with the
outdoors, each opening shall have a minimum free area
of 1 square inch per 4,000 Btuh of total appliance input.
The openings shall exchange directly, or by ducts, with
the outdoor spaces (crawl or attic) that freely exchange
with the outdoors. See Figure 2.
12" Max.
Water
Heater
Furnace
See
Notes
12" Max.
Round Duct
Diameter
100 sq. In 12 inches
100 sq. In 12 inches
100 sq. In 12 inches
100 sq. In 12 inches
120 sq. In 13 inches
140 sq. In 14 inches
160 sq. In 15 inches
Vent or
Chimney
Furnace
Alternate
Air Inlet
Figure 2. Combustion Air Drawn from a Crawl

Outdoor Air Using Vertical Ducts

If combustion air is taken from outdoors through vertical
ducts, the openings and ducts must have a minimum free
area of one square inch per 4,000 Btuh of total appliance
input. Attics or crawl spaces must connect freely with the
outdoors if they are the source of air for combustion and
ventilation. See Figure 3.
Ventilation Louvers
at each end of attic
Vent or
Chimney
See
Notes
Total Input Rating
(Btuh)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
Figure 3. Combustion Air Drawn from Outside

Outdoor Air Using Horizontal Ducts

If combustion air is taken from outdoors through horizontal
ducts, the openings and ducts must have a minimum free
area of one square inch per 2,000 Btuh of total appliance
input. Ducts must have cross - sectional area at least as
large as the free area of their respective openings to the
furnace space. See Figure 4 (page 8).
Ventilation Louvers
(each end of attic)
Outlet
Air NOTE: Air openings shall
each have a free area of
Water
not less than one square
Heater
inch per 4,000 Btuh of the
total input rating of all
Inlet Air
equipment in the enclosure.
Ventilation Louvers For
Unheated Crawl Space
Space or Vented Attic
Attic
Insulation
Ducts must
extend above
attic insulation.
Water
Furnace
Heater
Minimum Free Area
(Each Opening)
10 sq. In
15 sq. In
20 sq. In
25 sq. In
30 sq. In
35 sq. In
40 sq. In
Through Vertical Ducts
Air Duct must be
at least 1 sq. in.
per 4,000 Btuh of
total input rating.
Air Duct must be
at least 1 sq. in.
per 4,000 Btuh of
total input rating.
12" Max
Round Duct
Diameter
4 inches
5 inches
5 inches
6 inches
6 inches
7 inches
8 inches
7