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Vent or
. _J
%ES;en,ng
m
...............
=
! D
".
See
at least 100 sq. in. or E
_aotes
1 sq. in. per 1,000 Btuh
_
Water"_
_
of total input rating,
_
/Heath.,,_' II
whichever is greater,
i
Furnace
Openings must start at
i
_
no more than 12 inches
.'/
from the top and bottom
See
ef the enclosure.
Noteo_
_
_,
_
_
12" Max.
Total input Rating
Minimum Free Area
(Btuh)
•
(Each Opening)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
100 sq. In
100 sq. In
100 sq. In
100 sq. In
120 sq. In
140 sq. In
160 sq. In
Round Duct
Diameter
12inches
12inches
12inches
12inches
13inches
14 inches
15inches
Figure 1. Combustion
Air Drawn from inside
Vent
Chimne
Ventilation
Louvers
at each end of attic
Insulation
Air Duct must be
1 sq. in.
per 4,000 Btuh of
total input rating.
Tota! Input Rating
Minimum Free Area
(Btuh)
'
(Each Opening)
40,000
60,000
80,000
100,000
120,000
140,000
160,000
10 sq. In
15 sq. In
20 sq. In
25 sq. In
30 sq. In
35 sq. In
40 sq. In
Ducts must
extend above
attic insulation.
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
Figure 3. Combustion
Air Drawn from Outside
Through Vertical Ducts
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 (Figure 2).
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 (Figure 3).
Vent or
Chimney
Air Inlet
Ventilation Louvers
(each end of attic)
Outlet J*
Air
_Water
Heater
Inlet Air
/
NOTE: Air openings shall
each have a free area of
not less than one square
inch per 4,000 Btuh of the
total input rating of all
equipment in the enclosure.
--...
j
Ventilation
Louvers
For
Unheated
Crawl
Space
Figure 2. Combustion
Air Drawn from a Crawl
Space or Vented Attic
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 (Figure 4). Ducts must have cross - sectional area at
least as large as the free area of their respective openings
to the furnace space.
\
Vent or
3himney
Water
Heater
I_
Air Duct
J_
J
Air Ducts must be
at least 1 sq. in.
Furnace
per 2,000 Btuh of
total input rating.
-
Air Duct
=-
•
E
J
Total nput Rating
Minimum Free Area
Round Duct
(Btuh)
....
(Each Opening)
Diameter
40,000
20 sq. In
5 inches
60,000
30 sq. In
6 inches
80,000
40 sq. In
7 inches
100,000
50 sq. In
8 inches
120,000
60 sq. In
9 inches
140,000
70 sq. In
10 inches
160,000
80 sq. In
10 inches
Figure 4. Combustion
Air Drawn from Outside
Through Horizontal
Ducts
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