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All Air from Outdoors
Outdoor fresh air can be provided to a confined area either
directly or by the use of vertical and horizontal ducts. The
fresh air can be taken from the outdoors or from crawl or
attic spaces that freely communicate with the outdoors.
Attic or crawl spaces cannot be closed and must be
properly ventilated to the outside.
Ductwork must be of the same cross-sectional area as
the free area of the opening to which they connect. The
minimum dimension of rectangular air ducts cannot be less
than three inches.
The size of each of the two openings is determined by the
method in which the air is to be provided. Refer to Table
3 to calculate the minimum free area for each opening.
Figures 6, 7, and 8 are typical examples of each method.
Louvers and Grilles
In calculating free area for ventilation and combustion
air supply openings, consideration must be given to the
blocking effect of protection louvers, grilles, and screens.
These devices can reduce airflow, which in turn may
require larger openings to achieve the required minimum
free area. Screens must not be smaller than 1/4” mesh. If
the free area through a particular design of louver or grille
is known, it should be used in calculating the specified free
area of the opening. If the design and free area are not
known, it can be assumed that most wood louvers will allow
20 - 25% of free area while metal louvers and grilles will
allow 60 - 75% of free area.
Louvers and grilles must be locked open or interconnected
with the equipment so that they are opened automatically
during equipment operation.
Keep louvers and grilles clean and free of debris or other
obstructions.
Table 3
Minimum Free Area of Permanent Openings for Ventilation and
Combustion Air Supply - All Air from Outdoors Only.
Based on the total BTUH input rating for all gas utilizing equipment
within the confined space.
Minimum Free Area
Reference
Opening
Per Opening
Drawing
(sq. in.)
Source
Direct to
1 Square inch per 4000
Figure 6
outdoors
BTUH
Vertical ducts
Figure 7
1 Square inch per 4000
BTUH
1 Square inch per 2000
Horizontal ducts
Figure 8
BTUH
Example: A water heater with an input rate of 50,000 BTUH using
horizontal ducts would require each opening to have a minimum
free area of 25 square inches.
Minimum free area = 50,000 BTUH x 1 sq. in. / 2000 BTUH = 25
Sq. Inches.
These openings connect directly with the outdoors
*
through a ventilated attic, a ventilated crawl space, or
through an outside wall.
Consult the local codes of your area for specific ventilation
and combustion air requirements.
Gable vent
to outdoors
Install above
insulation
Confined
Outlet
Space
air to attic
1 Sq. inch per
4000 BTUH
Inlet air
Alternate
from the
Air Inlet
crawl
space
Open
1 sq. inch per
Foundation
4000 BTUH
Vent
Figure 6
All Air from Outdoors; Inlet Air from Ventilated
Crawl Space/Outlet Air to Ventilated Attic
Gable vent
to outdoors
Install above
insulation
Outlet air
Confined
to attic
Space
1 Sq. inch per
4000 BTUH
Inlet air duct
1 sq. inch per
4000 BTUH
12” maximum
Figure 7
All Air from Outdoors
Through Ventilated Attic
1 sq. inch
per
2000 BTUH
Confined
Outlet
Space
Outdoor
Air Ducts
Inlet
1 sq. Inch
Figure 8
per
All Air from Outdoors
2000 BTUH
Using Horizontal Ducts
Page 9
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