All air from inside the building, All air from outdoors, Louvers and grilles – American Water Heater 317994-002 User Manual

Page 12

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12

12” Max.

Permanent
openings to
the outside or
additional
rooms within
the building

Closet

or other

confined

space

12” Max.

FIGURE 4.

All Air from Inside the Building

When additional air is to be provided to the confined area

from additional room(s) within the building, the total volume

of the room(s) must be of sufficient size to properly provide

the necessary amount of fresh air to the water heater

and other gas utilization equipment in the area. If you are

unsure that the structure meets this requirement, contact

your local gas utility company or other qualified agency for

a safety inspection.

Each of the two openings shall have a minimum free area

of 1 square inch per 1,000 BTUH of the total input rating of

all gas utilization equipment in the confined area, but not

less than 100 square inches (Figure 5).

FIGURE 5.

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

4 to calculate the minimum free area for each opening.

Figures 6, 7, 8A and 8B 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 4
Minimum Free Area of Permanent Openings for

Ventilation and Combustion Air Supply - All Air from

Outdoors Only.
Based on total BTUH input rating for all utilizing

equipment within the confi ned space.

Opening Source

Minimum Free Area

Per Opening (sq. in.)

Reference Drawing

*Direct to outdoors

1 sq. in, per 4000 BTUH

Figure 6

Vertical Ducts

1 sq. in, per 4000 BTUH

Figure 7

Horizontal Ducts

1 sq. in, per 2000 BTUH

Figure 8A

Single Opening

1 sq. in, per 3000 BTUH

Figure 8B

Example: A water heater with an input rating 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. in.

*

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.

Confined
Space

Permanent
Openings

1 Square
inch/1000
BTUH
(min. 100
sq. in.)

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