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Combustion air and vent system requirements, Louvers and grilles, Figure 4. figure 5 – A.O. Smith 12 40GPC T 100 User Manual

Page 8

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COMBUSTION AIR AND VENT SYSTEM REQUIREMENTS

All Air from Inside the Building: When

additional air is to be provided to the confi ned

area from additional room(s) within the building,

the total volume of the room(s) must be of

suffi cient size to properly provide the necessary

amount of fresh air to the water heater and other

gas utilization equipment in the area. 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 confi ned area, but not less than 100 square

inches for each opening.

Figure 3.

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 the table below to calculate

the minimum free area for each opening.

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.

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 4

Vertical Ducts

1 sq. in, per 4000 BTUH

Figure 5

Horizontal Ducts

1 sq. in, per 2000 BTUH

Figure 6

Single Opening

1 sq. in, per 3000 BTUH

Figure 7

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

(MINIMUM

100 SQ. IN.
FOR EACH
OPENING)

Figure 4.

Figure 5.

ABLE VENT

TO OUTDOORS

INSTALL ABOVE

INSULATION

CONFINED

SPACE

ALTERNATE

AIR INLET

OUTLET

AIR TO

ATTIC 1 SQ.

INCH PER

4000 BTUH

INLET AIR FROM

THE CRAWL SPACE

OPEN

FOUNDATION

VENT

1 SQ. INCH PER

4000 BTUH

100 SQ. INCH MINIMUM (EACH)

ALL AIR FROM OUTDOORS: INLET AIR FROM VENTILATED

CRAWL SPACE/OUTLET AIR TO VENTILATED ATTIC

GABLE VENT
TO OUTDOORS

INSTALL ABOVE
INSULATION

CONFINED

SPACE

OUTLET AIR TO
ATTIC 1 SQ. INCH
PER 4000 BTUH -
100 SQ. INCH MIN.
(EACH)

INLET AIR DUCT
1 SQ. INCH PER
4000 BTUH - 100
SQ. INCH MIN.
(EACH)

12” MAXIMUM

ALL AIR FROM OUTDOORS THROUGH VENTILATED ATTIC

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