Reference | A.O. Smith 12 40GPC T 100, PCG6240T403NOV 100 specs

COMBUSTION AIR AND VENT SYSTEM REQUIREMENTS

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. 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 for each opening.

CONFINED

SPACE

PERMANENT

OPENINGS

1 SQUARE

INCH/1000

BTUH

(MINIMUM

100 SQ. IN.

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

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 conﬁned space.

	Minimum Free Area	Reference
Opening Source	Per Opening (sq. in.)	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

ABLE VENT

TO OUTDOORS

		INSTALL ABOVE
		INSULATION
	CONFINED	OUTLET
	SPACE	OUTLET
	SPACE	AIR TO
		AIR TO
		ATTIC 1 SQ.
		INCH PER
		4000 BTUH
	ALTERNATE	INLET AIR FROM
	AIR INLET	THE CRAWL SPACE

	1 SQ. INCH PER		OPEN
	4000 BTUH
			FOUNDATION
	100 SQ. INCH MINIMUM (EACH)
			VENT

ALL AIR FROM OUTDOORS: INLET AIR FROM VENTILATED

CRAWL SPACE/OUTLET AIR TO VENTILATED ATTIC

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

CONFINED SPACE

Figure 4.

GABLE VENT

TO OUTDOORS

INSTALL ABOVE

INSULATION

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)

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.

12” MAXIMUM

ALL AIR FROM OUTDOORS THROUGH VENTILATED ATTIC

Figure 5.