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 BTU/HR input rating for all utilizing equip- ment within the confined space.

	Minimum Free Area	Reference
Opening Source	Per Opening (sq. in.)	Drawing
* Direct to outdoors	1sq.in,per4000BTU/HR	Figure 6
Vertical ducts	1sq.in,per4000BTU/HR	Figure 7
Horizontal ducts	1sq.in,per2000BTU/HR	Figure 8A
Single Opening	1sq.in,per3000BTU/HR	Figure 8B

Example: A water heater with an input rate of 50,000 BTU/HR using horizontal ducts would require each opening to have a minimum free area of 25 square inches.

Minimum free area = 50,000 BTU/HR x 1 sq. in. / 2000 BTU/HR = 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 BTU/HR
Alternate	Inlet air
Alternate	from the
Air Inlet	crawl
	space
1 sq. inch per	Open
4000 BTU/HR	Foundation
	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
Confined	Outlet air
Confined	to attic
Space	1 Sq. inch per
	4000 BTU/HR
	Inlet air duct
	1 sq. inch per
	4000 BTU/HR

Figure 7	12” maximum
All Air from Outdoors
Through Ventilated Attic

	1 sq. inch
	per
	2000 BTU/HR
Confined	Outlet
Space
	Outdoor
	Air Ducts
	Inlet

Figure 8A	1 sq. Inch
All Air from Outdoors	per
Using Horizontal Ducts	2000 BTU/HR

318935-003 specifications

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