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
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 8, 9, and 10 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 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 8 |
Vertical ducts | 1sq.in,per4000BTU/HR | Figure 9 |
Horizontal ducts | 1sq.in,per2000BTU/HR | Figure 10 |
Single Opening | 1sq.in,per3000BTU/HR | Figure 11 |
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 |
Outlet |
air to attic |
1 Sq. inch per |
4000 BTU/HR |
Alternate |
Air Inlet |
Open |
Foundation |
Vent |
Gable vent |
to outdoors |
Install above |
insulation |
Outlet air |
to attic |
1 Sq. inch per |
4000 BTU/HR |
12” maximum
1 sq. inch |
per |
2000 BTU/HR |
Outlet |
Outdoor |
Air Ducts |
Inlet |
1 sq. Inch
per 2000 BTU/HR
9