12” Max.
free area of the opening to which they connect. The minimum dimension of rectangular air ducts cannot be less than three inches.
Permanent openings to the outside or additional rooms within the building
Closet |
or other |
confined |
space |
12” Max. |
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 15, 16, 17 and 18 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
FIGURE 13.
All Air from 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 14).
Confined
Space
Permanent
Openings
1 Square inch/1000
BTUH (min. 100 sq. in.)
FIGURE 14.
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
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 confined space.
| Minimum Free Area |
|
Opening Source | Per Opening (sq. in.) | Reference Drawing |
*Direct to outdoors | 1 sq. in, per 4000 BTUH | Figure 15 |
Vertical Ducts | 1 sq. in, per 4000 BTUH | Figure 16 |
Horizontal Ducts | 1 sq. in, per 2000 BTUH | Figure 17 |
Single Opening | 1 sq. in, per 3000 BTUH | Figure 18 |
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 venti- lated attic, a ventilated crawl space, or through an outside wall.
Consult the local codes of your area for specific ventilation and combustion air requirements.
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