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8
VENTILATION AND COMBUSTION AIR SUPPLY (CONT.)
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 3 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 1 to calculate the minimum free
area for each opening. Figures 6, 7, and 8 are typical examples of each
method.
CAUTION
INLET AIR CANNOT BE TAKEN FROM AN ATTIC EQUIPPED WITH
POWER VENTILATION.
LOUVERS AND GRILLES
In calculating free area for ventilation and combustion air supply openings,
consideration must be given to the blocking affect of protection louvers,
grilles, and screens. These devices can reduce air flow which in turn may
require larger openings to achieve the required minimum free area. Screens
must not be smaller than 1/4 inch 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 1
MINIMUM FREE AREA OF PERMANENT OPENINGS FOR
VENTILATION AND COMBUSTION AIR SUPPLY - ALL AIR
FROM OUTSIDE ONLY.
OPENING SOURCE MINIMUM FREE AREA PER
OPENING (SQ. IN.)
REFERENCE
DRAWING
DIRECT TO
OUTDOORS* 1 SQUARE INCH PER 4000 BTUH FIGURE 6
VERTICAL DUCTS 1 SQUARE INCH PER 4000 BTUH FIGURE 7
HORIZONTAL DUCTS 1 SQUARE INCH PER 2000 BTUH FIGURE 8
Example: A water heater with an input rate 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 Inches
* These openings may communicate directly with the outdoors through a
ventilated attic, a ventilated crawl space, or through an outside wall.