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Calculate minimum size of vent openings and ducts
vents use louvers or grilles to protect the opening. The louver or grill itself blocks some of the free area, so the open-
ing may need to be larger to meet the minimum free area requirements.
Each vent opening must be no smaller than 100 square inches .
Rectangular ducts must have a minimum dimension of no less than three inches .
All screens must have mesh ¼” or larger.
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Check that air source is clean and free of chemicals
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See graphics on next page.
Table 1
indoor spaces.
Opening Source Minimum Free Area
Direct to outdoors* 1 sq. in. per 4,000 BTU/hr (see Figure 1, 2)
1 sq. in. per 4,000 BTU/hr (see Figure 3)
Horizontal ducts 1 sq. in. per 2,000 BTU/hr (see Figure 4)
Single Opening 1 sq. in. per 3,000 BTU/hr (see Figure 5)
Two permanent openings
to another room**
1 sq. in. per 1,000 Btu/hr (see Figure 6)
Opening: 100 in.
2
MIN.
Minimum dimension of air openings:
no less than 3 in.
an outside wall.
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