Rinnai RH180 Installation Fundamentals page 25

Vent Guidelines: Combustion air
A confined space must have two combustion air openings. Size the combustion air
openings based on the Btu input for all gas utilizing equipment in the space and the
method by which combustion air is supplied.
Louvers and Grills:
When sizing the permanent opening:
Consideration must be taken for the design of the louvers or grills to maintain the required free area
required for all gas utilizing equipment in the space. If the free area of the louver or grill is not
available, assume wood louvers will have 25% free area and metal louvers / grills will have 75%
free area. Under no circumstance should the louver, grill or screen have openings smaller than ¼".
Example:
Wood: 10 inches x 12 inches x 0.25 = 30 square inches of free space.
Metal: 10 inches x 12 inches x 0.75 = 90 square inches of free space.
Combustion air provided to the appliance should not be taken from an area of the structure that
may produce a negative pressure (i.e. exhaust fans, powered ventilation fans).
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