Rheem GPH series Installation Instructions Manual page 12

B: Method 1
Provide two permanent openings,
one located within 12 inches of the
top and one located within 12
inches of the bottom of the
enclosure. Each opening shall
communicate directly, or by ducts,
with the outdoors or spaces (crawl
or attic) that freely communicate
with the outdoors.
a. Where directly communicating
with the outdoors or where
communicating to the outdoors
through VERTICAL DUCTS,
each opening shall have a
minimum free area of 1 square
inch for each 4000 BTUH of
total appliance input rating in the
enclosure. Here are typical duct
sizes:
VERTICAL OUTDOOR AIR
OPENING DIMENSIONS
BTUH Free Area
Input Each Opening
50,000 12.50 sq. inches
75,000 18.75 sq. inches
100,000 25.00 sq. inches
125,000 31.25 sq. inches
150,000 37.50 sq. inches
b. Where communicating with
outdoors through HORIZONTAL
DUCTS, each opening shall have
a minimum free area of 1 square
inch for each 2000 BTUH of
total input rating for all equipment
in the enclosure. Here are typical
duct sizes:
HORIZONTAL OUTDOOR AIR
OPENING DIMENSIONS
BTUH Free Area
Input Each Opening
50,000 25.00 sq. inches
75,000 37.50 sq. inches
100,000 50.00 sq. inches
125,000 62.50 sq. inches
150,000 75.00 sq. inches
12
B: Method 2
One permanent opening, located within
12 inches of the top of the enclosure,
shall be permitted where the
equipment has clearances of at least 1
inch from the sides and back and 6
inches from the front of the appliance.
The opening shall directly
communicate with the outdoors or
communicate through a vertical or
horizontal duct to the outdoors or
spaces (crawl or attic) that freely
communicate with the outdoors and
have a minimum free area of:
a. One square inch for each 3000
BTUH of the total input rating of all
equipment located in the enclosure,
AND
b. Not less than the sum of the areas of
all vent connectors in the confined
space.
FIGURE 8
Round
OUTSIDE AIR USING A HORIZONTAL INLET & OUTLET
Pipe Size
4"
5"
6"
7"
7"
Round
Pipe Size
6"
7"
8"
9"
10"
IMPORTANT: If the furnace is in a
location with an exhaust fan, there
must be sufficient ventilation to prevent
the exhaust fan from creating a
negative pressure in the room.
Combustion air openings must NOT
BE RESTRICTED in any manner.
CONSULT LOCAL CODES FOR
SPECIAL REQUIREMENTS.
Air opening in the furnace casing front,
return air grilles, and warm air registers
must not be obstructed.