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.
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 warmm air
registers must not be obstructed.
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.
12
FIGURE 9
OUTSIDE AIR USING A HORIZONTAL INLET & OUTLET
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:
VERTICAL OUTDOOR AIR
OPENING DIMENSIONS
BTUH Free Area Round
Input Each Opening Pipe Size
45,000 11.25 sq. inches 4(
50,000 12.50 sq. inches 4(
67,500 16.90 sq. inches 5(
75,000 18.75 sq. inches 5(
100,000 25.00 sq. inches 6(
125,000 31.25 sq. inches 7(
150,000 37.50 sq. inches 7(
HORIZONTAL OUTDOOR AIR
OPENING DIMENSIONS
BTUH Free Area Round
Input Each Opening Pipe Size
45,000 22.50 sq. inches 6(
50,000 25.00 sq. inches 6(
67,500 33.75 sq. inches 7(
75,000 37.50 sq. inches 7(
100,000 50.00 sq. inches 8(
125,000 62.50 sq. inches 9(
150,000 75.00 sq. inches 10(
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