1. All air from inside the structure:
a. Each opening MUST have at least 1 sq in. of free area per 1000 Btuh of the total input for all equipment within the confined space, but
not less than 100 sq in. per opening. (See Fig. 2.)
For Example:
GB3AAV FURNACE
INPUT BTUH
FREE AREA PER
OPENING (SQ IN.)
46,000
100
69,000
100
92,000
100
115,000
115
135,000
138
b. If the building is constructed unusually tight, a permanent opening directly communicating with the outdoors should be provided. This
opening should have a minimum free area of 1 sq in. per 5000 Btuh of total input rating for all equipment in the enclosure.
c. If the furnace is installed on a raised platform to provide a return-air plenum, and return air is taken directly from the hallway or space
adjacent to the furnace, all air for combustion must come from outdoors. (See Fig. 3.)
2. All air from outside the structure:
a. If combustion air is taken from outdoors through vertical ducts, the openings and ducts MUST have at least 1 sq in. of free area per 4000
Btuh of the total input for all equipment within the confined space.
For Example:
GB3AAV FURNACE
INPUT BTUH
FREE AREA PER
OPENING (SQ IN.)
ROUND PIPE
(IN. DIA)
46,000
11.5
4
69,000
17.3
5
92,000
23.0
6
115,000
28.8
7
135,000
34.5
7
b. If combustion air is taken from the outdoors through horizontal ducts, the openings and ducts MUST have at least 1 sq in. of free area
per 2000 Btuh of the total input for all equipment within the confined space.
Fig. 2—Air for Combustion and Ventilation
(Inside Air)
A93387
RETURN
AIR
6
″
MIN
(FRONT)
†
SUPPLY AIR
VENT THROUGH ROOF
(CATEGORY I)
1 SQ IN.
PER 1000
BTUH* IN DOOR
OR WALL
12
″
MAX
1 SQ IN.
PER 1000
BTUH* IN DOOR
OR WALL
12
″
MAX
INTERIOR
HEATED
SPACE
* Minimum opening size is 100 square in. with
minimum dimensions of 3-In.
†
Minimum of 3-In. when type B-1 vent is used.
UNCONFINED
SPACE
—5—
