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Step 1—Unconfined Space
An unconfined space has a volume of at least 50 cu ft for each
1000 Btuh total input for all appliances (furnaces, clothes dryers,
water heaters, etc.) in the space.
For example:
FURNACE
INPUT (BTUH)
MINIMUM WITH
7–1/2 FT CEILING (SQ. FT.)
44,000 294
66,000 441
88,000 587
110,000 734
132,000 881
154,000 1028
If the unconfined space is constructed unusually tight, air for
combustion and ventilation must come from either the outdoors or
spaces freely communicating with the outdoors. Combustion and
ventilation openings must be equivalent to those used for a
confined space (defined below). Return air must not be taken from
the room unless an equal or greater amount of air is supplied to the
room.
Step 2—Confined Space
A confined space has a volume less than 50 cu ft per 1000 Btuh
of total input ratings of all appliances installed in that space. A
confined space must have provisions for supplying air for com-
bustion, ventilation, and dilution of flue gases using one of the
following methods in Table 2 and Fig. 7 and 8.
NOTE: When determining the free area of an opening, the
blocking effect of louvers, grilles, and screens must be considered.
If the free area of the louver or grille design is unknown, assume
wood louvers have a 20 percent free area and metal louvers or
grilles have a 60 percent free area. Screens must not be smaller
than 1/4-in. mesh. Louvers and grilles must be constructed so they
cannot be closed.
The opening size depends upon whether air comes from outside of
the structure or an unconfined space inside the structure.
1. Air from inside the structure requires 2 openings (for struc-
tures not of unusually tight construction):
a. Each opening must have a minimum free area of not less
than 1 sq in per 1000 Btuh of total input rating for all gas
utilization equipment in the confined space, but not less
than 100 sq in. The minimum dimension of air openings
should be no smaller than 3 in. (See Table 2 and Fig. 7 ).
b. If building construction is unusually tight, a permanent
opening directly communicating with the outdoors shall be
provided. (See next section).
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 furnace, all air for combustion
must come from outdoors.
2. Air from outside the structure requires 1 of the following:
a. If combustion air is taken from outdoors through 2 vertical
ducts, the openings and ducts must have at least 1 sq in. of
free area per 4000 Btuh of total input for all equipment
within the confined space. (See Fig. 8 and Table 2.)
b. If combustion air is taken from outdoors through 2 hori-
zontal ducts, the openings and ducts must have at least 1 sq
in. of free area per 2000 Btuh of total input for all
equipment within the confined space. (See Fig. 8 and Table
2.)
Table 2–Minimum Free Area of Combustion Air Opening*
FURNACE
INPUT
(BTUH)
AIR FROM INDOOR
UNCONFINED SPACE
OUTDOOR AIR THROUGH
VERTICAL DUCTS
OUTDOOR AIR THROUGH
HORIZONTAL DUCTS
OUTDOOR AIR THROUGH
SINGLE DUCT
Free Area
of Opening
(Sq In.)
Free Area of
Opening and Duct
(Sq. In.)
Round Pipe
(in. Dia)
Free Area of
Opening and Duct
(sq In.)
Round Pipe
(in. Dia)
Free Area of
Opening and Duct
(Sq In.)
Round Pipe
(In. Dia)
44,000 100 11 4 22 6 14.7 5
66,000 100 16.5 5 33 7 22.0 6
88,000 100 22.0 6 44.0 8 29.3 7
110,000 110 27.5 6 55 9 36.7 7
132,000 132 33 7 66 10 44.0 8
154,000 154 38.5 7 77 10 51.3 8
* Free area shall be equal to or greater than the sum of the areas of all vent connectors in the confined space. Opening area must be increased if other gas appliances
in the space require combustion air.
Fig. 7—Confined Space: Air for Combustion and
Ventilation from an Unconfined Indoor Space
A02038
AIR
DUCTS
6″ MIN
(FRONT)
†
AIR DUCTS
VENT THROUGH ROOF
1 SQ IN.
PER 1000
BTUH
*
IN DOOR
OR WALL
12″ MAX
1 SQ IN.
PER 1000
BTUH
*
IN DOOR
OR WALL
12″ MAX
* Minimum opening size is 100 sq in. with
minimum dimensions of 3 in.
†
Minimum of 3 in. when type-B1 vent is used.
UNCONFINED
SPACE
CONFINED
SPACE
INTERIOR
HEATED
SPACE
7