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8
Unconfined Space
A water heater in an unconfined space uses indoor air for
combustion and requires at least 50 cubic feet for each
1,000 BTUH of the total input for all gas appliances. The
table below shows a few examples of the minimum square
footage (area) required for various BTUH inputs.
30,000
45,000
60,000
75,000
90,000
105,000
120,000
135,000
188
281
375
469
563
657
750
844
9 x 21
14 x 20
15 x 25
15 x 31
20 x 28
20 x 33
25 x 30
28 x 30
Typical Room
with 8' Ceiling
Minimum Square
Feet with
8' Ceiling
BTUH
Input
Table 2
IMPORTANT:
• The area must be open and be able to provide the
proper air requirements to the water heater. Areas that
are being used for storage or contain large objects may
not be suitable for water heater installation.
• Water heaters installed in open spaces in buildings with
unusually tight construction may still require outdoor
air to function properly. In this situation, outside air
openings should be sized the same as for a confined
space.
• Modern home construction usually requires supplying
outside air into the water heater area.
Confined Space
For the correct and proper operation of this water heater,
ample air must be supplied for the combustion, ventilation,
and dilution of flue gases. Small enclosures and confined
areas must have two permanent openings so that sufficient
fresh air can be drawn from outside of the enclosure. One
opening shall be within 12 inches of the top and one within
12 inches of the bottom of the enclosure as shown in
Figure 4.
The size of each opening (free area) is determined by the
total BTUH input of all gas utilization equipment (i.e., water
heaters, furnaces, clothes dryers, etc.) and the method by
which the air is provided. The BTUH input can be found on
the water heater data plate. Additional air can be provided
by two methods:
1. All air from inside the building.
2. All air from outdoors.
Figure 4
Opening Locations-
Confined Spaces
12” maximum
Permanent
openings to
the outside or
additional
rooms within
the building
Closet
or
other
confined
space
12” maximum
All Air from Inside the Building
When additional air is to be provided to the confined area
from additional room(s) within the building, the total volume
of the room(s) must be of sufficient size to properly provide
the necessary amount of fresh air to the water heater
and other gas utilization equipment in the area. If you are
unsure that the structure meets this requirement, contact
your local gas utility company or other qualified agency for
a safety inspection.
Each of the two openings shall have a minimum free area
of 1 square inch per 1,000 BTUH of the total input rating of
all gas utilization equipment in the confined area, but not
less than 100 square inches. (Figure 5.)
Confined
Space
Figure 5
All Air from Inside Building
Confined Space Installation
Permanent
Openings
1 square
inch/1000
BTUH
(minimum
100 sq. In.
each)