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A water heater in an unconfined space uses indoor air for
combustion and requires at least 50 cubic feet for each
1,000 BTU/HR of the total input for all gas appliances. The
table below shows a few examples of the minimum square
footage (area) required for various BTU/HR inputs.
• 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
• Modern home construction usually requires supplying
outside air into the water heater area.
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
The size of each opening (free area) is determined by the
total BTU/HR input of all gas utilization equipment (i.e.,
water heaters, furnaces, clothes dryers, etc.) and the
method by which the air is provided. The BTU/HR 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.
the outside or
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 BTU/HR of the total input rating
of all gas utilization equipment in the confined area, but not
less than 100 square inches (Figure 5).
9 x 21
14 x 20
15 x 25
15 x 31
20 x 28
20 x 33
25 x 30
28 x 30
with 8' Ceiling
All Air from Inside Building
Confined Space Installation
100 sq. In.)