# 45465K005
Page 5
Unconfined Space
An unconfined space is defined as “a space whose
volume is more than 50 cubic feet per 1000 BTU per hour
of the combined input rating of all appliances installed in
that space.” When a furnace is installed in an unconfined
space in a building, it can be assumed that the infiltration
will be sufficient to supply the required air. If the furnace is
installed in a ventilated attic or crawl space, it is assumed
that the infiltration is sufficient to supply the required air.
However, in a building of unusually tight construction,
additional outdoor air should be provided.
Confined Space
A confined space is defined as “a space whose volume is
less than 50 cubic feet per 1000 BTU per hour of the
combined input rating of all appliances installed in that
space.”
Confined Space Installation/Air from Inside Structure
If the furnace is installed in a confined space within the
building and combustion air is taken from a heated space,
the combustion air and ventilating air must enter and leave
the space through two permanent openings of equal area
(see Figure 3). One opening shall be located within 12" of
the ceiling and the other within 12" of the floor, each
having a free area of 1 square inch per 1000 BTU/HR of
total input rating of all appliances within the space and not
less than 100 square inches each.
Figure 3
Note: Each opening shall have a free area of at least one square
inch per 1000 BTU per hour of the total input rating of all
equipment in the enclosure, but not less than 100 square
inches.
Water
Heater
Chimney or
Gas Vent
Openings
Furnace
Equipment in Confined Space
All Air From Inside
Confined Space Installation/Air from Outside Structure
If air from outside is brought in for combustion, the confined
space shall be provided with two permanent openings. One
opening shall be within 12" of the ceiling and one opening
within 12" of the floor. Several methods can be used to
bring the outside air in through these openings. The open-
ings shall open directly or by ducts with the outdoors,
through spaces (crawl space or attic) that freely open to the
outdoors, or indirectly through vertical ducts (see Figures 4
and 5). If any of these methods are used, each opening
shall have a free area of 1 square inch per 4000 BTU/HR of
the total input rating of all appliances within the enclosure.
Another option is to use horizontal combustion ducts
(see Figure 6 on page 6). If horizontal combustion ducts
are run, 1 square inch per 2000 BTU/HR is required.
Figure 5
Figure 4
Equipment in Confined Space
All Air from Outside
(Inlet Air from Crawl Space and
Outlet Air to Ventilated Attic)
Chimney or
Gas Vent
Ventilation Louvers
(Each end of attic)
Water
Heater
Furnace
Outlet
Air
Inlet
Air
Ventilation
Louvers
(For unheated
crawl space)
Note: The inlet and outlet air opening shall each have a free area of
at least one square inch per 4000 BTU per hour of the total
input rating of all equipment in the enclosure.
Chimney or
Gas Vent
Ventilation Louvers
(Each end of attic)
Inlet Air
(Ends 12”
above bottom)
Outlet Air
Water
Heater
Furnace
Equipment in Confined Space
All Air from Outside
(All Air Through Ventilated Attic)
Note: The inlet and outlet air opening shall each have a free area of
at least one square inch per 4000 BTU per hour of the total
input rating of all equipment in the enclosure.
