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Air Requirements For One-Pipe
Installation
When air for combustion is to be taken from
around the furnace, a protective screen must be
installed over the combustion air intake opening.
This screen is provided with the furnace instal-
lation instructions and functions to prevent de-
bris from entering the combustion system. It
should be installed on the combustion air intake
collar or inlet PVC. If furnace location is such that
this opening might be unintentionally obstructed,
a 3" PVC elbow should be installed on the collar,
and the screen placed inside the inlet of the
elbow. See Figure 4.
Installation In An Unconfined Space
!
CAUTION:
"Tight" buildings (with weather strip-
ping and caulk to reduce infiltration),
may require special provisions for in-
troduction of outside air to ensure
satisfactory combustion and venting,
even though the furnace is located in
an unconfined space.
An unconfined space is an area including all
rooms not separated by doors with a volume
greater than 50 cubic feet per 1,000 Btuh of the
combined input rates of all appliances which draw
combustion air from that space. For example, a
space including a water heater rated at 45,000
Btuh and a furnace rated at 80,000 Btuh requires
a volume of 6,250 cubic feet [50 x (45 + 80) =
6,250] to be considered unconfined. If the space
has an 8 foot ceiling, the floor area of the space
must be 750 square feet (6,250 / 8 = 781). In
general, a furnace installed in an unconfined
space will not require outside air for combustion.
10
WARNING:
!
Furnaces installed with combustion air
drawn from a heated space which in-
cludes exhaust fans, fireplaces, or
other devices that may produce a nega-
tive pressure should be considered
confined space installations.
Installation In A Confined Space
A confined space is one which does not meet the
unconfined space volume requirements, and
typically involves installation in a small room. All
such installations must have specific provisions
for introduction of combustion and ventilation
air. Codes require that two openings be pro-
vided for this - one with bottom edge within 12"
of the floor and one with top edge within 12" of
the ceiling. The size and other criteria for these
openings must be per the following sections.
Combustion air openings must not be restricted
in any manner.
Furnaces installed in a confined space which
supply circulating air to areas outside of the
space must draw return air from outside the
space and must have return air ducts tightly
sealed to the furnace.
Air From Inside
Air for combustion and ventilation may be taken
from inside the building through an interior wall
if the building is not "tight" and if the total volume
of the furnace space and the space from which
air is drawn meets the volume requirements for
an unconfined space. In such cases, the two
openings in the wall must each have free area
of at least one square inch per 1000 Btuh of total
appliance input, but not less than 100 square
inches of free area. See Figure 5. For example,
if the combined input rate of all appliances is less
than or equal to 100,000 Btuh, each opening
must have a free area of at least 100 square
inches. If the combined input rate of all appli-
ances is 120,000 Btuh, each opening must have
a free area of at least 120 square inches. (See
Figure 5.)
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