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• Antistatic fabric softeners for clothes dry-
ers
• Masonry acid washing materials
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 installa-
tion instructions and functions to prevent debris
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 Unconfi ned Space
CAUTION:
“Tight” buildings (with weather strip-
ping and caulk to reduce infi ltration),
may require special provisions for
introduction of outside air to ensure
satisfactory combustion and venting,
even though the furnace is located in
an unconfi ned space.
An unconfi ned 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 ex-
ample, 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 unconfi ned.
If the space has an 8 foot ceiling, the fl oor area
of the space must be 750 square feet (6,250 /
8 = 781). In general, a furnace installed in an
unconfi ned space will not require outside air
for combustion.
WARNING:
Furnaces installed with combustion
air drawn from a heated space which
includes exhaust fans, fi replaces, or
other devices that may produce a nega-
tive pressure should be considered
confi ned space installations.
Installation In A Confi ned Space
A confi ned space is one which does not meet
the unconfi ned space volume requirements, and
typically involves installation in a small room. All
such installations must have specifi c provisions
for introduction of combustion and ventilation air.
Codes require that two openings be provided for
this - one with bottom edge within 12” of the fl oor
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 confi ned 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 unconfi ned 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 appli-
ances 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
appliances is 120,000 Btuh, each opening must
have a free area of at least 120 square inches.
(See Figure 5.)
Air Directly Through An Exterior Wall
If combustion air is provided directly through an
exterior wall, the two openings must each have
free area of at least one square inch per 4000
Btuh of total appliance input. (See Figure 6.)
Outdoor Air Through Vertical Openings or Ducts
If combustion air is provided through vertical
ducts or openings to attics or crawl spaces,
the two openings must each have free area of
at least one square inch per 4000 Btuh of total
appliance input. Ducts must have cross-sectional
areas at least as large as the free area of their
respective openings to the furnace space. At-
tics or crawl spaces must communicate freely
with the outdoors if they are the source of air
