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Combustion
Air Quality
The recommended
source of combustion air is
to use the outdoor air supply. However, the use
of indoor air in most applications
is acceptable
except as follows:
1.
If the furnace
is installed
in a confined
space it is recommended
that the necessary
combustion air come from the outdoors by
way of attic, crawl space, air duct, or direct
opening.
2.
If outdoor
combustion
air is used, there
must be no exposure to the installations or
substances
listed in Item 3 below.
.
o
o
o
o
The following types of installation
may re-
quire Outdoor
Air for combustion,
due to
chemical exposures:
Commercial
buildings
Buildings with indoor pools
Furnaces installed in laundry rooms
Furnaces
installed
in hobby
or craft
rooms
•
Furnaces installed near chemical storage
areas
Exposure
to the following
substances
in
the combustion air supply may also require
Outdoor
Air for combustion:
•
Permanent wave solutions
Chlorinated
waxes and cleaners
• Chlorine
based
swimming
pool chemi-
cals
Water softening chemicals
De-icing salts or chemicals
Carbon tetrachloride
Halogen type refrigerants
Cleaning solvents (such as perchloroethy-
lene)
Printing inks, paint removers, varnishes,
etc.
Hydrochloric
acid
Cements and glues
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
16
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 must be installed on the collar, and
the screen placed inside the inlet of the elbow.
See Figure 10.
Installation
In An Unconfined
Space
CAUTION:
"Tight"
buildings
(with weather
strip-
ping and caulk to reduce infiltration),
may require
special
provisions
for
introduction
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 ex-
ample, a space including a water heater rated
at 45,000 Btuh and a furnace rated at 75,000
Btuh requires a volume of 6,000 cubic feet [50 x
(45 + 75) = 6,000] to be considered unconfined.
If the space has an 8 foot ceiling, the floor area
of the space must be 750 square feet
(6,000
/ 8 = 750). In general, a furnace installed in an
unconfined space will not require outside air for
combustion.
WARNING:
Furnaces
installed
with combustion
air drawn from a heated space which
includes 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 provided for
this - one with bottom edge within 12"of the floor
and one with top edge within 12" of the ceiling.
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