14
CIRCULATING AIR REQUIREMENTS
WARNING:
Do not allow combustion products to enter the
circulating air supply. Failure to prevent the
circulation of combustion products into the
living space can create potentially hazardous
conditions including carbon monoxide
poisoning that could result in personal injury
or death.
All return ductwork must be secured to
the furnace with sheet metal screws. For
installations in confi ned spaces, all return
ductwork must be adequately sealed. When
return air is provided through the bottom of the
furnace, the joint between the furnace and the
return air plenum must be air tight.
The surface that the furnace is mounted on must
provide sound physical support of the furnace
with no gaps, cracks or sagging between the
furnace and the fl oor or platform.
Return air and circulating air ductwork must
not be connected to any other heat producing
device such as a fi replace insert, stove, etc.
This may result in fi re, explosion, carbon
monoxide poisoning, personal injury, or
property damage.
Plenums and Air Ducts
• Plenums and air ducts must be installed in accordance
with the Standard for the Installation of Air Conditioning
and Ventilating Systems (NFPA No. 90A) or the
Standard for the Installation of Warm Air Heating and
Air Conditioning Systems (NFPA No. 90B).
• Table 8 (page 38) contains the maximum airfl ow and
temperature rise data for each furnace input rate.
NOTE:
If the maximum airfl ow is 1,600 CFM or more,
it is recommended that two openings be used for return
air on upfl ow furnaces. Downfl ow furnaces can only
use one return opening.
• It is recommended that the outlet duct contain a
removable access panel. The opening should be
accessible when the furnace is installed in service and
shall be of a size that smoke or refelcted light may be
observed inside the casing to indicate the presence of
leaks in the heat exchanger. The cover for the opening
shall be attached in such a manner as to prevent leaks.
• If outside air is used as return air to the furnace for
ventilation or to improve indoor air quality, the system
must be designed so that the return air is not less than
60° F (15° C) during operation. If a combination of
indoor and outdoor air is used, the ducts and damper
system must be designed so that the return air supply
to the furnace is equal to the return air supply under
normal, indoor return air applications.
• When a cooling system is installed which uses the
furnace blower to provide airfl ow over the indoor coil,
the coil must be installed downstream (on the outlet
side) of the furnace or in parallel with the furnace.
• If a cooling system is installed in parallel with the
furnace, a damper must be installed to prevent chilled
air from entering the furnace and condensing on the
heat exchanger. If a manually operated damper is
installed, it must be designed so that the furnace will
not operate when the damper is in the cooling position
or when in heating position, the cooling system is
inoperable.
• It is good practice to seal all connections and joints
with industrial grade sealing tape or liquid sealant.
Requirements for sealing ductwork vary from region
to region. Consult with local codes for requirements
specifi c to your area.
Return Air Connections
In applications where the supply ducts carry heated air
to areas outside the space where the furnace is installed,
the return air must be delivered to the furnace by duct(s)
secured to the furnace casing, running full size and without
interruption.
• Upfl ow furnaces draw the return air from the base of
the furnace. A stand or return air duct must be supplied
to the furnace to provide the required return air.
• Downfl ow models draw the return air from the top of
the furnace. The minimum required clearance to the top
of the furnace is detailed on the furnace rating plate.
Additional clearance may be required depending upon
fi lter accessibility.
For each U.S.A. application, the home manufacturer
shall comply with all of the following conditions to have
acceptable return air systems for closet installed forced
air heating appliances:
• Regardless of the location, the return air opening
into the closet shall not be less than specifi ed in the
appliance’s listing.
• Means shall be provided to prevent inadvertent closure
by a fl at object placed over the return air opening when
it is located in the fl oor of the closet (versus the vertical
front or side wall).
• The cross-sectional area of the return duct system
leading into the closet shall not be less than 390 square
inches.
• The total free area of openings in the fl oor or ceiling
registers serving the return air duct system must be at
least 352 sq. in. At least one register should be located
where it is not likely to be covered by carpeting, boxes
and other objects.
• Materials located in the return duct system must have a
fl ame spread classifi cation of 200 or less. This includes
a closet door if the furnace is in a closet.
