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CIRCULATING AIR SUPPLY
Plenums and air ducts must be installed in
accordance with the Standard for the Installa-
tion of Air Conditioning and Ventilating Systems
(NFPA No. 90A) or the Standard for the Instal-
lation of Warm Air Heating and Air Conditioning
Systems (NFPA No. 90B).
If outside air is utilized as return air to the furnace
for ventilation or to improve indoor air quality, the
system must be designed so that the return air
to the furnace is not less than 50°F (10°C) during
heating 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 airflow over the
indoor coil, the coil must be installed down-
stream (on the outlet side) 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 con-
densing on the heat exchanger. If a manually
operated damper is installed, it must be de-
signed so that operation of the furnace is pre-
vented when the damper is in the cooling posi-
tion and operation of the cooling system is
prevented when the damper is in the heating
position.
Products of combustion must not be allowed to enter the return air ductwork or
the circulating air supply. Failure to prevent products of combustion from being
circulated 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 confined spaces, all return ductwork must be adequately sealed
and joints must be taped. When return air is provided through the bottom of the
furnace, the joint between the furnace and the return air plenum must be sealed.
The floor or platform on which the furnace is mounted must provide sound physical
support of the furnace with no gaps, cracks, or sagging between the furnace and
the floor or platform.
Return air and circulating air ductwork must not be connected to any other heat
producing device such as a fireplace insert, stove, etc.
8
Return Air
The return air ductwork may be connected to
any or all of the following: left side return, right
side return, or bottom return. Where maximum
airflow is 1800 CFM or more two openings
must be used.
Determining Nominal AC System
Capacity
In order to select the appropriate airflow for AC
operation the nominal system capacity must be
known. The nominal system capacity is
Upflow Models
Downflow Models
Figure 4. Protective Screen for
WARNING:
!
Protective Screen
One Pipe Installations
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