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Duct Work and Air Conditioning
Design and installation of the duct system should follow the current guidelines of the Air
Conditioning Contractors of America (ACCA) or the American Society of Heating,
Refrigeration and Air Conditioning Engineers, Inc. (ASHRAE). Refer to the Residential
Duct Systems, Manual D, from the ACCA, and the ASHRAE Handbook Fundamentals
volume, from ASHRAE, for recommended practices in duct system design and
installation. To obtain copies of these publications for a fee, contact the ACCA and the
ASHRAE at the addresses given in Appendix A of this manual.
All furnaces are tested over a range of external static pressure that simulates the airflow
resistance of the ductwork, fittings, and diffusers connected to the furnace for a typical
(average) duct system. The furnace blower and blower motor have been selected to
work successfully against the following range of duct system resistance.
Recommended range of duct system resistance for all models: 0.2 to 0.5 in. W.G.
external static pressure.
Due to the need to maintain an adequate supply of combustion and ventilation air, the
furnace shall not be installed in small room without return air duct system. A duct the full
size of the furnace return air opening shall extend to a location outside the furnace
room.
If the furnace is used in connection with summer air conditioning (cooling), the furnace
should be installed in parallel with, or on the upstream side of, the evaporator coil to
avoid water vapor condensation in the furnace heat exchanger. If the cooling unit is
installed in a parallel flow arrangement, dampers (or other means used to control
airflow) should be provided to prevent chilled air from entering the furnace. If such a
damper is manually operated, it must be equipped with a means to prevent operation of
either unit, unless the damper is placed in either the full heat or full cool position.
NOTICE: Return air grilles and supply registers in the air distribution system
should never be obstructed.
Air Filter Mounting
c CAUTION: If cutting openings in the furnace casing is required, DO NOT cut
any supporting clips or angles. Cut as close to the scribed line as possible, but
DO NOT cut the base clip or blower pan support angle.
Lowboy furnaces are factory-equipped with permanent-type, air filters located above the
blower compartment, in the rear of the furnaces.
NOTICE: Horizontal / counterflow furnaces are not factory-equipped with an air
filter or filter rack. Furthermore, the furnace blower compartment does not contain
enough free space to permit an air filter to be mounted within the furnace casing.
However, the installer should supply, or fabricate, a filter rack and mount an air filter in
the return air plenum above, or upstream of, the furnace blower compartment.