Installation with Return Ducts
A return air duct system is recommended. If the furnace is
installed in a confined space or closet, a return connection must
be run, full size, to a location outside the closet. The air duct in
the closet must be tight to avoid any entrance of air from the
closet into the circulating air.
Installation with an Evaporator Coil
IMPORTANT: When an air conditioning unit is used in
conjunction with the furnace, the evaporator coil must be
installed in the discharge (supply) air. Do not install an evaporator
coil in the return air; excessive condensation will occur within the
Installation without an Evaporator Coil
If a cooling coil is not installed with the furnace, then a removable
access panel should be provided in the supply plenum for
purposes of inspecting the heat exchanger. This opening must be
accessible when the furnace is installed. It must be large enough
that the heat exchanger can be viewed for possible openings
using light assistance or so that a probe can be inserted for
sampling the airstream. The cover for the opening must be leak
Never allow the products of combustion, including carbon
monoxide, to enter the return ductwork or circulation air
Circulating Air and Filters
Duct systems and register sizes must be properly designed for
the CFM and external static pressure rating of the furnace.
Ductwork should be designed in accordance with the
recommended methods of "Air Conditioning Contractors of
America" Manual D.
A duct system must be installed in accordance with Standards of
the National Board of Fire Underwriters for the Installation of Air
Conditioning, Warm Air Heating and Ventilating Systems.
Pamphlets No. 90A and 90B.
A closed return duct system must be used, with the return duct
connected to the furnace.
NOTE: Ductwork must never be attached to the back of the
For installations requiring more than 1,800 CFM, use a bottom
return or 2-sided return. Supply and return connections to the
furnace may be made with flexible joints to reduce noise
transmission. To prevent the blower from interfering with
combustion air or draft when a central return is used, a
connecting duct must be installed between the unit and the utility
room wall. A room, closet or alcove must not be used as a return
When the furnace is used in connection with a cooling unit, the
furnace should be installed in parallel with or on the upstream
side of the cooling unit to avoid condensation in the heating
element. With a parallel flow arrangement, the dampers or other
means used to control the flow of air must be adequate to
prevent chilled air from entering the furnace and, if manually
operated, must be equipped with means to prevent operation of
either unit unless the damper is in the full heat or cool position.
When the furnace is installed without a cooling coil, it is
recommended that a removable access panel be provided in the
outlet air duct. This opening shall be accessible when the furnace
is installed and shall be of such a size that the heat exchanger
can be viewed for a visual light inspection or enough space so
that a sampling probe can be inserted into the airstream. The
access panel must be made to prevent air leaks when the
furnace is in operation.
When the furnace is heating, the temperature of the return air
entering the furnace must be between 55°F and 100°F (13ºC and
When a furnace is installed so that supply ducts carry air
circulated by the furnace to areas outside the space containing
the furnace, the return air shall also be handled by a duct sealed
to the furnace casing and terminating outside the space
containing the furnace.
Filters—Read This Section Before Installing the Return
Filters must be used with this furnace. Discuss filter maintenance
with the building owner. Filters do not ship with this furnace, but
must be provided by the installer. Filters must comply with UL900
or CAN/ULCS111 standards. If the furnace is installed without
filters, the warranty will be voided.
NOTE: An undersized opening will cause reduced airflow. The
bottom return is set up as a knock out.
Depending on the installation and/or customer preference,
differing filter arrangements can be applied. Filters can be
installed in the central return register, or a side panel external
filter rack kit (upflows) or the ductwork above a downflow
furnace. As an alternative, a media air filter or electronic air
cleaner can be used as the primary filter.
Circulation Air Filters
One of the most common causes of a problem in a forced air
heating system is a blocked or dirty filter. Circulating air filters
must be inspected monthly for dirt accumulation and replaced if
necessary. Failure to maintain clean filters can cause premature
heat exchanger failure.
A new home may require more frequent replacement until all
construction dust and dirt is removed. Circulating air filters are to
be installed in the return air duct external to the furnace cabinet.
Minimum Filter Sizes for Disposable Filters
Disposable nominal 300 F.M. face velocity
Filter Size—sq in (cm