Control Module
Neutral 120 VAC
Hot 120 VAC
{
Air Cleaner
Optional
Accessories
Humidifier
Figure 21
Accessory Operation
Accessory Operation: The furnace control module ener-
gizes the humidifier whenever the induced draft blower is
energized (when an air cleaner is installed on the system,
the humidifier is not energized until the air cleaner is
energized). The control module energizes the air cleaner
whenever the air circulation blower is energized.
VlI. Circulating Air and Filters
DUCTWORK - AIR FLOW
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 Contrac-
tors of America” Manual D.
A duct system must be installed in accordance with Stan-
dards of the National Board of Fire Underwriters for the
Installation of Air Conditioning, Warm Air Heating and
Ventilating Systems. Pamphlets No. 90A and 90B. Ductwork
must never be attached to the back of the furnace.
A closed return duct system must be used, with the return
duct connected to the furnace. Supply and return con-
nections 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 air chamber.
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 visual light inspection or
such that a sampling probe can be inserted into the air-
stream. The access panel must be made to prevent air
leaks when the furnace is in operation.
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