650.75-N4U
035-14370-000
34
Unitary Products Group
OPERATION AND MAINTENANCE
SEQUENCE OF OPERATION
The following describes the sequence of operation of the fur-
nace. Refer to the schematic wiring diagrams in the back of
this manual for component location.
CONTINUOUS BLOWER
On cooling/heating thermostats with fan switch, when the fan
switch is set in the ON position, a circuit is completed
between terminals R and G of the thermostat. The blower
motor is energized through the low heat terminal and runs on
the selected speed. This allows constant air circulation at
lower flow rate.
INTERMITTENT BLOWER - COOLING
On cooling/heating thermostats with fan switch, when the fan
switch is set in the auto position and the thermostat calls for
cooling, a circuit is completed between the R, Y and G termi-
nals.
The motor is energized through the cool fan terminal and
runs on the selected speed. The fan off setting is fixed at 60
seconds for SEER enhancement.
HEATING CYCLE
When the system switch is set on HEAT and the fan is set on
AUTO, and the room thermostat calls for heat, a circuit is
completed between terminals R and W of the thermostat.
When the proper amount of combustion air is being provided,
a pressure switch activates the ignition control
The ignition control provides a 17-second warm-up period.
The gas valve then opens for seven seconds.
As gas starts to flow and ignition occurs, the flame sensor
begins its sensing function. If a flame is detected within seven
seconds after ignition, normal furnace operation continues
until the thermostat circuit between R and W is opened. After
flame is present for 30 seconds, the circulating blower is
energized.
When the thermostat circuit opens, the ignition control is de-
energized. With the ignition control de-energized, the gas
flow stops and the burner flames are extinguished. The ven-
ter continues to operate for 15 seconds after the gas flow
stops.
The blower motor continues to operate for the amount of time
set by the fan-off delay switches located on the control board.
The heating cycle is then complete, and the unit is ready for
the start of the next heating cycle.
If flame is not detected within the seven second sensing
period, the gas valve is de-energized. The control is
equipped with a re-try option. This provides a 60 second wait
following an unsuccessful ignition attempt (flame not
detected).
After the 60 second wait, the ignition sequence is restarted
with an additional 10 seconds of igniter warm-up time. If this
ignition attempt is unsuccessful, one more re-try will be made
before lockout.
A momentary loss of gas supply, flame blowout, or a shorted
or open condition in the flame probe circuit will be sensed
within 0.8 seconds.
The gas valve will deenergize and the control will restart the
ignition sequence immediately. Recycles will begin and the
burner will operate normally if the gas supply returns, or the
fault condition is corrected prior to the last ignition attempt.
Otherwise, the control will lockout.
The control will repeat the ignition sequence for a total of two
recycles if flame is lost within the first 10 seconds of estab-
lishment.
If flame is established for more than 10 seconds after ignition,
the control will clear the ignition attempt (retry) counter. If
flame is lost after 10 seconds, it will restart the ignition
sequence. This can occur a maximum of five times.
During burner operation, a momentary loss of power of 50
milliseconds or longer will drop out the main gas valve. When
the power is restored, the gas valve will remain de-energize
and a restart of the ignition sequence will begin immediately
Hot Surface Ignition System
.
Do not attempt to light this furnace by hand (with a
match or any other means). There may be a
potential shock hazard from the components of the
hot surface ignition system. The furnace can only
be lit automatically by its hot surface ignition sys-
tem.
