035-17468-001 Rev. A (801)
32
Unitary Products Group
OPERATION AND MAINTENANCE
SEQUENCE OF OPERATION
The following describes the sequence of operation of the fur-
nace as it relates to a furnace equipped with a variable speed
blower. Refer to the schematic wiring diagram for component
location on Page 39 or 40.
HEATING OPERATIONAL SEQUENCE
TWO STAGE THERMOSTAT
The ignition control in this furnace is unique to this product.
The control allows for a 24 volt or a 115 volt circulator input/
output. This allows for the replacement of the variable speed
motor with a multi-speed PSC motor in the event a variable
speed motor is not available at the time of servicing.
In a typical system, a call for first stage heat is initiated by
closing the W1 thermostat contacts. The inducer blower is
energized at high speed and the control waits for the low
pressure switch contacts to close. The inducer blower is
energized at low speed and the 120 VAC silicon carbide
ignitor is powered.
At the end of the ignitor warm-up time, the first stage of the
two stage gas valve is energized (low fire). Flame must be
detected within 7 seconds. If flame is detected, the delay-to-
fan-on period begins. After the delay-to-fan-on period ends,
the circulator fan is energized at low heat speed and the air
cleaner and humidifier are energized.
A call for second stage heat (W1 and W2) after a call for first
stage heat will energize the inducer at high speed and ener-
gize the circulator at high heat speed. The circulator will ener-
gize at high heat speed after 30 seconds with a call for
second stage heat. If the furnace has been operating in first
stage heat for more than 30 seconds and the control receives
a call for second stage heat, the blower will immediately
switch to high speed. The second stage pressure switch con-
tacts will close and energize the second stage gas valve (high
fire).
If a call for second stage heat occurs at the same time as a
call for first stage heat, the second stage heat is energized
immediately after flame rectification is sensed on first stage.
When the second stage of the thermostat is satisfied, the
inducer motor is reduced to low speed and the 2nd stage gas
valve is de-energized to 1st stage. The circulator remains on
high heat speed for 30 seconds then de-energizes to low
heat speed.
When the first stage of the thermostat is satisfied, the gas
valve is de-energized and the heat delay to fan off begins tim-
ing. The inducer will postpurge for an additional 15 seconds,
then the inducer and humidifier will de-energize. Upon com-
pletion of the heat delay-to-fan-off, the circulator and air
cleaner are de-energized. Control timing, refer to (Refer to
Figure 41 on Page 33).
If flame is not detected during the trial for ignition period, the
gas valve is de-energized, the ignitor is turned off and the
ignition control goes into the “re-try” sequence.
The ”re-try” sequence provides a 60 second wait with inducer
interpurge following an unsuccessful ignition attempt (flame
not detected). After this wait, the ignition sequence is
restarted with an additional 10 seconds of ignitor warm-up
time. Two retries will be attempted before the control goes
into system lockout. If flame is established for more than 10
seconds after ignition, the ignition controller will clear the igni-
tion attempt (or retry) counter.
If flame is detected, then lost, the ignition control will repeat
the initial ignition sequence. After 5 attempts to sustain flame,
the control will go into system lockout.
During burner operation, a momentary loss of power of 50
milliseconds or longer will de-energize the main gas valve.
When power is restored, the gas valve will remain
de-energized and a restart of the ignition sequence will begin
immediately.
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 de-energize and the
control will restart the ignition sequence.
Recycles will begin and the burner will operate normally if the
gas supply returns, or the fault condition is corrected, before
the last ignition attempt. Otherwise, the control will go into
system lockout.
If the control has gone into system lockout, it may be possible
to reset the control by a momentary power interruption of one
second or longer. Refer to the FURNACE CONTROL DIAG-
NOSTICS on Page 37 of this instruction for reset information.