035-17438-001 Rev. A (901)
Unitary Products Group
19
When the first stage of the thermostat is satisfied, the gas
valve is deenergized 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 deenergize. Upon com-
pletion of the heat delay-to-fan-off, the circulator and air
cleaner are deenergized. For control timing, refer to Figure
15 on page 20.
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 warmup
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 deenergize the main gas valve.
When power is restored, the gas valve will remain deener-
gized and a restart of the ignition sequence will begin imme-
diately.
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.
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 24 of this instruction for reset informa-
tion.
Single Stage Thermostat
If a single stage thermostat is used the unit will be controlled
through the CFM Timer board timer adjust pins (refer to Fig-
ure 9 on page 12). For the furnace to operate on High Fire
the timer jumper must be placed on the 10, 15 or 20 minute
adjustment terminals. On a call for heat the unit will operate
on low fire as detailed above. If the heating requirements
cause the cycle to exceed the jumper settings, the high fire
timer will energize the second stage heat. The second stage
operation will function as detailed above. The heating cycle
will be completed in high fire operation. If the timer jumper is
in the “TIME OFF” position with a single-stage thermostat, the
unit will stay in low fire operation.
