282911-YIM-B-0807
Unitary Products Group
13
SEQUENCE OF OPERATION
The unit is controlled by a conventional heating/cooling ther-
mostat common to this class of equipment.
HEATING
The control board begins a call for heat when W1 is ener-
gized (connected to R). The control ignores W2 until pilot
ignition has been established.
The control checks to see if the pressure switch is open. If the
pressure switch is closed, the control board flashes "3" on the
LED and waits indefinitely for it to open. When the pressure
switch is sensed as open, the control begins pressure switch
proving period. If the call for heat is lost, the control goes
back to Standby.
PRESSURE SWITCH PROVING
The control board energizes the induced draft motor (High
speed for 2 stage model) and waits for the low pressure
switch to close. When the low pressure switch closes, the
control begins Pre-purge period. If the call for heat is lost, the
control de-energizes the inducer without post-purge and
returns to standby.
If the low pressure switch does not close within 10 seconds of
inducer energizing, the control board flashes "2" on the LED.
If the pressure switch does not close within 5 minutes of
inducer energizing, the control shuts off the inducer for 30
seconds, then energizes the inducer for another 5 minute try
to close the pressure switch. This cycle continues indefinitely
until either the pressure switch is proved closed, or the call
for heat ends.
PRE-PURGE
The control board monitors the low pressure switch and
ensures it remains closed during pre-purge. If the pressure
switch opens, the control goes back to pressure switch prov-
ing mode. The control waits for a 15 second pre-purge
period, then begins the ignition trial
IGNITION TRIAL PERIOD
The control board energizes the pilot gas valve and spark
outputs for an 85 second Ignition trial. The control de-ener-
gizes the spark when flame is sensed and enters a flame sta-
bilization period.
If flame is not established within the ignition trial period, the
control de-energizes the spark and gas valve and begins an
inter-purge period before attempting another ignition trial.
If the call for heat is lost during an ignition trial period, the
control immediately de-energizes spark and gas. The control
runs the inducer motor through a post purge period before
de-energizing.
If the pressure switch is lost during an ignition trial period, the
control immediately de-energizes spark and gas. The control
begins pressure switch proving before an inter-purge and re-
ignition attempt.
PILOT FLAME STABILIZATION PERIOD
The control board de-energizes the spark output, and waits
for a 2 second flame stabilization period before energizing the
main gas valve.
If flame is lost during the flame stabilization period, the con-
trol board counts it as a flame loss and retries ignition or locks
out flashing a "5" on the LED.
HEAT BLOWER ON DELAY
The control board waits for 30 seconds and then energizes
the indoor blower heat speed. Blower on delay time begins at
the start of flame proving period in the trial for ignition.
If the thermostat demand for heat is removed, the control de-
energizes the gas valve, energizes the blower on heat speed
and initiates a post-purge and heat blower off delay.
MAIN BURNER OPERATION
High heat warm-up
Two stage models run high heat for the first 60 seconds fol-
lowing Pilot Flame Stabilization period regardless of W2
demand. If W2 is not energized at the end of this 60 second
period the control de energizes the high gas output and steps
the inducer to low speed. If W2 is energized the control
remains on high heat.
There is no high heat warm-up on single stage models
Low heat
The control board keeps the pilot gas valve, main gas valve
and induced draft motor energized while continuously moni-
toring the call for heat, low pressure switch, and flame status.
If the call for heat (W1) is lost, the control de-energizes the
gas valve and begins post purge.
If low pressure switch opens, the control de-energizes the
gas valve and begins pressure switch proving mode.
If flame is lost, the control de-energizes the gas valve within
2.0 second and counts the flame loss. If flame has been lost
more than 16 times within the same call for heat, the control
board locks out flashing "5" on the LED. If flame has been
lost less than 16 times, the control attempts re-ignition after a
300 second inter-purge period.
High heat
If the W2 terminal was energized more than 1 second before
W1 at the start of the call for heat, and remains continuously
energized through the call for heat, the control considers it to
be connected to a single stage thermostat and implements a
10 minute Auto staging feature. The 2nd stage thermostat
call is ignored until 10 minutes into steady heat (9 minutes
after high heat warm-up ended).
