Single Stage 50A50 Sequence Of Operation - American Standard *UD Series Operation Manual

Single Stage 50A50 and 50A55 Sequence of Operation
White-Rodgers Integrated Furnace
Controls 50A50-405/-406/-471/-472/
-473/-474/-571 and 50A55-571/-474
Models
When the disconnect 1 is in the "ON" position, power
is applied through the blower door interlock switch 6
to the control line voltage input terminal 8 and out of
the control to the primary side of the control transformer
"XFMR" 9. The low voltage side of the transformer
supplies 24 volts to the control through terminals
"TH" o and "TR" i. Control terminal "R" ; supplies 24
volts to the "R" terminal of the room thermostat.
Once power is applied, the control flashes the LED
"ON" for 1 second and performs a self check routine.
Following the normal system check, the control
flashes the LED light once per second continuously
(slow flash) while in stand-by.
On a call for heat, 24 volts is applied from the thermostat
terminal "W" to the "W" terminal ; on the control. The
control checks and confirms normally closed contacts at
the temperature cut out "TCO" f, the auxillary limit
(downflow and some upflow/horizontal models), the
flame roll-out fuse link (two fuse links are used on
downflow and upflow/horizontal models) f and j
and normally open contacts of the safety pressure
switch "PS" f. With all safety and control switches
in their proper position, the control will energize the
induced draft motor y and flash the LED light two times
per second continuously (fast flash) during a call for heat.
When the safety pressure switch "PS" f closes, the
control begins the ignition sequence. The hot surface
ignitor y is energized for several seconds (see note)
allowing the thermal element to heat up. The control
then switches on 24 volts to the gas valve "MV" termi-
nals #1 a. The redundant and main solenoids are
energized allowing gas flow and main burner ignition.
When flame current is detected by the control through
its terminal "FP" k, the 45 second indoor blower motor
delay on timing begins. Flame failure response time
is set for 2 seconds. After flame has been established
for ten seconds, the flame failure response time is
reset for 0.7 seconds. If flame current is not sensed by the
control k within the trial for ignition period (see note),
the redundant and main gas valve solenoids a are
de-energized. The control will begin a interpurge
cycle and adds additional seconds to the hot surface
ignitor warm-up timing (see note). The control
energizes the gas valve a for the second attempt to
establish main burner ignition. If flame current is not
sensed by the control on the 2nd retry within the trial
for ignition period (see note), the control will repeat the
previous cycle one additional time before locking out.
At the end of the indoor blower delay on time, line voltage
is applied at the controls terminal "HEAT" 5 energizing
the indoor blower motor at heating fan speed, supplying
warm air to the space.
When the thermostat is satisfied, the gas valve's redundant
and main solenoids a are de-energized, extinguishing
main burner flame. Once the control senses loss of
flame current (0.7 sec.) k, the induced draft motor t is
de-energized after a five second post purge cycle. The
indoor blower motor delay off timing begins. At the
completion of the fan delay off timing, the indoor
blower motor 5 is de-energized and the cycle
is complete.
Note: See Timing Chart for details or Integrated Furnace
Control Label.
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