Two Stage 50A51-405 Sequence Of Operation - American Standard *UD Series Operation Manual

Two Stage 50A51 Sequence of Operation
White-Rodgers Integrated Furnace
Control 50A51-405/-495 Models
When the service disconnect 1 is in the "ON" position,
power is applied through the blower door interlock
switch 6 to the controls line voltage input terminals 8
and out of the control to the primary side of the
control transformer "XFMR" u. The secondary side
of the control transformer supplies 24 volts to the
control through terminal "TH" and "TR" po. Control
terminal "R" ; supplies 24 volts to the "R" terminal
of the room thermostat.
Once power is applied, the control flashes the LED
light "ON" for one second and performs a self check
routine. Following the normal system check, the
control flashes the LED one time per second (slow flash)
continuously while in stand-by.
On a call for heat, 24 volts is applied from the room
thermostats "W1" terminal to the "W1" terminal z
on the control. The control checks and confirms
normally closed contacts at the temperature cut out
"TCO" j, 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) l and normally open contacts at
the safety pressure switch #1 j. With all safety and
control switches in their proper position, the control
will energize the induced draft motor on high speed w
and flashes the LED two times per second continuously
(fast flash) during a call for heat.
When safety pressure switch "#1" j closes, the
control switches the induced draft motor to low speed w
and begins the ignition sequence. The hot surface
|ignitor w is energized for several seconds (see note)
allowing the thermal element to heat up. The control
then switches 24 volts to its "MVL" and "MV COM" g
terminals to terminals #1 g and #2 f on the gas
valve. The redundant and main solenoids are energized
allowing gas flow and main burner ignition. When
flame current is sensed by the control through its
"FP" ; terminal, the 45 second indoor blower motor
time delay "ON" begins. Flame failure response time
is set for 2 seconds. After flame has been established
for 10 seconds, the flame failure response time is
reset for 0.7 seconds. If flamed current is not sensed by
the control within the trial for ignition period (see note),
the main valve low and redundant gas valve
solenoids g, 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 for the second
attempt to establish main burner ignition. If again
flame current is not sensed by the control within the
trial for ignition period (see note), the control will repeat
the previous cycle before locking out. At the end of
the indoor blower motor delay "ON" timing, line
voltage is applied at control terminal "HEAT LO" 6
energizing the indoor blower motor at low heat fan
speed, supplying warm air to the space.
If the temperature in the space continues to fall, the
thermostat second stage contacts "W2" close.
24 volts is switched from thermostat terminal "W2" to
the "W2" terminal z on the control. A 30 second,
2nd stage recognition time delay begins. At the end of
the 30 second delay, the induced draft motor is
switched to high speed w causing pressure switch #2 d
to close. When pressure switch #2 closes, 24 volts is
switched from control terminal "MVH" d to the gas
valve terminal #3 d energizing the second stage
solenoid allowing increased gas flow to the burners.
At the same time, the indoor blower motor is
switched to high heat fan speed 7.
When second stage thermostat contacts "W2" satisfy,
the induced draft motor is switched back to low speed w
causing pressure switch #2 d to open breaking the
circuit to the second stage gas valve solenoid d. Gas
flow is reduced to the burners. The indoor blower
motor will switch back to low heat fan speed after a
30 second delay 6.
When first stage thermostat contacts "W1" satisfy,
the main valve low and the redundant gas valve
solenoids g are de-energized extinguishing main
burner flame. Once the control senses the loss of
flame current (0.7 sec.) ;, the induced draft motor w
is de-energized after a five second post-purge cycle.
The indoor blower motor "OFF" timing begins. At the
end of the indoor blower motor "OFF" timing, the
indoor blower motor is de-energized and the cycle
is complete.
Note: See Timing Chart for details or Integrated Furnace
Control Label.
16