American Standard *UD Series Operation Manual page 13

White-Rodgers
50A50-405/406 Prior To Revision E4
If the condensate drain is blocked on condensing
(90%) furnace models, either by debris, improper
draining, or by freezing condensate, the pressure
switch contacts will open. When the Control senses
the pressure switch contacts are open, the gas valve
will be de-energized to remove the flame and the
system shutdown sequence initiated. The Control will
begin flashing the diagnostic LED 3 times to indicate
a pressure switch problem. The system will remain
shutdown until the condensate drain has been cleared
and the condensate flows freely. When the pressure
switch contacts close, a normal ignition sequence will
begin and the 3 flash fault will return to a steady fast
flash to indicate a normal call for heat.
During a normal heating cycle, if the 50A50-405/406
Rev. E4-E6/471/472/473/474/571 Control senses the
pressure switch contacts are closed and then open the
gas valve will be de-energized to remove the flame, if
the pressure switch does not reclose in 60 seconds the
induced draft motor will be de-energized for 30 seconds.
After the 30 second delay, a normal ignition sequence
will begin. On condensing (90%) models, the delay
allows any condensate, that may be blocking the
pressure switch sensing tube, time to drain. On 80%
models, the delay provides another attempt for the
induced draft motor to reach maximum speed and
close the pressure switch contacts.
Pressure Switch Problem
50A51-405 Two Stage and 50A51-505 Two Stage
Variable-Speed Systems
On systems using 50A51-405 two-stage and 50A51-505
two-stage variable-speed Controls, the first stage
pressure switch fault diagnostic operation is the same
as the 50A50-405/-406 Controls. (Prior to Revision E4.)
During a normal 1st stage heating cycle, when the
thermostat calls for 2nd stage heat, there will be a
30 second delay between 1st and 2nd stage heat. If
the thermostat calls for 1st and 2nd stage heat at the
same time, there will be a 10 minute delay between
1st and 2nd stage heat.
During the 1st stage ignition sequence, the induced
draft motor will always be energized to high speed. When
the Control senses the 1st stage pressure switch closed,
the induced draft motor is energized to low speed.
1 See Integrated Furnace Control Label or Timing Chart.
50A50/50A51 Fault Diagnostics
If the 1st stage pressure switch closes and then opens
during the ignitor warm-up period, the Control will
begin flashing the diagnostic LED 3 times to indicate
a pressure switch problem and wait 10 seconds for
the pressure switch contacts to close. If the 1st stage
pressure switch contacts close, a normal ignition
sequence will resume and the 3 flash fault will return
to a steady fast flash to indicate a normal call for heat.
If the 1st stage pressure switch contacts do not close in
10 seconds, the induced draft motor is energized to high
speed in an attempt to close the contacts. When the
contacts close, a normal ignition sequence will
resume and the induced draft motor will remain
energized on high speed until the flame is sensed for
a minimum of 10 seconds. The induced draft motor
will then be energized to low speed and the 3 flash
fault will return to steady fast flash to indicate a normal
call for heat. If the 1st stage pressure switch contacts
do not remain closed and the ignition sequence has
been recycled for the 4th time, the Control will go into
system lockout and begin flashing the diagnostic
LED 2 times to indicate a system lockout due to loss
of flame.
During a 2nd stage call for heat, if the 2nd stage
pressure switch contacts do not close within the
30 second delay between stages, the Control will
de-energize the gas valve to remove the flame and
begin flashing the diagnostic LED 3 times to indicate
a pressure switch problem. After the Control senses
the loss of flame, a 3 minute error timer will be
started and the shutdown sequence initiated as if
the call for heat were removed. The induced draft
motor will remain energized on high speed for a
5 second postpurge and the (selected) indoor blower
"off" delay timer will begin.
After the 3 minute error time delay, the Control will
restart the heating cycle if the thermostat is still
calling for 1st and 2nd stage heat. If the 2nd stage
pressure switch contacts still have not closed after the
10 minute delay between stages, the Control will
repeat the above shutdown sequence as long as the
thermostat calls for 2nd stage heat.
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