American Standard *UD Series Operation Manual page 14

White-Rodgers
Pressure Switch Problem
50A51-495/-506/-507 Two Stage Variable-Speed Systems
The induced draft motor will always be energized to
high speed. The induced draft motor will be energized
to low speed when the Control senses the 1st stage
pressure switch contacts have closed.
If the 1st stage pressure switch contacts open during
a normal 1st stage heating cycle, the gas valve will
be de-energized to remove the flame and the induced
draft motor will be energized to high speed for
10 minutes. If the 1st stage pressure switch contacts
close during the 10 minute high speed purge, the
Control will restart the ignition sequence. If the
1st stage pressure switch contacts remain closed after
10 minutes, the induced draft motor will be energized
to low speed and a normal heating operation will
continue. If the pressure switch contacts do not close
after 10 minutes, the induced draft motor will be
energized again to high speed for 10 minutes and the
diagnostic LED will begin flashing 3 times to indicate
a pressure switch problem. Recycling the induced
draft motor will continue as long as a 1st stage call
for heat exists and the Control will not go into lockout.
The 2nd stage pressure switch fault diagnostic
operation is the same as 50A51-405/-505 Controls.
White-Rodgers
COOL-H PARK PARK LINE-H
E17 E28 E27
HEAT-H XFMR-H
E16 E18 E14
K2 K3
K1
1
OFF ON
1
2
3
4
C4
E4
DS2
DS1
Y
CNT2789
50A50/50A51 Fault Diagnostics
50A55-571/-474 Self Diagnostic Features
E13
LINE-N
E4
E7
E8
E9
XFMR-N
E10
Diagnostic Indicator Flash Codes:
K5 CIR-N
E11
2 Flashes
3 Flashes
K6
4 Flashes
E1
5 Flashes
6 Flashes
7 Flashes
8 Flashes
FUSE 1
See Pages 14 and 15 for Sequence of Operation and
Wire Schematic.
W R G C
1 Fuse only on 474 Model and Dip Switches replace jumpers.
Thermal Protection Devices
At any time during a call for heat, if the Control senses
the high temperature limit (and/or aux. limit) or flame
roll out switch(es) are open, the gas valve is de-energized,
the induced draft motor is energized (low speed on
50A51 Controls), and the indoor blower is energized
to run on heat speed.
On models utilizing fusible link devices, the fusible
link is a single use device and must be replaced
if it has opened. However, if the temperature
the fusible link senses is approaching the trip
temperature but does not exceed it, the contacts may
open and close intermittently. The Control will initiate
another ignition sequence upon the closure of the
intermittent fusible link.
Flame Sensed With Gas Valve Off
At any time the Control senses a flame current signal
without a call for heat or when the gas valve is
de-energized, the induced draft motor is energized
(low speed on 50A51 Controls), and the indoor blower
is energized to run on heat speed.1 The Control will
go into system lockout and will not reset until this
fault is corrected. The diagnostic LED will begin
flashing 5 times to indicate the flame sensed with the
gas valve de-energized.
1 See Integrated Furnace Control Label or Timing Chart.
Slow Flash – Normal, No Call For Heat Present.
Fast Flash – Normal, Call For Heat Present.
Continuous On – Internal Control Failure.
– System Lockout (Retries or Recycles Exceeded).
– Pressure Switch Stuck Open or Closed.
– Open High Temperature Limit Switch.
– Flame Sensed Without Gas Valve.
– 115 Volt AC Power Reversed.
– Gas Valve Circuit Error.
– Low Flame Sense Signal.
13