Phase Protection; Thermistor Troubleshooting - Carrier 48LC Series Controls, Start-Up, Operation And Troubleshooting

PROBLEM
Heat will not turn on
Inadequate heating
Heat will not turn off

Phase Protection

The phase loss protection option will monitor the three-phase elec-
trical system to provide phase reversal and phase loss protection.
PHASE REVERSAL PROTECTION
If the control senses an incorrect phase relationship, the relay
(K1) will be de-energized (opening its contact). If the phase re-
lationship is correct, the relay will be energized. The control
has a self-bypass function after a pre-set time. If the control de-
termines that the three phases stay in a correct relationship for
10 consecutive minutes, the relay will stay energized regard-
less of the phase sequence of three inputs as long as 24 VAC
control voltage is applied. This self-bypass function will be re-
set if all three phases are restored in a phase loss event.
PHASE LOSS PROTECTION
If the reverse rotation board senses any one of the three phase
inputs has no AC voltage, the relay will be de-energized (open-
ing its contact). This protection is always active as long as
24 VAC control voltage is applied, and is not affected by the
self bypass function of the phase sequence monitoring func-
tion. However, in the event of phase loss, the relay will be re-
energized only if all three phases are restored and the three
phases are in the correct sequence.
A red LED is provided to indicate the function of the board.
See the table below.
LED STATUS
On continuously Relay contact closed (normal operation).
Relay contact open (phase loss or phase reversal
Blinking has occurred). No power will be supplied to the
control system.
Off 24 VAC control power not present (off).
Table 23 — Electric Heat Service Analysis
CAUSE
Active alarm.
Unit is NOT configured for heat.
No power to unit.
Unit is in minimum heat off-time, or minimum
cool-heat changeover time.
Thermostat or occupancy schedule setpoint not
calling for heating.
Heat forced off in Service Test mode.
No 24 VAC at heater contactor.
Open temperature limit switch on heater.
Dirty air filters.
Thermostat or occupancy schedule setpoint only
calling for W1.
Heat undersized for load.
Restricted airflow.
Too much outdoor air.
Limit switch cycles heaters.
Bad heater elements.
Unit is in minimum heat on-time.
Thermostat or occupancy schedule setpoint still
calling for heating.
Heat forced on in Service Test mode.
Heater contactor failed.
FUNCTION
Check active alarms using SystemVu™ Display.
Check heating configurations using the SystemVu Display.
Check power supply, fuses, wiring, and circuit breakers.
Check using SystemVu Display.
Check using SystemVu Display.
Check using SystemVu Display. Turn Service Test mode off.
Check transformer and circuit breaker.
Check auto-reset limit switches on heater.
Check manual-reset limit switch (LS) on indoor fan housing.
Check minimum airflow. Check limit switch when it is cool, replace
if open.
Replace air filters.
Allow time for W2 to energize or adjust setpoints.
Decrease load or increase size of heater.
Remove restriction. Check SAT compared to the SAT heating
limits.
Check economizer position and configuration. Adjust minimum
position.
Check rotation of blower and minimum airflow.
Power off unit and remove high voltage wires. Check resistance of
element, replace if open.
Check using SystemVu Display.
Check using SystemVu Display.
Check using SystemVu Display. Turn Service Test mode off.
Power off unit. Check contactor and replace if closed.

Thermistor Troubleshooting

The SystemVu™ controller uses thermistors to sense tempera-
tures used to control operation of the unit. Resistances at vari-
ous temperatures are listed in Table 24. Thermistor pin connec-
tion points are shown in the Major System Components sec-
tion. The general locations of the thermistors are shown in the
Major System Components section.
AIR TEMPERATURES
Air temperatures are measured with 10K thermistors. This in-
cludes supply-air temperature (SAT), outdoor-air temperature
(OAT), space temperature sensors (T55, T56, T59), and return
air temperature (RAT).
The outdoor air temperature (OAT) thermistors use a snap-
mount to attach through the unit sheet metal panels. The snap-
mount tabs must be flattened on the tip end of the sensor to re-
lease for removal from the panel. (See Fig. 23.) To reinstall,
make sure the snap-mount tabs extend out.
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REMEDY