61
A red LED is provided to indicate the function of the board.
See the table below.
Thermistor Troubleshooting —
The electronic con-
trol uses thermistors to sense temperatures used to control
operation of the unit. Resistances at various temperatures are
listed in Tables 50 and 51. Thermistor pin connection points are
shown in the Major System Components section. The general
locations of the thermistors are shown the Major System
Components section.
AIR TEMPERATURES — Air temperatures are measured
with 10 kilo-ohm thermistors. This includes supply-air temper-
ature (SAT), outdoor-air temperature (OAT), space temperature
sensors (T55, T56, T58), and return air temperature (RAT).
The supply air temperature (SAT) and outdoor air tempera-
ture (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 release for removal from the
panel. See Fig. 13. To reinstall, make sure the snap-mount tabs
extend out.
REFRIGERANT TEMPERATURES — Condenser coil tem-
peratures are measured with 5 kilo-ohm thermistors. These
measurements provide an approximate saturated condensing
temperature for each circuit (SCT.A, SCT.B). Ensure that ther-
mistors are placed at the correct location and are snapped se-
curely over the return bend so that contact is made between the
thermistor and the tube.
THERMISTOR/TEMPERATURE SENSOR CHECK — A
high quality digital volt-ohmmeter is required to perform this
check.
1. Connect the digital voltmeter across the appropriate ther-
mistor terminals at the J8 terminal strip on the Main Base
Board (see Major System Components section).
2. Using the voltage reading obtained, read the sensor tem-
perature from Tables 50 and 51.
3. To check thermistor accuracy, measure temperature at
probe location with an accurate thermocouple-type
temperature-measuring instrument. Insulate thermocou-
ple to avoid ambient temperatures from influencing
reading. Temperature measured by thermocouple and
temperature determined from thermistor voltage reading
should be close, within 5° F if care was taken in applying
thermocouple and taking readings.
Table 47 — Gas Heating Service Analysis
LED STATUS
FUNCTION
On Continuously
Relay contact closed (normal operation).
Blinking
Relay contact open (phase loss or phase
reversal has occurred) — No power will be
supplied to the control system.
Off
24 vac control power not present (off).
PROBLEM CAUSE
REMEDY
Burners Will Not Ignite.
Active alarm.
Check active alarms using ComfortLink™ Scrolling Marquee.
No power to unit.
Check power supply, fuses, wiring, and circuit breakers.
No power to IGC (Integrated Gas Control).
Check fuses and plugs.
Heaters off due to time guard to prevent short cycling. Check using ComfortLink Scrolling Marquee.
Thermostat or occupancy schedule set point not
calling for Cooling.
Check using ComfortLink Scrolling Marquee.
No gas at main burners.
Check gas line for air and purge as necessary. After purging gas
line of air, allow gas to dissipate for at least 5 minutes before
attempting to re-light unit.
Water in gas line.
Drain water and install drip.
Inadequate Heating.
Dirty air filters.
Replace air filters.
Gas input too low.
Check gas pressure at manifold. Refer to gas valve adjustment on
page 90.
Thermostat or occupancy schedule set point only
calling for W1.
Allow time for W2 to energize.
Unit undersized for load.
Decrease load or increase of size of unit.
Restricted airflow.
Remove restriction.
Too much outdoor air.
Check economizer position and configuration. Adjust minimum
position using ComfortLink Scrolling Marquee.
Limit switch cycles main burners.
Check rotation of blower, thermostat heat anticipator settings, and
temperature rise of unit. Adjust as needed.
Poor Flame
Characteristics.
Incomplete combustion (lack of combustion air)
results in: Aldehyde odors, CO, sooting flame, or
floating flame.
Check all screws around flue outlets and burner compartment.
Tighten as necessary.
Cracked heat exchanger, replace.
Unit is over-fired, reduce input. Adjust gas line or manifold
pressure.
Check vent for restriction. Clean as necessary.
Check orifice to burner alignment.
Burners Will Not Turn Off.
Unit is in Minimum on-time.
Check using ComfortLink Scrolling Marquee.
Unit running in Service Test mode.
Check using ComfortLink Scrolling Marquee.
Main gas valve stuck.
Turn off gas supply and unit power. Replace gas valve.
Summary of Contents for 48PG24
Page 68: ...68 Fig 14 Low Voltage Control Schematic 48PG20 28 Units without Humidi MiZer System a48 7627...
Page 69: ...69 Fig 15 Low Voltage Control Schematic 48PG20 28 Units with Humidi MiZer System a48 8227...
Page 70: ...70 Fig 16 Low Voltage Control Schematic 50PG20 28 Units without Humidi MiZer System a48 7778...
Page 71: ...71 Fig 17 Low Voltage Control Schematic 50PG20 28 Units with Humidi MiZer System a50 8101...
Page 72: ...72 Fig 18 Power Schematic Units without Humidi MiZer System a48 7628...
Page 73: ...73 Fig 19 Power Schematic Units with Humidi MiZer System a48 8228...
Page 74: ...74 Fig 20 Component Arrangement Units without Humidi MiZer System a48 8229...
Page 75: ...75 Fig 21 Component Arrangement Units with Humidi MiZer System a48 8230...
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