Carrier 48TC series, Service And Maintenance Instructions, Page: 44 / 112

44
MINIMUM HEATING ENTERING AIR TEMPERATURE
When operating on first stage heating, the minimum temperature
of air entering the dimpled heat exchanger is 50°F (10°C) continu -
ous and 45°F (7°C) intermittent for standard heat exchangers and
40°F (4°C) continuous and 35°F (2°C) intermittent for stainless
steel heat exchangers. To operate at lower mixed-air temperatures,
a field-supplied outdoor-air thermostat must be used to initiate
both stages of heat when the temperature is below the minimum
required temperature to ensure full fire operation. Wire the out -
door-air thermostat OALT (P/N HH22AG106) in series with the
second stage gas valve. See Fig. 58. Set the outdoor-air thermostat
at 35°F (2°C) for stainless steel heat exchangers or 45°F (4°C) for
standard heat exchangers. This temperature setting will bring on
the second stage of heat whenever the ambient temperature is be -
low the thermostat setpoint. Indoor comfort can be compromised
when heating is initiated using low entering air temperatures with
insufficient heating temperature rise.
Fig. 58 — OATL Connections
Troubleshooting Heating System
Refer to Tables 18 and 19 for additional troubleshooting topics.
CTB
W2
Thermostat
TH1
TH2
W1 W1
W2
OALT
Table 18 — Heating Service Troubleshooting
PROBLEM CAUSE REMEDY
Burners Will Not Ignite
Misaligned spark electrodes. Check flame ignition and sensor electrode positioning. Adjust as needed.
No gas at main burners.
Check gas line for air, purge as necessary. After purging gas line of air, allow gas
to dissipate for at least 5 minutes before attempting to relight unit.
Check gas valve.
Water in gas line. Drain water and install drip leg to trap water.
No power to furnace. Check power supply, fuses, wiring, and circuit breaker.
No 24-v power supply to control
circuit.
Check transformer. Transformers with internal overcurrent protection require a
cool down period before resetting.
Miswired or loose connections. Check all wiring and wire nut connections.
Burned-out heat anticipator in
thermostat.
Replace thermostat.
Broken thermostat wires. Run continuity check. Replace wires, if necessary.
Inadequate Heating
Dirty air filter. Clean or replace filter as necessary.
Gas input to unit too low.
Check gas pressure at manifold. Clock gas meter for input. If too low, increase
manifold pressure, or replace with correct orifices.
Unit undersized for application. Replace with proper unit or add additional unit.
Restricted airflow. Clean filter, replace filter, or remove any restrictions.
Blower speed too low.
Use high speed tap, increase fan speed, or install optional blower, as suitable for
individual units.
Limit switch cycles main burners.
Check rotation of blower, thermostat heat anticipator settings, and temperature
rise of unit. Adjust as needed.
Too much outdoor air.
Adjust minimum position.
Check economizer operation.
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.
Overfired unit — reduce input, change orifices, or 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 locked into Heating mode for
a one minute minimum.
Wait until mandatory one-minute time period has elapsed or reset power to unit.