Carrier 48NLT - Manual (Page 21)

Table 11 — Heating Troubleshooting Chart
SYMPTOM CAUSE REMEDY
IGN locked out. LED flashing Look for problems external to the ignitor module.
LED glowing continuously Replace IGN control.
Pilot will not light. No spark at electrode Check air gap between electrode tip and pilot target
Gap should be as shown in Fig. 15.
Readjust as necessary.
Clean moisture or dirt accumulation on electrode ceramic with
cloth.
Cracked ceramic — replace pilot electrode assembly
Check for loose or broken wiring at and between electronic
control head and electrode Replace wire or tighten connection
as necessary.
Check fuses or circuit breaker to ensure voltage to unit.
Check for 24 v between TH and COM. If you read 24 v and
above steps have been completed, replace electronic ignition
control.
Spark shorting out to main burner Realign electrode tip away from main burner but maintain
spark gap to pilot burner. See Fig. 15.
No gas at pilot burner Clean pilot orifice.
Check inlet pressure to gas value. Recommended operating
pressure 7-in. wg natural gas, 11-in. wg LP gas; 0.5 psig (13.6-
in. wg) max. pressure
Check for 24 v between terminals PV and COM If you read 24
V and above steps have been completed, replace gas valve.
Burners will not ignite. Water in gas line Drain — install water trap.
No power to furnace Check power supply, fuses, wiring or circuit breaker.
No 24-v power supply to control circuit Check transformer — replace if necessary.
Miswired or ioose connections Check all wiring and wirenut connections.
Dirty pilot — yellow flame Clean pilot orifice.
Pilot burning improperly — sharp blue flame Replace pilot.
Burned-out heat anticipator in thermostat Replace thermostat.
No gas at main burners 1. Check for 24 v between terminals MV and COM on control
head If you read 24 v, replace gas valve portion of control
head/gas valve assembly.
2. If 24 V is not present, check flame sensor for cracked ce­
ramic insulator or shorted sensor cable.
Broken thermostat wire Run continuity check to locate break.
Inadequate heating. Dirty air filter Clean or replace filter as necessary.
Gas input to furnace too low Check gas pressure at manifold. Clock gas meter for input. If
loo low, increase manifold pressure, or replace with correct
orifices.
Unit undersized tor application Replace with proper unit — or add additional unit
Restricted airflow Clean or replace filter — or remove any restriction.
Blower speed too low Use faster speed tap
Check temperature rise.
Limit switch cycles main burners Dirty air filters — clean or replace.
Registers closed, restricted ductwork — open or remove
restriction.
Check temperature rise.
Check heat anticipator setting on thermostat — readjust.
Poor flame
characteristics. Incomplete combustion results in;
Aldehyde odors, CO, sooting flame —
floating flame
Check all screws around flue outlets and burner compartment
— tighten.
LACK OF COMBUSTION AIR.
Cracked heat exchanger — replace.
Overfired furnace — reduce input, or change orifices.
Check vent for restriction — clean as required
Check orifice for burner alignment.
#
#
GR — Ground
LP — Liquid Propane
21