Troubleshooting Heating System - Carrier 48FC M20 Series Service And Maintenance Instructions

MINIMUM HEATING ENTERING AIR TEMPERATURE
When operating on first stage heating, the minimum temperature
of air entering the dimpled heat exchanger is 50°F continuous
and 45°F intermittent for standard heat exchangers and 40°F
continuous and 35°F intermittent for stainless steel heat ex-
changers. 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 re-
quired 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 for stainless steel heat exchangers or 45°F for
standard heat exchangers. This temperature setting will bring on
the second stage of heat whenever the ambient temperature is
below the thermostat set point. Indoor comfort may be
compromised when heating is initiated using low entering air
temperatures with insufficient heating temperature rise.
PROBLEM
Misaligned spark electrodes.
No gas at main burners.
Water in gas line.
Burners Will Not Ignite.
No power to furnace.
No 24-v power supply to control circuit.
Miswired or loose connections.
Burned-out heat anticipator in thermostat.
Broken thermostat wires.
Dirty air filter.
Gas input to unit too low.
Unit undersized for application.
Restricted airflow.
Inadequate Heating.
Blower speed too low.
Limit switch cycles main burners.
Too much outdoor air.
Incomplete combustion (lack of combustion
Poor Flame
air) results in: Aldehyde odors, CO, sooting
Characteristics.
flame, or floating flame.
Unit is locked into Heating mode for a one
Burners Will Not Turn Off.
minute minimum.
Table 10 — Heating Service Troubleshooting
CAUSE
Check flame ignition and sensor electrode positioning. Adjust as needed.
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.
Drain water and install drip leg to trap water.
Check power supply, fuses, wiring, and circuit breaker.
Check transformer. Transformers with internal overcurrent protection
require a cool down period before resetting.
Check all wiring and wire nut connections.
Replace thermostat.
Clean or replace filter as necessary.
Check gas pressure at manifold. Clock gas meter for input. If too low,
increase manifold pressure, or replace with correct orifices.
Replace with proper unit or add additional unit.
Clean filter, replace filter, or remove any restrictions.
Use high speed tap, increase fan speed, or install optional blower, as
suitable for individual units.
Check thermostat heat anticipator settings, and temperature rise of unit.
Adjust as needed.
Adjust minimum position.
Check economizer operation.
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.
Wait until mandatory one-minute time period has elapsed or reset power
to unit.
44
Thermostat
TH1
W1
TH2
W2
Fig. 58 — OATL Connections

Troubleshooting Heating System

Refer to Tables 10 and 11 for additional troubleshooting topics.
Run continuity check. Replace wires, if necessary.
OALT
REMEDY
UCB
W1
W2