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Troubleshooting
Table 12. Troubleshooting gas heat sections
Symptom
Probable Cause
Loss of power, fuse blown, or
circuit breaker tripped
Manual gas valve closed
Thermostat not calling for heat
Safety switch tripped
Unit locked out on flame failure;
Unit not running, no
"Alarm" LED illuminated on flame-
heat in building.
relay panel.
Combustion airflow switch
contacts may be open
Induced draft exhauster fan, if so
equipped, airflow switch contacts
may be open.
Unit only runs when
Combustion airflow switch
service access door on
contacts may be open.
piping vestibule is open
Flame sensor not sensing pilot
flame
Unit will not run, locks
out on flame failure;
"Alarm" LED
illuminated on flame
relay panel.
Low flame signal voltage
CO is above 400 ppm. Gas pressure not adjust properly. Adjust gas pressure. See
34
Recommended Action
• Check the fuse/breaker panel and reset or replace the device.
• Check the fuses if provided, in gas heat control panel and replace as needed.
Open the gas valve. There may be more than one manual gas valve in the piping system.
Be sure to check all valves.
• Repair or replace the thermostat, if defective.
• Set thermostat to heating mode.
Verify that one of the following safety switches has tripped:
• High Limit. This switch trips due to a high temperature condition in the gas heat
section. Allow the air handler fan to run and cool heat exchanger. The switch must be
manually reset when the heat exchanger cools.
• Airflow Interlock. This switch trips when there is no or very low airflow over the heat
exchanger. Repair the air handler fan, if necessary. Also, check to see if the unit is
running with a variable-frequency drive. The minimum speed may be set too low.
• High Pressure Gas. This switch trips if there is excessive gas pressure in the gas piping.
Adjust the gas pressure regulator or contact the local gas utility.
• Low Pressure Gas. This switch trips when there is very low gas pressure in the gas
piping. Adjust the gas pressure regulator or contact the local gas utility.
Press the Reset button to reset the flame relay and attempt to restart the gas heat
module/section. If the unit continues to trip-out, see the troubleshooting symptom
below.
• The combustion air motor may be overloaded. Press the Reset button on back side
of the combustion air blower motor.
• The combustion air blower motor may be defective. Replace the motor.
• The modulating gas valve end switch contacts may be open, indicating that the gas
valve is not fully closed. Look for jammed linkages and repair as necessary.
• Replace the modulating gas valve and actuator.
• Check airflow switch sensor tubes for clogging or obstruction. Remove debris to clear
tubes.
• Check exhaust flue for obstructions. Clear obstructions.
• Check inlet air damper for obstructions. Clear obstructions.
• Check airflow switch sensor tubes for clogging or obstruction. Remove debris to clear
tubes.
• Check exhaust flue for obstructions. Clear obstructions.
• Check if air balancing damper is closed. Open damper.
• Check 3-phase wiring to unit for correct phasing. Motor may be running backwards.
Check combustion air inlet (into piping vestibule) for obstructions blocking entrance of
combustion air. Remove obstructions or debris.
• The manual gas valve may be closed; open it.
• (3:1 and 10:1 turndown burners) UV flame sensor may be dirty; check the lens for
dirt, soot, and so on, and clean the lens, if necessary.
• Check the ignition cable and wiring for loose, frayed connections or broken wiring.
Repair as necessary.
• Pilot solenoid valve may not be opening.
• Check for voltage at the pilot solenoid valve.
• Check for sufficient gas pressure in the pilot line and adjust the pilot pressure
regulator.
• (20:1 Turndown Burners) Check flame rod for cleanliness, adequate grounding and
that it is properly located in the flame. Also ensure that temperature at flame rod
insulator is no greater than 500 F (260 C).
• Perform Ignition Interference Test per component manufacturers manual.
• Check flame signal voltage per procedure in Start-Up section.
• (3:1 and 10:1 turndown burners) UV flame sensor may be dirty; check lens for dirt,
soot, and so on and clean the lens, if necessary.
• The flame sensor may be defective; replace it.
• (20:1 turndown burners) Check flame rod for cleanliness, adequate grounding and
that it is properly located in the flame. Also ensure that temperature at flame rod
insulator is no greater than 500 F (260 C).
"Proper Gas Pressure" on page
19.
CLCH-SVX08A-EN
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