Fenwal Series 35-6 Ignition Control
Fault Conditions and LED Key
LED steady on Internal control fault
1 Flash Combustion airflow fault
2 Flashes Flame with no call for heat
3 Flashes Ignition lockout
LED flashes on for ¼ second and off for ¼ second dur-
ing fault condition. Pause between fault codes is three
seconds.
Service Checks
Flame current is the current which passes through the
flame from the sensor to ground. The minimum flame
current necessary to keep the system from lockout is 0.7
microamps. To measure flame current, connect analog
DC microammeter to the FC− and FC+ terminals per Fig-
ure K14. Meter should read 0.7 uA or higher. If the meter
reads below 0 on scale, meter leads are reversed. Discon-
nect power and reconnect meter leads for proper polarity.
Security Device Operation
1. A combustion air pressure switch is provided as part
of the control system to verify airflow through draft
inducer by monitoring the difference in pressure
between the draft inducer and the atmosphere. If
sufficient negative pressure is not present, indicating
lack of proper air movement through furnace heat
exchanger, the switch opens shutting off gas supply
through the ignition control module. On furnaces with
two-speed draft inducer operation, a dual air pres-
sure switch is used, monitoring high and low speed
pressures. The air pressure switches have fixed set-
tings and are not adjustable.
2. The furnace is equipped with manual reset rollout
switch(es) in the event of burner flame rollout. The
switch will open on temperature rise and shut off gas
supply through the ignition control module. Flame
rollout can be caused by insufficient airflow for the
burner firing rate (high gas pressure), blockage of the
vent system or in the furnace heat exchanger. The
furnace should not be placed back in operation until
the cause of rollout condition is identified. The rollout
switch can be reset by pressing the button on top of
the switch.
3. The furnace is equipped with a fixed temperature
high limit switch mounted on the vestibule panel that
shuts off gas to the heater through the ignition con-
trol module in the event of reduced circulating airflow
over the heat exchanger. Reduced airflow can be
caused by motor failure of the circulating air blower,
dirt, blockage or restriction of the air inlet or outlet to
the unit. The high limit switch will automatically reset
when the temperature drops to 30°F [16.7°C] below
the setpoint. Determine the cause of the reduced
airflow and correct.
4. A circulating blower air proving pressure switch is
installed that breaks power to the burner circuit to
disable the furnace in the event of loss of circulating
airflow over the heat exchanger. Loss of airflow can
be caused by a motor failure, broken fan drive belt or
restriction of the air inlet or outlet to the unit. A loss
in circulating airflow, if not detected early, can cause
the furnace to cycle on high limit which can cause
overheating and damage to internal components.
5. Above the furnace intake on the inlet air side, an
auxiliary manual reset high limit switch is installed set
at 120°F [49°C] to prevent the furnace from cycling
and causing potential damage should the airflow
interlocks fail on auto-reset high limit. Under normal
operation, the supply fan is delayed on shut down to
allow any head buildup to dissipate.
Maintenance
Furnace Module Inspection
Turn off all electrical power to the unit before inspection
and servicing.
1. The furnace should be inspected annually by a quali-
fied service agency. The condition of the burners,
heat exchanger, draft inducer, vent system, operat-
ing controls and wiring should be determined. Check
WARNING
A secure and effective functioning gas burner requires
sufficient combustion gas exhaust discharge. Disabling a
security device such as a pressure sensitive switch on a
gas device is dangerous and can be fatal. This can also
prevent proper functioning of the device and will result in
the guarantee being void. Do not attempt to disable the
pressure switch to place the heater in operation. Contact
a qualified service agency.
WARNING
The circulating blower air proving switch is adjustable
and must be set properly to avoid damage due to fur-
nace cycling on high limit.
IMPORTANT
Should power to the unit be disrupted during furnace
operation this may cause the manual reset limit to open
and prevent furnace from operating automatically when
power is turned back on.
VCES-VHC-IOM-1F – VHC-36, -42 & -50
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