Specifications

Page 17
The ignition control is located in the heating control box. It is
connected to the unit using jacks J58 and plugs P58. On a
heating demand, the ignition control is energized after com
bustion air blower prove switch (S18) closes. The ignition
control then allows 30 to 40 seconds for the combustion air
blower to vent exhaust gases from the burners. At the end of
the delay the ignition control activates the first stage operator
of the gas valve (low fire), the spark electrode, the flame sens
ing electrode and blower relay. Sparking stops after flame is
sensed. The combustion air blower continues to operate
throughout the heating demand. If the flame fails or if the
burners do not ignite, the ignition control will attempt to ignite
the burners up to two more times. If ignition cannot be ob
tained after the third attempt the control will lock out. The igni
tion control is not adjustable.
Electronic direct spark ignition with flame rectification sensing
is used on all GCS24 units. Flame signal strength ranges
from 8 to 20 microamps. The ignition control is manufactured
by Fenwal.
FIGURE 14
SPARK
HIGH VOLTAGE
POWER (24VAC INPUT)
TO GAS VALVE
(24VAC OUTPUT)
TO FLAME SENSOR
GROUND
TOP VIEW
FENWAL IGNITION CONTROL
The Fenwal control is illustrated in figure 14. The fourwire
harness, plugged directly into the jack at the lower corner of
the control, is used to connect the control to unit. Each of the
four jack terminals is identified by function. The spark elec
trode wire connects to the spark plugtype connector on top of
the control.
WARNING
SHOCK HAZARD. SPARK RELATED COM
PONENTS CONTAIN HIGH VOLTAGE WHICH CAN
CAUSE PERSONAL INJURY OR DEATH. DISCON
NECT POWER BEFORE SERVICING. CONTROL IS
NOT FIELD REPAIRABLE. UNSAFE OPERATION
WILL RESULT. IF THE CONTROL IS INOPERABLE,
SIMPLY REPLACE THE ENTIRE CONTROL.
The ignition control provides three main functions: gas valve
control, ignition and flame sensing. The ignition attempt se
quence provides three trials for ignition before locking out.
The unit will usually ignite on the first attempt. See figure 15 for
a normal ignition sequence with nominal timings for simplicity
Proper gas/air mixture is required for ignition on the first at
tempt. If there is any deviation, within tolerance of the unit, a
second or third trial may be necessary for ignition. The control
will lock out the heating system if ignition is not obtained within
three trials and the (indoor) blower will not start. Reset after
lockout requires only breaking and remaking thermostat de
mand. See figure 16 for the ignition attempt sequence with
retrials (minimal timings given for simplicity). Loss of flame
during a heating cycle is indicated by an absence of flame sig
nal (0 microamps). If this happens, the control will immediate
ly restart the ignition sequence and then lock out if ignition is
not gained after the third trial.
Specific timings for the Fenwal control are shown in figure 17.
FIGURE 15
NORMAL IGNITION SEQUENCE
TIMINGS NOMINAL
THERMOSTAT DEMAND
COMBUSTION AIR BLOWER
GAS VALVE
IGNITION SPARK
BLOWER
IGNITION TRIAL
SECONDS 0 30 35 0 110
ON / CLOSED
OFF / OPEN
END OF
DEMAND
1
COMBUSTION AIR PROVE SWITCH
5 570
FIGURE 16
RETRIALS - IGNITION ATTEMPT SEQUENCE - TIMINGS NOMINAL
THERMOSTAT DEMAND
COMBUSTION AIR BLOWER
GAS VALVE
IGNITION SPARK
BLOWER
IGNITION TRIAL
SECONDS 0 30 35 70
ON
OFF
65 100 105
RESET AT THERMOSTAT
BY BREAKING DEMAND
FENWAL LOCKOUT
COMBUSTION AIR PROVE SWITCH
5