43
STS
-
JZ
-
F / IQ
-
F I&O Manual Z83.20
-
2016
IM101220
RD: APR 2019
RL: 20C
22. FENWAL DSI:
SEQUENCE OF OPERATION / FLAME RECOVERY / SAFETY LOCKOUT
Power Up / Stand By
Upon applying 24 volts power to 24VAC, the control will reset, perform a self check routine, initi-
ate full time flame sensing, flash the diagnostic LED for up to four seconds, and enter the ther-
mostat scan state.
Heat Mode
When a call for heat is received from the thermostat supplying 24 volts to TH, the control checks
the pressure switch for normally open contacts. The combustion blower is then energized and
once the pressure switch contacts close, a 30 second purge delay begins. Following the purge
period the gas valve is energized and spark commences for the 15 second trial for ignition.
When flame is detected during the trial for ignition, spark is shutoff immediately and the gas
valve combustion blower remains energized. The thermostat, pressure switch, and main burner
flame are constantly monitored to assure the system continues to operate properly. When the
thermostat is satisfied and the demand for heat ends, the main valve is de
-
energized immed-
iately, the control senses the loss of flame signal and initiates a 30 second post
-
purge period be-
fore de
-
energizing the combustion blower.
Failure to Light
-
Lockout
(
THREE TRIAL MODEL
)
This three
-
try control will attempt two additional ignition trials with a 30 second inter
-
purge be-
tween trials, before going into
‘
soft
’
lockout. The valve relay will be de
-
energized immediately,
and the combustion blower will be turned off following the 30 second post purge period.
If the thermostat continues to call for heat after one hour the control will automatically reset and
attempt to ignite the burner again (three trials).
At any time less than the 1 hour auto
-
reset, recovery from lockout requires a manual reset by
either resetting the thermostat or removing 24 volts for a period of 5 seconds.
Flame Failure
-
Re
-
Ignition
If the established flame signal is lost while the burner is operating, the control will respond within
0.8 seconds. The HV spark will be energized for a trial ignition period in an attempt to relight the
burner. If the burner does not light the control will de
-
energize the gas valve. Two more attempts
will be made to relight the burner. If the burner does not relight the control will go into
‘
soft
’
lock-
out as noted above in
“
Failure to Light
”.
If flame is re
-
established, normal operation resumes.
Combustion Airflow Problems
-
Lockout
Combustion air flow is continually monitored during an ignition sequence by the air flow switch
(PSW). If during the initial call for heat the pressure contacts are in the closed position for 30
seconds without an output to the Combustion Blower, an air flow fault will be declared and the
control will remain in this mode with the combustion blower off.
If the air flow switch remains open for more than 30 seconds after the combustion blower out-
put (L1 & IND) is energized, an air flow fault will be declared and the control will stay in this
mode with the combustion blower on, waiting for the air flow switch to close.
When proper air flow is detected from the air flow switch input (PSW) the control begins the
pre
-
purge period followed with a 15 second ignition sequence.
If the air flow signal is lost while the burner is firing, the control will immediately de
-
energize the
gas valve and the combustion blower will remain on. If the call for heat remains, the control will
wait for proper air flow to return. If proper air flow is not detected after 30 seconds an air flow
fault signal will be declared. If proper air flow is detected at any time, a normal sequence will
begin with the pre
-
purge period.
