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9. 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, initiate full
time flame sensing, and enter the thermostat 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 com-
bustion blower remains energized. The thermostat, pressure switch, and main burner flame are con-
stantly monitored to assure the system continues to operate properly.
For the first two minutes, the heater operates on Low-fire
•
After 2 minutes, it runs on Hi-Fire for next 5 minutes, irrespective of Modulating signal from the ther-
•
mostat
After total of 7 minutes, the heater responds to the modulating signal( 2-10V) from the thermostat
•
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 before de-
energizing the combustion blower.
Failure to Ignite - Lockout (
This three-try control will attempt two additional ignition trials with a 30 second inter-purge between tri-
als, before going into 'soft' lockout. The valve relay will be de-energized immediately, and the combus-
tion 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 re-
setting the thermostat or removing 24 volts for a period of 5 seconds.
Flame Failure - Re-cycle
If the established flame signal is lost while the burner is operating, the control will go to recycle mode.
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' lockout 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 output (L1 &
IND) is energized, an air flow fault will be declared and the control will stay in this mode with the com-
bustion 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 pe-
riod.
)
THREE TRIAL MODEL
21
HE+ Series Modulating Supplemental Manual
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