Honeywell S8910U Installation Instructions And Safety Information page 13

Purge/Prepurge
When the S8910U is used in a fan-assisted combustion
system, the combustion air blower starts on a call for heat
from the thermostat. On proof of airflow, the air proving
switch closes and energizes the S8910U. When the
S8910U is used in an atmospheric system, the call for heat
energizes the module.
START
PREPURGE/
IGNITER
WARMUP
TRIAL FOR
IGNITION
MAIN BURNER
OPERATION
END
CALL FOR HEAT (24V
TERMINAL POWERED)
INTERNAL SAFE START
CHECK BEGINS.
NO FLAME
SIMULATING
FAILURE
PREPURGE
IGNITER WARMS UP
GAS CONTROL OPENS
FOR TIMED TRIAL
FOR IGNITION.
IGNITER OFF;
FLAME SENSING BEGINS.
FLAME
PRESENT
GAS CONTROL STAYS
FLAME
OPEN (MAIN BURNER
RUNS).
CALL FOR HEAT
IS SATISFIED.
GAS CONTROL CLOSES
(MAIN BURNER OFF).
Fig. 7. S8910U normal operating sequence.
S8910U UNIVERSAL HOT SURFACE IGNITION MODULE
In either case, the S8910U first initiates a 32-second delay
to allow system prepurge.
Igniter Warmup
After prepurge, the S8910U energizes the igniter to start
the igniter warmup. The module energizes the hot surface
igniter for a 34-second warmup period, during which the
gas control is closed.
FLAME
SYSTEM WILL
SIMULATING
NOT START
FAILURE
GAS
NO
CONTROL
FLAME
CLOSES
ONE TRIAL
(TABS A AND C)
GAS
CONTROL
FAILURE
CLOSES
THREE TRIALS
(TABS B AND D)
13
ONE TRIAL
(TABS A
AND C)
.LOCKOUT OCCURS;
SYSTEM MUST BE
RESET.
LESS
THAN
THREE
THREE TRIALS
(TABS B AND D)
HOW MANY IGNITION
TRIALS COMPLETED THIS
CALL FOR HEAT?
THREE
LOCKOUT OCCURS;
SYSTEM MUST BE
RESET.
BETWEEN
TRIAL
PURGE
M8527A
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