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S8921D HOT SURFACE IGNITION MODULE
OPERATION
The S8921D is a direct ignition control used with 120 Vac
timed warmup hot surface igniter. The control provides
operating control of the appliance and shuts off all gas flow
on ignition failure or loss of main burner flame in heating
appliances. As part of system control, the module also
controls a combustion blower and monitors the system air
proving switch.
Module operation is in four phases—igniter warmup,
combustion air proving, trial for ignition, and burner
operation. The S8921D provides five trials for ignition
before lockout on each call for heat. Table 1 shows the
S8921D sequence for a normal heating cycle and Table 2
shows the sequence when the burner fails to light. See
Application section and control label for specific S8921D
model timings.
Table 1. S8921D Sequence on Normal Heating Cycle.
THERMOSTAT CALL FOR HEAT
IGNITER ON
AIRFLOW SWITCH CHECK
COMBUSTION AIR BLOWER ON
PROOF OF AIR FLOW
GAS CONTROL OPEN
PROOF OF FLAME
1
APPLICATION SECTION AND S8921D
MODEL LABEL.
Table 2. S8921D Sequence on Failure to Light.
THERMOSTAT
CALL FOR HEAT
IGNITER ON
AIRFLOW
SWITCH CHECK
COMBUSTION
AIR BLOWER ON
PROOF OF AIR
FLOW
GAS CONTROL
OPEN
PROOF OF
FLAME
LOCKOUT
1
APPLICATION SECTION AND S8921D
MODEL LABEL.
69-0924—2
1
M7596
1
M7597
6
Igniter Warmup
When a call for heat starts, the S8921D checks the air
proving switch to ensure it is in the no air position. Then
the S8921D energizes the igniter to start the igniter
warmup. During this warmup period, the gas control is
closed and the combustion blower is not energized.
Combustion Air Proving
After the igniter warmup, the combustion air blower is
energized. When the air proving switch confirms proper
combustion air flow, the prepurge timing begins. When
prepurge is complete, the ignition trial begins.
Trial For Ignition
The gas control opens for the ignition trial time. The hot
surface igniter stays powered for the ignition activation
period and then the igniter turns off. Near the end of the
ignition trial time, the flame rectification sensing circuit
determines if the main burner flame is present. If so, the
gas control remains open and the burner operation phase
begins.
Burner Operation
When the main burner is lit, a flame rectification circuit is
completed between the flame sensor (flame rod) and the
main burner (burner ground). The S8921D flame sensing
circuit detects the flame current and holds open the gas
control. The main flame is monitored continuously during
the call for heat.
Safety Shutdown
Failure to Light
If flame is not sensed by the end of the first timed trial for
ignition, the gas control closes and the module initiates a
second ignition cycle. If flame is not established, another
ignition cycle is initiated. If flame is still not established
following the fifth trial, the gas control closes and the
module locks out. It must be manually reset by removing
power or setting the thermostat below room temperature
for at least 30 seconds.
If the burner goes out during the run cycle, the gas control
closes and the module checks for the number of ignition
trials performed during the current call for heat. If the
number is less than five, the module initiates an ignition
warmup and trial for ignition. After the fifth trial during a
single call for heat, the module locks out. The LED flashes
in a 3-flash mode to indicate ignition failure. The module
resets automatically after one hour or it can be manually
reset following the lockout.
Airflow Problems
If the air proving switch is jumpered or stuck in the closed
(air proven) position, the S8921D will NOT energize the
igniter or the combustion blower or begin a trial for ignition.
The LED goes into a 1-flash mode until the air proving
switch moves to the proper position or the call for heat
ends. If the air proving switch fails to confirm proper
combustion air flow within the customer selected time
interval after the combustion blower is energized (or within
the customer selected time interval after the air proving
switch has opened during a heating cycle), the S8192D
goes into lockout with the LED in the 2-flash mode, and
the combustion air blower turns off.
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