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Honeywell S8921D, Installation Instructions Manual, Page: 6 / 12

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Honeywell S8921D Installation Instructions Manual Download Page 6

69-0924—2

6

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.

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

PREPURGE

IGNITION WARMUP

TRIAL FOR IGNITION

COMBUSTION 

BLOWER SPINUP

NORMAL RUN

M7596

1

1     APPLICATION SECTION AND S8921D
       MODEL LABEL.

THERMOSTAT 
CALL FOR HEAT

IGNITER ON

AIRFLOW 
SWITCH CHECK

COMBUSTION 
AIR BLOWER ON

PROOF OF AIR 
FLOW

GAS CONTROL 
OPEN

PROOF OF 
FLAME

LOCKOUT

PREPURGE

IGNITION WARMUP

TRIAL FOR IGNITION

COMBUSTION BLOWER SPINUP

BETWEEN TRIAL DELAY

PREPURGE

LOCKOUT

RESET

IGNITION WARMUP

TRIAL FOR IGNITION

COMBUSTION BLOWER SPINUP

BETWEEN TRIAL DELAY

M7597

1

1     APPLICATION SECTION AND S8921D
       MODEL LABEL.

SEQUENCE REPEATES 3 MORE TIMES

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.

Summary of Contents for S8921D

Page 1: ... 60 Hz Contact Rating at 120 Vac Hot Surface Igniter 5A Combustion Air Blower 3 FLA 10 LRA IMPORTANT The S8921D is to be used only in a 60 Hz system Be sure the system is not 50 Hz Hot Surface Igniter Norton Model 271 or equivalent NOTE If an igniter other than a Norton Model 271 is used the igniter must meet the following minimum specifications required over the life of the igniter Igniter must r...

Page 2: ...A 4 enclosure is recommended for the ignition module see the Electronic Ignition Service manual 70 6604 Corrosive Chemicals Corrosive chemicals can attack the module and gas control eventually causing a failure If chemicals are used for routine cleaning avoid contact with the controls Where chemicals are suspended in air as in some industrial or agricultural applications NEMA 4 enclosure is recom ...

Page 3: ...ly before doing any work on the unit Disconnect and tag the wires from the old module Label each wire as it is removed Remove the old module from its mounting location Mount New Ignition Module Mount the new module in the same location as the old module Protect the module from exposure to water moisture corrosive chemicals and excessive dust and grease Make certain that the ambient temperature at ...

Page 4: ...ormer has adequate VA The ignition module requires 0 4A at 24 Vac Add the current draws of all other devices in the control circuit including the pilot and main valves in the gas control and multiply by 24 to determine the total VA requirement of these components Add this total to 9 6 VA for the ignition module The result is the minimum transformer VA rating Use a Class II trans former when replac...

Page 5: ... gas control is energized 5 Gas control is energized for the trial for ignition time This time can be measured by connecting a voltmeter range set to measure approximately 24 Vac across the gas valve terminals 6 After the trial for ignition the between trial delay begins and the combustion air blower is turned off 7 The second trial begins with the hot surface igniter warmup followed by the combus...

Page 6: ...nition 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 i...

Page 7: ...onsiderations are Cycling frequency Appliances that may cycle 20 000 times annually should be checked monthly Intermittent use Appliances that are used seasonally should be checked before shutdown and again before the next use Consequence of unexpected shutdown Where the cost of an unexpected shutdown would be high the system should be checked more often Dusty wet or corrosive environment Since th...

Page 8: ... of module Re establish call for heat Check for 24 Vac across VALVE and VALVE GND terminals during trial for ignition Note Voltage present only during trial for ignition after air proving switch confims combustion air flow Replace module Check replace wiring to gas control Check replace gas control Note Ignition module will continue ignition cycle to lockout after 5 trials for ignition Troubleshoo...

Page 9: ...em Checks Step 1 Check wiring harness Make sure 1 Connections to the ignition module air proving switch and gas control are clean and tight 2 Hot surface ignition cable provides good electrical continuity Step 2 Check ignition system grounding Nuisance shutdowns are often caused by a poor or erratic ground A common ground is required for the module flame sensor and main burner 1 Check for good met...

Page 10: ...69 0924 2 10 S8921D HOT SURFACE IGNITION MODULE ...

Page 11: ...69 0924 2 11 S8921D HOT SURFACE IGNITION MODULE ...

Page 12: ...ted Honeywell Limitée 155 Gordon Baker Road North York Ontario M2H 3N7 69 0924 2 J S Rev 6 98 Home and Building Control Honeywell Inc Honeywell Plaza P O Box 524 Minneapolis MN 55408 0524 www honeywell com Printed in U S A on recycled paper containing at least 10 post consumer paper fibers ...

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