Operation
Normal Operation
The S8901 is powered by a 24 Vac transformer and is activated when the thermostat calls for heat. Operation is as follows (refer to the Normal Operating Sequence for Standard Models).
All models check for a false flame condition (short to ground) on a call for heat. Module will lock out for 1 hour if false flame condition is present. Reset could be manually done from the thermostat. If no false flame condition is present, operation continues.
On models with a fan terminal, the combustion air blower will start. On proof of airflow, the air proving switch closes, and the prepurge timing will begin, timing dependent on the model.
On models without a fan terminal, prepurge timing will begin on a call for heat, timing dependent on the model.
Following prepurge, the module energizes the hot surface igniter while keeping the gas valve closed, depending on the warmup time period.
After the warmup period, the gas control opens for a timed trial for ignition, allowing gas flow to the main burner.
Near the end of the trial for ignition period, the igniter shuts off to allow the flame sensor to detect the presence of the main burner flame.
If the main burner lights, a flame sensing circuit is completed through the flame to the burner ground. A minimum of 0.8 uA needs to be sensed. This current flow sets the safety lockout timer to the reset (normal) condition. Should the current flow be interrupted; i.e., flame-out condition, the trial for ignition begins again.
The S8901 will keep the gas control main valve open as long as there is a call for heat and there is enough flame current sensed. If, however, the safety lockout timing period ends before the main burner lights or the flame sensor establishes enough current, the system will go into safety lockout for 1 hour (if exceeded number of trials).
When the system goes into safety lockout, the gas control circuit is interrupted, and the alarm circuit is completed and LED will flash 2 times. The system will stay locked out for one hour or until it is reset by moving the thermostat set point below room temperature so there is no call for heat for at least 1 minute.
For a summary of the normal operating sequences across all standard models, refer to the Normal Operating Sequence for Standard Models.
Failed trial for ignition
If the main burner flame is not lit and sensed before the end of the ignition trial time, the control shuts off the gas valve (100% shutoff) and either retries or locks out for 1 hour before trying again (model dependent). If it goes into lockout, the control will remain in lockout for 1 hour or until power to the control is cycled by the system thermostat or by removing and restoring system power.
Normal Operating Sequence for Standard Models
