General Operational Sequence - A.O. Smith 100 Series Service Handbook

Safety Shutdown:
1. If the lockout interlocks open or a sensor designated as a safety limit are read as defective, Control System will lockout and
the blower motor will be de-energized.
If these open during the firing period, all fuel valves will be de-energized, the system will complete postpurge, and will lockout
indicated by an alarm.
2. If the main flame is not detected at the end of the last recycle attempt of the main flame establishing period, all fuel valves will
be de-energized, the device will complete postpurge, and will lockout indicated by an alarm.
3. If the flame sensing signal is lost during the run period (if lockout is selected), all fuel valves will be de-energized within 4 seconds
after the loss of the flame signal, the device will complete postpurge, and will lockout indicate by an alarm.
4. Manual reset is required following any safety shutdown. Manual reset may be accomplished by pressing the push button on the
device, pressing the remote reset wired into connector J10, or through an attached display.
Interrupting power to Control System will cause electrical resets, but does not reset a lockout condition.

GENERAL OPERATIONAL SEQUENCE

Initiate
The R7910 enters the Initiate sequence on Initial Power up or:
• Voltage fluctuations vary less than 20VAC or greater than 30VAC.
• Frequency fluctuations vary +/-5% (57 to 63 Hz).
• If Demand, LCI, or Stat interrupt (open) during the Prepurge Period.
• After the reset button is pressed or fault is cleared at the displays.
The Initiate sequence also delays the burner motor from being energized and de-energized from an intermittent AC line input or
control input.
If an AC problem exists for more than 240 seconds a lockout will occur.
Hydronic/Central Heating (XB Boilers)
Start-up sequence central heating request (system in standby):
1. Heat request detected (On Setpoint - On Hysteresis).
2. The CH pump is switched on.
3. After a system Safe Start Check, the Blower (fan) is switched on after a dynamic ILK switch test (if enabled).
4. After the ILK switch is closed and the purge rate proving fan RPM is achieved (or High Fire Switch is closed) - prepurge time is
started.
5. When the purge time is complete, the purge fan RPM is changed to the Lightoff Rate or if used, the damper motor is driven to
the Low Fire Position.
6. As soon as the fan-rpm is equal to the light-off rpm (or the Low Fire Switch closes), the Trial for Ignition or Pre-Ignition Time is
started.
7. Pre-Ignition Time will energize the ignitor and check for flame.
8. Trial for Ignition. Specifics for timings and device actions are defined by A. O. Smith.
9. The ignition and the gas valve are switched on.
10. The ignition is turned off at the end of the direct burner ignition period, or for a system that does use a pilot, at the end (or
optionally at the middle) of the Pilot Flame Establishing Period (PFEP). For an interrupted pilot system this is followed by a Main
Flame Establishing Period (MFEP) where the pilot ignites the main burner. For an intermittent pilot there is no MFEP.
11. The fan is kept at the lightoff rate during the stabilization timer, if any.
12. Before the release to modulation, the fan is switched to minimum RPM for the CH Forced Rate and Slow Start Enable, if the
water is colder than the threshold.
13. At the end of the CH-heat request the burner is switched off and the fan stays on until post purge is complete.
14. A new CH-request is blocked for the forced off time set by the Anti Short Cycle (if enabled).
15. The pump stays on during the pump overrun time.
16. At the end of the pump overrun time the pump will be switched off.
Servicing should only be performed by a Qualified Service Agent 41