Sequence Of Operation - State Water Heaters 120 Series Service Handbook

seQuence of operation

Read the Sequence of operation below before attempting to diagnose or repair any operational problems.
Refer to the Features And Components on page 8 for the location of various water heater components
described below. See the Sequence of operation Flow Chart on page 55 also.
1. When the control system is first powered, during boot up, it will display manufacturer information and
Control System software revisions. After a few moments the Control System LCd, which is part of the
uIM (see page 37), will display the default screen known as the "desktop Screen." Manufacturer and water
heater model information will be visible in the Title Bar at the top of the LCd. See desktop Screen on
page 46.
2. If the Control System determines that the actual water temperature inside the storage tank is below
the programmed operating Set Point minus the differential setting, a heating cycle is activated. See
Temperatures on page 49.
3. The Control System then performs selected diagnostic system checks that include:
• The Blocked Exhaust switch and the Blocked Intake Air switch contacts must be confirmed closed.
• The ECo (Energy Cut out) switch contacts must be confirmed closed.
• The Blower Prover switch contacts must be confirmed open.
4. If any of the selected diagnostic checks fail, the Control System will lock out and display a specific fault
message on the LCd, see Figure 31 on page 29 and Fault Messages on page 58.
5. If all diagnostic system checks pass, the Control System energizes the Combustion Blower for the Pre-
Purge operating state. See operating States on page 48.
6. The Control System must confirm the Blower Prover switch contacts close after the Combustion Blower is
energized. If the Blower Prover switch contacts do not close the Control System will lock out and display
the Blower Prover Open fault message on the LCd.
7. If the Blower Prover switch contacts are confirmed closed the Control System energizes the Igniter for the
Igniter Warm up operating state. See operating States on page 48.
8. The Control System monitors Igniter current and must sense a minimum of 0.5 AC amps during the
Igniter Warm up operating state. If minimum current is not sensed the Control System will lock out and
display the Low Igniter Current fault message on the LCd.
9. If Igniter current is confirmed at or above the required minimum the Control System energizes the 24 Volt
Gas Valve allowing gas to flow to the Burner.
10. The Control System de-energizes the Igniter.
11. The Control System monitors flame sensing current to confirm a flame is present at the Burner. See
Flame Sensing operation on page 18. The Control System must sense a minimum of 1.0 µA through the
Flame Sensor during the Ignition Verification operating state. See operating States on page 48.
12. If the flame sensing current is not at or above 1.0 µA the Control System will try for ignition up to three
times. After three failed trials for ignition the Control System will lock out and display the Ignition Failure
fault message on the LCd.
13. If the flame sensing current is established at or above 1.0 µA the Control System enters the Heating
operating state. See operating States on page 48.
14. The Control System will continue heating the water until the operating Set Point is reached. At this point,
the Control System will de-energize the 24 Volt Gas Valve and enter the Post-Purge operating state. See
operating States on page 48.
15. The Combustion Blower will run for the duration of the Post-Purge operating state to purge the water
heater of all combustion gases. When the Post-Purge operating state is complete, the Combustion Blower
is de-energized and will coast to a stop.
16. The Control System now enters the Standby operating state while continuing to monitor water temperature
from the Temperature Probe and the state of other system devices. If the tank temperature drops below
the operating Set Point minus the differential setting, the Control System will automatically return to Step
2 and repeat the operating sequence.
54 Servicing should only be performed by a Qualified Service Agent