American Water Heater GAS-FIRED COMMERCIAL WATER HEATER - Manual (Page 34)

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1. The electronic thermostat controls the 24-volt circuit to the ignition control.
When the thermostat contacts close, a 24-volt circuit is completed from the
transformer through the thermostat relay to the ignition control module to
initiate the ignition sequence.
2. On flue damper models, the damper reaches then full open position, the
end switches in the damper close, completing the 24-volt circuit to the
ignition module.
3. The ignition control sends 24-volt power to the pilot valve “PV” terminals on
the gas valve allowing pilot gas to flow to the pilot. The ignition control also
simultaneously sends high voltage low current electricity through the
electrode wire to the pilot electrode causing sparks at the pilot electrode to
ignite the pilot gas. If the pilot fails to ignite within 90 seconds, the ignition
control stops the pilot gas and sparking for 15 seconds to allow gas to
dissipate. The ignition control will then attempt 2 (two) more ignition trials
as stated above. If the pilot does not ignite after 3 trials, the ignition control
will then go into “lockout mode”, and the ignition system will remain shut
down until the control is reset by interrupting power to the ignition module.
4. When the pilot gas ignites, the flame is sensed by the electrode or a flame
sensing rod. The flame sensing signal received by the ignition control
causes the sparking to stop and the main gas valve to open. The main
burners ignite from the pilot flame. The pilot flame is continually monitored
by the flame sensing circuit. If for any reason, the pilot flame is not sensed
by the electrode or flame sensing rod, the main gas valve closes, the spark
electrode is reenergized, and the ignition trial period is reestablished. The
same sequence occurs during a power or gas supply interruption.
5. The main burners continue to operate until the water temperature in the
tank increases enough to cause the thermostat contacts to open. When
the thermostat contacts open the 24-volt power is interrupted to the ignition
control module and the gas valve closes. The flue damper (on “D” models)
closes.
6. If for some reason thermostat contacts fail to open, then the high
temperature limit (ECO) contacts in the thermostat sensor open and
interrupts power to the pilot valve causing the pilot to extinguish. The
ignition module senses the pilot outage and attempts to relight the pilot by
reestablishing the sparking at the electrode as stated in paragraph 3
above. Since the pilot valve circuit is open, the pilot will not ignite and the
system will go into “lockout mode” after 3 ignition trials. In the “lockout
mode”, the ignition system will remain shutdown until the control is reset by
interrupting power to the ignition module. The ECO will not reset to close
the contacts until the tank water temperature drops to between 100°F
(38°C) and 160°F (71°C). If the ECO contacts open, the cause for the high
water temperature should be determined before resetting the ignition
system.
Troubleshooting Procedure for Electronic Thermostat
If water temperature is too hot, proceed as follows:
Index