Bradford White U-TW4-75S-76R-3N - Manual (Page 13)

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Normal Heating Sequence (cont’d)
7. Main Burner Ignition: After pilot flame is sensed, the gas control acti-
vates the main valve for main burner ignition. The gas control will ig-
nore flame and pressure switch signals for 3 seconds to allow for the
main burner to stabilize.
8. Steady State Operation: During steady state operation, the gas control
monitors:
Thermostat Temperature Sensor: When the setpoint temperature is sat-
isfied, the gas control is shutdown, and the blower will post-purge for 15
seconds. The gas control LED flashes a short flash once every 4 seconds
(Idle status code).
Pressure switch/blower temperature switch: If either switch opens, the
pilot valve and the main valve are shut down. The blower continues to
run for 30 seconds attempting to close the circuit. The gas control LED
flashes 3 times with 3 second pause.
Flame Sensor: If flame is lost, the pilot and main valves are shutdown.
The blower runs for 15 seconds. The gas control attempts to re-light the
pilot 4 times. If unsuccessful, the blower is shutdown, and the gas con-
trol proceeds to a 5 minute lockout. The gas control re-attempts to light
the pilot starting at Normal Heating Sequence #2.
9. Thermostat Satisfies: Gas control LED flashes once every 4 seconds.
10.Burner Off.
11.Blower post-purge (15 seconds).
Abnormal Operation
1. Flammable Vapor Sensor Fault:
A) If the resistance is greater than 70,000 ohms: The gas control imme-
diately turns off all outputs. The gas control waits and monitors resis-
tance for 30 seconds. If the resistance is greater than 65,000 ohms af-
ter 30 seconds, the gas control proceeds to verify resistive delay for 2
minutes and flashes 7 times then 1 time with a three second pause. This
process is repeated 5 times until the control either returns to normal op-
eration or proceeds to flammable vapor lockout.
Sequence of Operation
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