Bradford White U-TW4-75S-76R-3N - Manual (Page 12)

Page 12
Power Up Sequence
1. Start-up: Upon power up, the gas control runs a safe start check with a
typical delay of 5 seconds.
2. Flammable Vapor Verification: The gas control verifies that the Flamma-
ble Vapor Sensor is in the proper operating range prior to energizing any
components. If the sensor is within the proper range, the gas control
resumes normal operation. If the Flammable Vapor Sensor is out of
range, the gas control LED immediately flashes 7 times with a 3 second
pause.
Normal Heating Sequence
1. Thermostat Calls for Heat: Prior to energizing the blower, the gas control
checks the safety circuit to insure the circuit is open. In the safety circuit
the pressure switch is normally open and the blower temperature switch
is normally closed.
2. If the safety circuit is closed, the gas control waits 4 seconds, the gas
control LED flashes 2 times with a 3 second pause. The gas control
waits 2 minutes. Then, the blower runs for 30 seconds. This cycle re-
peats until the safety circuit opens.
3. Blower energizes.
4. Pressure switch proves blower/vent system operation: If the pressure
switch does not close within 30 seconds, the gas control LED flashes 3
times with 3 second pause. The blower runs for 30 seconds every 2 min-
utes trying to get the pressure switch or blower temperature switch to
close. This cycle repeats as long as there is a call for heat.
5. Blower pre-purge period (15 seconds).
6. Trial for Pilot Ignition (90 seconds): The gas control lights the pilot by
activating the spark igniter and gas flow to the pilot burner. If flame is
not sensed within 90 seconds, the spark igniter and gas flow are deacti-
vated. The blower will post-purge, and the gas control LED flashes 6
times with a 3 second pause.
Sequence of Operation
12