Sequence Of Operations - Bradford White EF120T4003NA3 Service Manual

1. Thermostat calls for heat.
2. Combustion blower starts at a reduced rpm for a "soft" start light off.
3. Blower pre-purge period of approximately 15 seconds.
4. Ignition control board runs an internal verification safety check for approximately 15 seconds.
5. Trial for ignition (approximately 5 seconds per trial, 3 trials total):
a. Spark establishing period (3 seconds), gas valve opens, sparks from spark rod to ground rod, igniting
the fuel air mixture at the burner surface.
b. Burner on, flame proving period (2 seconds). Requires a minimum of 1 microamp through the flame
sense rod to prove flame.
c. If the blocked vent pressure switch contacts (normally closed) are open, then the ignition sequence
will not start and error code "67" (pressure switch failed to close/open or vent safety switch failed to
close/open) will flash once on the digital display. The unit will then go into pre-purge/"Hold" while the
unit is waiting for the issue to be corrected. If the issue continues to occur, the digital display with flash
error code "137" (pressure switch is open, or vent safety switch is open) while the unit is waiting to
restart the normal sequence of operation.
Once the flame signal is verified, the blower will remain at the "soft" start RPM for 5 seconds to stabilize
6.
the flame.
7. Steady state operation – Burner continues to operate until:
a. The thermostat circuit opens, gas valve closes, and blower continues to operate for 30 second post-
purge period.
b. If the blocked vent pressure switch contacts open (normally closed) while the burner is on, then the
gas valve closes, and the unit will retry a normal sequence of operation. If issue remains on restart,
the unit will go into recycle as described in 5c.
8. The RPM of the blower will modulate depending on the heater demand. The control allows for a turn down
ratio (TDR) of approximately 3:1. Generally, the blower will operate at higher speeds if the thermostat
remains far from the setpoint and at lower speeds as the thermostat approaches the setpoint.
9. Thermostat is satisfied.
10. Gas valve closes and burner is extinguished.
11. Blower post purge for 30 seconds at maximum RPM.

Sequence of Operations

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