281 Issue 1 - Service Manual - BX3
11
3.3 Spark Igniter
The Spark Igniter consists of a spark probe and earth probe used to create a fixed spark gap.
HCDSI 24 -1 generates a spark from the sparker canister across the gap of the sparker probe.
This spark gap is located beneath the burner port at one end of the burner rack. Once ignition
has occurred and the flame is sensed, the spark igniter stops sparking.
3.4 Flame Sensor
The Flame Sensor consists of a single metal probe and is located beneath the burner at the
opposite end of the burner rack from the Spark Igniter. When flame is present at this burner, a
small electric current passes from the flame sensor through the flame to the closest ground or
earth point (either the Burner or Heat Exchanger). HCDSI 24 -1 control monitors the Flame
Sensor current level. If it is within an allowable range, the controls then allow for normal
operation.
3.5 Combustion Fan Operation
The combustion fans primary function is to “draw” air and flame through the heat exchanger. If
this combustion airflow is reduced, the combustion quality will be reduced and sooting may
occur. If the airflow is reduced considerably, the flames will not be drawn into the heat
exchanger. (Fortunately, there are measuring devices to prevent these occurrences from causing
an unsafe condition). The combustion fan operates at a fixed fan speed, and is switched on and
off by the HCDSI 24 -1 control. When the fan is in operation, the suction effect of the fan creates
a ( inside the heat exchanger from the heat exchanger inlet ports all the way through to
the combustion fan box on which the combustion fan is mounted.
During start-up, the combustion fan performs a pre-purge for approximately 15 seconds. This
ensures there is no un-burnt fuel in the heat exchanger. During this phase, the required quantity
of air for adequate combustion is confirmed by a vacuum pressure switch. If the airflow is not
sufficient, the vacuum pressure will be too low, and the pressure switch will not close circuit. A
pressure switch error will be logged and no ignition will occur.
During normal operation the vacuum pressure generated by the combustion fan is continually
monitored by the pressure switch. If this vacuum pressure drops too low, the pressure switch will
go open circuit, the burners will the shut down and an error will be logged.
During a normal shut down sequence, the burners will shut down, and the combustion fan will
switch off after 5 seconds.
Combustion Fan Motor
Resistance Reading
Fime Winding Resistance @20
o
C
24.3 Ohms
