FloAire INDIRECT FIRED BENT TUBE MODULE - Manual (Page 24)

24
Flame Safety
Power-vent Blower
Speed Controller*
Flame Safety Control
The second system to understand is the Flame Safety Control. The
FSC is present only to monitor the flame, NOT to control
temperature. The FSC uses a sensor mounted at the intake of the
upper-most firing tube of the furnace to sense the existence of a
flame. The FSC controls the opening of the redundant solenoid gas
valves and the operation of the spark igniter to initiate a flame upon
start up. When there is a call for heat, the LED on the FSC is
energized indicating that the unit has power. Then, there is a one
minute pre-purge in which the power-vent blower on the furnace is
sent to high speed to exhaust any gas in the Heat-exchanger/Control
Cabinet that may be present prior to trial for ignition. As soon as the
pre-purge has initiated, the FSC checks that airflow is sensed by the power-vent airflow switch* and that
the High Limit and Roll-out switches are not tripped. (*NOTE: If while trouble-shooting the unit it is
necessary to jumper-out this airflow switch, the jumpers must be applied to the contacts immediately
after the power-vent motor is turned on and the FSC begins to check for airflow, otherwise the unit will go
into lock-out mode.) Upon successful sensing of induced power-vent airflow and continuity of
temperature limit and Roll-out switches, the FSC initiates a 15 second ignition sequence. During this
ignition sequence, the FSC opens the On/Off gas valves and allows gas to pass through to the Gas
Manifold. At the same moment, the spark igniter begins to spark, causing the electrode on the burner to
ignite the gas. This results in a flame at the lowest firing tube of the furnace which immediately ignites the
flow of gas in each succeeding firing tube moving vertically until the entire furnace is lit. When the sensor
detects the flame at the intake of the upper-most firing tube the FSC continues to power the On/Off gas
valve until there is a loss of flame presence. This is the normal operating mode.
Modulating Stage Sequence
As mentioned in the previous sections, every unit is equipped with a modulating furnace(s) located
furthest downstream at the discharge of the heater module. The modulating stage(s) operates differently
than the other On/Off staged furnaces in that instead of being “On” or “Off”, the gas flow to this furnace is
modulated up and down to account for varying calls for heat
during the unit’s operating period. In addition, the speed of its
power-vent blower is varied as the gas flow changes in order to
maintain constant combustion efficiency over the entire firing
range. The modulating furnaces Power-vent blower(s) is
controlled by a speed controller that varies the output voltage
to the motor based on a 0-10Vdc signal from the FVFAB. There
is one Speed Controller board per modulating furnace. The
Output voltage (True RMS) to the motor varies non-linearly
between 120Vac @ 10Vdc for high fire and 81.6Vac @ 0Vdc
for low fire. *NOTE: Red dip switches located on the modulating
furnaces Power-vent Blower Speed Controller are de-activated
and should not be adjusted.