83
Flame Safety Control (FSC)
The Flame Safety Control (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 solenoid gas valve, 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. 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 before 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.
Upon successful sensing of induced power vent airflow, 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 valve 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. This 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 uppermost 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.
Figure 37 - Ignition Sequence
Modulating Stage Sequence
The modulating stage operates differently than other On/Off staged furnaces. 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 to
maintain constant combustion efficiency over the entire firing range.
The modulating furnace power vent blower is controlled by an on-board speed controller, located on the
MUA control board. Voltage to the motor is based on a
0-10V DC
signal. The output voltage (True RMS) to
the motor varies non-linearly between
120V AC
@
10V DC
for high fire and
86.5V AC
@
0V DC
for low
fire.
MUA Board and High Fire Start
The MUA board compares a difference between two sensor values and set points; or compares a
0-10V
DC
or
0-20
mA
signal from an analog control to the modulating furnace. The signal is linearized such that
input voltage is directly proportional to amount of gas being delivered to the modulating valve.
To ensure proper light-off in all conditions, the MUA board contains software that forces the modulating
furnace to light at high-fire when that furnace’s main gas valve is first opened. There is a built-in timer that
allows it to send a constant
10V DC
signal to the modulating gas valve, and power vent blower speed
controller. This will force the furnace into high-fire for a period of
17-seconds
after the initial spark is sent
by the FSC. After this forced high-fire light-off period has expired, the modulating furnace’s power vent
blower and modulating gas valve will receive a modulating signal from the MUA board.
Interval
Description
Time (Min:Sec)
(Non-Linear Scale)
Initial Call
for Heat
1 Min.
Pre-Purge
15 Sec. Trial
for Ignition
1 Min.
Inter-Purge
15 Sec. Trial
for Ignition
1 Min.
Inter-Purge
15 Sec. Trial
for Ignition
2 Min.
Post-Purge
1 Hr
Lockout
Repeat
Cycle
0:00
1:00
1:15
2:15
2:30
3:30
3:45
5:45
End Cycle
