Typical Sequence of Operation (3-1 gas burner)
for MicroTech III Control Systems
When 120V power is furnished through the system on/off
switch (S1), through the burner on/off switch (S3), and through
the high limit control (FLC), terminal #6 on the flame safeguard
(FSG) is powered on a call for heat .
Whenever power is restored to the flame safeguard, the flame
safeguard will go through a 10 second initiation period before
the prepurge period will begin . The burner air control valve will
be at minimum position during off cycles.
Upon a call for heat, the controller will close digital output
(EXPB-DO1) and energize the R20A relay . Once the normally
open contacts of the R20A relay close, 120V power is supplied
to terminal # 6 on the FSG . The FSG then energizes its
terminal #4, which powers the burner combustion air blower
motor (BM) and starts the FSG prepurge cycle . Blower
operation is sensed by the Air Switch (AS), which makes
terminal FSG-6 to FSG-7 during the prepurge cycle .
After completion of the FSG prepurge period there will be a 10
second trial for ignition during which terminal #8 (combination
gas valve - GV1) and terminal #10 (ignition transformer - IT)
will be energized. If flame is being detected through the
flame rod (FD) at the completion of the 10 second trial for
ignition period, terminal #10 (ignition transformer - IT) will
be de-energized and terminal #9 (main gas valves - GV4
and GV5) will be energized and the control system will be
allowed to control the firing rate once the heating stage timer
(default 5 minutes) has passed. After the flame has lit and
been proven and the heating stage time has passed, the
controller will modulate (VM1) to the required firing rate via
analog output EXPB-AO_X7. In the event the flame fails to
ignite or the flame safeguard fails to detect its flame within
10 seconds, terminals #4, 8, 9, and 10 will be de-energized,
thus de-energizing the burner . The FSG will then lockout and
would require manual resetting . If the FSG lockout occurs,
FSG terminal #3 will energize the R24 alarm input status relay
which will ‘make’ a digital input to the controller (EXPB-DI_X4) .
When this digital input is ‘made’ the controller will drive VM1
to the closed position . At the same time the EXPB-DO2 will
open, the prepurge sequence will be disabled and reset . If the
FSG terminal # 8 de-energizes relay R22 (EXPB-DI_X1) after
having it turned on and the FSG is not off on safety lockout, the
controller will drive VM1 to the closed position .
If an attempt is made to reset the FSG or if an automatic
restart is initiated after flame failure, the earlier described
FSG prepurge cycle will be repeated . If the unit overheats, the
high limit control (FLC) will cycle the burner, limiting furnace
temperature to the limit control set point. The flame safeguard
contains ‘LEDS’ (lower left corner) that will glow to indicate
operation .
Figure 9: Piping Schematic
Start-Up and Operating Procedures
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IM 684-6 • ROOFTOP SYSTEMS
