McQuay OM 920-1
113
Operator’s Guide
AO_X7 for prepurge. When the burner air valve reaches the full open position switch (LS2)
will 'make' and provide a digital input to the controller (EXPB-DI_X3). This digital input will
initiate a 20 second (adjustable) timing period in the controller. At the completion of the
timing period, the controller will begin to drive the burner air valve to its minimum (low fire)
position. When the valve reaches the minimum position switch LS1 will 'make' and provide a
digital input to the controller (EXPB-DI_X2) indicating the controller's prepurge sequence is
complete. As soon as the FSG prepurge time expires FSG terminal #8 will energize relay R22
which will turn on a digital input to the controller (EXPB-DI-X1). As soon as this digital input
is 'made' the controller will close digital output (EXPB-DO2) allowing the combination gas
valve(s) (GV1) to be energized.
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 depending on burner size) 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, de-energize digital output EXPB-DO2
and the prepurge sequence will be disabled and reset. If the FSG terminal # 8 de-energizes
R22 (EXPB-DI_X1) after having it turned on and the FSG is not off on safety lockout, the
prepurge sequence will start over. If an attempt is made to restart the burner by resetting the
FSG or if an automatic restart is initiated after flame failure the earlier described prepurge
cycle with the wide open air valve 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.
Typical Sequence of Operation (3-1 Gas)
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.
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 require manual resetting. If the FSG lockout occurs,
FSG terminal #3 will energize the R24 alarm input status relay which will 'make' a digital