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Sequence of Operation
2:1 Staged Sequence
1. Exhaust Fan Contact (S1) Manually Closed
(optional)
• Power passes through N.C. contact on exhaust fan
overload (ST2 OL), which is closed if exhaust fan
(M2) has not overloaded
• Power passes to exhaust fan starter (ST2)
• N.O. contact on exhaust fan starter (ST2) is
energized and closed
• Power passes to and energizes exhaust fan (M2)
2. Supply Fan Contact (S2) Manually Closed
• Power passes through N.C. field-supplied fire
contact (FSC)
• Power passes through optional N.O. contact on
exhaust fan starter (ST2), which is closed when the
optional exhaust starter (ST2) is activated
• Power passes through N.C. contact on supply
starter overload (ST1 OL), which is closed if the
supply fan has not overloaded
• Power passes through N.C. contact on optional
freeze protection timer (RT4), which is closed if the
temperature has remained above the set point
• Power passes to and energizes optional inlet
damper (D1), which opens
• Power passes through optional damper limit switch
(DL1), which is energized and closed if the optional
inlet damper is open. It may take several minutes
for the damper to fully open and for the damper
limit switch to close
• Power passes to and energizes fan relay (RF)
• Power passes through N.O. contact on fan relay
(RF), which is closed once the fan relay (RF) is
activated
• Power passes to and energizes starter relay (ST1)
• N.O. contact on supply fan starter (ST1) is
energized and closed
• Supply fan (M1) starts
3. Heat Contact (S4) Manually Closed
• Power passes through N.O. fan relay (RF), which is
energized and closed if the supply fan (M1) is on
• Power passes through N.C. contact on optional
inlet air sensor (TS4), which is closed if inlet air
temperature is below the set point
• Power passes to and energizes the heat relay (RH)
• N.O. contact on heat relay (RH) closes
• 24 VAC is supplied to stage controller (SC1)
• If the discharge temperature is less than the set
point on the discharge air sensor (TS2) and the
high temperature limit control (HLC1) has not
been tripped, the N.O. contact for furnace stage
controller will close
• Power will be supplied to the ignition controller
(IC1), which will begin its sequence of operation
4. Ignition Controller (IC1) Sequence of Operation
• The N.O. contact on air proving switch (PS2) is open
• The ignition controller (IC1) energizes the
combustion blower relay (CR)
• N.O. contact on combustion blower relay (CR)
closes
• Power passes to and energizes the combustion
blower (CM)
• The N.O. contact on air proving switch (PS2) closes
• The ignition controller (IC1) begins a 15 second
pre-purge
• The high fire relay (RT3) is energized and the N.O.
contact on high fire relay (RT3) closes
• The main gas valve (MV) fully opens (100%)
• Igniter begins sparking
• 10 second trial for ignition begins
• The furnace will light at high fire (100%)
• When the flame is detected, the igniter stops
sparking
• The furnace will remain at high fire (100%) for at
least 10 seconds
• High fire contact (RT3) will open
• Furnace stages to maintain the discharge air
temperature set point (SC1)
5. High Fire – Low Fire Sequence of Operation
• The furnace lights at high fire (100%) and remains
at high fire for 10 seconds
• If the discharge temperature is above the discharge
air sensor (TS2) set point, the N.O. furnace stage 2
controller contact (SC2), will open and the furnace
will go to low fire (50%)
• If the furnace is at low fire (50%) and the discharge
temperature is above the discharge air sensor
(TS2) set point, the furnace stage 1 contact (SC1)
will open and the furnace will shut down
• If the furnace is at low fire (50%) and the discharge
temperature is below the discharge air sensor (TS2)
set point, the furnace stage 2 contact (SC2) will
close and the furnace will go to high fire
6. Optional Evaporative Cooling Contact (S4)
Closed*
• N.O. contact on fan relay (RF) is energized and
closed
• Power passes through N.O. contact on optional
inlet air sensor (TS4), which is energized and
closed if the inlet air temperature is above the set
point
• Power passes to and energizes cool relay (RC)
• N.O. contact on cool relay (RC) is energized and
closed
• Power passes to evaporative cooling pump (P1)
*If DX or chilled water coils are used rather than an
evaporative cooler, the cooling sequence of operation
will depend on the coil controls. Cooling coil controls are
supplied by others.
®
Model IG / IGX Make-Up Air
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