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Sequence of Operation
2:1 Modulation 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. contact on fan relay
(RF), which is energized and closed if the supply
fan (M1) is on
• Power passes through the N.C. contact on optional
inlet air sensor (TS4), which is closed if inlet air
temperature is below set point
• Power passes to and energizes the heat relay (RH)
• Power passes to N.C. contact on high temperature
limit control (HLC1), which remains closed if it has
not been tripped
• N.O. contact on heat relay (RH) closes
• 24 VAC is supplied to ignition controller (IC1) and
amplifier (AMP)
• If the discharge air sensor (TS2) reading is less
than the temperature selector (TS3) setting, the
amplifier (AMP) sends a call for heating to the
ignition controller (IC1)
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%) and the
modulating gas valve (MOD) opens to high fire
• 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 modulates to maintain the temperature
selector (TS3) set point
5. High Fire – Low Fire Sequence of Operation
• The furnace lights at and remains at high fire
(100%) for 10 seconds
• If the discharge temperature sensor (TS2) reading
is above the temperature selector (TS3) set point,
and furnace is not at low fire, the amplifier will
adjust the modulating gas valve (MOD) down until
the discharge temperature sensor (TS2) reading
equals the temperature selector (TS3) setting
• If the discharge temperature sensor (TS2) reading
is below the temperature selector (TS3) set point,
and the furnace is not at high fire, the amplifier will
adjust the modulating gas valve (MOD) up until the
discharge temperature sensor (TS2) reading equals
the temperature selector (TS3) setting
• If the furnace is at low fire and the discharge
temperature sensor (TS2) reading is above the
temperature selector (TS3) set point, the amplifier
will end the call for heat and the ignition controller
(IC1) will shut down the furnace
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)
®
Model IG / IGX Make-Up Air
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