Sequence Of Operation - International comfort products F9MXT0401410A Service And Technical Support Manual

SERVICE AND TECHNICAL SUPPORT MANUAL

Sequence of Operation

NOTE: Furnace control must be grounded for proper operation
or control will lockout. Control is grounded through green wire
connected to gas valve and burner bracket screw. Using the
schematic diagram in Figure 15, follow the sequence of
operation through the different modes. Read and follow the
wiring diagram very carefully.
NOTE: If a power interruption occurs during a call for heat
(W/W1 or W/W1−and−W2), the control will run the blower for
the selected heat off−delay period two seconds after power is
restored, if the thermostat is still calling for gas heating. The
green LED light will flash a status code 1+2 during this period,
after which the LED will switch to a heartbeat, as long as no
faults are detected. After this period, the furnace will respond to
the thermostat normally.
The blower door must be installed for power to be conducted
through the blower door interlock switch ILK to the furnace
control CPU, transformer TRAN, inducer motor IDM, blower
motor BLWM, hot−surface igniter HSI, and gas valve GV.
Two−Stage Heating with Single−Stage Thermostat
NOTE: The thermostat type switch (TT) selects either the
two−stage thermostat operation mode when ON, (see item 2)
or the single stage thermostat operation mode when OFF in
response to a call for heat. When the W2 thermostat terminal is
energized it will always cause high−heat operation when the R
to W circuit is closed, regardless of the setting of the thermostat
type switch. This furnace can operate as a two−stage furnace
with a single−stage thermostat because the furnace control
CPU includes a programmed sequence of controlled operation,
which selects low−heat for the first 12 minutes of operation then
switches to high heat operation.
If the power is interrupted, the control CPU will operate in low
heat for 12 minutes then it will switch to high−heat, as long as
the thermostat continues to call for heat.
The wall thermostat "calls for heat", closing the R to W circuit.
The furnace control performs a self−check, verifies the
low−heat and high−heat pressure switch contacts LPS and
HPS are open, and starts the inducer motor IDM in
high−speed. The HUM terminal is energized for a 115V
humidifier (if used).
a. Inducer Prepurge Period (see Figure 15 schematic
diagram)
(1.) Low heat (W/W1) the inducer motor IDM comes up
to high−speed, the low−heat pressure switch LPS
closes. After the low−heat pressure switch closes
the furnace control CPU will begin a 15−second
prepurge period, and continue to run the inducer
motor IDM at high−speed.
(2.) High Heat (W/W1+W2) the inducer motor IDM
comes up to high speed and the high−heat
pressure switch relay HPSR is de−energized to
close the NC contact. When sufficient pressure is
available, the high−heat pressure switch HPS
closes, and the high−heat gas valve solenoid
GV−HI is energized. After LPS closes, 24VAC
power is supplied for a field−installed humidifier at
the HUM 24VAC terminal and the furnace control
CPU begins a 15−second prepurge period. If the
high−heat pressure switch HPS fails to close and
the low−heat pressure switch LPS closes, the
furnace will operate at low−heat gas flow rate until
the high−heat pressure switch closes for a
maximum of 2 minutes after ignition. If HPS doesn't
close after 2 minutes from LPS closing, GV is turned
off and heat off delay is completed. Then a new
cycle will begin if a call for heat remains.
b. Igniter Warm−Up−At the end of the prepurge period,
the (Hot Surface Igniter) HSI is energized for a
17−second igniter warm−up period.
26
d. Flame−Proving−When the burner flame is proved at
e. Blower On Delay−If the burner flame is proven, the
g. Switching from High− to Low−Heat−The furnace
h. Heat−Off Delay−When the thermostat is satisfied, the
15. Two−Stage Thermostat and Two−Stage Heating (See
Figure 17 − Figure 22 for thermostat connections)
Specifications subject to change without notice.
Gas Furnace: (F/G)9MXT
c. Trial−for−Ignition Sequence−When
warm−up period is completed the main gas valve relay
contacts GVR close to energize the gas valve solenoid
GV−M, the gas valve opens. The gas valve solenoid
GV−M permits gas flow to the burners where it is ignited
by the HSI. 5 sec after the GVR closes, a 2−sec flame
proving period begins. The HSI igniter will remain
energized until the flame is sensed or until the 2−sec
flame proving period begins. If the furnace control CPU
operates in high−heat operation, the high−heat gas
valve solenoid GV−HI is also energized.
the flame−proving sensor electrode FSE, the inducer
motor IDM switches to low−speed unless running at
high−heat, and the furnace control CPU begins the
blower−ON delay period and continues to hold the gas
valve GV−M open. If the burner flame is not proved
within two seconds, the control CPU will close the gas
valve GV−M, and the control CPU will repeat the ignition
sequence for up to three more Trials−For−Ignition
before going to Ignition− Lockout. Lockout will be reset
automatically after three hours, or by momentarily
interrupting 115 vac power to the furnace, or by
interrupting 24 vac power at 24VAC or COM to the
furnace control CPU (not at W/W1, G, R, etc.). If flame
is proved when flame should not be present, the furnace
control CPU will lock out of Gas−Heating mode and
operate the inducer motor IDM on high speed until flame
is no longer proved.
blower on delay for low−heat and high−heat are as
follows:
Low−Heat−45 seconds after the gas valve GV−M is
energized the blower motor (BLWM) is energized at LO
HEAT speed.
High−Heat−25 seconds after the gas valve GV−M is
energized the BLWM is energized at HI HEAT speed.
Simultaneously, the electronic air cleaner (EAC)
terminal is energized and remains energized as long as
the BLWM is energized.
f. Switching from Low−to High−Heat−If the furnace
control CPU switches from low−heat to high−heat, the
furnace control CPU will switch the inducer motor IDM
speed from low to high. The high−heat pressure switch
relay HPSR is de−energized to close the NC contact.
When sufficient pressure is available the high−heat
pressure switch HPS closes, and the high−heat gas
valve solenoid GV−HI is energized. The blower motor
BLWM will switch to HI HEAT speed 5 seconds after the
furnace control CPU switches from low−heat to
high−heat.
control CPU will not switch from high−heat to low−heat
while the thermostat R−to−W circuit is closed when
using a single stage thermostat.
R to W circuit is opened, de−energizing the gas valve
GV−M, stopping gas flow to the burners, and
de−energizing the humidifier terminal HUM 24VAC. The
inducer motor IDM will remain energized for a
15−second post−purge period then turn off, also turning
off HUM for 115V humidifier. The blower motor BLWM
and air cleaner terminal EAC will remain energized for
90, 120, 150, or 180 seconds (depending on selection
at heat−off delay switches). The furnace control CPU is
factory−set for a 120−second heat−off delay.
the igniter
440 04 4321 08