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/W1ïandïW2), the control will run the blower for
the selected heat offïdelay 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, hotïsurface igniter HSI, and gas valve GV.
TwoïStage Heating with SingleïStage Thermostat
NOTE: The thermostat type switch (TT) selects either the
twoïstage 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 highïheat operation when the R
to W circuit is closed, regardless of the setting of the thermostat
type switch. This furnace can operate as a twoïstage furnace
with a singleïstage thermostat because the furnace control
CPU includes a programmed sequence of controlled operation,
which selects lowïheat 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 highïheat, 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 selfïcheck, verifies the
lowïheat and highïheat pressure switch contacts LPS and
HPS are open, and starts the inducer motor IDM in
highïspeed. 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 highïspeed, the lowïheat pressure switch LPS
closes. After the lowïheat pressure switch closes
the furnace control CPU will begin a 15ïsecond
prepurge period, and continue to run the inducer
motor IDM at highïspeed.
(2.) High Heat (W/W1+W2) the inducer motor IDM
comes up to high speed and the highïheat
pressure switch relay HPSR is deïenergized to
close the NC contact. When sufficient pressure is
available, the highïheat pressure switch HPS
closes, and the highïheat gas valve solenoid
GVïHI is energized. After LPS closes, 24VAC
power is supplied for a fieldïinstalled humidifier at
the HUM 24VAC terminal and the furnace control
CPU begins a 15ïsecond prepurge period. If the
highïheat pressure switch HPS fails to close and
the lowïheat pressure switch LPS closes, the
furnace will operate at lowïheat gas flow rate until
the highïheat 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 WarmïUpïAt the end of the prepurge period,
the (Hot Surface Igniter) HSI is energized for a
17ïsecond igniter warmïup period.
440 04 4321 02
c. TrialïforïIgnition
d. FlameïProvingïWhen the burner flame is proved at
e. Blower On DelayïIf the burner flame is proven, the
f. Switching from Lowïto HighïHeatïIf the furnace
g. Switching from Highï to LowïHeatïThe furnace
h. HeatïOff DelayïWhen the thermostat is satisfied, the
Specifications subject to change without notice.
Gas Furnace: (F/G)9MXT
SequenceïWhen
warmïup period is completed the main gas valve relay
contacts GVR close to energize the gas valve solenoid
GVïM, the gas valve opens. The gas valve solenoid
GVïM permits gas flow to the burners where it is ignited
by the HSI. 5 sec after the GVR closes, a 2ïsec flame
proving period begins. The HSI igniter will remain
energized until the flame is sensed or until the 2ïsec
flame proving period begins. If the furnace control CPU
operates in highïheat operation, the highïheat gas
valve solenoid GVïHI is also energized.
the flameïproving sensor electrode FSE, the inducer
motor IDM switches to lowïspeed unless running at
highïheat, and the furnace control CPU begins the
blowerïON delay period and continues to hold the gas
valve GVïM open. If the burner flame is not proved
within two seconds, the control CPU will close the gas
valve GVïM, and the control CPU will repeat the ignition
sequence for up to three more TrialsïForïIgnition
before going to Ignitionï 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 GasïHeating mode and
operate the inducer motor IDM on high speed until flame
is no longer proved.
blower on delay for lowïheat and highïheat are as
follows:
LowïHeatï45 seconds after the gas valve GVïM is
energized the blower motor (BLWM) is energized at LO
HEAT speed.
HighïHeatï25 seconds after the gas valve GVïM is
energized the BLWM is energized at HI HEAT speed.
Simultaneously, the electronic air cleaner (EAC 1 AMP)
terminal is energized and remains energized as long as
the BLWM is energized.
control CPU switches from lowïheat to highïheat, the
furnace control CPU will switch the inducer motor IDM
speed from low to high. The highïheat pressure switch
relay HPSR is deïenergized to close the NC contact.
When sufficient pressure is available the highïheat
pressure switch HPS closes, and the highïheat gas
valve solenoid GVïHI is energized. The blower motor
BLWM will switch to HI HEAT speed 5 seconds after the
furnace control CPU switches from lowïheat to
highïheat.
control CPU will not switch from highïheat to lowïheat
while the thermostat RïtoïW circuit is closed when
using a single stage thermostat.
R to W circuit is opened, deïenergizing the gas valve
GVïM, stopping gas flow to the burners, and
deïenergizing the humidifier terminal HUM 24VAC. The
inducer motor IDM will remain energized for a
15ïsecond postïpurge period then turn off, also turning
off HUM for 115V humidifier. The blower motor BLWM
and air cleaner terminal EAC 1 AMP will remain
energized for 90, 120, 150, or 180 seconds (depending
on selection at heatïoff delay switches). The furnace
control CPU is factoryïset for a 120ïsecond heatïoff
delay.
the igniter
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