International comfort products F8MVL Installation Instructions Manual page 50

HPS are open, and starts the inducer motor IDM in
high−speed.
a. Inducer Prepurge Period
(1.) If the furnace control CPU selects low−heat op-
eration the inducer motor IDM comes up to
speed, the low−heat pressure switch LPS
closes, and the furnace control CPU begins a
15−second prepurge period. If the low−heat
pressure switch LPS fails to remain closed the
inducer motor IDM will remain running at high−
speed. After the low−heat pressure switch re−
closes the furnace control CPU will begin a
15−second prepurge period, and continue to run
the inducer motor IDM at high−speed.
(2.) If the furnace control CPU selects high−heat
operation, the inducer motor IDM remains run-
ning at high−speed, and the high−heat pressure
switch relay HPSR is de−energized to close the
NC contact. When sufficient pressure is avail-
able the high−heat pressure switch HPS closes,
and the high−heat gas valve solenoid GV−HI is
energized. The furnace control CPU begins a
15−second prepurge period after the low−heat
pressure switch LPS closes. 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 maxim-
um of 2 minutes after ignition.
b. Igniter Warm−Up −At the end of the prepurge peri-
od, the Hot−Surface Igniter HSI is energized for a
17−second igniter warm−up period.
c. Trial−For−Ignition Sequence −When the igniter
warm−up period is completed the main gas valve
relay contact GVR closes to energize the gas valve
solenoid GV−M. The gas valve solenoid GV−M per-
mits gas flow to the burners where it is ignited by the
HSI. Five seconds after the GVR closes, a 2−second
flame proving period begins. The HSI igniter will re-
main energized until the flame is sensed or until the
2−second flame proving period begins. If the furnace
control CPU selects high−heat operation, the high−
heat gas valve solenoid GV−HI is also energized.
d. Flame−Proving − When the burner flame is proved
at the flame−proving sensor electrode FSE, the in-
ducer motor IDM switches to low−speed unless the
furnace is operating in 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 SEC1 or SEC2 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.
e. Blower−On delay − If the burner flame is proven the
blower−ON delays for low−heat and high−heat are
as follows:
Low−heat − 45 seconds after the gas valve GV−M
is opened the blower motor BLWM is turned ON at
low−heat airflow.
High−heat − 25 seconds after the gas valve GV−M
50
2. Two−Stage Thermostat and Two−Stage Heating
See Figure 27−Figure 33 for thermostat connections.
NOTE: In this mode the low−heat only switch SW1−2 must be
ON to select the low−heat only operation mode in response to
closing the thermostat R−to−W1 circuit. Closing the thermostat
R−to−
operation, regardless of the setting of the low−heat only
switch.
The wall thermostat "calls for heat", closing the R−to−W1
circuit for low−heat or closing the R−to−W1−and−W2 circuits
for high−heat. 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 start up and shut down functions and delays described in
item 1. above apply to the 2−stage heating mode as well,
except for switching from low− to high−Heat and vice versa.
Specifications are subject to change without notice.
is opened the BLWM is turned ON at high−heat air-
flow. Simultaneously, the humidifier terminal HUM
and electronic air cleaner terminal EAC−1 are ener-
gized and remain energized throughout the heating
cycle.
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 transition to high−heat
airflow five seconds after the furnace control CPU
switches from low−heat to high−heat.
g. Switching from High− to Low−Heat −The furnace
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.
h. Blower−Off Delay −When the thermostat is satis-
fied, the 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. The
inducer motor IDM will remain energized for a
5−second post−purge period. The blower motor BL-
WM and air cleaner terminal EAC−1 will remain en-
ergized at low−heat airflow or transition to low−heat
airflow for 90, 120, 150, or 180 seconds (depending
on selection at blower−OFF delay switches). The
furnace control CPU is factory−set for a 120−second
blower−OFF delay.
W1−and−W2 circuits always
a. Switching from Low− to High−Heat − If the ther-
mostat R−to−W1 circuit is closed and the R−to−W2
circuit closes, the furnace control CPU will switch the
inducer motor IDM speed from low to high. The
high−heat pressure switch relay HPSR is de−ener-
gized to close the NC contact. When sufficient pres-
sure 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 transition
to high−heat airflow five seconds after the R−to−W2
circuit closes.
b. Switching from High− to Low−Heat −If the thermo-
stat R−to− W2 circuit opens, and the R−to−W1 cir-
cuit remains closed, the furnace control CPU will
switch the inducer motor IDM speed from high to
low. The high−heat pressure switch relay HPSR is
energized to open the NC contact and de−energize
the high−heat gas valve solenoid GV−HI. When the
inducer motor IDM reduces pressure sufficiently, the
high−heat pressure switch HPS will open. The gas
valve solenoid GV−M will remain energized as long
as the low−heat pressure switch LPS remains
causes high−heat
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