Bryant 986TB Installation, Start-Up, Operating And Service And Maintenance Instructions page 87

(1.) If the furnace control CPU selects low−heat opera-
tion the inducer motor IDM comes up to speed, the
low−heat pressure switch LPS closes, and the fur-
nace control CPU begins a 15−sec 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−sec prepurge period, and continue to run the
inducer motor IDM at high−speed.
(2.) If the furnace control CPU selects high−heat opera-
tion, the inducer motor IDM remains running at
high−speed, and the high−heat pressure switch re-
lay 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 furnace
control CPU begins a 15−sec 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 fur-
nace will operate at low−heat gas flow rate until the
high−heat pressure switch closes for a maximum of
2 minutes after ignition.
b. Igniter Warm−Up −At the end of the prepurge period, the
Hot−Surface Igniter HSI is energized for a 17−sec 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 permits gas flow to the burners
where it is ignited by the HSI. Five sec after the GVR
closes, a 2−sec flame proving period begins. The HSI ig-
niter will remain energized until the flame is sensed or until
the 2−sec flame proving period begins. If the furnace con-
trol 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 inducer 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 sec, 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 Igni-
tion−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 fol-
lows:
Low−heat − 45 sec after the gas valve GV−M is opened
the blower motor BLWM is turned ON at low−heat airflow.
High−heat − 25 sec after the gas valve GV−M is opened
the BLWM is turned ON at high−heat airflow. Simultan-
eously, the humidifier terminal HUM and electronic air
cleaner terminal EAC−1 are energized and remain ener-
gized throughout the heating cycle.
f. Switching from Low− to High−Heat − If the furnace con-
trol 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 suffi-
cient 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 sec after the furnace control CPU
switches from low−heat to high−heat.
g. Switching from High− to Low−Heat −The furnace con-
trol 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 satisfied, 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 re-
main energized for a 15−sec post−purge period. The
blower motor BLWM and air cleaner terminal EAC−1 will
remain energized at low−heat airflow or transition to low−
heat airflow for 90, 120, 150, or 180 sec (depending on se-
lection at blower−OFF delay switches). The furnace con-
trol CPU is factory−set for a 120−sec blower−OFF delay.
2. Two−Stage Thermostat and Two−Stage Heating
See Fig. 38 and 39 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− W1−and−W2 circuits always causes high−heat 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.
a. Switching from Low− to High−Heat − If the thermostat
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−energized to close the NC contact.
When sufficient pressure is available the high−heat pres-
sure switch HPS closes, and the high−heat gas valve solen-
oid GV−HI is energized. The blower motor BLWM will
transition to high−heat airflow five sec after the R−to−W2
circuit closes.
b. Switching from High− to Low−Heat −If the thermostat
R−to− W2 circuit opens, and the R−to−W1 circuit remains
closed, the furnace control CPU will switch the inducer
motor IDM speed from high to low. The high−heat pres-
sure 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 suffi-
ciently, 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 closed. The
blower motor BLWM will transition to low−heat airflow
five sec after the R−to−W2 circuit opens.
3. Cooling mode
The thermostat "calls for cooling".
a. Single−Speed Cooling−
See Fig. 38 and 39 for thermostat connections
The thermostat closes the R−to−G−and−Y circuits. The
R−to− Y circuit starts the outdoor unit, and the R−to−
G−and−Y/Y2 circuits start the furnace blower motor
BLWM on cooling airflow. Cooling airflow is based on the
A/C selection shown in Fig. 64. The electronic air cleaner
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