Bryant 66120 Installation, Start-Up, Operating And Service And Maintenance Instructions page 86

a. Inducer Prepurge Period
(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
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 opera-
tion, the inducer motor IDM remains running at
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.
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 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- -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 permits 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 remain 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 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 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 is
opened the BLWM is turned ON at high- -heat airflow.
Simultaneously, the humidifier terminal HUM and elec-
tronic air cleaner terminal EAC- -1 are energized 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 trans-
ition 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 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
remain energized for a 15- -second 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 seconds (depending
on selection at blower- -OFF delay switches). The furnace
control CPU is factory- -set for a 120- -second 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
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 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 seconds 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
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