Bryant Bruant 4 Way Gas 355AAV - Manual (Page 47)

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ignited by the Hot Surface Igniter HSI. Five seconds after
the GVR closes, a 2--second flame 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.
4. Flame--Proving -- When burner flame is proved at the
flameproving sensor electrode FSE, 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 furnace control CPU will repeat
the ignition sequence for up to three more Trials--For--Igni-
tion before going to Ignition--Lockout. Lockout will be re-
set automatically after three hours, by momentarily inter-
rupting 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.
5. Inducer Speed Change -- If the cycle starts in low--heat,
the furnace control CPU reduces the inducer speed slightly
after flame sense. If cycle starts in high--heat, the furnace
control CPU increases the inducer speed 15 seconds after
flame sense. The reduction in speed in low--heat is to optim-
ize combustion for maximum efficiency.
6. Blower--On delay -- If the burner flame is proven, the
blower--ON delay for low--heat and high--heat are as fol-
lows:
Low--heat--60 seconds after the gas valve GV--M is opened,
the BLWM is turned ON at low--heat airflow.
High--heat--35 seconds after 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 energized
throughout the heating cycle.
7. Switching From Low-- To High-- Heat -- If the furnace
control CPU switches from low--heat to high--heat, the fur-
nace control CPU will de--energize the the high--heat pres-
sure switch relay HPSR to close the NC contact and slowly
increase the inducer motor speed until the high--heat pres-
sure switch HPS closes. When the high--heat pressure
switch HPS closes, the high--heat gas valve solenoid GV--
HI is energized and the inducer motor RPM is noted by the
furnace control CPU. The RPM is used to evaluate vent sys-
tem resistance. This evaluation is then used to determine the
required RPM necessary to operate the inducer motor in
high--heat mode. The blower motor BLWM will transition
to high--heat airflow five seconds after the furnace control
CPU switches from low--heat to high--heat.
8. 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.
9. 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 hu-
midifier terminal HUM. The inducer motor IDM will re-
main 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 con-
trol CPU is factory--set for a 120--second blower--OFF
delay.
TWO--STAGE THERMOSTAT AND TWO--
STAGE HEATING
See Fig. 57 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 and verifies the
low--heat and high--heat pressure switch contacts LPS and HPS are
open.
The start--up and shutdown functions and delays described in Item
1. above apply to 2--stage heating mode as well, except for
switching from low-- to high--heat and vice versa.
1. Switching From Low-- To High-- Heat--If the thermostat R
to W1 circuit is closed and the R toW2 circuit closes, the
furnace control CPU will de--energize the high--heat
pressure switch relay HPSR to close the NC contact and
slowly increase the inducer motor speed until the high--heat
pressure switch HPS closes. When the high--heat pressure
switch closes, the highheat gas valve solenoid GV--HI is
energized and the inducer motor RPM is noted by the
furnace control CPU. The RPM is used to evaluate vent
system resistance. This evaluation is then used to determine
the required RPM necessary to operate the inducer motor in
high--heat mode. The blower motor BLWM will transition
to high--heat airflow five seconds after the R to W2 circuit
closes.
2. 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 energize the high--heat
pressure switch relay HPSR to open the NC contact and
slowly decrease the inducer motor speed to the required
low--heat RPM. When the high--heat pressure switch HPS
opens, the high--heat gas valve solenoid GV--HI is
de--energized. 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 closed. The blower motor BLWM will transition to
low--heat airflow five seconds after the R to W2 circuit
opens.
COOLING MODE
The thermostat “calls for cooling”.
1. Single--Speed Cooling (See Fig. 32 and Fig. 51 for
thermostat connections.)
The thermostat closes R--to--G--and--Y circuits. The R--to--Y
circuit starts the outdoor unit, and 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. 47.
The electronic air cleaner terminal EAC--1 is energized with
115--v when blower motor BLWM is operating.
When the thermostat is satisfied, the R--to--G--and--Y
circuits are opened. The outdoor unit will stop, and furnace
blower motor BLWM will continue operating at cooling
airflow for an additional 90 sec. Jumper Y/Y2 to DHUM to
reduce the cooling off--delay to 5 seconds. (See Fig. 37.)
2. Single--Stage Thermostat and Two--Speed Cooling
(Adaptive Mode)
(See Fig. 58 for thermostat connections.)
This furnace can operate a two--speed cooling unit with a
single--stage thermostat because the furnace control CPU
includes a programmed adaptive sequence of controlled
355A