Bryant Condensing Gas 355CAV - Manual (Page 48)

48
and operate the inducer motor IDM at full speed until
flame is no longer proved.
5. Inducer Speed Change — If the cycle starts in
mediumheat, 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 after flame sense. The reduction in speed in
medium--heat is to optimize combustion for maximum
efficiency.
6. Blower--On delay — If the burner flame is proven the
blower--ON delay for medium--heat and high--heat are as
follows:
Medium--heat — 60 seconds after the gas valve GV--M is
opened the blower motor BLWM is turned ON at low-- or
medium--heat airflow.
High--heat — 35 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 re-
main energized throughout the heating cycle.
7. Switching from Medium-- to Low--Heat — If the fur-
nace control switches from medium--heat to low--heat, the
furnace control will turn the blower ON at low--heat air-
flow, energize the HPSR relay to open the NC contact, and
slowly decrease the inducer motor speed. When the HPSR
relay is energized and the NC contact opens the throttling
valve TV is deenergized and the gas flow reduces to low--
heat rate.
Switching from Low-- to Medium--Heat — If the fur-
nace control CPU switches from low--heat to medium--
heat, the furnace control CPU will de--energize the HPSR
relay to close the NC contact and slowly increase the in-
ducer motor speed until the medium--heat pressure switch
MPS closes. When the medium--heat pressure switch MPS
closes, the throttling valve solenoid TV 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 medium--
heat and high--heat mode. The blower motor BLWM will
transition to medium--heat airflow five seconds after the
furnace control CPU switches from low--heat to medium--
heat.
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 HPSR relay to
close the NC contact and slowly increase the inducer mo-
tor speed until the medium--heat pressure switch MPS
closes. When the medium--heat pressure switch MPS
closes, the throttling valve solenoid TV 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 medium--
and high--heat mode. The blower motor BLWM will tran-
sition to high--heat airflow five seconds after the furnace
control CPU switches from low--heat to high--heat. As the
inducer RPM gradually increases the high--heat pressure
switch HPS closes and the gas valve solenoid GV--HI is
energized.
Switching from Medium-- to High--Heat — If the fur-
nace control CPU switches from medium--heat to high--
heat, the furnace control CPU will gradually increase the
inducer motor speed to the required high--heat RPM. The
blower motor BLWM will transition to high--heat airflow
five seconds after the furnace control CPU switches from
medium--heat to highheat. As the inducer RPM gradually
increases the high--heat pressure switch HPS closes and
the gas valve solenoid GV--HI is energized.
Switching from High-- to Medium-- or Low--Heat —
The furnace control CPU will not switch from high--heat
to medium-- or low--heat while the thermostat R to W cir-
cuit is closed when using a single--stage thermostat.
8. 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--energiz-
ing the throttling valve TV, and 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 blow-
er--OFF delay switches). The furnace control CPU is facto-
ry--set for a 120--second blower-- OFF delay.
Two--Stage Thermostat and Two--Stage Low / High Heating
See Fig. 58 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 furnace will start up in either medium--, or high--heat. The
furnace will operate in low--heat after starting and operating for 1
minute at medium--heat before transitioning to low--heat.
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 medium--heat pressure switch contacts
LPS and MPS are open, then de--energizes the HPSR relay to
close the NC contact.
The start up and shut down functions and delays described above
apply to the 2--stage low/high heating mode as well, except for
switching from high-- to low--heat.
1. 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 gradually decrease
the inducer motor speed to the required medium--heat
RPM. When the inducer motor IDM reduces pressure
sufficiently, the highheat pressure switch HPS will open
and the high--heat gas valve solenoid GV--HI will be
de--energized. The gas valve solenoid GV--M will remain
energized as long as the low--heat pressure switch LPS
remains closed. When the inducer motor speed gets within
15% of the required medium--heat RPM the furnace
control CPU will start a 5 second blower airflow change
delay. After the 5 second blower airflow change delay is
completed the blower airflow will transition to
low--heatairflow. At this point the furnace control CPU
will energize the HPSR relay to open the NC contact and
slowly decrease the inducer motor speed to the required
low--heat RPM. When the HPSR relay is energized and
the NC contact opens the throttling valve TV is
de--energized and the gas flow reduces to low--heat rate.
When the inducer motor IDM reduces pressure
sufficiently, the medium--heat pressure switch MPS will
open.
Two--Stage Thermostat and Two--Stage Medium/High
Heating
See Fig. 58 for thermostat connections.
NOTE: In this mode the medium--heat only switch SW4--2 must
be ON to select the medium--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 medium--heat only
switch.
355C
A
V