Two-Stage Thermostat And Two-Stage Low/High Heating; Two-Stage Thermostat And Two-Stage Medium/High Heating - Bryant 355CAV A Series Troubleshooting Manual

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
furnace control CPU will de-energize the HPSR relay to
close the NC contact and slowly increase the inducer 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- and
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. 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 furnace
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 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 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 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
throttling valve TV, and humidifier terminal HUM. The
inducer motor IDM will remain energized for a 15-second
post-purge period.
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
LOW/HIGH HEATING
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
The blower motor BLWM and air
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.
a. 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 high-heat 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-heat airflow. 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.
3. TWO-STAGE THERMOSTAT AND TWO-STAGE
MEDIUM/HIGH HEATING
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.
The wall thermostat "calls for heat", closing the R to W1 circuit
for medium-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 medium/high heating mode as well, except
for switching from high- to medium-heat.
a. Switching from High- to Medium-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 high-heat 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 medium-
heat airflow.
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