Bryant Condensing Gas 355CAV User Manual

Page 49

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.

1. 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 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 medium--heat airflow.

Cooling Mode
The thermostat “calls for cooling.”

2. Single--Speed Cooling

See Fig. 29 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. 47.
The electronic air cleaner terminal EAC--1 is energized
with 115 vac when the 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 the

furnace blower motor BLWM will continue operating at
cooling airflow for an additional 90 seconds. Jumper
Y/Y2 to DHUM to reduce the cooling off--delay to 5

seconds. (See Fig. 33.)

3. Single--Stage Thermostat and Two--Speed Cooling

(Adaptive Mode)
See Fig. 59 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

operation, which selects low--cooling or high--cooling
operation. This selection is based upon the stored history

of the length of previous cooling period of the
single--stage thermostat.

NOTE: The air conditioning relay disable jumper ACRDJ must
be connected to enable the adaptive cooling mode in response to
a call for cooling. (See Fig. 33.) When in place the furnace
control CPU can turn on the air conditioning relay ACR to
energize the Y/Y2 terminal and switch the outdoor unit to
high--cooling.
The furnace control CPU can start up the cooling unit in either

low-- or high--cooling. If starting up in low--cooling, the furnace

control CPU determines the low--cooling on--time (from 0 to 20
minutes) which is permitted before switching to high--cooling.
If the power is interrupted, the stored history is erased and the
furnace control CPU will select low--cooling for up to 20 minutes

and then energize the air conditioning relay ACR to energize the

Y/Y2 terminal and switch the outdoor unit to high--cooling, as
long as the thermostat continues to call for cooling. Subsequent

selection is based on stored history of the thermostat cycle times.

The wall thermostat “calls for cooling”, closing the R to G and--
Y circuits. The R to Y1 circuit starts the outdoor unit on

low--cooling speed, and the R to G--and--Y1 circuits starts the
furnace blower motor BLWM at low--cooling airflow which is the

true on--board CF selection as shown in Fig. 47.

If the furnace control CPU switches from low--cooling to high
cooling, the furnace control CPU will energize the air

conditioning relay ACR. When the air conditioning relay ACR is
energized the R to Y1--and--Y2 circuits switch the outdoor unit to

high--cooling speed, and the R to G--and--Y1-- and--Y/Y2 circuits

transition the furnace blower motor BLWM to high--cooling
airflow. High--cooling airflow is based on the A/C selection

shown in Fig. 47.
NOTE: When transitioning from low--cooling to high--cooling
the outdoor unit compressor will shut down for 1 minute while
the furnace blower motor BLWM transitions to run at
high--cooling airflow.
The electronic air cleaner terminal EAC--1 is energized with 115
vac whenever the blower motor BLWM is operating.

When the thermostat is satisfied, the R to G--and--Y circuit are

opened. The outdoor unit stops, and the furnace blower BLWM

and electronic air cleaner terminal EAC--1 will remain energized
for an additional 90 seconds. Jumper Y1 to DHUM to reduce the

cooling off--delay to 5 seconds. (See Fig. 33.)

4. Two--Stage Thermostat and Two--Speed Cooling

See Fig. 58 for thermostat connections.

NOTE: The air conditioning relay disable jumper ACRDJ must
be disconnected to allow thermostat control of the outdoor unit
staging. (See Fig. 33.)
The thermostat closes the R to G--and--Y1 circuits for low cooling

or closes the R to G--and--Y1--and--Y2 circuits for high cooling.
The R to Y1 circuit starts the outdoor unit on low cooling speed,

and the R to G--and--Y1 circuit starts the furnace blower motor

BLWM at low--cooling airflow which is the true on--board CF
selection as shown in Fig. 47. The R to Y1--and--Y2 circuits start

the outdoor unit on high--cooling speed, and the R to
G--and--Y/Y2 circuits start the furnace blower motor BLWM at

high--cooling airflow. High--cooling airflow is based on the A/C

selection shown in Fig. 47.
The electronic air cleaner terminal EAC--1 is energized with 115

vac whenever the blower motor BLWM is operating.
When the thermostat is satisfied, the R to G--and--Y1 or R to

G--and--Y1--and--Y2 circuits are opened. The outdoor unit stops,
and the furnace blower BLWM and electronic air cleaner terminal

EAC--1 will remain energized for an additional 90 seconds.

Jumper Y1 to DHUM to reduce the cooling off--delay to 5
seconds. (See Fig. 33.)
Thermidistat Mode
See Fig. 52--55 for thermostat connections.
The dehumidification output, DHUM on the Thermidistat should
be connected to the furnace control thermostat terminal DHUM.

When there is a dehumidify demand, the DHUM input is

activated, which means 24 vac signal is removed from the
DHUM input terminal. In other words, the DHUM input logic is

reversed. The DHUM input is turned ON when no dehumidify
demand exists. Once 24 vac is detected by the furnace control on

the DHUM input, the furnace control operates in Thermidistat
mode. If the DHUM input is low for more than 48 hours, the

furnace control reverts back to non--Thermidistat mode.
The cooling operation described above also applies to operation
with a Thermidistat. The exceptions are listed below:

1. Low cooling – When the R to G--and--Y1 circuit is closed

and there is a demand for dehumidification, the furnace

blower motor BLWM will drop the blower airflow to 86%
of low cooling airflow which is the true on--board CF
selection as shown in Fig. 47.

355C