Bryant 355CAV User Manual

Page 50

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 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

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.

2. High cooling – When the R to G--and Y/Y2 circuit is

closed and there is a demand for dehumidification, the
furnace blower motor BLWM will drop the blower airflow

to 86% of high--cooling airflow. High--cooling airflow is
based on the A/C selection shown in Fig. 47.

3. Cooling off--delay – When the “call for cooling” is satisfied

and there is a demand for dehumidification, the cooling

355C