Bryant 355MAV Troubleshooting Manual page 5

3. Hot surface ignitor is energized for 15 sec, then de-
energized.
4. Main blower motor operates for 20 sec at low speed,
operates for 20 sec at high speed, then turns off.
5. After component operation test is completed, 1 or more
fault codes (11, 22, 41, or 42) will flash. See service label
on back of main furnace door or Fig. 1 for explanation of
fault codes.
NOTE: To repeat component test, turn setup switch SW-6 to OFF
and then back to ON.
After component test, perform the following:
1. Release blower panel access door switch and turn setup
switch SW-6 to OFF position.
2. If applicable, replace blower access panel and check LED
status by removing plug in blower access panel.
3. Reinstall main furnace door if all LEDs are off, indicating
furnace is ready to operate when a signal from thermostat is
received.
IX. BYPASS HUMIDIFIER MODE
When setup switch SW-3 BPH is in ON position, RPM calculated
for low heat is multiplied by 1.15 for all furnace model sizes. This
compensates for increased return-air temperature caused by by-
passed air supply.
X. DEHUMIDIFICATION MODE
A dehumidification input is provided via a 1/4-in. male quick-
connect terminal labeled DEHUM, located next to the transformer
secondary connections. The DEHUM input acts differently de-
pending on which style of variable speed furnace control you have.
The older style variable speed furnace control (shown in Fig. 1)
DOES NOT have a DE connection while the newer style variable
speed furnace control (shown in Fig. 1) has a DE connection. The
newer style variable speed furnace control is expected to be
available by mid 1998. Both of these variable speed furnace
controls function the same except the DEHUM logic is reversed.
This logic reversal has come about from historical use of a
standard humidistat to do dehumidification since the contacts open
on high humidity, thus removing the 24-v signal to initiate
dehumidification.
On the older style variable speed furnace controls, a field supplied
relay is required between the thermidistat or humidistat control and
furnace. The relay coil is connected between DHUM output on the
thermidistat control and C terminal on the furnace control or the
OM
humidistat output and C terminal on the furnace control. Its
OM
normally closed contact is connected between R and DEHUM
terminals on the furnace control. (See Fig. 1 and Fig. 2). When a
dehumidify demand exists, relay is de-energized, and normally
closed contacts supply 24-v to the furnace DEHUM terminal. As
a result, the furnace control reduces the blower airflow by 15
percent to 340 CFM per ton during continuous fan or cooling
operation.
On newer style variable speed furnace controls, a field supplied
relay IS NOT required. The DEHUM output on the thermidistat
control or the humidistat output is connected directly to the
DEHUM terminal on the furnace control. In addition, the DE
jumper located next to the DEHUM terminal must be removed to
enable the DEHUM input. (See Fig. 1 and Fig. 3). When a
dehumidify demand exists, the furnace control reduces the blower
airflow by 21 percent to 315 CFM per ton during continuous fan
or cooling operation.
THERMIDISTAT
24 VAC HOT
DEHUMIDIFY
24 VAC COMM
Fig.2—Without DE Connection
THERMIDISTAT
24 VAC HOT
DEHUMIDIFY
24 VAC COMM
Fig. 3—With DE Connection
This blower speed reduction compensates for high humidity
conditions during cooling operation on all furnace model sizes.
Reference Thermidistat™ Control Installation, Start-up, and Op-
erating Instructions for further details.
XI. ZONE MODE
When setup switch SW-5 MZ is in ON position, blower motor
control is the same as above except with the following exceptions:
1. While blower is operating in either low heat or continuous
fan, the coast down calibration is performed once a minute
to update blower RPM for zone damper position changes
during a given cycle.
NOTE: Boards with date codes of 9506 or later will not perform
a coast down calibration if blower pulse width to blower motor is
greater than 60 percent. This prevents nuisance faults from
occurring when a high continuous fan CFM has been selected.
2. While blower is operating in either high heat or cooling,
blower pulse width to blower motor is frozen when blower
RPM is within 10 percent of calculated blower RPM for 5
sec.
—5—
VARIABLE-SPEED
CONDENSING
FURNACE
R
DHUM
C
VARIABLE-SPEED
CONDENSING
FURNACE
R
DHUM
NOTE 1
C
Com
NOTE 1 - Remove DE Connection
To Enable DEHUM Input
R
DEHUM
Com
A98294
R
DEHUM
DE
A98295