Bryant 373LAV User Manual

Page 14

1. Heating mode

When the thermostat "calls for heat," R-W circuit closes.
The furnace control performs a self-check, verifies the
pressure switch contacts are open, and starts the inducer
motor.

a. Prepurge period—As the inducer motor comes up to

speed, the pressure switch contacts close to begin a
15-sec prepurge period.

b. Igniter warm-up—At the end of the prepurge period, the

igniter is energized for a 17-sec igniter warm-up period.

c. Ignition sequence—When the igniter warm-up period is

completed, the gas valve opens, permitting gas flow to
the burners where it is ignited. After 5 sec, the igniter is
de-energized and a 2-sec flame-sensing period begins.

d. HUM terminal is energized with the gas valve. See

Accessories in Electrical Connections section.

e. Flame-sensing—When burner flame is sensed, the con-

trol begins the blower on delay period and continues
holding the gas valve open. If burner flame is not sensed,
the control closes the gas valve and repeats the ignition
cycle.

NOTE:

Ignition sequence will repeat 3 additional times before a

lockout occurs. Lockout automatically resets after 3 hrs, or can be
manually reset by turning 115v off (not at thermostat) for 3 sec
minimum, then on again.

f. Blower on delay—Forty-five sec after burner flame is

proven, the blower motor is energized on heating speed.
Simultaneously, the humidifier and electronic air cleaner
terminals (HUM and C

-24V for humidifier, EAC-1

and EAC-2 for electronic air cleaner) are energized.

g. Blower off delay—When the thermostat is satisfied, the

circuit between R and W is broken, de-energizing the gas
valve and stopping gas flow to the burners. The blower
motor, and EAC remain energized for 135 sec.

h. Post-purge—The inducer motor remains energized 5 sec

after the burners are extinguished.

2. Cooling mode

When the thermostat "calls for cooling," R-G and R-Y
circuits close. The R-Y circuit starts the outdoor condensing
unit and combined R-Y and R-G circuits start the furnace
blower motor on cooling speed. The EAC-1 terminal is
energized with 115v whenever the blower is operating on
cooling speed.

When the thermostat is satisfied, R-G and R-Y circuits are
broken. Furnace blower and EAC continue operating on
cooling speed for an additional 90 sec.

3. Continuous blower mode

NOTE:

EAC-1 terminal is energized with 115v whenever blower

is operating.

When the R-G circuit is made, the blower motor operates on
heating speed. During a call for heat, the blower stops,
allowing the furnace heat exchangers to heat up more
quickly, then restarts at the end of the 45-sec blower on
delay period.

The blower reverts to continuous operation after the heating
cycle is completed.

When the thermostat "calls for cooling," the blower oper-
ates on cooling speed. When the thermostat is satisfied, the
blower operates an additional 90 sec before reverting back
to continuous operation on heating speed.

4. Heat pump mode

When installed with a heat pump, the furnace control
automatically changes the timing sequence to avoid long
blower off time during demand defrost cycles. When the
W-Y or W-Y-G thermostat inputs are received at the same
time, the control changes the blower to heating speed or
starts the blower if it was off, and begins a heating cycle.
The blower remains on until the end of the prepurge period,
then shuts off until the end of the ignition warm up and trial
for ignition periods (a total of 24 sec). The blower then
comes back on at heating speed.

When the W input signal disappears, the control begins the
normal inducer post-purge period and the blower changes to
cooling speed after a 1-sec delay. If the W-Y-G signals
disappear at the same time, the blower remains on for the
selected heating blower off delay period and the inducer
goes through its normal post-purge period. If the W-Y
inputs should disappear, leaving the G signal input, the
control goes into continuous blower and the inducer re-
mains on for the normal post-purge period.

Anytime the control senses false flame, the control locks
out of the heating mode. This occurs because the control
cannot sense the W input due to the false flame signal and,
as a result, sees only the Y input and goes into cooling mode
blower off delay. All other control functions remain in
standard format.

NOTE:

EAC-1 terminal is energized whenever blower operates.

HUM terminal is only energized when gas valve is energized.

Start-up Procedures

1. Purge gas lines—After all connections have been made,

purge the lines and check for leaks.

WARNING:

Never purge a gas line into a combustion

chamber. Never use matches, candles, flame, or other
sources of ignition to check for gas leakage. Use a
soap-and-water solution to check for gas leaks. Failure to
follow this warning could result in fire, explosion, per-
sonal injury, or death.

2. Component test—The furnace control allows all compo-

nents, except gas valve, to be run for a short period of time.

This feature helps diagnose a system problem in case of a
component failure. To initiate component test procedure,
short (jumper) the TEST 3/16-in. quick connect terminal on
control (adjacent to diagnostic light) and the C

-24V

terminal on furnace thermostat connection block for ap-
proximately 2 sec. (See Fig. 14.)

NOTE:

Component test feature will not operate if any thermostat

signal is present at control.

Component test sequence is as follows.

a. Momentarily jumper TEST and C

-24V terminals until

LED goes off.

b. LED will display previous status 4 times.

c. Inducer motor starts and continues to run for entire

component test.

d. Hot surface igniter is energized for 15 sec, then de-

energized.

e. Blower motor operates on cooling speed for 10 sec, then

stops.

f. Blower motor operates on heating speed for 10 sec, then

stops.

g. Inducer motor stops.

—14—

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