51
90--sec period, after which the LED will be ON continuous, as
long as no faults are detected. After the 90--sec period, the furnace
will respond to the thermostat normally.
The blower door must be installed for power to be conducted
through the blower door interlock switch ILK to the furnace
control CPU, transformer TRAN, inducer motor IDM, blower
motor BLWM, hot--surface igniter HSI, and gas valve GV.
1. Heating
(See Fig. 26 for thermostat connections.) The wall
thermostat ¡åcalls for heat,¡å closing the R--to--W circuit.
The furnace control performs a self--check, verifies the
pressure switch contacts PRS are open, and starts the
inducer motor IDM.
a. Inducer Prepurge Period-- As the inducer motor
IDM comes up to speed, the pressure switch contacts
PRS close, 24 VAC power is supplied for a field
installed humidifier at the HUM terminal and the
control begins a 15 --sec prepurge period.
b. Igniter Warm--Up-- At the end of the prepurge period,
the Hot--Surface igniter HSI is energized for a
17--second igniter warm-- up period.
c. Trial--for--Ignition Sequence-- When the igniter
warm-- up period is completed, the main gas valve
relay contacts GVR close to energize the gas valve GV,
the gas valve opens, The gas valve GV permits gas
flow to the burners where it is ignited by the HSI. Five
seconds after the GVR closes, a 2 --second flame
proving period begins. The HSI igniter will remain
energized until the flame is sensed or until the
2--second flame proving period begins.
d. Flame--Proving-- When the burner flame is proved at
the flame--proving sensor electrode FSE, the furnace
control CPU begins the blower--ON delay period and
continues to hold the gas valve GV open. If the burner
flame is not proved within two seconds, the control
CPU will close the gas valve GV, and the control CPU
will repeat the ignition sequence for up to three more
Trials--For--Ignition before going to Ignition--Lockout.
Lockout will be reset automatically after three hours
or by momentarily interrupting 115 vac power to the
furnace, or by interrupting 24 vac power at SEC1 or
SEC2 to the furnace control CPU (not at W, G, R,
etc.).
If flame is proved when flame should not be present,
the furnace control CPU will lock out of Gas-- Heating
mode and operate the inducer motor IDM until flame
is no longer proved.
e. Blower--On Delay-- If the burner flame is proven, the
blower motor is energized on HEAT speed 25 sec after
the gas valve GV is energized.
Simultaneously, the electronic air cleaner terminal
EAC--1 is energized and remains energized as long as
the blower motor BL WM is energized.
f. Blower--Off Delay-- When the thermostat is satisfied,
the R--to--W circuit is opened, de--energizing the gas
valve GV, stopping gas flow to the burners, and
de-- energizing the humidifier terminal HUM. The
inducer motor IDM will remain energized for a
5--second post--purge period. The blower motor
BLWM and air cleaner terminal EAC--1 will remain
energized for 90, 120, 150, or 180 seconds (depending
on the blower--OFF delay selection). The furnace
control CPU is factory --set for a 120--second
blower--OFF delay.
2. Cooling Mode
(See Fig. 26 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 circuits start
the furnace blower motor BLWM on COOL speed. 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 on
the COOL speed for an additional 90 sec. Cut jumper J2 to
reduce the cooling off--delay to 5 sec. (See Fig. 25.)
3. Continuous Blower Mode
When the R--to--G circuit is closed by the thermostat, the
blower motor BLWM will operate on continuous-- blower
speed (same as HEAT speed). Terminal EAC--1 is
energized as long as the blower motor BL WM is
energized.
During a call for heat, the blower BLWM will stop during
igniter warm--up (17 sec), ignition, and blower--ON delay
(25 sec), allowing the furnace heat exchangers to heat up
more quickly, then restarts at the end of the blower--ON
delay period at HEAT speed.
When the thermostat ¡åcalls for cooling¡å, the blower
motor BLWM will operate at COOL speed. When the
thermostat is satisfied, the blower motor BLWM will
operate an additional 90 sec, on COOL speed before
reverting back to continuous blower speed.
When the R--to--G circuit is opened, the blower motor
BLWM will continue operating for an additional 5 sec, if
no other function requires blower motor BLWM
operation.
4. Heat Pump
When installed with a heat pump, the furnace control
automatically changes the timing sequence to avoid long
blower off times during demand defrost cycles. When the
R--to--Wand-- Y or R--to--W--and--Y--and--G circuits are
energized the furnace control CPU will continue to turn on
the blower motor BLWM at HEAT speed, and begin a
heating cycle. The blower motor BLWM will remain on
until the end of the prepurge period, then shut off for 24
sec then come back on at HEAT speed. When theWinput
signal disappears, the furnace control begins a normal
inducer post--purge period and the blower switches to
COOL speed after a 3 sec delay . If the
R--to--W--and--Y--and--G signals disappear at the same
time, the blower motor BLWM will remain on for the
selected blower-- OFF delay period. If the R--to--W--and--Y
signals disappear, leaving the G signal, the blower motor
BLWM will continue running the blower motor at HEAT
speed after the selected blower--OFF delay period is
completed.
Step 4 —Wiring Diagrams
Refer to Fig. 24 and 45 for wiring diagrams.
Step 5 —Troubleshooting
Refer to the service label. (See Fig. 50–Service Label.)
The Troubleshooting Guide can be a useful tool in isolating
furnace operation problems. Beginning with the word Start,
answer each question and follow the appropriate arrow to the next
item.
The Guide will help to identify the problem or failed component.
After replacing any component, verify correct operation
sequence.
58ST
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