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SEQUENCE OF OPERATION
CAUTION: Furnace control must be grounded for
proper operation, or control will lock out. Control is
grounded through green/yellow wire routed to gas valve
and burner box screw. Failure to follow this caution will
result in intermittent unit operation.
Using schematic diagram, follow sequence of operation through
different modes. (See Fig. 30.) Read and follow wiring diagram
carefully.
NOTE: If a power interruption occurs during a call for heat (W),
the control will start a 90-second blower-only ON period two
seconds after power is restored, if the thermostat is still calling for
gas heating. The red LED light will flash code 12 during the
90-second period, after which the LED will be ON continuous, as
long as no faults are detected. After the 90-second 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.
I. HEATING MODE
(See Fig. 27 for thermostat connections.)
The wall thermostat ″calls for heat″, 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-second 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 Hot Surface Igniter
HSI. Five seconds after the GVR closes, a 2-second
flame period begins. The HSI igniter will remain ener-
gized 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, by momentarily
interrupting 115 vac power to the furnace, or by inter-
rupting 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 66 seconds
(040 through 120 sizes) or 45 seconds (140 size) 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 BLWM 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 15-second (040 through 120
sizes) or 5-second (140 size) 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.
II. COOLING MODE
(See Fig. 27 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 seconds. Cut jumper J2 to reduce the cooling
off-delay to 5 seconds.
III. 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 BLWM is energized.
During a call for heat, the blower BLWM will stop during igniter
warm-up (17 seconds), ignition (7 seconds), and blower-ON delay
(66 or 45 seconds for 040 through 120 sizes or for 140 size),
allowing the furnace heat exchangers to heat up more quickly, then
restarts at the end of the blower-ON delay period at HEAT speed.
In heating, the furnace control CPU will continue running the
blower motor BLWM at HEAT speed after the selected blower-
OFF delay period is completed.
When the thermostat ″calls for cooling″, the blower motor BLWM
will operate at COOL speed. When the thermostat is satisfied, the
blower motor BLWM will operate an additional 90 seconds on
COOL speed before reverting back to HEAT speed. Jumper J2 can
be cut to reduce the cooling off-delay to 5 seconds. (See Fig. 31.)
When the R-to-G circuit is opened, the blower motor BLWM will
continue operating for an additional 5 seconds, if no other function
requires blower motor BLWM operation.
IV. HEAT PUMP MODE
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-W-and-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 seconds
then come back on at HEAT speed. When the W input signal
disappears, the furnace control begins a normal inducer post-purge
period and the blower switches to COOL speed after a 3 second
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 BLWM at HEAT speed after the
selected blower-OFF delay period is completed.
—35—
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