15. Reinstall vent connector on furnace vent elbow. Securely
fasten vent connector to vent elbow with 2 field-supplied,
corrosion-resistant, sheet metal screws located 180° apart.
16. Replace blower access door only, if it was removed.
17. Set thermostat above room temperature and check furnace for
proper operation.
18. Verify blower airflow and speed changes between heating and
cooling.
FIRE OR EXPLOSION HAZARD
Failure to follow this warning could result in personal injury,
death and/or property damage.
Never use a match or other open flame to check for gas leaks.
Use a soap-and-water solution.
19. Check for gas leaks.
20. Replace outer access door.
Step 3—Sequence of Operation
NOTE:
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 manifold bracket screw.
Using the schematic diagram in Fig. 56, follow the sequence of
operation through the different modes. Read and follow the wiring
diagram very 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 Amber 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.
1.
Heating
(See Fig. 25 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
to begin 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,
and 24 vac power is supplied for a field-installed humidi-
fier at the HUM terminal. 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
auto-
matically 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 seconds 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 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 (depend-
ing on the blower-OFF delay selection). The furnace
control CPU is factory-set for a 120-second blower-OFF
delay.
Fig. 59—Cleaning Heat Exchanger Cell
A91252
48
