24
XI. Sequence of Operation
Normal Heating Sequence
1. Thermostat calls for heat.
2. The vent blower is energized.
3. The ignitor is energized and is allowed to preheat for
17 seconds.
4. The gas valve is energized delivering gas to the
burners and starting combustion.
5. The control checks for a signal from the flame
sensor within seven seconds after the gas valve is
energized. Gas will only continue to flow if a signal
is present.
6. The control waits 30 seconds and turns on the air
circulation blower to the speed that was selected for
heating operation.
7. The thermostat is satisfied and opens.
8. The control de-energizes the gas valve.
9. After a five or 15 second delay while flue products
are purged from the furnace heat exchanger, the
vent blower motor is de-energized.
10. The air circulation blower has an adjustable delay-off
timing of 60, 90, 120 or 180 seconds (starting from
the time the gas valve closes). This allows more heat
from the furnace to be transferred to the conditioned
space. After this time has elapsed, the blower will be
de-energized.
Abnormal Operation - Integrated Ignition
Control
If the furnace operation is questionable, look through
the observation window in the blower access door and
make a note of the number of flashes in sequence
between pauses. See Section IX for diagnosis.
1 .
Internal Control Failure with Integrated Ignition
Control
If, during a self test, the control cycles determines
an internal fault, the control stops and the diagnos-
tic light will
light continuously.
The control should
be replaced.
2. System Lockout
If flame was not sensed during the first 6 seconds
after the gas valve was energized, the control turns
off the gas. There will then be a 60 second delay
with the combustion blower energized to purge the
heat exchanger. The ignitor will then be energized
and preheated for 27 seconds. The gas valve will
be energized. If flame is not sensed in 6 seconds
the gas valve will be de-energized and another
purge will occur. The control will cycle the gas valve
a total of
three tries
before it determines it cannot
establish measurable combustion and enters a
locked out state. If flame is sensed but lost after 10
seconds, the control will recycle
four more times
before locking out. The diagnostic light will indicate
this condition with
one short flash
followed by a
longer off. The control can be reset and brought out
of lockout mode by turning the thermostat off and
then back on. The control will automatically reset
after one hour.
3. Pressure Switch Stuck Closed
If the control senses the pressure switch is closed
when the induced draft blower is off, it waits until
the fault is corrected. The light code for this prob-
lem is
two short flashes
followed by a longer
pause. The probable cause is faulty pressure switch
or wiring.
4. Pressure Switch Stuck Open
If, after the induced draft blower is energized, the
pressure switch does not close, the control will keep
the blower on and wait for the switch to close. The
diagnostic light code for this problem is
three short
flashes
followed by a pause. The probable cause is
either: disconnected hose to the pressure switch,
faulty pressure switch or wiring, or restricted air
intake or flue piping.
5. Open Limit Control
If the limit control opens, the air circulation blower
will be turned on until the limit closes. The induced
draft blower will remain on for the first fifteen sec-
onds of this period. The diagnostic light code for
this is
four short flashes
followed by a pause. The
probable cause is either: low conditioned air flow
due to dirty filter or resistance in duct work, faulty
limit, faulty blower, or blower speed set too low.
6. Open Rollout Control
If the rollout control opens, the air circulation blower
will be energized until the limit is manually reset.
The induced draft blower will remain on for the first
fifteen seconds of this period. The diagnostic light
code for this is
five short flashes
followed by a
pause. Probable cause is insufficient combustion
air, restricted flue passage or restricted heat ex-
changer.
7. Flame Sensed with No Call for Heat
If the control senses a flame when the gas valve is
de-energized it will run the air circulation blower
and the induced draft blower continuously. The diag-
nostic light code for this is
continuous light flash-
ing.
Probable cause is miswiring.
Normal Cooling Sequence - Integrated
Ignition Control
With the room thermostat in the FAN-AUTO position,
the indoor air circulation blower and outdoor condens-
ing unit will be energized when a call for cooling occurs
and the indoor air circulation blower will come on follow-
ing a five second delay. When the call for cooling ends,
the outdoor condensing unit will be de-energized. The
indoor air circulation blower will continue to run for 45
seconds.
