92
SEQUENCE OF OPERATION
UNIT OPERATION HAZARD
Failure to follow this caution may result in intermittent unit
operation.
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.
CAUTION
!
Using the schematic diagram, follow the sequence of operation
through the different modes. See Fig.
72
. Read and follow the
wiring diagram very carefully!!
NOTE
: If a power interruption occurs during a call for heat
(W/W1 or W/W1-and-W2), 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.
Communicating Control and Modulating Heating
(Adaptive Mode)
Best comfort will be attained when a communicating wall control
is used with this product. Wiring and set--up instructions are
provided with the communicating control. See the furnace data
sheet accessory section for help in selecting the appropriate
communicating control for this furnace.
When a communicating control is used, the furnace will modulate
through its full operation range, or can be limited via the minimum
and maximum CFM configurations.
Operation of the furnace at the beginning and end of each heating
cycle will be the same as detailed below in the Single--Stage
Thermostat section EXCEPT that the communicating control will
send
modulating
rate
command
signals
through
the
communication bus rather than energizing the 24--v thermostat
terminals.
Note that the R to W/W1 circuit signal will be
controlled by the COMMR relay on the furnace control. See the
wiring diagram in Fig. 72 .
Single-Stage Thermostat and Modulating Heating
(Adaptive Mode)
See Fig. 36 and 38 for thermostat connections
.
NOTE
:
Minimum-heat only switch SW1-2 selects the
minimum-heat
only
operation
mode
when
ON.
Intermediate-heat only switch SW4-2 selects intermediate-heat
only operation mode when ON. If both switches are ON the
furnace control will default to intermediate-heat. If either or both
switches are ON, the furnace control will operate at two-stages
only as referenced in Two Stage Thermostat section below. If both
switches are OFF the furnace control will operate in the adaptive
heating mode in response to a call for heat. See Fig. 65. When the
W2 thermostat terminal is energized it will always cause
maximum-heat operation as long as the R to W circuit is closed,
regardless of the setting of the minimum-heat or intermediate-heat
only switches.
This furnace can operate as a modulating furnace with a
single-stage thermostat because the furnace control CPU includes a
programmed adaptive sequence of controlled operation, which
selects
a
modulated
rate
between
minimum-
and
maximum-heat
.
This selection is based upon the stored history of
the length of previous gas-heating periods of the single-stage
thermostat.
The furnace will start up in either intermediate-, or maximum-heat.
The furnace will transition and operate at minimum-heat or the
calculated modulating rate after starting and operating for 45
seconds at intermediate-heat. The furnace control CPU defaults to
minimum-heat on the first thermostat cycle and calculates the
modulating rate that the furnace should run at for 19 minutes on
subsequent heat cycles.
If the power is interrupted, the stored modulating rate is erased and
the furnace control will select intermediate-heat for 45 seconds,
minimum-heat for 19 minutes and then switch to maximum-heat,
as long as the thermostat continues to call for heat. The furnace
control CPU then uses this information to calculate the modulating
rate the furnace will operate at on the next heating cycle. If the
calculated modulating rate is between 40% - 99% then the furnace
control CPU will operate for 45 seconds at intermediate-heat, and
then operate at the calculated modulating rate for up to 19 minutes
and then switch to maximum-heat as long as the thermostat
continues to call for heat. If the calculated modulating rate is
100% then the furnace control will only operate at maximum-heat
until the thermostat is satisfied.
The wall thermostat ”calls for heat”, closing the R to W circuit.
The furnace control CPU performs a self-check, verifies the low,
medium, and high pressure switch contacts LPS, MPS, and HPS
are open, then de-energizes the PSR relay to close the NC contact.
1.
Inducer Prepurge Period
-
The furnace control CPU turns
on inducer motor IDM and slowly increases the inducer
motor speed. After the low pressure switch LPS closes the
furnace control CPU continues to increase the inducer mo-
tor speed until the medium pressure switch MPS closes.
When the medium pressure switch MPS closes, inducer
motor RPM is noted by the furnace control CPU, and a
25-second prepurge period begins. The RPM is used to
evaluate vent system resistance.
This evaluation is then
used to determine the required RPM necessary to operate
the inducer motor during pre-purge, the first 45 seconds of
intermediate-heat, or any modulating rate that the furnace
will transition to after the blower on-delay is completed.
NOTE
:
The heat cycle can start in either intermediate- or
maximum-heat.
If a maximum-heat cycle is initiated, the
furnace control CPU will continue to increase the inducer motor
speed after the medium pressure switch MPS closes. When the
medium pressure switch closes, inducer motor RPM is noted by
the furnace control CPU, and a 25-second prepurge period begins.
The RPM is used to evaluate vent system restriction.
This
evaluation is then used to determine the required RPM necessary
to operate the inducer motor in maximum-heat pre-purge, or
maximum-heat. The high pressure switch HPS should be closed
before ignition but the furnace control ignores this input until after
ignition occurs.
2.
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
3.
Trial-For-Ignition Sequence
-
When the igniter warm-up
period is completed the main gas valve relay contact GVR
closes to energize the gas valve solenoid GV (pin 5). The
gas valve solenoid GV (pin 5) permits gas flow to the
burners where it is ignited. Five seconds after the GVR
closes, a 2-second Flame-Proving period begins. The HSI
