SERVICE AND TECHNICAL SUPPORT MANUAL
Gas Furnace: (F/G)9MAE
Specifications subject to change without notice.
34
440 04 4700 00
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
manifold bracket screw.
CAUTION
!
Using the schematic diagram (See
Figure 15
), 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/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 1+2 during the 90-second period, after
which the LED flash a heartbeat (bright
−
dim), 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 setup 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 capacity 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
Figure 15
.
Single-Stage Thermostat and Modulating Heating
(Adaptive Mode)
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
Figure 15
) 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 motor 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. The
gas valve solenoid GV 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 igniter
will remain energized until flame is sensed or until the
2-second flame proving period begins.
4.
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 furnace control CPU will
