Sequence Of Operation - Carrier F96CTN Installation, Start-Up, Operating, Service And Maintenance Instructions

F96CTN and G96CTN (Series A): Installation, Start-up, Operating, Service and Maintenance Instructions

SEQUENCE OF OPERATION

NOTE: Furnace control must be grounded for proper operation or else
control will lock out. Control is grounded through green/yellow wire
routed to gas valve and burner box screw. Using the schematic diagram
in Fig. 73, 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-sec blower-only ON period
two sec after power is restored, if the thermostat is still calling for gas
heating. The Amber LED light will flash a 1+2 during the 90-sec period,
after which the LED will be flash a heartbeat, as long as no faults are
detected. After the 90-sec 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. Two-Stage Heating (Adaptive Mode) with Single-Stage
Thermostat
See Fig. 38 and Fig. 39 for thermostat connections
NOTE: The low-heat only switch SW1-2 selects either the low-heat
only operation mode when ON, (see item 2. below) or the adaptive
heating mode when OFF in response to a call for heat. See
the W2 thermostat terminal is energized it will always cause high-heat
operation when the R-to-W circuit is closed, regardless of the setting of
the low-heat only switch. This furnace can operate as a two-stage
furnace with a single-stage thermostat because the furnace control CPU
includes a programmed adaptive sequence of controlled operation,
which selects low-heat or high-heat operation. 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 low- or high-heat. If the furnace starts
up in low-heat, the control CPU determines the low-heat on-time (from 0
to 16 minutes) which is permitted before switching to high-heat.
If the power is interrupted, the stored history is erased and the control
CPU will select low-heat for up to 16 minutes and then switch to
high-heat, as long as the thermostat continues to call for heat.
Subsequent selection is based on stored history of the thermostat cycle
times.
The wall thermostat "calls for heat", closing the R-to-W circuit. The
furnace control performs a self-check, verifies the low-heat and
high-heat pressure switch contacts LPS and HPS are open, and starts the
inducer motor IDM in high-speed.
a. Inducer Prepurge Period
(1.) If the furnace control CPU selects low-heat operation the
inducer motor IDM comes up to speed, the low-heat pressure
switch LPS closes, and the furnace control CPU begins a 15-sec
prepurge period. If the low-heat pressure switch LPS fails to
remain closed the inducer motor IDM will remain running at
high-speed. After the low-heat pressure switch re-closes the
furnace control CPU will begin a 15-sec prepurge period, and
continue to run the inducer motor IDM at high-speed.
(2.)If the furnace control CPU selects high-heat operation, the
inducer motor IDM remains running at high-speed, and the
high-heat pressure switch relay HPSR is de-energized to close
the NC contact. When sufficient pressure is available the
high-heat pressure switch HPS closes, and the high-heat gas
valve solenoid GV-HI is energized. The furnace control CPU
begins a 15-sec prepurge period after the low-heat pressure
switch LPS closes. If the high-heat pressure switch HPS fails to
close and the low-heat pressure switch LPS closes, the furnace
will operate at low-heat gas flow rate until the high-heat pressure
switch closes for a maximum of 2 minutes after ignition.
Manufacturer reserves the right to change, at any time, specifications and designs without notice and without obligations.
b. Igniter Warm-Up -At the end of the prepurge period, the
Hot-Surface Igniter HSI is energized for a 17-sec igniter
warm-up period.
c. 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-M. The gas valve solenoid
GV-M permits gas flow to the burners where it is ignited by the
HSI. Five sec after the GVR closes, a 2-sec flame proving period
begins. The HSI igniter will remain energized until the flame is
sensed or until the 2-sec flame proving period begins. If the
furnace control CPU selects high-heat operation, the high-heat
gas valve solenoid GV-HI is also energized.
d. Flame-Proving - When the burner flame is proved at the
flame-proving sensor electrode FSE, the inducer motor IDM
switches to low-speed unless the furnace is operating in
high-heat, and the furnace control CPU begins the blower-ON
delay period and continues to hold the gas valve GV-M open. If
the burner flame is not proved within two sec, the control CPU
will close the gas valve GV-M, 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, 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/W1, G, R,
Fig. 60. When
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 on high speed until flame is no
longer proved.
e. Blower-On delay - If the burner flame is proven the blower-ON
delays for low-heat and high-heat are as follows:
Low-heat - 45 sec after the gas valve GV-M is opened the
blower motor BLWM is turned ON at low-heat airflow.
High-heat - 25 sec after the gas valve GV-M is opened the
BLWM is turned ON at high-heat airflow. Simultaneously, the
humidifier terminal HUM and electronic air cleaner terminal
EAC-1 are energized and remain energized throughout the
heating cycle.
f. Switching from Low- to High-Heat - If the furnace control
CPU switches from low-heat to high-heat, the furnace control
CPU will switch the inducer motor IDM speed from low to high.
The high-heat pressure switch relay HPSR is de-energized to
close the NC contact. When sufficient pressure is available the
high-heat pressure switch HPS closes, and the high-heat gas
valve solenoid GV-HI is energized. The blower motor BLWM
will transition to high-heat airflow five sec after the furnace
control CPU switches from low-heat to high-heat.
g. Switching from High- to Low-Heat -The furnace control CPU
will not switch from high-heat to low-heat while the thermostat
R-to-W circuit is closed when using a single-stage thermostat.
h. Blower-Off Delay -When the thermostat is satisfied, the R to W
circuit is opened, de-energizing the gas valve GV-M, stopping
gas flow to the burners, and de-energizing the humidifier
terminal HUM. The inducer motor IDM will remain energized
for a 15-sec post-purge period. The blower motor BLWM and air
cleaner terminal EAC-1 will remain energized at low-heat
airflow or transition to low-heat airflow for 90, 120, 150, or 180
sec (depending on selection at blower-OFF delay switches). The
furnace control CPU is factory-set for a 120-sec blower-OFF
delay.
2. Two-Stage Thermostat and Two-Stage Heating
See Fig. 38
NOTE: In this mode the low-heat only switch SW1-2 must be ON to
select the low-heat only operation mode in response to closing the
thermostat R-to-W1 circuit. Closing the thermostat R-to- W1-and-W2
71
and Fig. 39 for thermostat connections.