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PG96VTA: Installation, Start-up, Operating and 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.
72, 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 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. 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-second 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-second 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-second 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
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-second 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 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. 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 seconds, 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
Fig.
59. When
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, 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 seconds after the gas valve GV-M is opened the
blower motor BLWM is turned ON at low-heat airflow.
High-heat - 25 seconds 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 seconds 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-second 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
seconds (depending on selection at blower-OFF delay switches).
The furnace control CPU is factory-set for a 120-second
blower-OFF delay.
69
until the high-heat pressure switch closes for a maximum of
2 minutes after ignition.
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