Two- -Stage Thermostat And Two- -Stage Heating - Carrier PG9UAA Troubleshooting Manual

resistance. This evaluation is then used to determine the
required RPM necessary to operate the inducer motor
during the low- -heat prepurge period and low- -heat
mode.
NOTE: The heat cycle can start in either high- - or low- -heat. If a
high- -heat cycle is initiated, the furnace control CPU will
de- -energize the high- -heat pressure switch relay HPSR to close the
NC contact and continues to increase the inducer motor speed after
the low- -heat pressure switch LPS closes. When the high- -heat
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 resistance. This evaluation is then
used to determine the required RPM necessary to operate the
inducer motor in high- -heat mode.
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. After 5 seconds,
the igniter HSI is de- -energized and a 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 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 will be reset automatically
after three hours, 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 at full speed until
flame is no longer proved.
e. Inducer Speed Change – If the cycle starts in low- -heat,
the furnace control CPU reduces the inducer speed
slightly after flame sense. If the cycle starts in high- -
heat, the furnace control CPU increases the inducer
speed 15 seconds after flame sense. The reduction in
speed in low- -heat is to optimize combustion for
maximum efficiency.
f. Blower- -On delay – If the burner flame is proven the
blower- -ON delay for low- -heat and high- -heat are as
follows:
Low- -heat - - 30 seconds after the gas valve GV-M is
opened the blower motor BLWM is turned ON at
low-heat airflow.
High-heat – 35 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.
g. Switching from Low- - to High- -Heat - - If the furnace
control CPU switches from low-heat to high-heat, the
furnace control CPU will de-energize the high-heat
pressure switch relay HPSR to close the NC contact and
slowly increase the inducer motor speed until the
high-heat pressure switch HPS closes. When the
high-heat pressure switch HPS closes, the high-heat gas
valve solenoid GV-HI is energized and the inducer
motor RPM is noted by the furnace control CPU. 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 in high-heat
mode. The blower motor BLWM will transition to
high-heat airflow five seconds after the furnace control
CPU switches from low-heat to high-heat.
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 us-
ing 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 re-
main 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.
2. TWO- -STAGE THERMOSTAT AND TWO- -STAGE
HEATING
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 circuits always causes
regardless of the setting of the low- -heat only switch.
The wall thermostat "calls for heat", closing the R to W1 circuit for
low- -heat or closing the R to W1- -and- -W2 circuits for high- -heat.
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.
The start up and shut down functions and delays described in item
1. above apply to the 2- -stage heating mode as well, except for
switching from low- - to high- -Heat and vice versa.
a. Switching from Low- - to High- -Heat - - If the
thermostat R to W1 circuit is closed and the R to W2
circuit closes, the furnace control CPUwill de-energize
the high-heat pressure switch relay HPSR to close the
NC contact and slowly increase the inducer motor speed
until the high-heat pressure switch HPS closes. When
the high-heat pressure switch HPS closes, the high-heat
gas valve solenoid GV-HI is energized and the inducer
motor RPM is noted by the furnace control CPU. 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 in high-heat
mode. The blower motor BLWM will transition to
high-heat airflow five seconds after the furnace control
CPU switches from low-heat to high-heat.
b. Switching from High- - to Low- -Heat - - If the
thermostat R to W2 circuit opens, and the R to W1
circuit remains closed, the furnace control CPU will
energize the high-heat pressure switch relay HPSR to
open the NC contact and slowly decrease the inducer
motor speed to the required low-heat RPM . When the
high-heat pressure switch HPS opens, the high-heat gas
valve solenoid GV-HI is de-energized. When the
inducer motor IDM reduces pressure sufficiently, the
high-heat pressure switch HPS will open. The gas valve
solenoid GV-M will remain energized as long as the
low-heat pressure switch LPS remains closed. The
3
high- -heat operation,