37
2.
Igniter Warm--Up
--At end of the prepurge period, the Hot
Surface Igniter HSI is energized for a 17--sec igniter
warm--up period.
3.
Trial--For--Ignition
Sequence
--When
the
igniter
warm--up period is completed, the main gas valve relay
contacts GVR close to energize the gas valve GV, the gas
valve opens. The gas valve GV permits gas flow to the
burners where it is ignited by the Hot Surface Igniter HSI.
Five seconds after the GVR closes, a 2--second flame
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.
4.
Flame--Proving
--When 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 furnace 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 on high speed until
flame is no longer proved.
5.
Inducer Speed Change
--If the cycle starts in low--heat,
the furnace control CPU reduces the inducer speed slightly
after flame sense. If 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.
6.
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 BLWM is turned ON at low--heat airflow.
High--heat--35 seconds after 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.
7.
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 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.
8.
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.
9.
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.
TWO--STAGE THERMOSTAT AND TWO--STAGE
HEATING
See Fig. 49 for thermostat connections.
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 high--heat operation,
regardless of the setting of the low--heat--only switch.
The wall thermostat “calls for heat,” closing the R toW1 circuit
for low--heat or closing the R to W1--and--W2 circuits for
high--heat.
The furnace control performs a self--check and
verifies the low heat and high--heat pressure switch contacts LPS
and HPS are open.
The start--up and shutdown functions and delays described in
item 1. above apply to 2--stage heating mode as well, except for
switching from low-- to high--heat and vice versa.
1.
Switching From Low-- To High-- Heat
--If the thermostat
R to W1 circuit is closed and the R toW2 circuit closes, 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 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 R to W2 circuit closes.
2.
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 blower motor BLWM will
transition to low--heat airflow five seconds after the R to
W2 circuit opens.
COOLING MODE
The thermostat “calls for cooling.”
1.
Single--Speed Cooling
(See Fig. 19 for thermostat connections.)
The thermostat closes R--to--G--and--Y circuits. The
R--to--Y
circuit
starts
the
outdoor
unit,
and
R--to--G--and--Y/Y2 circuits start the furnace blower motor
BLWM on cooling airflow. Cooling airflow is based on
the A/C selection shown in Fig. 39.
The electronic air cleaner terminal EAC--1 is energized
with 115--v when blower motor BLWM is operating.
When the thermostat is satisfied, the R--to--G--and--Y
circuits are opened. The outdoor unit will stop, and
furnace blower motor BLWM will continue operating at
cooling airflow for an additional 90 sec. Jumper Y/Y2 to
58U
VB
