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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. Heating

See Fig. 37 and 38 for thermostat connections.

The wall thermostat “calls for heat,” closing the R--to--W
circuit. The furnace control performs a self--check, verifies
the pressure switch contacts LPS and HPS are open, and

starts the inducer motor IDM.

Inducer Prepurge Period

-- As the inducer motor IDM

comes up to speed, the collector box pressure switch con-
tacts LPS close to begin a 15--sec prepurge period. The in-
ducer housing pressure switch contacts HPS are ignored

until one minute after the gas valve closes.

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.

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, and 24 vac power is supplied for a field--installed
humidifier at the HUM terminal. The gas valve GV 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 be-
gins. The HSI igniter will remain energized until the flame

is sensed or until the 2--sec flame proving period begins.

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 sec, the control CPU will close the gas
valve GV, and the control CPU will repeat the ignition se-

quence for up to three more Trials--For--Ignition before go-
ing to Ignition--Lockout.

Lockout will be reset

automati-

cally 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, 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 until flame is no longer
proved.

Blower--On Delay

-- If the burner flame is proven, the

blower motor is energized on HEAT speed 25 sec after the

gas valve GV is energized.
Simultaneously, the electronic air cleaner terminal EAC--1
is energized and remains energized as long as the blower

motor BLWM is energized.

Blower--Off Delay

-- When the thermostat is satisfied, the

R--to--W circuit is opened, de--energizing the gas valve GV,
stopping gas flow to the burners, and de--energizing the hu-

midifier terminal HUM. The inducer motor IDM will re-
main energized for a 15--sec post--purge period. The blow-
er motor BLWM and air cleaner terminal EAC--1 will

remain energized for 90, 120, 150, or 180 sec (depending
on the blower--OFF delay selection). The furnace control

CPU is factory--set for a 120--sec blower--OFF delay.

Cooling Mode

The thermostat “calls for cooling.”

a. Single--Speed Cooling

See Fig. 37 and 38 for thermostat connections

The thermostat closes the R--to--G--and--Y circuits. The
R--to-- Y circuit starts the outdoor unit, and the R--to--
G--and--Y circuits start the furnace blower motor BLWM

on COOL speed.

The electronic air cleaner terminal EAC--1 is energized
with 115 vac when the blower motor BLWM is operating.

When the thermostat is satisfied, the R--to--G--and--Y cir-
cuits are opened. The outdoor unit will stop, and the fur-
nace blower motor BLWM will continue operating on the

COOL speed for an additional 90 sec.

Continuous Blower Mode

When the R--to--G circuit is closed by the thermostat, the
blower motor BLWM will operate on continuous--blower
speed (FAN speed). Terminal EAC--1 is energized as long as

the blower motor BLWM is energized. During a call for
heat, the blower BLWM will stop during igniter warm--up

(17 sec), ignition (7 sec), and blower--ON delay (25 sec),
allowing the furnace heat exchangers to heat up more quick-
ly, then restarts at the end of the blower--ON delay period at

HEAT speed.
In heating, the furnace control CPU will hold the blower

motor BLWM at HEAT speed during the selected blower--
OFF delay period before reverting to continuous--blower

speed.
When the thermostat “calls for high--cooling”, the blower
motor BLWM will operate at COOL speed. When the ther-

mostat is satisfied, the blower motor BLWM will operate an
additional 90 sec on COOL speed before reverting back to

continuous--blower speed.
When the R--to--G circuit is opened, the blower motor
BLWM will continue operating for an additional 5 sec, if no

other function requires blower motor BLWM operation.

Heat pump

See Fig. 37 and 38 for thermostat connections. When in-
stalled with a heat pump, the furnace control automatically

changes the timing sequence to avoid long blower off times
during demand defrost cycles. When the R--to--W--and--Y or
R--to--W--and--Y--and--G circuits are energized the furnace

control CPU will continue to turn on the blower motor
BLWM at HEAT speed, and begin a heating cycle. The

blower motor BLWM will remain on until the end of the
prepurge period, then shut off for 24 sec then come back on
at HEAT speed. When the W input signal disappears, the

furnace control begins a normal inducer post--purge period
and the blower switches to COOL speed after a 3 sec delay.

If the R--to--W--and--Y--and--G signals disappear at the same
time, the blower motor BLWM will remain on for the se-
lected blower-- OFF delay period. If the R--to--W--and--Y

signals disappear, leaving the G signal, the blower motor
BLWM will continue running the blower motor at HEAT

speed after the selected blower--OFF delay period is com-
pleted.

Component Self-Test

Refer to page 67 for instructions.