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The blower door must be installed for power to be conducted
through blower door interlock switch ILK to furnace control CPU,
transformer TRAN, inducer motor IDM, blower motor BLWM,
hot surface ignitor HSI, and gas valve GV.

1. Adaptive Heating Mode—Single-Stage Thermostat with

2-Stage Heating

(See Fig. 9 for thermostat connections.)

NOTE:

With high-heat-only switch SW-1 off, low-heat-only

switch SW-2 selects either the low-heat-only operation mode when
on (see item 2 below), or adaptive heating mode when off in
response to a call for heat. (See Fig. 11.) When high-heat-only
switch (SW-1) is in ON position, it always causes high-gas-heat
operation when R-W/W1 circuit is closed, regardless of setting of
low-heat-only switch (SW-2).

This furnace can operate as a 2-stage furnace with a
single-stage thermostat because furnace control CPU in-
cludes a programmed adaptive sequence of controlled
operation, which selects low-gas-heat or high-gas-heat op-
eration. This selection is based upon the stored history of
the length of previous gas heating on/off periods of the
single-stage thermostat.

The furnace will start up in either low- or high-gas heat. If
the furnace starts up in low-gas heat, the control CPU
determines the low-gas heat on time (from 0 to 16 minutes)
which is permitted before switching to high-gas heat.

If power is interrupted, the stored history is erased. When
this happens, the control CPU selects low-gas heat for up to
16 minutes and then switches to high-gas heat, as long as
the thermostat continues to call for heat. Subsequent selec-
tion is based on stored history of thermostat cycle times.

When wall thermostat "calls for heat", R-W/W1 circuit
closes. The furnace control performs a self check, verifies
the low-heat and high-heat pressure switch contacts LPS
and HPS are open, and starts inducer motor IDM in low
speed or high speed as appropriate.

a. Inducer prepurge period—As inducer motor IDM comes

up to low speed or high speed, the low-heat pressure
switch contacts LPS (or LPS and HPS) close to begin a
15 sec prepurge period.

b. Ignitor warm-up—At end of prepurge period, the hot

surface ignitor HSI is energized for a 17-sec ignitor
warm-up period.

c. Trial-for-ignition sequence—When ignitor warm-up pe-

riod is completed, the main gas valve relay contacts
MGVR-1 and -2 close to energize the low-heat gas valve
solenoid GV. The gas valve opens, and 24-v power is
supplied for a field-installed humidifier at terminals
HUM and C

. The low-heat gas valve solenoid GV

permits gas flow to the burners where it is ignited. After
5 sec, the ignitor HSI is de-energized, and a 2-sec
flame-proving period begins.

If high-heat gas valve solenoid GV is also energized
after normally closed high-heat pressure switch relay
HPSR closes and after inducer motor IDM goes to high
speed and provides sufficient pressure to close high-heat
pressure switch HPS. HPSR is open while furnace is
powered in standby mode. If high-heat pressure switch
HPS fails to close and low-heat pressure switch LPS
closes, furnace operates at low-heat gas flow rate until
high-heat pressure switch closes.

d. Flame-proving—When burner flame is proved at flame-

proving sensor electrode FSE, the control CPU begins
blower on delay period and continues to hold gas valve
GV open. If burner flame is not proved within 2 sec, the
control CPU closes gas valve GV, and the control CPU
repeats the ignition sequence for up to 3 more trials-for-
ignition before going to ignition lockout. LOCKOUT IS
RESET AUTOMATICALLY after 3 hr, or by momen-
tarily interrupting 115-v power to furnace, or by inter-
rupting 24-v power at SEC1 or SEC2 to the control CPU
(not at W/W1, G, R, etc.). Opening thermostat R-W
circuit will not reset an ignition lockout.

If flame is proved when flame should not be present, the
control CPU locks out of gas heating mode and operates
inducer motor IDM on high speed until flame is no
longer proved.

TABLE 6—SETUP SWITCH DESCRIPTION

SETUP

SWITCH NO.

NORMAL

POSITION

DESCRIPTION

OF USE

SW-1

Only High-Gas Heat

OFF (Staged Gas

Heat)

Turn switch on to obtain only high-gas-heat operation on any call
for heat regardless of whether R-W/W1, or R-W/W1, -W2 is
closed. SW-1 overrides SW-2.

SW-2

Low-Gas Heat (Adaptive Mode)

OFF (Single-Stage Thermostat)

Turn switch off for installations with single-stage thermostats; con-
trol selects low-gas-heat or high-gas-heat operation based on pre-
vious cycles. Turn switch on for installations with 2-stage thermo-
stats to permit only low-gas-heat operation in response to closing
R-W/W1. High-gas heat is supplied only when R to W/W1 and W2
are closed.

SW-3 and SW4

ON, OFF

Switches control gas heating mode blower off delay. (See Table
7.)

Fig. 11—Control Center

A93348

PR2

COM

PR1

EAC-2

EAC-1

I-

COO