Operating instructions
heating. The red LED flashes code 12 during 90-sec period, after
which LED will be on continuously as long as no faults are
detected. After 90-sec period, furnace responds to thermostat
normally.
Blower door must be installed for power to be conducted through
blower door interlock switch ILK to furnace control CPU, trans-
former TRAN, inducer motor IDM, blower motor BLWM, hot
surface ignitor HSI, and gas valve GV.
1. Adaptive Heating Mode—Single-Stage Thermostat and
2-Stage Heating
(See Fig. 16 for thermostat connections.)
NOTE: With high-heat-only switch SW-1 off, low-heat-only
switch SW-2 selects either low-heat-only operation mode when on
(see item 2. below), or adaptive heating mode when off in response
to "call for heat." (See Fig. 17.) When high-heat-only switch SW-1
is on, it always causes high-gas-heat operation when R-W/W1
circuit is closed, regardless of the 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 stored history of the
length of previous gas heating on/off periods of single-stage
thermostat.
The furnace starts up in either low- or high-gas heat. If
furnace starts up in low-gas heat, control CPU determines
low-gas heat on time (from 0 to 16 minutes) which is
permitted before switching to high-gas heat.
If power is interrupted, stored history is erased, and control
CPU selects low-gas heat for up to 16 minutes and then
switches to high-gas heat as long as thermostat continues to
"call for heat." Subsequent selection is based on stored
history of thermostat cycle times.
When the wall thermostat "calls for heat," R-W1 circuit
closes. The furnace control performs a self-check, verifies
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 the end of prepurge period, 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, main gas valve relay contacts
MGVR-1 and -2 close to energize low-heat gas valve
solenoid GV, gas valve opens, and 24-v power is
supplied for a field-installed humidifier at terminals
HUM and Com. Low-heat gas valve solenoid GV
permits gas flow to the burners where it is ignited. After
5 sec, ignitor HSI is de-energized, and a 2-sec flame-
proving period begins.
If furnace control CPU selects high-gas-heat operation,
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 pres-
sure switch HPS. HPSR is open while furnace is pow-
ered 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, control CPU begins
blower on delay period and continues to hold gas valve
GV open. If burner flame is not proved within 2 sec,
control CPU closes gas valve GV, and control CPU
repeats 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 control CPU
(not at W/W1, G, R, etc.). Opening thermostat R-W
circuit will not reset ignition lockout.
If flame is proved when flame should not be present,
control CPU locks out of gas heating mode and operates
inducer motor IDM on high speed until flame is no
longer proved.
e. Blower on delay—If burner flame is proven, 45 sec after
gas valve GV is opened blower motor BLWM is
energized on appropriate heating speed, low-gas-heat or
high-gas-heat speed. Simultaneously, EAC terminals
EAC-1 and EAC-2 are energized with 115v and remain
energized as long as blower motor BLWM is energized.
f. Switching from low- to high-gas heat—If furnace control
CPU switches from low-gas heat to high-gas heat,
control CPU switches inducer motor IDM speed from
low to high. The high-heat pressure switch relay HPSR
closes. When inducer motor IDM provides sufficient
pressure to close high-heat pressure switch HPS, high-
heat gas valve solenoid GV is energized. Blower motor
BLWM switches speed for high-gas heat 5 sec after
control CPU switches from low-gas heat to high-gas
heat.
g. Switching from high- to low-gas heat—Control CPU
will not switch from high-gas heat to low-gas heat while
thermostat R-W circuit is closed when a single-stage
thermostat is used.
h. Blower off delay—When thermostat is satisfied, R-W
circuit is opened, de-energizing gas valve GV, stopping
gas flow to burners, and de-energizing humidifier termi-
nals HUM and Com. Inducer motor IDM remains
energized for a 5-sec post-purge period. Blower motor
BLWM and EAC terminals EAC-1 and EAC-2 remain
energized for 90, 135, 180, or 225 sec (depending on
selection at blower off delay switches SW-3 and SW-4).
Furnace control CPU is factory set for a 135-sec blower
off delay.
2. Non-Adaptive Heating Mode—Two-Stage Thermostat and
2-Stage Heating
(See Fig. 16 for thermostat connections.)
NOTE: The low-heat-only switch SW-2 ON selects low-heat-
only operation mode in response to closing thermostat R-W/W1
circuit. When high-heat-only switch SW-1 is off, closing thermo-
stat R to W1-and-W2 circuits is required for high-gas-heat
operation. When high-heat-only switch SW-1 is on, it always
causes high-gas-heat operation when R-W/W1 circuit is closed,
regardless of setting of low-heat-only switch SW-2 and regardless
of whether R-W2 circuit is closed or 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-gas heat and vice
versa.
a. When wall thermostat "calls for heat," R-W/W1 circuit
closes for low-gas heat or R to W1-and-W2 circuits
close for high-gas heat. The furnace control performs a
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