14. Reinstall internal vent pipe, if applicable.
15. Reinstall vent connector on furnace vent elbow. Securely
fasten vent connector to vent elbow with 2 field-supplied,
corrosion-resistant, sheet metal screws located 180° apart.
16. Replace blower access door only.
17. Set thermostat above room temperature and check furnace for
proper operation.
18. Verify blower airflow and speed changes between heating and
cooling.
19. Check for gas leaks.
Never use a match or other open flame to check for gas leaks.
Use a soap-and-water solution. A failure to follow this
warning could result in fire, personal injury, or death.
20. Replace outer access door.
Step 3—Sequence of Operation
NOTE:
Furnace control must be grounded for proper operation or
control will lock out. Control is grounded through green/yellow
wire routed to gas valve and burner box screw.
NOTE:
If a power interruption occurs during a call for heat
(W/W1 or W/W1-and-W2), the control will start a 90-second
blower-only ON period two seconds after power is restored, if the
thermostat is still calling for gas heating. The amber LED light will
flash code 12 during the 90-second period, after which the LED
will be ON continuous, as long as no faults are detected. After the
90-second 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. Single-Stage Thermostat and Two-Stage Heating (Adap-
tive mode)
See Fig. 22 or 31 for thermostat connections
NOTE:
The low-heat only switch SW1-2 selects either the
low-heat only operation mode when ON, (see item 2. below) or the
adaptive heating mode when OFF in response to a call for heat.
(See Table 6.) When the W2 thermostat terminal is energized it
will always cause high-heat operation when the R to W circuit is
closed, regardless of the setting of the low-heat only switch.
This furnace can operate as a two-stage furnace with a single-stage
thermostat because the furnace control CPU includes a pro-
grammed adaptive sequence of controlled operation, which selects
low-heat or high-heat operation. This selection is based upon the
stored history of the length of previous gas-heating periods of the
single-stage thermostat.
The furnace will start up in either low- or high-heat. If the furnace
starts up in low-heat, the control CPU determines the low-heat
on-time (from 0 to 16 minutes) which is permitted before switch-
ing to high-heat.
If the power is interrupted, the stored history is erased and the
control CPU will select low-heat for up to 16 minutes and then
switch to high-heat, as long as the thermostat continues to call for
heat. Subsequent selection is based on stored history of the
thermostat cycle times.
The wall thermostat
″
calls for heat
″
, closing the R to W circuit.
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.
a. Inducer Prepurge Period -If the furnace control CPU
selects low-heat operation the inducer motor IDM comes
up to speed, the low-heat pressure switch LPS closes, and
the furnace control CPU begins a 15-second prepurge
period. If the low-heat pressure switch LPS fails to remain
closed the inducer motor IDM will remain running at
high-speed. After the low-heat pressure switch re-closes the
furnace control CPU will begin a 15-second prepurge
period, and continue to run the inducer motor IDM at
high-speed.
If the furnace control CPU selects high-heat operation, the
inducer motor IDM remains running at high-speed, and the
high-heat pressure switch relay HPSR is de-energized to
close the NC contact. When sufficient pressure is available
the high-heat pressure switch HPS closes, and the high-heat
gas valve solenoid GV-HI is energized. The furnace control
CPU begins a 15-second prepurge period after the low-heat
pressure switch LPS closes. If the high-heat pressure switch
HPS fails to close and the low-heat pressure switch LPS
closes, the furnace will operate at low-heat gas flow rate
until the high-heat pressure switch closes for a maximum of
2 minutes after ignition.
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 inducer motor
IDM switches to low-speed unless the furnace is operating
in high-heat, and 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 auto-
matically 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.
e. Blower-On delay - If the burner flame is proven the
blower-ON delay for low-heat and high-heat are as follows:
(1.) Low-heat - 45 seconds after the gas valve GV-M is
opened the blower motor BLWM is turned ON at
low-heat airflow.
(2.) High-heat - 25 seconds after the gas valve GV-M is
opened the BLWM is turned ON at high-heat airflow.
Simultaneously, the humidifier terminal HUM and elec-
tronic air cleaner terminal EAC-1 are energized and remain
energized throughout the heating cycle.
f. Switching from Low- to High-Heat - If the furnace
control CPU switches from low-heat to high-heat, the
furnace control CPU will switch the inducer motor IDM
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