48
Purge Gas Lines
If not previously done, purge the lines after all connections have
been made and check for leaks.
FIRE AND EXPLOSION HAZARD
Failure to follow this warning could result in a fire,
explosion, personal injury, or death.
Never purge a gas line into a combustion chamber. Never
test for gas leaks with an open flame. Use a commercially
available soap solution made specifically for the detection
of leaks to check all connections.
!
WARNING
Step 3 — Sequence of Operation
UNIT OPERATION HAZARD
Failure to follow this caution may result in intermittent unit
operation.
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.
CAUTION
!
Using the schematic diagram (See Fig. 48), follow the sequence of
operation through the different modes. Read and follow the wiring
diagram very carefully. ! !
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, throttling valve TV, and gas
valve GV.
Single--Stage Thermostat and Step--Modulating
Heating (Adaptive Mode)
See Fig. 29 or Fig. 59 for thermostat connections.
NOTE
: Low--heat only switch SW1--2 selects the low--heat only
operation mode when ON. Medium--heat only switch SW4--2
selects medium--heat only operation mode when ON. If both
switches are ON the furnace control will default to medium--heat. If
either or both switches are ON the furnace control will operate at
two--stages only as referenced in Section 2 below. If both switches
are OFF the furnace control will operate in the adaptive heating
mode in response to a call for heat. (See Fig. 33.) When the W2
thermostat terminal is energized it will always cause high--heat
operation as long as the R to W circuit is closed, regardless of the
setting of the low--heat or medium--heat only switches.
This furnace can operate as a step--modulating furnace with a
single--stage thermostat because the furnace control CPU includes a
programmed adaptive sequence of controlled operation, which
selects low--heat, medium--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 medium--, or high--heat. The
furnace will operate in low--heat after starting and operating for 1
minute at medium--heat before transitioning to low--heat. The
furnace control CPU determines the combined low--heat and
medium--heat on--time (from 0 to 16 minutes) which is permitted
before switching to high heat.
If the power is interrupted, the stored history is erased and the
furnace control CPU will select medium--heat for 1 minute,
low--heat for 15 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 CPU performs a self--check, verifies the low--heat
and medium--heat pressure switch contacts LPS and MPS are open,
then de--energizes the HPSR relay to close the NC contact.
1. Inducer Prepurge Period -- The furnace control CPU turns
on inducer motor IDM and slowly increases the inducer
motor speed. After the low--heat pressure switch LPS closes
the furnace control CPU continues to increase the inducer
motor speed until the medium--heat pressure switch MPS
closes. When the medium--heat pressure switch MPS closes,
throttling valve TV is energized, inducer motor RPM is
noted by the furnace control CPU, and a 25--second
prepurge period begins. 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
during medium--heat prepurge, the first minute of
medium--heat mode, and low--heat mode.
NOTE
: The heat cycle can start in either high-- or medium--heat. If
a high--heat cycle is initiated, the furnace control CPU will
continue to increase the inducer motor speed after the
medium--heat pressure switch MPS closes. When the medium--heat
pressure switch closes, throttling valve TV is energized, inducer
motor RPM is noted by the furnace control CPU, and a 25--second
prepurge period begins. 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
pre--purge, and high--heat mode.
2. 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.
3. 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. Five seconds after the GVR closes, a 2--
second Flame--Proving period begins. The HSI igniter will
remain energized until 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 energized when the
high--heat pressure switch HPS closes.
4. 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--M open. If the burner flame is not
proved within two seconds, the furnace 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 at full speed until flame
is no longer proved.
58MV
C
