-- 4 9 --
HEATING
MODE
(See Fig. 30 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 PRS are open, and starts the
inducer motor IDM.
a. Inducer Prepurge Period -- As the inducer motor
IDM comes up to speed, the pressure switch contacts
PRS close, 24 vac power is supplied for a field
installed humidifier at the HUM terminal and the
control begins a 15-second prepurge period.
b. Igniter Warm-Up -- At the end of the prepurge period,
the Hot-Surface Igniter HIS 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
contacts GVR close to energize the gas valve GV, the
gas valve opens. The gas valve GV permits gas flow to
the burners where it is ignited by the Hot Surface
Igniter HSI. Five seconds after the GVR closes, a
2-second flame period begins. The HSI igniter will
remain energized until the flame is sensed or until the
2-second flame proving period begins.
d. 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 seconds, the control
CPU will close the gas valve GV, 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 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,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.
e. Blower-On Delay -- If the burner flame is proven, the
blower motor is energized on HEAT speed 66 seconds
(040 through 120 sizes) or 45 seconds (140 size) 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.
f. 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 humidifier terminal HUM. The
inducer motor IDM will remain energized for a
15-second (040 through 120 sizes) or 5-second (140
size) post-purge period. The blower motor BLWM and
air cleaner terminal EAC--1 will remain energized for
90, 120, 150, or 180 seconds (depending on the
blower-OFF delay selection). The furnace control CPU
is factory-set for a 120-second blower-OFF delay.
COOLING MODE
(See Fig. 30 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 circuits are
opened. The outdoor unit will stop, and the furnace blower motor
BLWM will continue operating on the COOL speed for an
additional 90 seconds. Cut jumper J2 to reduce the cooling
off-delay to 5 seconds.
CONTINUOUS BLOWER MODE
When the R-to-G circuit is closed by the thermostat, the blower
motor BLWM will operate on continuous-blower speed (same as
HEAT 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 seconds), ignition (7 seconds), and
blower-ON delay (66 or 45 seconds for 040 through 120 sizes or
for 140 size), allowing the furnace heat exchangers to heat up
more quickly, then restarts at the end of the blower-ON delay
period at HEAT speed. In heating, the furnace control CPU will
continue running the blower motor BLWM at HEAT speed after
the selected blower-OFF delay period is completed.
When the thermostat “calls for cooling”, the blower motor
BLWM will operate at COOL speed. When the thermostat is
satisfied, the blower motor BLWM will operate an additional
90 seconds on COOL speed before reverting back to HEAT
speed. Jumper J2 can be cut to reduce the cooling off-delay to 5
seconds. (See Fig. 33.) When the R-to-G circuit is opened, the
blower motor BLWM will continue operating for an additional
5 seconds, if no other function requires blower motor BLWM
operation.
HEAT PUMP MODE
When installed 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 seconds 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 second 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
selected 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 BLWM at HEAT speed after
the selected blower-OFF delay period is completed.
Control initiates a 90-sec blower only on period before starting
another heat pump cycle if there is a power interruption. Anytime
control senses false flame, control locks out of heating mode.
This reaction occurs because control ignores W input due to false
flame signal and, as a result, sees only Y input and goes into
cooling mode blower off delay. All other control functions remain
in standard format.
NOTE: EAC--1 terminal is energized whenever blower operates.
HUM terminal is only energized when gas valve is energized.
COMPONENT TEST
NOTE: The furnace control component test allows all
components to run for a short time; except the gas valve and
humidifier terminal HUM are not energized for safety reasons.
The EAC--1 terminal is energized when the blower is energized.
This feature helps diagnose a system problem in case of a
component failure. The component test feature will not operate if
any thermostat signal is present at the control and not until all
time delays are completed.
58MCB
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