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HEAT ASSISTED LIMIT CONTROL
(HALC)
Downflow and downflow/horizontal
units use a heat assisted limit control
located in the blower compartment.
This limit monitors the blower
compartment air temperature. If the
air temperature becomes too high it
opens breaking 24 volt power to shut
down the gas valve. This protects the
blower motor and filter from
dangerously high temperatures. It
resets automatically after the air
temperature drops. This limit is set at
the factory and cannot be adjusted. It
is a 24 volt control.
Operation. A normal temperature
sensitive limit reacts too slowly
because of the nature of this furnace
and heat exchanger. This limit uses
a 24 volt heater to partially heat the
limit switch. This accelerates the limit
response to sudden temperature
increases in the blower compart-
ment. The internal heater also slows
the limit reset time to avoid short
cycling on this limit. The heater is
electrically powered anytime the gas
valve is energized from the gas valve
relay on the IFC board.
If the HALC opens during furnace
operation it de-energizes the gas
valve. The main blower and
induced draft blower operate in an
attempt to cool the system and
reset the HALC if a call for heat is
initiated and the furnace doesn't
ignite on the first try.
NOTE: Beginning with furnace date
code F3198, the HALC contacts are
now in series with the induced draft
blower motor.
If the HALC opens its contacts, the
inducer stops and the pressure
switch opens. This causes another
heat cycle and triggers a count by
the IFC. After four pressure switch
cycles, the control goes into soft lock
out. It also shows in the diagnostic
light flashes.
Check. The following voltage test
can be used.
1. Turn power on to the furnace.
2. Use a voltmeter to check for 24
vac across the limit contacts.
3. A 24 vac reading indicates an
open limit switch.
4. A zero (0) vac reading indicates
a closed switch.
FIGURE 107
HEAT ASSISTED
LIMIT CONTROL
Use an Ohmmeter to check the limit
heater.
1. Turn off all power to the furnace.
2. Disconnect a wire from one of
the heater terminals. (3/16"
quick connect terminals)
3. Measure the Ohm resistance
across the heater.
4. A resistance within the given
ranges indicates a good heater.
HEAT ASSISTED LIMIT CONTROL
(HALC) (-)GTA/(-)GSA UNITS ONLY
SETTING TEMPERATURES
105,000 BTU UNIT
100°, RESET 90°
2.0 WATT, 225-325 OHMS
120,000 BTU -GTA UNIT
105°, RESET 95°
2.0 WATT, 225-325 OHMS
75,000 BTU -GTA UNIT
105°, RESET 95°
1.5 WATT, 325-425 OHMS
ALL OTHER UNITS
110°, RESET 100°
1.5 WATT, 325-425 OHMS
5. Replace the HALC if the Ohm
reading is out of the range.
The reasons for an open HALC are
the same as listed for the main limit
control above.
OVER TEMPERATURE SWITCH
There are two different over
temperature switches on the
90 Plus furnace. One switch mounts
on a bracket on the center panel just
above the burners. It carries the part
number 47-22861-01. It's a 24 volt
control. The part number and
setting are marked on the control.
Only -GSA downflow units have
this switch.
CENTER PANEL MOUNTED
OVER TEMPERATURE SWITCH
TEMPERATURE SETTING
(-)GSA/(-)GTA UNITS ONLY
(ALL SIZES): 350°
Cannot be reset until temperature
cools to 240°
The second over temperature switch
mounts on the burner supports just
above the burners on upflow units
and just under the burners on
downflow units. It carries the part
number 4 7-22861-02. It is also a
24-volt control. The part number and
setting are marked on the control.
One, two or three of these over
temperature switches can be
found depending on furnace
model and input. This switch is
marked with a blue dot.
FIGURE 108
OVER TEMPERATURE SWITCH
91
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