HEATASSISTEDLIMIT
CONTROL
(HALC)
All upflow/horizontal, downflow, Check. Use an Ohmmeter to check
downflow/horizontal and dedicated
the limit contacts.
horizontal models have a heat
1. Turn off all power to the furnace.
assisted limit control located in the
2. Disconnect a wire from one of
blower compartment. This limit
the limit switch terminals.
monitors the blower compartment air
temperature. If the air temperature
3. Measure the Ohm resistance
becomes too high it opens and shuts
across the limit contacts.
down the gas
valve. This protects the
4.
A zero Ohm reading indicates a
blower motor and filter from
closed limit switch.
dangerously high temperatures. It
5. If the resistance is infinite, the
resets automatically after the air
temperature drops. This limit is set at
limit switch is open.
the factory and cannot be adjusted.
The limit can be checked with power
on using a
volt meter. A voltage
sensitive limit reacts too slowly
Operation. A normal temperature-
reading indicates an open switch. A
zero
volt reading confirms a closed
because of the nature of the furnace
and heat exchanger. This limit uses a
switch.
24-volt heater to partially heat the
Use an Ohmmeter to check the limit
limit switch. This accelerates limit
heater.
response to sudden temperature
1. Turn off all power to the furnace.
increases in the blower compartment.
The heater is electrically powered
2. Disconnect a wire from one of
anytime during a heating call from the
the heater terminals.
thermostat. The HALC contacts are in
3. Measure the Ohm resistance
series with the gas
valve.
across the heater.
4.
A resistance within the given
FIGURE 95
ranges indicates a good heater.
HEAT ASSISTED
5. Replace the HALC if the Ohm
LIMIT CONTROL (HALC)
reading is out of the range.
The reasons for an open HALC are
the same as for the main limit control
listed above.
NOTE: High voltages could cause
nuisance problems. Contact the
Service Department for a replace-
ment 30
volt HALC for field
installations.
DO NOT USE THIS
FIELD REPLACEMENT HALC AT
NORMAL VOLTAGES.
TABLE29
HEAT ASSISTED LIMIT SWITCH SETTING TEMPERATURES
UNIT INPUT OPEN RESET
WATTS(OHMS)
04(-)AU 45,000 105° 95°
1.5 (325-425)
05(-)AU 50,000 105° 95°
1.5 (325-425)
06(-)AU 67,500 105°
95°
1.5 (325-425)
07(-)AM 75,000 105° 95°
1.5 (325-425)
07(-)AU 75,000 105° 95° 1.5 (325-425)
10(-)AU 100,000 110° 100°
1.5 (325-425)
10(-)AM 100,000 105°
95°
2.0 (225-325)
10(-)BR 1.5 (325-425)
12(-)AR 125,000
105° 95°
2.0 (225-325)
15(-)AR 150,000 110° 100° 2.0 (225-325)
OVERTEMPERATURE
SWITCH
Operation. Over temperature
switches mount just above the
burners at the entry to the heat
exchanger. If they detect any heat
backing out into the furnace
vestibule, they open to shut down the
burners. These switches are in the
low voltage circuitry and must be
manually reset. If one opens, the
main blower and induced draft
blower run.
Standing pilot units have one
over
temperature switch mounted just
above the burners. Upflow/horizontal
hot surface ignition units have one
located above the burners and one at
either side of the burner brackets.
Dedicated horizontal units only have
two located at either side of the
burner brackets.
STANDING PILOT UNITS
One Over Temperature Switch
UPFLOW HSI UNITS
One Over Temperature Switch
UPFLOW/HORIZONTAL
Three Over Temperature Switches
DOWN FLOW/HORIZONTAL
Three Over Temperature Switches
DEDICATED HORIZONTAL
Two Over Temperature Switches
Over temperature switches protect
the unit against two situations:
1. A blocked heat exchanger.
2. A lack of combustion air.
If an
over temperature switch opens
ALWAYS check for a proper supply
of combustion air. The burner flame
doesn't have to back out to trip this
switch. It trips from temperature
produced in the furnace vestibule due
to lack of combustion air.
Check. Use an Ohmmeter to check
the
over temperature switch contacts.
1. Turn off all power to the furnace.
2. Measure the resistance of the
switch contacts.
3. An infinite resistance indicates
an open switch.
4. Press the reset button to reset
the switch.
5. The resistance should now be
zero Ohms.
Replace the switch if pressing the
button does not reset it.
This switch opens at 350°F.
J
91
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