Service Manual

SERVICING
55
S-3B HEATING ANTICIPATOR/ CYLCE RATE
Older heating thermostats may have an adjustable antici-
pator. This is a wire wound adjustable heater which is en-
ergized during the "ON" cycle to help prevent overheating
of the conditioned space.
The anticipator is a part of the thermostat and if it should
fail for any reason, the thermostat must be replaced.
Other thermostats have a cycle rate adjustment to adjust
cycles per hour. This should be set to match system type.
S-4 CHECKING TRANSFORMER AND CONTROL
CIRCUIT
A step-down transformer 120 volt primary to 24 volt sec-
ondary, 40 VA (Heating and Cooling Models) supplies ample
capacity of power for either operation.
WARNING
HIGH VOLTAGE
D
ISCONNECT ALL POWER BEFORE SERVICING OR
CHANGING ANY ELECTRICAL WIRING.
MULTIPLE POWER
SOURCES MAY BE PRESENT.
FAILURE TO DO SO MAY CAUSE
PROPERTY DAMAGE, PERSONAL INJURY OR DEATH.
1. Remove blower compartment door to gain access to
the thermostat low voltage wires located at the fur-
nace integrated control module.
2. Remove the thermostat low voltage wires at the fur-
nace integrated control module terminals.
With Power On (and Door Interlock Switch closed
LINE VOLTAGE NOW PRESENT
WARNING
3. Use a voltmeter, check voltage across terminals R
and C. Must read 24 VAC.
4. No voltage indicates faulty transformer, open fuse,
bad wiring, bad splice, or open door interlock switch.
5. Check transformer primary voltage at incoming line
voltage connections, fuse, splices, and blower door
interlock switch.
6. If line voltage is available to the primary side of
transformer and not at secondary side, the trans-
former is inoperative. Replace.
7. After completing check and/or replacement of trans-
former and check and/or repair of control circuit, re-
install blower compartment door.
8. Turn on electrical power and verify proper unit opera-
tion.
S-300 CHECKING PRIMARY LIMIT CONTROL
All modulating furnaces use a nonadjustable, automatic
reset, bi-metal type limit control. The primary limit con-
trol is located on the front panel of the furnace between
heat exchanger tubes. This is a normally closed control.
WARNING
HIGH VOLTAGE
D
ISCONNECT ALL POWER BEFORE SERVICING OR
CHANGING ANY ELECTRICAL WIRING.
MULTIPLE POWER
SOURCES MAY BE PRESENT.
FAILURE TO DO SO MAY CAUSE
PROPERTY DAMAGE, PERSONAL INJURY OR DEATH.
1. Turn off all voltage sources
2. Remove the two wires from the limit switch
3. Using an ohmmeter, check for a closed circuit across
the two limit contacts (this will be the same reading
that you see by touching your two meter leads to-
gether)
S-301 CHECKING AUXILIARY LIMIT CONTROL
Automatic Reset Auxiliary Limit Located in Blower Hous-
ing
Modulating furnaces use an auxiliary limit(s) (automatic
reset) control connected in series with the main limit switch.
If its temperature should be exceeded, it will open, inter-
rupting the voltage in the limit circuit. The auxiliary limit
is located on the side of the blower housing. This is a nor-
mally closed switch. To check auxiliary limits; perform the
following steps.
WARNING
HIGH VOLTAGE
D
ISCONNECT ALL POWER BEFORE SERVICING OR
CHANGING ANY ELECTRICAL WIRING.
MULTIPLE POWER
SOURCES MAY BE PRESENT.
FAILURE TO DO SO MAY CAUSE
PROPERTY DAMAGE, PERSONAL INJURY OR DEATH.
1. Turn off all power sources
2. Remove the two wires from the auxiliary limit switch
3. Using an ohmmeter, check for a closed circuit across
the two limit contacts (this will be the same reading
that you see by touching your two meter leads to-
gether)