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SERVICING
7. Locate the 3-circuit connector in the control/end bell. Using
an ohmmeter, check the resistance between each terminal
in the connector. If the resistance is 100kΩ or greater, the
control/end bell is functioning properly. Replace the control/
end bell if the resistance is lower than 100kΩ.
8. Reassemble motor and control/end bell in reverse of
disassembly. Replace blower assembly into the furnace.
MOTOR CHECKS
HIGH VOLTAGE!
Disconnect ALL power before servicing
or installing. Multiple power sources
may be present. Failure to do so may
cause property damage, personal injury
or death.
1. Disconnect power to the furnace.
NOTE: Motor contains capacitors that can hold a charge for
several minutes after disconnecting power. Wait 5 minutes
after removing power to allow capacitors to discharge.
2. Disassemble motor as described in steps 2 through 4
above.
3. Locate the 3-circuit harness from the motor. Using an
ohmmeter, measure the resistance between each motor
phase winding. The resistance levels should be equal.
Replace the motor if the resistance levels are unequal,
open circuited or short circuited.
4. Measure the resistance between each motor phase winding
and the motor shell. Replace the motor if any phase winding
is short circuited to the motor shell.
5. Reassemble motor and control/end bell in reverse of
disassembly. Replace blower assembly into the furnace.
CHECKING DUCT STATIC
The maximum and minimum allowable external static pres-
sures are found in the specification section. These tables also
show the amount of air being delivered at a given static by a
given motor speed or pulley adjustment.
The furnace motor cannot deliver proper air quantities (CFM)
against statics other than those listed.
Too great of an external static pressure will result in insuffi-
cient air that can cause excessive temperature rise, resulting
in limit tripping, etc. Whereas not enough static may result in
motor overloading.
To determine proper air movement, proceed as follows:
1. With clean filters in the furnace, use a draft gauge (inclined
manometer) to measure the static pressure of the return
duct at the inlet of the furnace. (Negative Pressure)
2. Measure the static pressure of the supply duct. (Positive
Pressure)
3. Add the two (2) readings together for total external static
pressure.
NOTE: Both readings may be taken simultaneously and read
directly on the manometer if so desired. If an air conditioner
coil or Electronic Air Cleaner is used in conjunction with the
furnace, the readings must also include theses components,
as shown in the following drawing.
4. Consult proper tables for the quantity of air.
If the total external static pressure exceeds the minimum or
maximum allowable statics, check for closed dampers, reg-
isters, undersized and/or oversized poorly laid out duct work.
Checking Static Pressure
CHECKING TEMPERATURE RISE
The more air (CFM) being delivered through a given furnace,
the less the rise will be; so the less air (CFM) being delivered,
the greater the rise. The temperature rise should be adjusted
in accordance to a given furnace specifications and its exter-
nal static pressure. An incorrect temperature rise may result
in condensing in or overheating of the heat exchanger. An
airflow and temperature rise table is provided in the blower
performance specification section. Determine and adjust
temperature rise as follows:
.
59
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