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8-2.6 Defective Thermocouples
Most temperature controllers and excess temperature limit interlocks are provided
with thermocouples for sensing. These sensors are subject to drift over time.
Compare known reading at sensor inside furnace to controller display. If it varies
grossly, sensor may have to be replaced. If sensor is damaged or broken open,
the controller display may give thermocouple error codes (see controller manual for
proper error code meanings).
8-2.7
On Electrically Heated Furnaces - Improper Line Voltage
Voltage at the furnace should be measured to determine if an excessive line drop is
causing reduced power input to the heating elements on an electrically heated
furnace. This could be caused by too many devices connected to the same circuit
or by undersized wiring between the furnace and the power source. Measure the
voltage with the furnace heating elements on and all other equipment on the same
circuit operating.
8-2.8
On Gas Heated Furnaces, Burner or Gas Pressure Adjustment
Contact your local gas company's service engineers. It is recommended to have
them check all installations as they are aware of the many variables which can
affect your operation. Correct gas pressure is essential. Measure gas pressure at
the beginning of the furnace gas pipe train to ensure pressure agrees with the
furnace nameplate requirement. If pressure drops as burner approaches high fire,
there is insufficient gas supply.
8-3 EXCESS TEMPERATURE LIMIT INTERLOCK ACTUATION
Depending upon the specific device used, it may be tripped by either an excessively
high furnace temperature or a sensing element failure. Before placing the equipment
back into operation, it should be determined what caused the excess temperature limit
interlock to actuate and the condition be corrected. See the excess temperature limit
interlock manufacturer's literature for the proper operation and adjustment of the
control used. CAUTION: If the furnace is equipped for use with flammable solvents or
vapors, the excess temperature limit interlock is factory set at the maximum
temperature allowed and should not be set to a higher temperature.
8-4 THE MOTOR STARTER OVERLOADS TRIPPED
All line voltage motors have current sensitive protective features in the motor starter.
On magnetic motor starters, the "motor running" pilot light will not remain on if the
overloads have tripped. The reset button is located on the motor starter inside the
control panel. Measure motor amperage and compare to nameplate. If over
amperage, determine cause and correct. If motor is only slightly over amperage, the
overload can be increased to compensate. Motor amperage of blowers will drop as the
furnace heats up and the air thins out reducing the load on the motor.
8-5
ON GAS HEATED FURNACES, PILOT WILL NOT LIGHT
A safe-start timer is built into the flame safety relay which will shut the system down if
the pilot is not proven in approximately 10 seconds. If this timer trips, press the reset
button on the flame safety relay near the gas burner. When the burner is inaccessible,
a reset button will be on the control panel. See TROUBLE SHOOTING GAS HEATED
EQUIPMENT procedures which is included in your manual if you have a gas heated
furnace.
