THP-400A, THP-500A and THP-550 Installation and Service Manual
Troubleshooting
Problem Probable Cause Remedy
Blower does not operate Inadequate or no
voltage present
Check power supply to unit. 115-V must be available.
Check Rating Information Table on page 5.
Fuse loose or blown Check control fuse, FU-02.
Blower switch not
in ON position
Turn ON.
Blower switch defective With switch in the ON position, check for 115-V at
Terminal 3. Replace switch if voltage is not present.
Low-temperature limit
switch not at proper setting
Timer: 3-min
Temperature: 40° F
To Reset: Turn burner and blower switches OFF and
then back ON.
Defective low-temperature
limit switch
Check wiring connections. Check for 115-V at
Terminal 5. If no voltage is present, replace switch.
Defective motor Check wiring condition and connections from motor
starter to motor. If motor is receiving proper voltage and
wiring is in good condition, the motor is defective and
must be replaced.
Blower runs, burner
does not ignite
Burner switch not
in ON position
Turn ON.
Defective burner switch With burner switch in the ON position, check voltage at
Terminal 5. If 115-V are present at Terminal 5 and not
at Terminal 6, the burner switch is defective.
Insufficient airflow Check unit's intake and discharge for obstructions.
Inspect fan blade and motor for proper operation.
Adjust airflow control according to the instructions on
page 19.
Remove airflow tubing and inspect for obstructions.
Reconnect tubing.
If voltage is present at Terminal 7 but not at Terminal 8,
replace the airflow switch.
High-temperature limit
switch out of adjustment
Press reset button on switch. If unit continues to shut
down, the unit is either overfired, or the limit switch is
defective. Contact TEMP-HEAT.
Adjustable high-
temperature limit switch
not at proper setting
Set at 240° F. If unit continues to shut down, the unit is
either overfired or the limit switch is defective. Contact
TEMP-HEAT.
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