FURNACE SAFETY CONTROL DEVICES
The following Safety Devices are included in the design of the furnace (* Depending on Age of the Unit)
A.
HIGH-TEMPERATURE LIMIT CONTROLS
- This is an automatic reset control that provides
over-temperature protection due to high temperatures typically caused by reduced airflow. If
high-limit switch trips occur, the equipment must be inspected to diagnose and resolve failures or
faults. Do not replace controls without ensuring that there are no other associated or underlying
causes. Always replace defective controls with UPG Source 1 Parts. The high-temperature limit
control is located on the furnace vestibule panel near the gas valve. The limit detects rising
temperature in the airstream and opens at a specific temperature.
Associated fault codes are either 4 (four) or 11 (eleven) RED FLASHES.
a. 4 RED FLASHES: indicates that the main limit switch has opened its normally closed
contacts. The control operates the supply air blower and inducer while the open limit condition
exists. If the limit switch has not closed within 5 min, the control operates as if the blower is
not functioning. The control starts a hard lockout and begins to flash the 11 Red Flashes error
code. Power must be cycled off and on to reset the control after the problem is corrected.
b. 11 RED FLASHES: indicates that the main limit switch has opened its normally closed
contacts and has remained open for more than 5 min. This condition is usually caused by a
failed blower motor or blower wheel. The control enters a hard lockout and power must be
cycled off and on to reset the control after the problem is corrected.
Diagnostics and Trouble Shooting Guide.
This fault may be caused by one or more of the
following, and each should be verified as to being either a potential cause or not:
a. A dirty filter – check and replace as necessary.
b. Improperly sized duct system or too many supply or return registers closed or blocked off
– check and open or balance supply and return airflow as necessary.
c. Improper indoor blower motor speed selection - Proper gas manifold pressure must be set
and firing rate verified. Indoor heating blower motor speed must be set to maintain proper
temperature rise as indicated on the unit rating plate. The heating blower speed is set at
the factory to the default blower speed, which is the blower speed that delivers the correct
airflow for proper heating operation in most applications. The default heating blower speed
for each model is shown in its instruction manual. In certain circumstances, it may be
necessary to change the heating blower speed to a different motor speed. Not all motor
speeds are appropriate for gas heating operation. Also, do not energize more than one
motor speed at a time (PSC motors only) or damage to the motor will result. A change in
motor speed will typically require a remeasurement of the Temperature Rise (temperature
difference between the return air and the supply air) from the furnace. Follow the
instructions for Measurement of Temperature Rise in the instruction manual.
d. Loose limit switch wiring or faulty limit switch – verify field wiring to the unit is properly
grounded, re-verify ground wires and conductor connectors are tight and secure, and that
the limit switch exhibits proper electrical continuity, and is not shorted to chassis.
e. Faulty Blower Motor - Confirm line and in some cases low voltage (depending on motor
type) to the motor to confirm if motor should be operating or not. Replace faulty blower
motor as necessary.
