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Safety Features
SAFETY FEATURES
LIMIT CONTROL/HALC
The high limit cut-off temperature is set at the factory and
cannot be adjusted. The temperature setting prevents the air
temperature leaving the furnace from exceeding the maximum
outlet air temperature, which, if exceeded, will shut the furnace
down.
There are several reasons for a limit switch to open and almost
always involve low airflow through the furnace.
1. A dirty or restricted air filter.
2. A dirty or restricted cooling coil.
3. Undersized or restricted return air system.
4. Undersized or restricted supply air system.
5. A problem affecting the main blower:
A. A wrong speed tap selection.
B. Failing motor bearings.
C. Low voltage to the motor.
D. Dirty blower wheel.
E. Wrong motor rotation.
F. Blower wheel slipping on the
F. motor shaft.
6. Overfiring the furnace with too much gas pressure.
7. Ventilation problems.
8. Failed blower motor.
FLAME ROLL-OUT SAFETY SWITCHES
Furnaces are equipped with safety switches to protect against
flame roll-out conditions in the burner compartment, which,
if tripped, will terminate the heating cycle. In the event of a
flame roll-out condition, the switch will shut the furnace down.
Switches for the furnaces are located on either side of the
burner cover plate. If a switch is tripped, it must be manually
reset. DO NOT jumper or reset this switch. If this switch should
trip, a qualified installer, service agency or the gas supplier
should be called to diagnose and/or correct the source of trip-
ping. If this unit is mounted in a closet, the door must be closed
when making this check.
PRESSURE SWITCH
This furnace is equipped with a normally-open pressure switch
that monitors pressure conditions within the furnace vent system
during the heating cycle.
There are several reasons for the pressure switch not to close.
1. An inoperative induced draft blower.
2. A loose or leaky pressure switch hose.
3. A blockage in the vent.
4. Severe downdrafts canceling the draft from the inducer fan.
5. A leaky gasket at the induced draft blower.
6. Improperly sized or installed vent.
The pressure switch contacts must open before the unit can go
through another heating cycle.
See diagnostic chart in this book for diagnostic recommenda-
tions.
WARNING
DO NOT BYPASS, JUMPER, OR REMOVE ANY SAFETY
SWITCH FROM THE FURNACE CONTROL CIRCUIT. IF
A SAFETY SWITCH CAUSES THE FURNACE TO SHUT
DOWN OR OPERATE INTERMITTENTLY, IT IS AN INDI-
CATION OF A POTENTIAL SAFETY HAZARD THAT MUST
BE ADDRESSED BY A QUALIFIED TECHNICIAN, SER-
VICE AGENCY OR THE GAS SUPPLIER. DO NOT RESET
SAFETY CONTROLS WITHOUT CORRECTIVE ACTION
AND/OR VERIFICATION OF PROPER SAFE OPERATION
BY A QUALIFIED INSTALLER, SERVICE AGENCY OR
THE GAS SUPPLIER.
REPLACE ANY SAFETY CONTROL COMPONENT ONLY
WITH IDENTICAL OEM REPLACEMENT PARTS. WHEN
A NEW SAFETY SWITCH IS INSTALLED, IT MUST BE
TESTED FOR A MINIMUM OF 15 MINUTES WITH THE
FURNACE OPERATING AT MAXIMUM INPUT RATE AND
WITH BOTH BLOWER AND BURNER DOOR INSTALLED.
IF THE FURNACE IS INSTALLED IN A CLOSET, THE
CLOSET DOOR MUST ALSO BE CLOSED FOR THIS
TEST. REPEAT THE TEST AT THE MINIMUM INPUT
RATE IF THE FURNACE IS A MULTI-STAGE FURNACE.
!
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