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4 8 | Bosch 96% AFUE Gas Furnace
12 Operational Checks
DANGER: FIRE, EXPLOSION, ELECTRICAL SHOCK, AND
CARBON MONOXIDE POISONING HAZARD
To avoid personal injury or death. Do not remove any
internal component covers or attempt any adjustment.
Electrical compartments are contained in both
compartments. Contact a qualifi ed service agent at once if
an abnormal fl ame appearance should develop.
Burner Flame
The burner fl ames should be inspected with the burner access panel installed.
Flames should be stable, quiet, soft, and blue (dust may cause orange tips but
they must not be yellow). Flames should extend directly outward from the burners
without curling, fl oating,or lifting off , and should enter heat-exchanger tube.
Flames must not impinge on the sides of the heat exchanger fi ring tubes.
Check the burner flames for:
1. Stable, soft and blue
2. Not curling, floating, or lifting off
Figure 40
Burner Flame
13 Safety Circuit Description
13.1 General
A number of safety circuits are employed to ensure safe and proper furnace
operation. These circuits serve to control any potential safety hazards and serve
as inputs in the monitoring and diagnosis of abnormal function. These circuits are
continuously monitored during furnace operation by the integrated control module.
13.2 Integrated Control Module
The integrated control module is an electronic device which controls all furnace
operations. Responding to the thermostat, the module initiates and controls
normal furnace operation, and monitors and addresses all safety circuits. If
a potential safety concern is detected, the module will take the necessary
precautions and provide diagnostic information through an LED.
13.3 Chamber Limit Switch - Fixed
The chamber limit switch - fi xed control is located on the partition panel and
monitors heat exchanger compartment temperature (it is an automatic reset
temperature sensor). The limit guards against the overheating resulting from
insuffi cient air passing over the heat exchanger.
Data subject to change
13.4 Fan Mounted Limit Switch
The fan mounted limit switches are located beside the blower housing. They are
temperature sensors which are automatically reset. They are used to prevent
overheating by hot air when the furnace is installed horizontally.
13.5 Rollout Switch - Resettable
The rollout switch - resettable controls are mounted on the burner/manifold
assembly and monitor the burner fl ame. They are manual-reset temperature
sensors. This limit guards against burner fl ames not being properly drawn into the
heat exchanger.
13.6 Overfl ow Protection Pressure Switch (Overfl ow Switch)
Overfl ow switch is a diff erential pressure switch. The shape and dimension of the
overfl ow switch is similar to the two other pressure switches except it has two
pressure ports, one in gray color (negative) and the other in black color (positive).
The overfl ow switch is normally closed. When condensate hoses/trap is blocked
and condensate is stopped fl owing to drain system, the level of condensate inside
the condensate collector box will rise. When the condensate reaches too high
of a level in condensate collector box, the overfl ow switch will open and shut
off the furnace. Make sure that black port (positive) is conn- ected to the lower
position tap on the condensate collector box and gray port (negative) to higher
tap of condensate box. [add the following in warning box]: The manufacturer's
default setting is for upfl ow and horiz- ontal right installations only. For horizontal
left installation, hoses for overfl ow switch are required to switch (see horizontal
left installation). Conn- ecting hoses incorrectly will result in failure to protect
condensate overfl ow.
13.7 Pressure Switches
The pressure switches are normally open, negative air pressure activated switches.
They monitor the airfl ow (combustion air and fl ue products) through the heat
exchanger via pressure taps located on the induced draft blower. These switches
guard against insuffi cient airfl ow (combustion air and fl ue products) through the
heat exchanger.
13.8 Flame Sensor
The fl ame sensor is a probe mounted to the burner/manifold assembly which uses
the principle of fl ame rectifi cation to determine the presence or absence of fl ame.
WARNING: FIRE, EXPLOSION, OR CARBON MONOXIDE
POISONING HAZARD
Replace ONLY with the same model number or as specifi ed
by the manufacturer.
Installation, Operation & Maintenance
04.2019 | Bosch Thermotechnology Corp.
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