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Service Reference Manual UNIT COMPONENTS
5-23
SRM-HW/HWC 2/99
Blocked Intake
Blocked intake is any condition that blocks or restricts the flow of intake
(combustion) air to a point where the pressure fall reaches the calibrated
open setting of the pressure switch.
Potential Causes:
• Rodents
• Spiders
• Nest building in the intake opening
• Debris or trash that has been pulled into the intake opening
Symptoms:
• Excessive negative pressure in the burner box area. The furnace
cycles on and off quite frequently, typically within 60 seconds or so.
• Inability of unit to stay on for a complete call for heat.
For a blocked intake or blocked flue, connect the incline manometer as
shown on in Figure 5-20 on page 5-27 for the specific model groups.
Never try to adjust a pressure switch or jumper the
switch to allow the furnace to operate. Doing so
could allow the furnace to operate under hazardous
conditions, potentially causing property damage,
personal injury or death.
WARNING
DO NOT try to repair a pressure switch. Use an
identical replacement. The use of an improperly
repaired or malfunctioning pressure switch could
result in property damage, personal injury or death.
WARNING
The measured static pressure reading should be compared to the pres-
sure switch specifications. If the measured static pressure meets or
exceeds the specifications and switch contacts will not transfer, check the
following items:
• Switch out of calibration
• Defective pressure switch
• Moisture in pressure switch tubing
• Incorrect switch installed