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PRESSURE SWITCHES
The water heaters covered in this Service Handbook are factory equipped with three pressure switches
(including the Low Gas Pressure Switch). "Top View" Figures on Page 6 shows the location of the two
pressures switches on top of the water heater. This section of the Service Handbook covers pressure switch
construction, operation and the test procedures used to diagnose operational problems associated with
pressure switches. The pressure switches are identified with colored labels; Blocked Air Intake Switch is Blue
& Blocked Exhaust Switch is Red.
CONTROL SYSTEM MONITORING
The Control System monitors the "state" of the pressure switch contacts individually through two separate
circuits, see "Wiring Diagram" on Page 50. The state of the switch refers to whether the switch contacts
are open or closed.
At the beginning of a heating sequence, before the Combustion Blower is energized, the Control System
enters the Input Verification operating state. During Input Verification the Control System monitors the two
pressure switches to ensure their contacts are in the correct "normal" state. The contacts for the Blocked Air
Intake Switch and Blocked Exhaust Switches must be closed.
If any of the pressure switch contacts are not in their correct normal state during Input Verification the Control
System will lock out and display a fault message on the UIM indicating which pressure switch caused the
fault condition. If all system checks pass during the Input Verification operating state the Control System
enters the Pre-Purge operating state and energizes the Combustion Blower. The Heater Status Screen and
functionality of the water heater can be used for troubleshooting.
If either the Blocked Air Intake Switch or Blocked Exhaust Switch contacts open at any time during a heating
sequence the Control System will lock out and display Blocked Air Intake or Blocked Exhaust on the UIM.
Wires connecting to air switches are labeled near the terminals. Ensure no mis-wiring among terminals.
12
Servicing should only be performed by a Qualified Service Technician
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