Air Pressure Switch Testing - A.O. Smith BTX-100 Service Handbook

ULTRA HIGH EFFICIENCY POWER VENT/POWER DIRECT VENT - SERVICE MANUAL

AIR PRESSURE SWITCH TESTING

Air pressure switch testing involves three procedures:
• Normal State Test
• Operational State Test
• Pressure Test
Normal State Test
Turn off power and disconnect the wires to the pressure switches. Wrap electrical tape
around the wire ends to prevent shorts. A continuity test is performed, using an ohm meter,
between the wiring terminals of the switch. The Blower Prover switch has normally open
contacts. The Blocked Intake and Blocked Exhaust switches have normally closed contacts.
If this test confirms the Blower Prover switch contacts are open and the Blocked Intake/
Exhaust switch contacts are closed - the switches have passed the test. If test results
indicate the Blower Prover switch contacts are closed - the switch must be replaced. If either
the Blocked Intake or Exhaust switch contacts are open - the switch must be replaced.
Operational State Test
The operational test is also a continuity test performed with an ohm meter between the
pressure switch wiring terminals. However, this test is performed while the blower is running
during the pre-purge mode. Ensure the wires to the air pressure switches are disconnected
and that electrical tape is wrapped around the wire ends to prevent shorts. Perform a
continuity test between the wiring terminals on the air pressure switches with the blower
running (water heater in pre-purge mode).
During this test the normally open contacts of the Blower Prover switch should close. The
Blower Prover switch "activates" (closes its contacts) during normal operation to verify or
"prove" that the blower is running. If the contacts remain open during this test the third test,
the pressure test, must be performed to determine if the pressure switch is defective. During
this test the Blocked Intake and the Blocked Exhaust switch contacts should remain closed,
if the contacts open during this test the third test, the pressure test, must be performed to
determine if the pressure switch is defective.
Technical Literature Department
Servicing should only be performed by a Qualified Service Agent
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