Contact States - A.O. Smith INTELLI-VENT Troubleshooting Manual

Air Pressure Switch Construction
Normal State Activated State
(open switch contacts) (closed switch contacts)
Wiring Terminals
Sensing Port
Switch Contacts
Internal Linkage
Flexible Diaphragm Vent Port
Air Pressure Switch Operation
Air pressure switches activate in response to pressure sensed through a tube. One end of this
tube attaches to the pressure switch sensing port, the other end attaches to a pressure
sensing port somewhere on the water heater, IE: the vent system, blower assembly.
A flexible diaphragm divides the body of the switch into two air chambers. The sensing port is
attached to one air chamber. The other air chamber is vented to the atmosphere which allows
the diaphragm to move. Internal linkage attaches the switch contacts to the diaphragm; with
sufficient pressure applied, diaphragm movement will activate the switch contacts.

Contact States:

The "state" of the contacts refers to whether or not the switch contacts are open or closed.
Closed contacts will allow electricity to flow through the switch because there is a continuous
"conductive" path through the switch contacts. Open contacts will not allow electricity to flow,
there is no "continuity" through the switch contacts. The state of the contacts, open or closed,
can be verified/confirmed with an ohm meter. An ohm meter is required to perform conclusive
diagnostic tests on air pressure switches.
With air pressure switches the term "normal state" refers to whether or not the switch
contacts are open or closed when there is no pressure applied to the switch. Switches can be
normally open or normally closed, abbreviated NO or NC in technical literature.
The "activated state" of a switch is the opposite of the normal state. A normally open switch
in it's activated state is closed. A normally closed switch in it's activated state is open.
Air pressure switches are described by their normal state and by their "contact action" which is
covered on the following page.
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