A.O. Smith INTELLI-VENT, Troubleshooting Manual

Page: 37 / 46

Air Pressure Switch Operation (cont)
Contact Actions:
The “action” of the contacts refers to what causes the switch to activate, to change from it’s
“normal state” to its “activated state” (see page 2). On an air pressure switch this can be either
a rise or fall in pressure. As the illustrations below indicate, locating the sensing port above or
below the flexible diaphragm facilitates these operational differences. The contact action is
further described to convey whether the contacts will open or close when the pressure sensed
is sufficient to activate the switch contacts, IE: “close on a fall in pressure.”
Pressure Switch Descriptions:
Air pressure switches are described and referred to by their Contact State and Contact Action.
IE: the switch on the left side in the illustration below is a “normally open, close on a fall”
pressure switch. See page 7 for more information about other types of switches.
Service Note:
Because one of the two air chambers is vented to the atmosphere, the ambient (in the
surrounding area) air pressure is critical. Ambient air pressure can push or pull on the
diaphragm through the vent port. If the ambient air pressure is extremely high (positive) or low
(negative) it may cause erratic pressure switch operation.
Direct vent installations will not eliminate this potential problem.
Tools Required for Testing
A volt/ohm multi-meter - Fieldpiece HS36 DMM (digital multi-meter) or equivalent.
A digital manometer is also required to measure pressure and perform conclusive diagnostic
tests. Specifications: range; -15.0" W.C. to +15.0" W.C. resolution; 0.01" W.C. Brands/models
recommended that meet these specifications include: UEI model EM-200 and Extech model
406850 dual pressure reading digital manometers.
Sensing Port Above Diaphragm
(a fall in pressure or “vacuum” activates the switch)
Flexible Diaphragm
Normally Open, Close on a Fall Normally Open, Close on a Rise
Sensing Port Beneath Diaphragm
(a rise in pressure activates the switch)
Vent Port
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