Air Pressure Switch Construction; Air Pressure Switch Operation - A.O. Smith INTELLI-VENT Troubleshooting Manual

INTELLI-VENT TROUBLESHOOTING GUIDE

AIR PRESSURE SWITCH CONSTRUCTION

The switch below has normally open switch contacts that close on a fall in pressure.
Normal State Activated State
Wiring terminals
Sensing port
Switch contacts
Diaphragm
Vent port

AIR PRESSURE SWITCH OPERATION

Air pressure switches activate in response to changes in air pressure sensed through a
plastic tube attached to a sensing port on the water vent system and/or blower housing.
The diaphragm divides the body of the switch into two air chambers. The air chamber the
sensing port is attached to is sealed. The other chamber is vented to the atmosphere which
allows the diaphragm to move up and down. There is linkage that attaches the switch
contacts to the diaphragm so that when the diaphragm moves the contacts are activated.
The term "activate" means to change state. If the contacts are normally open, activating the
contacts will cause them to close. If the contacts are normally closed, activating the
contacts will cause them to open.
Service Note:
Because one of the two air chambers is vented to the atmosphere, the ambient air
pressure in the installed space is critical. If the ambient air pressure is below atmospheric
(outdoor) air pressure, it will pull on the pressure switch diaphragm. If the ambient air
pressure is above atmospheric it will push on the pressure switch diaphragm. In either
condition the pressure switch will not operate as designed if the pressure imbalance is
severe enough. Correcting an air pressure imbalance between the installation space and
the outdoors can be achieved by providing adequate make up air openings.
Direct vent installations will not eliminate this potential problem.
Technical Training Department Ashland City, Tennessee © 2005
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