A.O. Smith INTELLI-VENT, Troubleshooting Manual, Page: 35 / 46

Tuesday, January 16, 2007
Air Pressure Switches
This bulletin has been prepared to explain and illustrate the construction, operation, and testing
procedures for various types of air pressure switches used on residential and commercial tank
type water heaters.
Air pressure switches are used on all “fan assisted” gas fired water heaters in our product line.
Air pressure switches are used to verify that operational conditions are being met; IE: the blower
is running or the vent/combustion air piping length is not excessive or obstructed.
Air pressure switches are usually wired together with other switches into a “series” circuit (see
page 8). The ignition control constantly monitors the “state” of the air pressure switch “circuit” -
whether it is open or closed. There will always be at least one air pressure switch that has
normally open contacts (see pages 7 & 8). As the blower starts and comes up to speed the
normally open contacts of this switch should “activate” (contacts will close) in response to the
pressure it senses – as long as there are no adverse operating conditions.
Normal Operating Sequence - Pressure Switch Circuit
(Power-Vent, Power Direct-Vent, Cyclone, Master-Fit BTN/BTI models)
1. At the beginning of a heating cycle the ignition control will verify the air pressure
switch circuit is open before the control energizes the blower motor.
2. After successfully completing the above open circuit verification step, the ignition control will
energize the blower, the control will then check the air pressure switch circuit a second time
after the blower is energized. This time the pressure switch circuit must be verified closed.
The ignitor would not be energized unless this circuit closes after the blower is energized.
3. The ignition control monitors this circuit during the entire heating cycle to ensure it remains
closed. If this circuit opens (even for a split second) during the heating cycle; the control will
close the gas valve and stop the heating cycle.
By monitoring the air pressure switch circuit an ignition control can perform critical operational
checks such as:
1. Verify normally open air pressure switch contacts are not stuck closed or jumpered.
2. Verify the combustion blower or inducer fan is running.
3. Verify the vent/combustion air piping length is not excessive and/or obstructed.
Paul Stewart
Technical Training Department
A. O. Smith Water Products Company
Ashland City, Tennessee
Technical Bulletin TB-A023-06
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