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Air Pressure Switches
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
AHSA02307
1 of 8 Air Pressure Switch Construction
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 its activated state is closed. A normally closed switch in its 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.
Vent Port
Sensing Port
Flexible Diaphragm
Wiring Terminals
Switch Contacts
Normal State
(open switch contacts)
Activated State
(closed switch contacts)
Internal Linkage
AHSA02307
2 of 8 Air Pressure Switch Operation (cont)
Contact Actions:
The action of the contacts refers to what causes the switch to activate, to change from its
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
AHSA02307
3 of 8 Testing the performance of air pressure switches involves three procedures.
First Procedure
is a normal state test to determine if the switch contacts are open or closed in the normal
state (see page 2) without pressure applied. Power is turned off for this test. The wires to the switch are
disconnected. A continuity test is then performed (ohm meter) between the wiring terminals on the switch. If the
switch is a normally open switch (see page 2) the continuity test should indicate no continuity - open. If the
result of this test indicated the contacts were closed, the switch must be replaced. Likewise if a normally closed
switchs contacts tested open in its normal state it would also have to be replaced. Note: gaining access to the
switch and its wiring terminals may involve removing the switch, opening a control box etc.
THE SECOND AND THIRD PROCEDURES ARE PERFORMED WITH THE BLOWER RUNNING.
Second Procedure
is an operational test to determine if the switch contacts are open or closed while the
blower is running and pressure is applied to the switch. The wires to the switch remain disconnected and a
continuity test is performed as in the first procedure above. Ensure the wires ends do not touch/short to ground.
Ensure the sensing tube is properly connected at both ends and test for continuity with the blower running at full
speed. Note the condition of the switch contacts; whether open or closed.
Normally open air pressure switches must close during operation and normally closed air pressure switches must
remain closed throughout the heating cycle on A. O. Smith fan assisted water heaters. This information can
usually be found in the sequence of operation section of the owners manual and/or service manual for the
product. Contact the A. O. Smith technical information center (800 527-1953) for more information.
Normally open switches: If the air pressure switch being tested is a normally open switch, it must close its
contacts (activate) before ignition can occur. If the continuity test indicates a normally open switchs contacts
have closed while the blower is running, the switch is operating correctly. If the continuity test indicates the
switch contacts are remaining open while the blower is running the third pressure test procedure will have to be
performed. This will determine if the switch is defective or if the switch contacts are failing to close be