A.O. Smith BTH 120-250(A) User Manual download pdf

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

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

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Summary of Contents

Page 1 - Technical Bulletin TB-A023-06

Tuesday, January 16, 2007Air Pressure SwitchesThis bulletin has been prepared to explain and illustrate the construction, operation, and testing proce

Page 2 - Air Pressure Switch Operation

Air Pressure Switch ConstructionAir Pressure Switch OperationAir pressure switches activate in response to pressure sensed through a tube. One end of

Page 3 - Tools Required for Testing

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 “no

Page 4 - Air Pressure Switch Testing

Testing the performance of air pressure switches involves three procedures.First Procedure is a normal state test to determine if the switch contacts

Page 5 - AHSA02307 5 of 8

Third Procedure is a pressure test and requires a digital manometer capable of reading positive and negative pressures. A good range of operation woul

Page 6

Third Procedure - Normally Open Switches (cont):Pressure reading taken DOES NOT reach the activation pressure (cont):Ensure the sensing tube is connec

Page 7 - Normal State Activated State

Air Pressure Switch TypesNormally Open = NONormally Closed = NCContact States Contact ActionsClose on a rise in pressureClose on a fall in pressureOpe

Page 8 - (BTH 120, 250, 300, 400 only)

Air Pressure Switch CircuitResidential Power-Vent & Power-Direct-Vent Water HeatersAir Pressure Switch CircuitCyclone Water Heater14Blocked Exhaus