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What does a vacuum switch do?
The vacuum or pressure switch measures the vacuum created by a blower and verifies the blower is operating at speed.
 How to replace pressure switch
water heater pressure switch Pressure switch / same as vacuum switch
What does a pressure switch or vacuum switch do?
The vacuum or pressure switch measures the vacuum created by a blower and verifies the blower is operating at speed.
When blower is running at correct speed, the pressure switch closes a circuit that can trigger a different operation.

-On a power vented water heater, the vacuum switch is a safety device that prevents possible explosion. If the blower is not running or running too slow, the pressure switch will not close circuit to the gas control valve and water heater will not turn on gas to the burner. This ensures full burning of fuel and prevents explosive gas from accumulating inside vent pipe.

Contruction, oeration, and testing for pressure switch .pdf
How the power vent water heater works:

The gas valve thermostat senses that water inside tank is below set point and closes a circuit that turns on the 120 volt blower located on top of water heater.
-The blower pulls air up through the vent pipe and begins a cycle of releasing toxic combustion by-products, including NO SO CO CO2 and acidic water vapor, into the atmosphere, where it creates an exciting new future for the planet. Alternative for some: Connect solar panels directly to electric water heater
-The vacuum safety switch is connected to blower via rubber tube. The rush of air through the vent pipe creates vacuum pressure.

At beginning of a call for heat, the vacuum switch is OPEN. The vacuum switch must close, like an ordinary on-off switch and complete a circuit that notifies gas control that blower is operating correctly.
-The vacuum or pressure switch measures the vacuum created by the blower and verifies the blower is operating at speed. This ensures the blower can evacuate combustion gasses and support complete burning of fuel. This prevents explosion hazard caused by ignitable fuel gathering in vent pipe. The efficiency of gas water heater combustion is typically 60-61%, with remaining 39-40% of heat going up the flue pipe. Calculate efficiency
-Once the blower operation is confirmed by the vacuum switch, it closes (turns on) a small circuit that signals the gas valve thermostat that the blower is working. 
-If the pilot light is ON, it heats the thermocouple or thermopile. The thermocouple or thermopile creates a small current that travels to gas control valve. If the pilot light and thermocouple are working correctly, and the pressure switch has confirmed the blower is on, the gas control valve releases gas to the burner. The pilot light ignites the fuel. 
-If the blower is not operating, or running at lower speed because of a brownout or lack of voltage or malfunction, the vacuum switch does not receive enough vacuum pressure and will not close, and the ignition sequence will not take place. Likewise if the pilot flame is out, or misaligned with the thermocouple or thermopile, or the thermopile or thermocouple are not functioning, then no current arrives at the gas control valve, and ignition will not take place.
-The blower has a resettable thermal safety switch that monitors vent temperatures. If temperature on vent pipe exceed 180, the thermal switch cuts off power and blower stops running. The vacuum switch resets to open position and gas valve thermostat turns off gas supply.
-If temperature of water inside tank exceed high limit set point of thermostat, then gas control valve turns off all gas. The gas control valve may require replacement. Some gas control valves can be reset once repairs have been made to correct cause of overheating. If the thermostat comes out of calibration, and is no longer performing, it requires replacement of gas control valve.

-When temperature inside tank reaches the set point, for example 120F, the gas control valve thermostat turns off all power, and the water heater goes into standby mode until water temperature drops below thermostat set point.

-Generally, to check if blower is good and pressure switch is bad, you bypass the pressure switch and connect blower directly to 120 volt outlet, running a pigtail to the blower or plugging blower directly into outlet.
" In troubleshooting pressure switches, you usually just jumper across it so it looks like a solid wire to the control.  Do this only when the unit is trying to fire up, so it is in the correct sequence. ~~ If you get no continuity however, it sounds like the contacts in the switch are shot and the switch should be replaced. Jumpering across it will let you know if that is what's keeping the heater from firing up. Do NOT leave the jumper in place as you're disabling a safety device." Source

Pressure switches fail when a few drops of water condense in the small tube the connects to the switch, or some other internal problem with contacts happens.  Using a volt meter on the switch while gently blowing /sucking on the tube will confirm it's operation... or lack of operation.
Pressure switch troubleshoot
Test pressure switch
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Pressure switch replacement
Bradford White pressure switch replacement
Check pressure switch

How to troubleshoot gas water heater
Troubleshoot power vent water heater
"How to check the vacuum line
Requires the use of a magnahelic gauge.
1. Place inline “T” fitting between vacuum switch and vacuum line from the blower motor.
2. Turn heater on and adjust so water heater turns on. Blower motor turns on..
3. Check  pressure on the magnahelic gauge. If gauge shows 0.6 to 1.5 inches water column, then blower motor is inducing the proper draft and vacuum lines are not obstructed.
4. If gauge does not read between 0.6 and 1.5 inches water column -
a. Move the inline “T” fitting and place it between the blower motor and the vacuum tube at the top of the heater.
b. Check the pressure on the magnahelic gauge. If the gauge shows 0.6 and 1.5 inches water column, then the blower motor is inducing the proper draft and vacuum lines are obstructed. Replace the heater.
 c. If the gauge does not read between 0.6 and 1.5 inches water column, then the blower motor is not inducing the proper draft. Replace the blower motor."

  Resource: Power vent troublshoot / pdf
Buy: magnahelic gauge
water heater pressure switch Pressure switch/ Vacuum switch for water heaters
There are 3 major water heater manufacturers, AO Smith, Bradford White, Rheem, that make all the other brands from Kenmore, Craftmaster, Whirlpool, American, Reliant (AO Smith) to Reliance (Rheem)
Buy water heater pressure switch:
Rheem water heater pressure switch
Bradford White pressure switch
AO Smith pressure switch

Pressure switch replacement
Bradford White pressure switch replacement
water heater blowers Blowers: Fasco, AO Smith, Bradford White, Rheem power vent water heater exhaust draft-induced blowers
Buy blower
Water heater blowers at Amazon
Other types blowers
Fasco multi-purpose blowers
AO Smith multi-purpose motors and blowers
Furnace blowers

14 things needed for gas water heater & .pdf resources
What you need for operation of gas water heater.
General troubleshoot
1) Basic parts and operation
2) Adequate incoming air supply ... issues: closed doors, tightly insulated house, very hot attic.
3) Adequate air supply getting to burner... issues: dirty vent screen, dirty environment, maintenance.
4) Thermal cutoff  ... TCO ... issues: failed part, maintenance, vent.

5) Flammable vapor lockout .... issues: exposure to vapors, bad FV sensor, failed FV system,
6) Fuel supply to gas valve thermostat ... natural gas and propane are different ... issues: wrong gas, low pressure, moisture in pipe, undersized gas meter, supply line too small for BTU rating.
7) Gas valve thermostat regulates fuel supply ... issues: not enough gas, failed thermostat, failed gas control valve.

8) Fuel supply from thermostat to burner ... issues: maintenance, thermocouple-thermopile, gas control valve.
9) Clean and replace burner ... issue: periodic maintenance requires burner inspection, cleaning, and possible replacement

10) Functioning thermopile-thermocouple-pilot light ... issues: misalignment, failure of part.
11) Ignition of fuel supply by pilot light or hot surface igniter ... issues: pilot light, piezo igniter, electrode.

12) Power vent water heater troubleshoot ... issues: pressure switch, blower, blower sequence from gas control, outlet polarity
13) Venting combustion byproduct .... issues: vent in disrepair, other vents in house drawing air

14) Misc troubleshoot manuals
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