Gas Boiler Pressure Switch Diagram Help!!

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  #1  
Old 01-03-19, 10:26 PM
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Gas Boiler Pressure Switch Diagram Help!!

Hi can someone explain to me how the attached diagram functions with the attached pressure switch.

This is a small Weil Mclain cast iron boiler with a hot surface igniter, and a small exhaust blower for flue gas, it's an 80% boiler. So the Pressure switch has the terminals. Normally Open, normally closed and Common. When the fan is running all 3 terminals have 24 volts. Is that correct???

When the fan stops, I still get 24 volts on all the terminals. Also, what I saw what the fan stops, but it takes about 1-2 seconds maybe 3 for the gas to shut off and I get some flame rolls out.

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Last edited by PJmax; 01-03-19 at 10:50 PM. Reason: added labeled diagram
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  #2  
Old 01-03-19, 11:01 PM
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Yes..... there will be 24vac on all three terminals.
The pressure switch is setup so that you can't deliberately bypass it.

Here's the breakdown..... follow the wire colors. Blue is common. On a call for heat.....R (24v) goes thru red to black to thermostat green to supply the relay coil. Now the relay is closed.

When the inducer starts...... R now goes thru pink thru the closed relay contacts thru green to keep the coil powered thru black and thru yellow to power ignition module.

When the pressure switch doesn't work properly people try bypassing it. That would mean connecting black (C) to yellow (NO) . You've now taken the red (NC) out of the equation and the relay doesn't get 24v.

Yes...... if you disable the inducer (unplug it) while the system is in a call for heat the gas valve will stay open a few seconds until the pressure switch opens. Why would you be doing that ?
 
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Old 01-04-19, 02:08 AM
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What would the voltage be when the circulator is running the fan went off on limit cause it reached it's 180 degree water temp at the boiler but the thermostat is still not satisfied, what would the voltages be on the 3 terminals of the pressure switch in that instance??

There have been a few times I have shut the boiler off at the service switch, the blower has not come on, but the hot surface igniter gets hot and the gas fires and it will actually run without the fan running. Suggestions???

Also, on seem tonight I cause it starting fine, running then when it goes off on limit or thermostat satisfy, the fan stops, but the gas valve keeps going for a 1-2-maybe 3 seconds, and it causes flame rollback. Any thoughts?? I have not touched any of the wiring order, just cleaned some tabs to make sure everything was getting a good connection.
 
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Old 01-04-19, 04:50 PM
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The limit switch shuts off the 120v to the inducer which causes the pressure switch to open and the ignition module is no longer powered. The NO (yellow line) will go dead. The circuit is setup that the burner will continue for several seconds until the pressure switch toggles.

Boilers are not my specialty. Control wiring is though. I'm not really sure why that boiler was setup that way except to save on parts and I guess dollars. Your question about roll out is a valid question and something that would need to be addressed by the manufacturer.
 
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Old 01-05-19, 10:46 AM
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Thanks for the help PJ, it was a ad pressure switch
 
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Old 01-05-19, 11:00 AM
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Ok..... pretty rare though.
 
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Old 01-13-19, 03:34 PM
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I've heard that but your help was much appreciated in a couple minutes used the meter saw the problem and supply house down the street had the switch and got it up and running. House was my sisters so forum helped on costs. Thanks again PJ!!
 
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Old 01-13-19, 04:04 PM
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You're welcome. Thanks for letting us know how you made out.
 
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