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40 yr Burnham gas water boiler burners popping when gas valve closes, lights off fine

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wcs5050
wcs5050 Member Posts: 131
in THE MAIN WALL
Does anyone know what causes this? At the end of the heat call, when the gas valve closes, light popping can be heard as the flame extinguishes along the various tubes. It has an old millivolt gas valve. Could this be a manifold pressure issue? Boiler is functioning fine, and this is nowhere near a problem, but I am curious.
The worst I ever encountered in this category was on an old modine that had been modified (by others) from standing to intermittent pilot, where it ignited far too explosively, then extinguished with a loud whappp.

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  • icesailor
    icesailor Member Posts: 7,265
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    My outdoor gas grill does that when I shut both burners off when I am done with it. Its done it since I bought it.
  • j a_2
    j a_2 Member Posts: 1,801
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    I am thinking that valve is passing gas!!!!!!! I have seen that on a few boilers as well….I would just make sure the valve is closing fully….Put a digital manometer on the output and cycle the valve generally speaking 3.5 when its open and 0 when it closed...
  • nicholas bonham-carter
    nicholas bonham-carter Member Posts: 8,578
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    What piping distance is between the gas valve, and the burner? If the pipe is longer than usual, some gas may be trapped in the pipe, and expand with the warmth, flowing out into the burner tubes, and being ignited.--NBC
  • unclejohn
    unclejohn Member Posts: 1,833
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    There is a mixture or percent of gas to air that is explosive. I think it's 50% gas /air. That old valve is probably a diaphragm valve and closing a little slower then it should and you get a pop when the gas / air get to a certain percent. When I went to a gas training school a long time ago they took a coffee can with the bottom cut out and the plastic top with a tube in it. They filled the can with gas and lit the tube and it burned as the guy talk about the dangers of gas, after about 10 minutes the top of the coffee can hit the ceiling with a bang. That he said was the customers roof, so always check for leaks with soap bubbles. It was a good presentation.
  • Tim McElwain
    Tim McElwain Member Posts: 4,625
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    What you have is called "extinction pop" it can usually be cured by closing the air shutter slightly. It sometimes begins to show up on older burners as the port loading on the burners changes. Unless it is really loud and disturbing it is not a problem.

    For those who offered some gas info, the correct Upper and Lower flammability limits on natural gas is 4.5% gas to 95.5% air the lower flammability limit and 14.5% gas to 85.5% air the upper flammability limit. The ideal burning speed or ultimate is 10% gas to 90% air.

    What is the make and model number of the gas valve?

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