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Honeywell L408J1009 Vaporstat POP sound

I just installed a Honeywell L408J1009 Vaporstat (purchased March 2019) to replace my original PA404A that wouldn't do better lower than 1.5-2.5psi.

It's been running the past few days as expected - not going over 1psi (12oz main, 8oz diff), but there is a "POP" sound at the start and end of the cycle and the gauge (0-5psi) pointer jumps. The normal microswitch click can be heard as well.

According to this thread, it may indicate a problem with the control:

But after reading through, I'm not sure if it's a symptom of a pending failure or if I should disregard if it's still functioning as configured.

Any advice would be appreciated as I hope I still may have time for a warranty replacement.



  • Fred
    Fred Member Posts: 8,542
    I would request a replacement unit, if I were you. Just about every unit that starts that popping sound, in the metal diaphragm ends up with a complete failure of the unit. It's not normal.
  • mikeg2015
    mikeg2015 Member Posts: 1,194
    Mine started doing this, I think it inverted and damaged to disc when boiler when t under vacuum. It won’t operate correctly now. Kept popping when I took it off and tested it with a manometer and little hand pump bulb. It’s a expensive paperweight. It’s over a year old so not sure what I can do with it.