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Weil Mclain 155 Backfires

Timothy1234
Timothy1234 Member Posts: 3
This boiler works normally except occasionally when transitioning from high fire to low fire, the spark ignites before purge, resulting in a  small “backfire” to the air silencer. Does anyone know the cause of this?
Mad Dog_2

Comments

  • Tim_D
    Tim_D Member Posts: 128
    What model boiler and what fuel type? This is almost always improper combustion set up but once the backfiring starts you may need to inspect the burner, blower and venturi for damage.
    HVACNUTMad Dog_2
  • HVACNUT
    HVACNUT Member Posts: 5,829
    Ultra?
    What series?
    When was the last time it was serviced or had maintenance?
    Mad Dog_2
  • Timothy1234
    Timothy1234 Member Posts: 3

    This is an Ultra 2 155 with a yearly maintenance/cleaning and a brand new blower. It backfires about 1 in 10 starts but at other times, it goes into an overheat hard fault - the recirc pump isn't turning on when this happens. Restarting the unit fixes this situation for a day or so, but then it reoccurs. Could this be a software problem in the controller? Ive checked the supply and outlet thermocouples and they're fine. The recirc pump isnt getting voltage from the controller when it goes i to the hard fault. The backfiring doesnt cause a fault which is pretty scary!

    Mad Dog_2
  • Mad Dog_2
    Mad Dog_2 Member Posts: 6,911

    Have the WM rep pay a visit. That's what these guys DO! Have your combustion Analyser ready to go. Mad

    HVACNUT
  • HVACNUT
    HVACNUT Member Posts: 5,829

    A tech with an analyzer should be able to solve the combustion issue. Don't wait on that. And if you're diagnosis is correct then it seems like a faulty U-control.

  • kcopp
    kcopp Member Posts: 4,431

    I am willing to bet the mesh burner is starting to seperate inside.

  • Tim_D
    Tim_D Member Posts: 128

    Backfiring in an Ultra is pretty rare. I would be looking at all of the basics, ignitor, burner and burner mesh, signs of flue gas recirculation (can damage the venturi and fan blades) burner and front cover gaskets, gas pressure and combustion set up. Regarding the high limit lock out. This is either a flow issue or a control settings issue. Go into the U-control and get to the diagnistics screen, errors, lockout history 1, from there you can see the actual temperatures, what mode it was in, what inputs and outputs were enabled, etc. This should help you narrow things up. If it appears to be a settings issue go to system settings and make sure that nothing is set above 180 (Some of the factory defaults are set to 190) and make sure the on/pff differentials are both set to 5 degrees.

  • Timothy1234
    Timothy1234 Member Posts: 3

    Problem solved! Previous burner replacement by tech failed to bend retainers up and two screws had loosened over time. New hardware, proper installation, and a closer inspection was all it took. Thanks for the many good suggestions!

    EdTheHeaterManflat_twinTim_D