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Vitodens 100W randomly failing no errors

jtredwood1 Member Posts: 1
I'm at a complete loss, and have had 3 certified contractors who have tried to service/clean, replaced igniter and gaskets...all still resulting in the same result: the boiler randomly just doesn't fire.  There is no error/fault code, it just sits there at about 64 degrees fahrenheit, and doesn't fire even though all the thermostats are on and there is no hot water.

The only remediation is to restart the boiler, sometimes once it's enough, sometimes I need to restart 5-6 times, sometimes I need to shut it down and try again in an hour.

I've spent roughly 2 grand on parts and labor based on different certified contractors' advice without any permanent fix.

Any suggestions? I'm at a loss, and almost at the point where I want to replace the whole dang thing. I don't want to just randomly keep replacing random parts until it works.


  • psb75
    psb75 Member Posts: 755
    Has anyone texted Viessmanns tech support line and established a case number for this problem? That would be a real good starting point,=.
  • GGross
    GGross Member Posts: 798
    Vito 100, will not fire and has no error code. That means the flow switch is not made, your heat exchanger is most likely airlocked since this is intermittent. when you restart the boiler the pump tries to turn on briefly (if internal) and any diverting valve inside the boiler operates as well, this pressure change is likely moving enough air through the heat exchanger that sometimes you get flow. Need to see the piping of the boiler, pictures from a good distance back showing as much as possible. With no error code present any viessmann tech should know to check flow first.
  • psb75
    psb75 Member Posts: 755
    Now...you have something to check...FIRST. Air-lock. V. Tech Support will be impressed that you did--if nothing else.
  • I agree with @GGross as I’ve had the same experience with V. boilers. 
    Often, there’s no purge assembly installed to properly flush the system and burp the boiler; additional valves, hose bibbs and a bypass piped in. It’s not difficult and very satisfying once the boiler turns on. 
    8.33 lbs./gal. x 60 min./hr. x 20°ΔT = 10,000 BTU's/hour

    Two btu per sq ft for degree difference for a slab