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Buderus GB142-24 doesn't want to light, flashing 6A

MrScott Member Posts: 15
Homeowner here with a Buderus GB142-24 that doesn't want to light, but once it gets started it's fine.
Over time the occasional flashing 6A turned into every time. Went to clean it, found the heat exchanger was a solid mass of lava rock and eroded fins.
Replaced the HX and a clicking circ pump.
Put in a sufficient expansion tank, right side up per the manufacturer.  Previous was undersized or failed, as indicated by pressure spikes pushing fluid out the relief valve (replaced corroded valve). 
Replaced leaking-when-cold dielectric unions. 
Got a flashing 6A.
Replaced the ionization rod. 
Got a flashing 6A.
Replaced the burner control.
Got a flashing 6A.
Replaced the gas valve (differential gauge on order).
Got a flashing 6A.
Checked gas inlet pressure; 10.7inch H2O at full burn. 
Helping it start with an unlit propane torch for a few seconds at the point in the cycle where the HSI comes on:  lights and runs, first time, every time.  Which tells me it's not the steady-state fuel flow, and the (short) exhaust isn't plugged. 

The old valve inlet screen was clean and clear of debris. 

What would you look at next, the blower? The propane supply?  3/4" pipe reduced to 1/2" about 15' and a few fittings from the boiler; has internal and external flex pipes. I suspect the external may be a little too restrictive, pressure drop when the solenoid valve opens?  But to 10.5 inch H2O, well within specs. 

Kind of seems like one of:
1.  The propane pressure maybe sags a bit at start-up?  Could try replacing the internal (stainless, just saw the TSB note) and using a larger external flex pipe, maybe add a small plenum to reduce the pressure drop when the valve opens. 
2.  (New) valve needs adjustment?  I tried putting the old one back, same behavior.  Differential gauge on order. 
3.  Blower is worn? 

The ignition timing might be just a bit too short on these, so under less than ideal conditions it doesn't reach ignition.  


    HVACNUT Member Posts: 5,751
    See the second paragraph in the preface. The life you save might be your own.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,703
    Hot surface igniter ?
    I have enough experience to know , that I dont know it all
  • chucke
    chucke Member Posts: 1
    Going thru same thing, and replacing UBA-3.5 with the 3.6. Not sure what’s better about the 3.6 but my UBA-3 starts blinking continuously when the 6A hard stop occurs. And of course it’s only over $800. So diagnose by elimination before considering the investment. I’m about to throw in the towel & go Veissmann
  • MrScott
    MrScott Member Posts: 15
    edited March 2022
    Replaced the blower and it worked.
    What I think was happening:
    - Old blower maybe has a bit of drag and runs a bit slow, but not obviously so.
    - For safety, the ignition timing is fairly tight.

    It cycles through purge / ignition attempt / purge.  During ignition attempts:
    - The ignition timing does not vary to compensate for the slower blower.
    - Propane valve turns on, however due to the slow blower and long tubes, the propane doesn't get into the combustion chamber quickly enough to reach an ignitable mix before the controller times out.

    Giving it a bit of propane got it to ignitable, however that's a test or very temporary workaround, not a long term solution.

    The replacement blower is faster, so it gets an ignitable mix and lights without timing out.

    I suspect that these are engineered without much timing margin for ignition, so a 5..10%? (haven't measured) blower slowdown will stop them from igniting reliably.