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

MrScott

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

See the second paragraph in the preface. The life you save might be your own.

Big Ed_4

Hot surface igniter ?

chucke

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

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.

MrScott

It ran OK for a while but is back to 6A and 6L. Looking at air to water heat pumps.

BTW better to put a few fingers across the air intake (choke it) than add propane.

Jamie Hall

Has someone with combustion test equipment actually been there and adjusted the burners? The fact that you can get it to run just fine by choking it or adding a bit of propane (same difference) tells me that the burner air to fuel ratio adjustment is out of range, and unless it is computer controlled (highly unlikely) somewhere there is an adjustment to get it in range…

A lot cheaper than replacing the unit.

SuperTech

That particular boiler is super high maintenance and extremely problematic, especially when burning propane and not setup properly using a digital combustion analyzer. That's an absolutely critical part of getting it to run properly.

If you have a hard time finding someone who is qualified to get this boiler to work properly good luck finding anyone who can work on an air to water heat pump...you must love spending time and money working on your own equipment.

GB1424s have been discontinued for a long time for a good reason. I'd consider getting something more reliable and easy to service.

Alan (California Radiant) Forbes

In their service manual, Buderus wants you to use a manometer to adjust the gas/air mix and only in part-load operation.

MrScott

Ah-HA! I smelled a bit of propane near the blower motor while helping it to light: there's a shaft seal that has popped out! Will investigate tonight to find out what it takes to access it, is it failed or just out of place, what kind of glue (if any) does it need to stay in place.

I had replaced the blower before and kept the part; I'll take that one apart first, to see whether it's the same issue and in case something goes SNAP.

As service people heard that I worked on it and didn't want to touch it, I actually bought a combustion analyzer. It showed CO2 and O2 as fine — when it was running. However the adjustment doesn't matter when the seals are moving around on you and changing the airflow!

Zman

CO2 and O2 are essentially the same measurement. The analyzer is reading one (usually O2) and calculating the other. What is your CO?

As you have discovered over the last couple years, it is easy to throw good money on top of bad (not to mention your time) with that particular model.

As for air to water heat pumps, they may be a good idea if you have a low temp radiant system. Probably not a good fit with high temps.

Big Ed_4

The GB142 is a work horse ….