Viessmann Vitodens 100 WB1A from 2008 broken. Ideas?

[DavidLewis]

Hi all,

I came down in the morning to my Viessmann's fan running flat out, but no heating, and a completely blank display (so no error code!) and no lights illuminated.

With these symptoms, I assumed it was the PCB so bought a brand-new-sealed-old-stock one and fitted it this morning. My mistake.... it showed F0 error for a few seconds and illuminated the 'reset' light, before going dead like the previous one Investigation shows that one of the soldered-in 400mA PCB fuses has blown, in the area of the power supply on the circuit board.

So, it's clear something is blowing up the PCBs. Is anyone familiar with this particular problem and these symptoms on this boiler?

I'm loathe to throw good money after bad... the boiler is 16 years old, but I would quite like to repair it if it's economic to do so.

Thanks for any ideas!

 

[omph]

Are there any water leaks ?
Try disconnecting the cables for each boiler part, and reconnect one at a time testing along the way to see if any particular component is giving you issues.

You could save shed load of money on heating/dhw if you upgraded though. Boilers have come a very long way in the last 15years

 

[DavidLewis]

No water leaks.

I've tried reconnecting each cable one at a time after replacing the fuse, but I think the damage to the PCBs has already been done. I'm not going to sacrifice a third PCB in this way...

It was a pretty nice condensing system boiler. Dual temperature, hot water priority. Modulates down to a CH flow temperature of 50 degrees (80 degrees for fast DHW reheat). I doubt there's much efficiency savings to be had from a new boiler, to be honest.

 

[JohnD]

Are you hoping to get 20 years out of it?

A new one will slot straight in.

 

[omph]

No water leaks.

I've tried reconnecting each cable one at a time after replacing the fuse, but I think the damage to the PCBs has already been done. I'm not going to sacrifice a third PCB in this way...

It was a pretty nice condensing system boiler. Dual temperature, hot water priority. Modulates down to a CH flow temperature of 50 degrees (80 degrees for fast DHW reheat). I doubt there's much efficiency savings to be had from a new boiler, to be honest.

Not so fast. I am running a viessmann 100 2021 version with weather comp on a 0.5 curve running 24/7 using ~40% less gas than my old setup which only ran 6 hours a day. It has flow temps of about 33c @ 0c, running at 97%+ efficiency. A constant internal temp of 23-23.5c and DHW goes all the way down to 30c. A modulation ratio of 1:10 and I could go on... - The viessmann 200-w is even better.

 

[omph]

P.S. since you got an F0, it could be the pump thats shorting.

 

[DavidLewis]

Are you hoping to get 20 years out of it?

A new one will slot straight in.

Ha! Put like that, it is tempting!

The thing is, we were half thinking about moving the boiler location to the garage as part of a kitchen knock-through project. But not until next year...

Would be good to get another six months out of the old Viessman for this reason.

 

[DavidLewis]

P.S. since you got an F0, it could be the pump thats shorting.

Could be. Although the pump on these is not modulating, and is probably (I haven't investigated) switched directly by one of the mains relays on the board. So it's difficult to see how a shorted pump would cause the damage I'm seeing to the PCB.

But thanks for the idea - I'll investigate further.

 

[omph]

Could be. Although the pump on these is not modulating, and is probably (I haven't investigated) switched directly by one of the mains relays on the board. So it's difficult to see how a shorted pump would cause the damage I'm seeing to the PCB.

But thanks for the idea - I'll investigate further.

You could do some continuity and insulation resistance tests for each of the parts if you've got the tools. That should save you the need to buy another pcb.

 

[JohnD]

I don't know anything about boilers.

But electrical faults in equipment made of metal that also contains water are commonly caused by a water bit dripping on an electric bit.

 

[DavidLewis]

OK, it's working again!

It turns out the fault was the water pressure sensor. It is a 5V-powered electronic sensor which had failed short-circuit, overloading the 5V power supply part of the PCB. The 400mA soldered-in fuse protecting the circuit had blown, which caused all other parts (such as the display, the temperature sensors, etc.) powered from that 5V supply to also stop working.

When I fitted the new PCB, the same thing obviously happened to it.

When I found the blown PCB fuse, that pretty much narrowed the fault to a sensor, and not any of the mains-powered components. The other few sensors measured correctly (helpfully, their characteristics were in the boiler's installation manual), but I couldn't test the unpowered water pressure sensor, so I guessed it was the culprit. I soldered in a new 400mA fuse, disconnected the pressure sensor, and the boiler came to life when I powered it up, displaying the correct water temperature, before flagging a water pressure sensor F-code. As expected, what with it being disconnected

Connecting a 1K-ohm potentiometer in place of the water pressure sensor allowed the boiler to read 2 bar, and fire up as normal, with no fault codes.

Thanks for your suggestions, which encouraged me to think logically and do some more detailed investigation.

Now to start choosing a replacement boiler for its new location next year!