Viessman Vitodesn 100 banging

Alucidnation · Oct 16, 2019

Hi guys,

I have a Viessman Vitodens 100 heat only open vent boiler that was fitted around 4 years ago.

It has been serviced when required and overall runs great and very efficient.

However, at the end of last year we had an extension built and the flue had to be extended around 2 meters, rather than straight out as it was originally.

Problem is, every time it fires up, we get a massive "thump" or banging on start up.

After having the original installer back, and one other several times, no one seems to know what is causing it, and have now basically washed their hands of it.

One seemed to suggest it was because of delayed ignition but didnt know what he could do about it.

Does anyone have any pointers or suggestions?

Thanks everyone

townfanjon · Oct 16, 2019

We have to be careful on gas advise given out , if this did the banging from the VERY first time the boiler fired it obviously sounds like its the flue , if it started a while after it could be a couple of things .

Alucidnation · Oct 16, 2019

townfanjon said:
We have to be careful on gas advise given out , if this did the banging from the VERY first time the boiler fired it obviously sounds like its the flue , if it started a while after it could be a couple of things .

Hi, yes totally understand about the gas thing, and i am not expecting anyone to give any advice on that aspect.

It was more fact finding really, as to why it does this, as two people couldn't find the problem and didn't really want to help.

And it is, 99% of the time when it fires up, and has only started happening since the flue was fitted.

gingalig · Oct 16, 2019

Look up viessmann registered engineers in your local area via the website, it's as issue that should be easily resolvable with the correct set up.

townfanjon · Oct 16, 2019

Alucidnation said:
Hi, yes totally understand about the gas thing, and i am not expecting anyone to give any advice on that aspect.

It was more fact finding really, as to why it does this, as two people couldn't find the problem and didn't really want to help.

And it is, 99% of the time when it fires up, and has only started happening since the flue was fitted.

Post a few pics of the flue, where its route is , where it terminates etc

Alucidnation · Oct 21, 2019

Hi Guys,

Sorry for the late reply!

Pictures of flue

Originally it went straight out the back to the outside, but we had an extension built there so the gas engineer re directed it to the front of the building.

townfanjon · Oct 21, 2019

Alucidnation said:
Hi Guys,

Sorry for the late reply!

Pictures of flue

Originally it went straight out the back to the outside, but we had an extension built there so the gas engineer re directed it to the front of the building.

Cheers , does the flue terminate againt anything , wall , tree etc.
Did the engineer leave you with the analyser results when he fitted the extension .
Just on another note , do not panel the flue “ in “ without providing hatches which are used to check the flue during a service .

Alucidnation · Oct 21, 2019

townfanjon said:
Cheers , does the flue terminate againt anything , wall , tree etc.
Did the engineer leave you with the analyser results when he fitted the extension .
Just on another note , do not panel the flue “ in “ without providing hatches which are used to check the flue during a service .

The flue terminates up the wall on the right to the outside air.

I have just looked through the folder and it appears he didn't leave a copy of any test results. If i manage to get them from him, i'll post up a picture.

And yes, the flue boxing will be completely removable, but that won't be done until the kitchen is redone in the next couple of years or so!

townfanjon · Oct 21, 2019

It is a weird one this , it may just be a coincidence but theres a couple of parts I would change in the boiler , just to rule them out , but deep down you have to say its something to do with the flue .
If you ever get to the bottom of it please post back on here .

gingalig · Oct 22, 2019

Alucidnation said:
Hi Guys,

Sorry for the late reply!

Pictures of flue

Originally it went straight out the back to the outside, but we had an extension built there so the gas engineer re directed it to the front of the building.

Your engineer needs to speak to Viessmann technical, looking at the pics and the flue length it should be a quick fix.

Alucidnation · Nov 2, 2019

Hi again.

I have been digging around on the internet (as you do), and as you guys have said, it may not always be that straight forward, but it may be to do with the gas/air ratios due to the new flue length, or worse case a new gas valve.

Thing is, the severity of the banging is variable and in some instances does not happen at all.

The engineer is coming on Monday, but, as with his previous efforts, will probably say it's normal!

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