Viessmann Vitodens 100 Boiler which moans

My boiler moans every so often and even though I've had the (very expensive) Viessmann engineers in twice, they can't fix it.
The moaning is more often in winter (when the heating is on?) but even in summer, it happens every couple of hours. The moaning lasts about 15 seconds on each occasion and is loud enough to be very disconcerting and bothers the neighbours.
Apart from the moaning, the boiler works very well.
Has anyone heard of this type of problem and can suggest a solution?

Have the viessmann guys changed anything?

It's possibly an air gas mixture issue at low rate. This will need an RGI to investigate.
Another possibility is that there is poor circulation in the system and this is causing noise.

It is very difficult to come to any conclusion without hearing the sound. What is moaning to me may not be moaning to you.

Very often its a gas mixture problem. I am not permitted to suggest remedies for this on an open forum. All good engineers will know anyway.

Those who do repairs for Viessmann are either one of a very few salaried people or independent service agents who are just engineers who like the easy flow of jobs albeit at rather low paid rates. It does not mean that they are necessarily very good ( although some are! ).

I would have advised you that a good independent is likely to be better than a Viess engineer.

Presumably you have gone for a fixed price repair and thats what you will get but obviously you need to pester them to fix it.

Its only a boiler and CAN be repaired by a competent engineer!

Tony Glazier

could be auto by pass and probably is