Water descaling

...above all because an annual maintenance according to DIN is mandatory.

Sure, it is a private household and no one will check it, but after all, it concerns a food product.

And the costs are not insignificant – in our former rental apartment it was 350 euros, but the system was also larger. But I would estimate at least 150 euros...

Not nice that apparently professionally knowledgeable people just keep such things quiet.

150€ in what period?

Per year of course...plus the salt tablets!

But who has a system like that serviced every year? I find that excessive and unnecessary

What you find should be irrelevant when giving such advice - at least you should be able to say 6& also justify why that is the case.

The manufacturers and the DIN standard prescribe it - which may of course be in the interest of the industry, which also partly determines the standards, but this is still about a food product.

Try to explain why you find it unnecessary - maybe it will be convincing.

If it's only about the hygienic aspect, you should start somewhere other than a water softening system. In my opinion, if it is properly designed and set, it does not require annual maintenance. With some models, you can even program a forced regeneration. And thus, clogging is almost completely excluded. And let's be honest, who actually has everything maintained according to regulations?