Water damage due to heating. Warranty according to VOB

Based on an excerpt from 2018 of my drinking water, two values are close to the limit for the "standard" heat exchanger. With these values, it would recommend the more expensive nickel-brazed one to avoid such corrosion.

to even have a chance of recourse, the builder would have had to know about the water values and it would have to be part of the contract (e.g., you knew about the poor water and included the excerpt from the waterworks in the contract). furthermore, the value could have deteriorated in recent years. it sounds strange, since normally a heat exchanger does not rust through after 4-5 years. these parts last significantly longer! normally there is a sacrificial anode inside, which is checked and if necessary replaced during maintenance. something definitely smells fishy here. you probably somehow damaged the part yourself. in the future, have a professional company do the maintenance. at least then you have someone you can hold accountable because wear parts must not cause other damages (e.g., sacrificial anode gone, tank rusted through) if the inspection and replacement of these parts was commissioned (which makes sense).