1) Dirty water implies degraded metal inside unit:
The anode rod protects metal parts. Once anode is depleted, metal parts react with water.
2) Open following troubleshoot pdf showing Ariston partshttp://waterheatertimer.org/pdf/How-to-Troubleshoot-Bosch-Ariston-Water-Heater.pdf
3) Ariston manual: Read manual cover to cover, make sure water heater receives yearly maintenance as outline in manual.http://waterheatertimer.org/pdf/Bosch-GL_English.pdf
4) Information on anode rodshttp://waterheatertimer.org/Replace-anode-rod.html
5) For Bosch gas and electric tankless: Open following link for troubleshooting resources, including Bosch contact information,
typical manuals, error codes, tech support, parts, fuel-saver
calculator, service bulletins, water heater forum, fixya links,
disclaimers, and links for finding service technician.
6) Add a comment with your experience so others with same-similar problem can benefit.
Also take advantage of fixya expert assistance live.
For a price, expert works with you while you work on water heater or any do-it-yourself project.
Fixya is always less expensive than a service call.