All tankless systems should have a set of flushing valves so that it can be delimed. It could also be used to sterilize the unit afterwards by substituting water and bleach, I supposed (you'd be best to ask the manufacturer first, though). Essentially, you'd disconnect from the supply and outlet pipes (normally just by turning the valves), plumb in the auxiliary pump with a bucket of mild acid and pump that around through the heat exchanger (essentially, all of the guts of the unit) to remove any mineral buildup. Then, you could clear out that solution, and replace with the chlorine (bleach) and water solution, then let it run again for awhile. This would sterilize it and remove any mineral buildup. how long that would last would depend on what's in the lines coming to it. You may need to do a similar thing to the filter housing, as that can trap organic materials that may be in the water and then let stuff grow in it, so the housing, and old filter may be a breeding ground for it that then colonizes also in the nice warm environs of the tankless WH. Filters are great places to grow nasty stuff.