I've searched Google and your forum for information about our problem, but am finding no leads. So, I'm hoping maybe I can get some insight here.
We have a two year old Tempra electric water heater. It works great most of the time, but then the hot water flow begins to reduce and after a few days, there is no longer any hot water. After actuating the valves, opening the drains and running the overflow, we can dump slimy black and light brown fungus-like debris out. It's slime only, nothing hard. The biggest pieces are almost as large as a silver dollar. After a few drains, we get lots of small pieces, about pea sized, but still the same flat, slimy stuff. After doing this, the unit will work fine again for a few days to a few months.
Today's episode went the same, except we could not get it to start flowing again. I ran compressed air through the overflow drain while the hot valves on the nearest bathtub was open, and the right-hand service valve was open below the unit. About 16 ozs. of translucent brownish water with a few small black chunks came out. After that, we buttoned it up and it is running fine.
This unit is in our guest house, which is occupied by my grandfather. His water usage is one shower a day, a few loads of laundry per week and washing his dishes by hand. So, it's not a lot of flow. The house is two years old, plumbed in PEX. We are on well water. The main house has a tank heater and no black "fungus" problems. The water was tested when we purchased the property three years ago and was found to be "excellent on all counts." I do not have the results at hand, but it's not hard, nor soft. It is a bit aggressive (slightly acidic) however, as it was eating at the main house's copper plumbing. (We are in the process of switching it over to PEX as well.) We are rural, near Houston, TX. Nobody nearby seems to have any experience with tankless units, so I fear we are on our own in solving this problem. At the well house, we have an OmniFilter (https://www.filtersfast.com/Omni-Fil...ter-system.asp ) and change the filter yearly.
If you need any more information that I'm leaving out, I'll gladly provide it.
Thanks for any insight. This is a confusing and frustrating problem for us (and Grandpa!!)
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.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013