Scale, hardwater, well water with sand in it, debris from flushing hot water tanks, debris from dip tube failures in the hot water heater, all play havoc with all modern fixtures. 

The hot water heater is 21 years old, however, I had the tanked inspected, drained and flushed a few months ago by a licensed plumber and there were no leaks or other issues.Can anyone provide a solution that would be satisfactory to an insurance company and won't require me to replace the existing hot water heater?  

Stained ceilings are often a tell-tale sign of leakageThe interior walls of the property must not have chipped or peeling paintPlumbing and Sanitation The property must have a fixed water basin, flushing toilet and shower/bath tubThere must be no plumbing or water leaksThere has to be hot and cold running water in both the kitchen and bathroom The bathroom must have either a window or exhaust fanThe water heater must have a discharge tube that drains to a floor pan, outside, sink, or other appropriate place. 

The biggest issues are overloading the dishwasher, washer and dryer, and/or flushing things down the toilet that weren't eaten first.

My favorite was the young woman flushing kitty litter down the toilet only to clog the sewer in the basement. $2000 to dig up & replace the old pipes just to remove the cement like sludge. 

The first thing I usually do when I buy a new property, I call my fence guy and I put 5' fence around the back yard and all the way to the front part of the house, flush with the house front side (That if there is no fence at all).

This is the simplest method, but it also means your tenants are more likely to abuse the utilities by leaving windows open with the heat or A/C running, leaving lights on, ignoring the toilet that constantly flushes on its own, etc.3.

If it's a new build you better plan on staying there for the next 2 weeks to flush out all of the problems and screw ups that the builder put in there.  

The plumber believes it is an issue with use - flushing too much, or things that shouldn’t be flushed.

I carry a water key with me at all times and the water meter at the street was easy to turn on to allow flushing away of this deposits and then bleaching each toilet.