I have a 400 gal and a 600 gal gas water heater. The heat exchangers have rusted out and been replaced. My problem is, a lot of the rust has entered the water system. It is fine until I shut the water down, then the system has to be flushed for 3-4 hours before the water runs clear. The sediment that comes out is extremely fine. It disappears in you hand when you touch it. I've had the water tested and there is no danger. Does anyone know how to clean the system. I have drained the system with no luck. It is a 5 story science building, when I shut it down the sediment causes problems with the lab experiments.