Hello. I hope you have resolved your issue by now 4fuzzies. We just had the exact same problem (Well, I say that not knowing precisely what the sediment in your line looked like.) We noticed the other day that there was sediment in the hot water line coming from the tankless (Rinnai brand) water heater. It is only 2 years old. We have another water heater in the house and all the lines connected through that were fine so we knew it wasn't dirty water coming in the house. We finally discovered that the problem was with the expansion tank that was connected to the tankless heater. When the old water tank was removed a couple years ago, the contractors chose not to replace the existing expansion tank. Well, apparently, the expansion tank has a bladder and perhaps because of the cold weather it had cracked. It was essentially disintegrating and entering the water stream. Expansion tank replaced = no more sediment. I'd love to know if your solution, assuming you found one, was the same. Good luck!