Hi Eagle,
Sounds like we have the exact same problem, mine is tankless too! I thought as one of the other post said that it might be sand in the pipes left from when the house was built but this has been going on for years and isn't getting better. So I think I've rule that out. I also was going to flush the system but never get around to it and that would only be a temporary solution. I think HUH, Master Plumber, has it right it's caused by hard water and it scaling off. I've had my water tested a number of times and it's a little on the hard side so maybe that's it. I thought of putting a filter on the output of the hot water coil but I don't know if the filter would take the heat and coil will in time clog up and fail. I now thinking the answer is to treat the water to reduce the hardness.
Bill
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