On 2008-04-18, Dan Musicant wrote:

I shut off the water at the cold water shutoff, remove the stem from
the hot water valve at the kitchen sink and see one, two or three
tiny grains of sand, flaked iron, who knows what it is, but it's
very tiny.

There's a screen on the water inlet on your tankless hot water heater.
You may wish to shut off the water supply to your tankless, remove the
screen and check it. If you see the same particles there, you know
they are not from your water heater but probably from your old water
piping. Obviously the particles at the sink would be from piping
after the water heater, since the particles can't make it past the
screen.

Another thing you may try is to flush the hot water line, although you
may have done this after installing the new water heater and/or
faucet. You remove the aerator from the kitchen faucet, turn the
faucet to full on, and let it turn for a couple minutes. Without the
aerator the flow is a bit chaotic, so expect water to splash around a
bit. After flushing you could clean the valve seat as you have been
doing, and then see if the problem still recurs.

Good luck.

Cheers, Wayne