Seeing a similar issue on a recent install of a tankless water heater. I'm considering putting a whole house filter after the tankless to capture precipitate.

1 Any idea on how quickly this would get clogged?

2 would/should I flush the lines / heater first

3 is there build up in the tank as well (I've seen several threads that indicate there is either build up in they tank or in the lines - not sure if this based on the type of heat exchanger - but that's my guess)

Many thanks