No, not scaling an organization.
Scale that builds up on hot water heater plumbing.
Filters in tankless water heaters are usually water softeners meant to prevent scale build up (that might be what's happening with your heater): https://www.uswatersystems.com/blog/2016/11/preventing-scale-in-tankless-water-heaters/
'For all of the convenience of on-demand hot water, there’s one especially annoying issue: de-scaling. With an ordinary tank-style water heater, the homeowner might go for years without draining the tank to remove sediment. It’s not advisable, but it is possible. With a tankless water heater, descaling is required at least once a year.'
'Minerals can build up in the small lines, and that can reduce water quality and pressure. If left unattended, scale can damage the whole appliance. If water pressure is low, chances are there’s debris in the system. Flushing it should solve the problem, and that’s a professional job with some models.'
Tankless water heaters need to be flushed regularly. Hence the need for valves, which none of our heaters has. What I should do is buy the valves, install them and flush the heater regularly. But there are too many other things to do at the moment, so I'll leave that for later. Unless it's convenient to do at the same time as the current changes?
As for filtering bigger debris, such as rust, we'd only need a regular spin-down or mesh filter appropriate for the size of those particles. This would be installed in the cold water inlet line, before it enters the water heater. I suspect that these filters are expensive. Do you think we could cannibalize one of the spin down filters from the water filtration panel? The little mesh pre-filter you have on the panel might work too, depending on mesh size.
How to Flush a Tankless Water Heater (with vinegar and a sump pump):
This one includes a water pre-filter: https://www.youtube.com/watch?v=T4FaqGod3VU
This one includes instructions for also cleaning the air filter: http://www.instructables.com/id/How-to-Flush-a-Tankless-Water-Heater-and-Clean-the/
Sediment pre-filter on Amazon: https://www.amazon.com/Pre-Sediment-Filter-Electric-Tankless-Heater/dp/B017BSWJ1G
Eccotemp Descaling Kit (includes sump pump, descaler, hoses): https://www.eccotemp.com/accessories-1/ (we can do the same thing what pumps and hoses we already have + vinegar)
More pre- sediment filters:
Water Heater valves:
The valve kits above are both 3/4" but our water heaters have 1/2" NPT inlets and outlets, so we'll adapters to 3/4".
There's also supposed to be a pressure relief valve, which the kits above include.