Since small RO systems are so prevalent, are there any designs available to use one for water supply for a small hydronic system that uses a tankless water heater?

We have extremely hard water, and a salt-based water softener, and we currently use RO for drinking water.  Our dhw supply uses a tankless Takagi heater that is 7 years old, works fine, but generates enough particulates to clog our Grohe faucet aerators once a month.  Sometimes it is much worse, depending on the volume of water we use and the quality of the water softener recharge.

Our existing heat source for radiant heat is an 80 gal hot water heater, and it is being replaced with a Navien CH-240 combination unit, that will also replace the Takagi and supply dhw.  That may help with the particulates in the dhw because Navien's design uses a heat exchanger internal to the unit.  The radiant heat water is heated by gas, then heat is transferred to the dhw side.  So we want to keep the water in the radiant heat loop as pure as possible to prevent scale and any particulates that could clog the Navien filter.

I've been looking for a design that would use a small RO system plus a small pump to provide the makeup water.  The CH-240's inlet for domestic water is separate from the inlet for makeup water, and uses an automatic feeder valve.

The heating and potable water sides do not mix. Once your heating side is filled you will not need much if any makeup water. If you have an issue with your softener you should address that issue separately.

Who ever sells and/or installs the Navien should have the answers and provide annual maintenance.

The RO treated water would also be used for initial fill after installing the Navien, or any time the system needs repairs and cleaning. I think the heater needs to be drained in order to clean the filter, although the manual is not real clear abou that.

Having been on a well with a TDS over 2000 and hardness of over 45 I would recommend filling the system with the R/O. The trick would be using a separate R/O system so there would be no chance of cross contamination. I would think besides the initial fill the R/O should be fine. I would just use a larger tank at least 5 gallons so the pressure and volume would be there when it needed any to make up.

RO water is corrosive unless corrosion inhibitors are added.