My BRS RODI unit came with an auto shut off valve, so the solenoid wasn’t needed. The shut off valve is activated once the float valve cuts off flow.
Although an RODI auto shut off is good, it is not optimal for continuously filling a reservoir. The setup you describe keeps the reservoir full all the time, which is nice, but the constant cycling of your RODI unit will have a negative effect on your DI due to TDS creep. You may not have this issue if your water source is more pure.
Generally speaking the best way to fill a reservoir is to flush the RO membrane and then do a long fill on the reservoir. Setting the flushing aside, to achieve this we utilize two techniques to fill reservoirs. Either a timer or dual float switches hooked to a controller or relay.
The easiest method is a simple countdown timer switch commonly available for around $10. Turn the dial to the time it takes to fill the reservoir and it will energize the solenoid until the time runs out and then cut power to the solenoid and close the solenoid. The downside is this method requires you to turn the unit on as there's little automation.
Next would be two float switches programmed to a controller to fill the reservoir when the low switch is activated and cut off the solenoid when the high switch is activated.
Finally, the most involved setup would be two float switches installed like the previous method, but wired to a relay, acting in a similar fashion as a controller would. I say more involved simply because you have to wire everything up correctly.
The benefit, aside from the TDS creep, is you now have fail safes in the event the mechanical float valve fails by breaking or debris or growth inhibits the valve to fully close. I personally would not rely solely on a mechanical valve.
