As Russ noted, systems that turn on/off a lot, you get a TDS "leak". The RO/DI should run for long periods of time, otherwise you will be putting high TDS water (for a short period of time) through your DI.
For this and several reasons, do NOT run your RO/DI directly to your tank/sump with a shutoff. Run it to a storage tank of some kind, and THEN run a topoff to your tank. Periodically, fill up the storage tank.
Bill
215g FOWLR... and anemones, GSP, gorgonians... carp, that isn't FO!
"I killed my first SW Fish in 1971..."