I think this is a great discussion. While I mainly use hot water for showers and washing clothes but I also use a lot of hot water when cooking and washing dishes. Since heated waste water would have potential contact with my food I would rather not use RODI waste water for this purpose. The sealed water container depends on its location and method of introducing RODI waste water into the system. The diagram below I made up to show my radiant heating loop setup. The loop is part of the hot water system in the house which has a ~40PSI line pressure. The loop of hot water has equalized pressure with the hot water tank so when the pump is activated to heat the tank it is recirculating water without creating a difference in the water system line pressure. The bronze re-circulation pump I am using and similar pumps cannot run dry. If you were going to have an inline storage tank for RODI waste water where in the loop would it be or would you have it on the cold water supply side? One issue with inserting water back into the water system of the house is the line pressure of the water system. In order to put water back into they system from another source its pressure would need to be higher or equalized to the line pressure to allow water flow which is why a sealed tank would be needed unless a specialized pump and valve system was added to slowly feed water from an unsealed tank into the water supply line/loop. I am interested to hear how you would set this up as it is a really interesting discussion.