An onboard closed-loop water system means that the carbon and DI water system is built into the machine. This is good news in that you can connect any type of water to the washer (tap, RO, or DI) and the onboard system will purify it as necessary. Because the carbon and resin bottles are smaller, they will be depleted faster with lower quality feed water. This means they will need to be replaced more frequently, if tap or RO water is connected to the machine, than DI. So, we would recommend using DI water if available for maximum performance and longevity.