I finally broke down and brought an ro/di unit and have been using it for about three months now. I have noticed that the tds on my product water has started going up when i first start the unit the product tds goes from 0 to 24ppm quickly and then in about a minute or two drops back to 0ppm. Should i just go ahead and replace the di now or wait until the tds stays above 0ppm? Oh and the unit does do an automatic back flush for 15seconds.