Originally Posted by
Entropy
I have a softener on the house, but my RO unit pulls water from before the softener. I am not sure how much a diffference it makes, but the basic difference (as I understand it) is that a softener exchanges calcium and magnesium for sodium. Calcium and magnesium will both clog your RO membrane faster so it is better to soften the water first, BUT sodium is harder for the RO membrane to stop (more gets by) so your final TDS number (or feed to the DI if you have one) is bigger with soft water. So it looks like you get to choose between your RO membrane clogging sooner or your DI being exhausted sooner. Since the DI is reusable in some cases, and cheaper in all cases it is probably best to use the soft water. I don't have DI so for me I run it before the softener to get a lower TDS number.
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