I suppose it does highly depend on just how "hard" your water may be that would make soft water a better alternative.
In theory one recharging of my softener should provide 1000 gallons of soft water.
My water hardness is 35 grains.
I have a 35,000 grain softener ergo a 1000 gallons of water between cycles more or less.
Fourteen years ago I filled the my homemade unit straight from the well, but for all those years a flat plate exchanger wasn't in the equation either.
Not sure just how "rusty" our water might be compared to others though.
From the Mayo Clinic:
http://www.mayoclinic.org/healthy-living/nutrition-and-healthy-eating/expert-answers/sodium/faq-20058469Several other sources also confirm that soft water does have added sodium, however it is an extremely low level as defined by the FDA.
Whether this "extremely low level" is high enough to be more detrimental to a OWB system compared to "hard" water is something that could be debated for weeks I suppose.
I did see in the NC brochure that they recommend soft water while I've seen at least one other manufacturer strongly advise against soft water. However none of them define at what grain level is considered "hard". Like I've pointed out, our water is 35 grains of hardness while I've heard of other parts of the country being several times higher than that, the higher the grain level, the more salt will be found in soft water as salt ions replace the calcium and magnesium ions which makes the water hard.
I'm seriously considering trying a few of those "softener" cartridges from Central Boiler, they are more of a chemical filter that doesn't contain salt I think.