Water Softener Sizing - SFR vs capacity
Hello all. I apologize in advance for posting yet another water softener sizing thread, but I think I have a unique dilemma. It seems to stem that both the SFR and the softening capacity are both proportional to the volume of resin. Here is the problem:
White scale is ruining our glassware, and it is all over the stainless steel dishwasher and plumbing fixtures. Moreover, we have a Rinnai tankless water heater (9.8gpm) that we are very happy with, but I suspect is very susceptible to this stuff.
Despite our symptoms, by labwork, our water doesn't seem that terrible;
Municipal supply; Hardness ~ 10 grains per gallon; ph = 7.3; TDS 153 ppm, iron 0.
We have two adults and three kids (ages 6,4, and 2), so by conventional calculations, we should use a minimum of 300 gallons per day. Checking my water bills in the last 18 months, I only use about 130 gallons per day -- perhaps because my kids are so young and we have high efficiency appliances. So, even at the higher usage estimate, using a 7-9 day regeneration cycle, I need only about 24,000 grains of capacity.
On the other hand, we have a 4 full-bathrooms and 2 half-bathrooms. The master shower has an overhead showerhead and two body jets showerheads. We are considering adding a third powder room and a jacuzzi tub in the distant future. (About 31-34 wsfu.) According to the International Plumbing Code, with the number of fixtures in my house, I should have flow downstream from the softener of 25 gallons per minute. (Which probably isn't realistic anyway since I only have about 50psi on a 3/4 inch supply.)
It would take maybe 4 cuft of resin to acheive that flow rate. But I would have to set my salt dose so low, it would be a huge waste of water.
With 1.5 cuft of resin, it seems like I would have more than enough capacity, but not nearly enough flow.
If I go with a compromise of 2.5cuft of resin, I am concerned that I would have both insufficient flow, and also way too much capacity and wasted water.
All this, and I would prefer a system flexible enough to handle the changing the water usage of my family as the kids grow up. Am I thinking about this correctly? Would there be any advantage to a twin system in this situation? (I can't think of any.)
Any thoughts would be greatly appreciated.