That is a tough call... you can size for the two people in the house now or size for the normal occupancy of the house.

Negligible iron might need to be considered in the calculations so I'd like to know what measurement in ppm the negligible iron is.

With 2 people a 1 cu ft / tank twin would be regenerating every 7 days or so and switching over to the other tank. When the occupancy goes up to 4 people you'd be regenerating every 4 days or so and then switching tanks. That would be at a 6 lb / cu ft of resin salt dose which is pretty efficient.

As the occupancy increases you wouldn't need to change any settings on the softener... just need to add salt more often.

A 1 cu ft / tank twin will give you a 9 gpm SFR which should be sufficient.

A Fleck 9100SXT based 2.0 cu ft (1 cu ft each tank) softener would be worth considering.

Thank you! I'm going to get get independent test results before I purchase. What factor do you use to figure out the grains for iron & manganese? I've seen anywhere from 3-5.

Hardness in gpg + ((iron ppm or mg/l * 4) + (manganese ppm or mg/l *2)) = compensated hardness (round up to next higher gpg).

Always size to the max hardness.

I decided not to do the lab test and bought one of those home test kits. I got 23 gpg for hardness and 0.5 PPM for iron. I also retested with a aquarium kit had laying around and got the same results. Of course not accurate compared to a $250 lab test but I think it should be sufficient.

Based on that. It looks like 1.5 cu ft could be better or do you think I can get by with 1.0 for each tank? I'm going to get the 9100sxt, do you have any programming tips for it?

Appreciate the help,
My

If you're satisfied with the home test...

A 2.0 cu ft twin tank (1 cube/tank) will work nicely. Get a top basket for the 9100, the Noryl bypass (not the SS one) and gravel under bed regardless of what the seller says. You'll need to choose the appropriate yoke to connect to your softener loop..

Set capacity @ 20k, (compensated) hardness @ 25gpg, and 6 lbs salt dose. Should regen every 6 or 7 days with 2 people. As occupancy increases it will regen more often. With 4 people probably every 3 or 4 days between regenerations at he same efficient salt dose.

Even though iron is slight I'd start a routine application of a little Iron Out or similar product once a month.

Thank you. Got every ordered! Also, I'm planning to use potassium chloride instead of salt. Do I have to change anything for that?

You might need to increase the salt dose if you use KCl. Get a pack of hardness test strips and test that the water holds soft right up to regeneration. If it doesn't, increase the salt dose to 6.5 lbs.

For resin cleaning, once a month use 1/4 cup of Iron Out, Super Iron Out, Rust Pro etc. dissolved into a gallon or two of warm water and pour it into the 4" round tube
in the brine tank where the float is and then flush that with a quart of clear water. Then do a manual regeneration.
Repeat for the other tank.