I can't comment on the size you need, but since you are in the beginning stages, consider a unit that will use potassium chloride instead of salt. Our home Depot has it. It is more expensive but oh so much healthier for you and your family. I learned about it here on GW in another forum, California Gardening. I had wanted to use our gray water from our home to assist in watering plants during our drought, but the forum informed me that I would kill my plants with that salty water. Imagine what drinking and using it does to US! So it's easy with our unit to use Potassium Chloride instead of salt. Ours is a Rain Soft unit, professionally installed by Rain Soft. It's a whole house softener. There are 2 of us living in a 2900 sq. ft home with guests now and then. Here is a link to learn about Potassium Chloride. I am not a rep for them!. http://www.watertechonline.com/articles/considering-potassium-chloride-for-softening

Suzi - You are confusing salt with sodium. A gray water system would use the regen water (which DOES contain salt in high concentrations) for your plants and certainly that would damage them. Softened water contains slightly elevated sodium (about 1/3 of what is in milk for comparison, which is only a problem if you have been placed on a severely sodium-restricted diet by a physician). Additionally, using KCl rather than NaCl is possible with ANY softener with a simple programming adjustment - no specialized equipment required. Don't "imagine" and spread misinformation. Know the facts.

jwith - Your calculations are off. I suspect it is because you are assuming around 32000 grain capacity for one cubic ft of resin, which is only true when it is new. Once in operation, capacity is determined by salt dosage. Since you have iron and manganese, you will need to regen at 8 lb salt/cf resin, which will give you 24000 grains capacity per cf resin.

Additionally, when you have iron/manganese time is your enemy. To prevent fouling, the softener should be regenerated every four days. Given than, the difference inn salt usage between a 1.0 cf softener and a 1.5 cf softener will be 30 lb/month, 360 lb/yr. If you are don't expect 10 gpm usage to be frequent and sustained (and that amount of flow in a 2-person household would be unusual), I would go with the smaller softener.

The problem
with iron-laden water in a softener is that softener resin has a great affinity
for iron and does not release it easily.
Iron will slowly foul the softener resin from the inside out. Consequently, future-proofing for 15-20 years may not be a reasonable expectation. With with right precautions, 10 years can be achieved. Extra measures need to be taken to remove the
iron. For this you have several options.

1. The easiest
   option may be to simply use Iron-Remover salt.
2. Alternatively,
   you can use Super Iron Out. You may
   choose to layer this in with your salt.
   If you choose this option, use 1/4 C per 40 lb bag of salt.
3. Super Iron
   Out is more effective if used monthly with the following procedure:
   
4. A liquid
   iron removal system such as Res Up can be installed in your brine tank to feed
   citric acid to the brine to aid with iron removal.

My
preferred methods are 1 or 4 because they employ citric acid, which is a much
safer chemical than those in Super Iron Out (sodium hydrosulfite and sodium
metabisulfite). The choice is yours -
pick a method and follow it religiously and your softener will give you many
years of reliable service.

Thanks aliceinwonderland_id. I think my calculations were about the same as yours. I assumed 20k Grains/cf at 6 lb/cf and 7 day-override. I was hoping my iron content (<1ppm), wasn't significant enough to require 4 day regeneration.

I actually created an excel spreadsheet based on the qualitywaterassicates.com sizing chart. The spreadsheet fixes a few bugs with the other chart and allows you to control all kinds of sizing and efficiency parameters.

I think we would have a significant waste of salt with 8 lb/cf and 4-day override. Based on our water usage (1500 GPD) and a 1 cf system, we would regenerate the unit back to 24k Grains every 4 days. During this time we would only use (1500 GPD * 4 Days) 6k Grains. We would essentially be wasting 18kG or 75% of a the salt. This amounts to 540 lbs of wasted salt a year. If we went to a 1.5cf we would waste nearly 910lbs of salt. At $8 per 40lbs salt, that would be a waste of $108 to $182 annually.

If I extend my DO to 7 and reduce the dose to 6lb/cf of salt, do you think my resin would last 5 years? I'd rather replace the resin every 5 years, then buy and haul an extra 500lbs of salt each year.

Thanks for your suggestions about cleaning the iron from the resin. I'll probably use your number 1 suggestion and pay the extra $3 per 40lbs of salt. The biggest reason for getting the water softener is to be done with Super-Iron-Out. My wife and I hate this stuff, specifically because the irritation caused by the chemical dust.

There is a flaw on that site with the iron/manganese calculation.

Since other water conditions factor in, the only way to know if your softener will make it 5 years with longer than 4 days between regens is to try it - many folks have found themselves in trouble after a year or two.

I should have said this before, but the best possible way to use the least salt, get long life from your softener and resin, and allow yourself a larger softener for additional flow, would be to install iron removal upstream of the softener. Then you can use a 1.5 cuft softener, set your salt dosage at 6 lb/cf and go a little over two weeks between regens, cutting salt usage to about 17 lb per month. Your up-front cost will be higher, but your maintenance costs will reduce significantly, not to mention the 500 lb of salt per year you won't have to carry.