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Water Softener Sizing - SFR vs capacity

Posted by efreilly (My Page) on
Fri, Apr 15, 11 at 20:36

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.

Follow-Up Postings:

RE: Water Softener Sizing - SFR vs capacity

The maximum flow rate through a 3/4" line is 15gpm. The pressure it is delivered at needs to be considered also. There will be about a 10-15 psi pressure drop through the softener. A Fleck 7000 or Clack WS1 (1.5cuft) 45k system would provide about 16gpm with this pressure drop. The 25gpm you stated is the max flow rate with all fixtures running. A general rule is 1/2 that flow rate is required for typical usage.
Either of these systems at your present water usage would regenerate about 30 days. The system should not sit that long in between regens for fear of hardness bleed through at low flows. Both of these system have a 14 day override to regenerate. As your families water use increases this can be reset to normal operation. At a 6# salt dose you would use about 13# per month and have a salt efficiency of 3000 grains per pound. At $8 for a 50# bag (Dura Cube or Hardi Cube, I strongly recommend to use high grade salt) this would equate to $25 yearly salt usage.
There would not be any advantage to a twin tank system in this application because of the water usage and water chemistry. Low hardness can greatly effect the efficiency of tankless water heaters. Most manufacturers recommend treatment of water supplies with high mineral content.
1300 grains of daily hard water x 365 = 474,500.
7000 grains = 1# of dissolved mineral. In 1 yrs time your home currently has 67#'s of dissolved mineral build up. This is why even low hardness can cause problems in a home.
I hope this helps,

RE: Water Softener Sizing - SFR vs capacity

14 days between regenerations compromises resin life and with your water use channeling would be a concern.

Minimizing risk of channeling

If I have a long(-ish) regeneration time (10-14 days), would any of these minimize channeling risk?

-- upflow regeneration?
-- turbulator?
-- gravel base?
-- fine mesh resin?

I've read your responses in previous threads and really appreciate you both taking the time to respond. It's such a great resource to have guys like you point out the issues. From what I can tell with the quotes I've gotten locally, you guys are going to save me thousands and I'll probably end up with a much more efficient system.

RE: Water Softener Sizing - SFR vs capacity

No, and none will mitigate channeling.

RE: Water Softener Sizing - SFR vs capacity

Even if I try to go with rjh2o's lower flow rate of 16gpm, as I read the specs on the resins, they all are similar in that you can only go up to about 6gpm per cuft. If I only get 1.5cuft of resin as you suggested, won't my flow be way too little (~9 gpm) compared to the desired flow? If I try crank it as high as you say, won't I get major leakage of hardness into the water supply?

Also, If I set the regeneration time lower--say 7-8 days, can't I just set the salt dose really really low? Like say 3-4? What would be the downside to that? Would it just use a lot of water?

Also, I must be conceptualizing channelling wrong. If the turbulator physically mixes the resin, how can that not have some benefit? I don't mean that to sound incredulous, I just don't understand it.

RE: Water Softener Sizing - SFR vs capacity

You can regenerate every 7 days and set it to 4#'s. You would just use more water. Channeling occurs when low flow rates are run through resin beds. These low flows do not create enough turbulance through the resin bed and consequently the hard water forms channels through the media and hard water to home.
None of the options you mentioned are designed to reduce channeling they are options to help regeneration, iron removal and salt efficiency. Although for your application up-flow brining would be very beneficial. As I stated you are only going to get 15gpm flow rate with your plumbing. Max flow on the unit I stated is 16gpm. Normal water usage is 8-12gpm. With the figures you supplied your present gpm daily flow rates are about 5gpm in a 10 hour day.

RE: Water Softener Sizing - SFR vs capacity

What resin can handle 15-16 gpm with only 1.5 cuft without bleeding hardness through? Even at 8-12 gpm, the specs of the resins I've looked at 1.5 cuft wouldn't quite cut it:

Ionac C-249 1-6 gpm/ft3
Ionac C-266 NS 1-6 gpm/ft3
Purolite C-100E 1-5 gpm/ft3
Purolite SST-60 1-7.5 gpm/ft3
Resintech CG8 2-10 gpm/ft3

The Resintech CG8 looks like the only one that would handle 15gpm in 1.5cuft. I know nothing about it, though. Does anyone else have experience with it? Would it work for what I'm doing?

That's why I thought I needed 2.5 cuft -- to be able to handle the high flow at peak times. But while 2.5 cuft might handle the high flow, That's way too much capacity isn't it? That would leave me two options:

1) Use a ton of water with a really low salt dose, or
2) Go way too long (like a month) between regenerations.

I know the valve can handle it, I'm concerned about the resin. Shouldn't it be sized to handle the PEAK usage and not the average?

RE: Water Softener Sizing - SFR vs capacity

"I know the valve can handle it, I'm concerned about the resin. Shouldn't it be sized to handle the PEAK usage and not the average?"

Yes, and since you're paying for soft water you should get soft (zero hardness) water 100% of the time.

You have a common problem and there is no magic bullet solution. If you size the softener for your actual water use @ 1.5 cubes as rjh20 suggests then hardness will leak through every time you exceed the SFR of 1.5 cubes of resin in a 10x54 tank and in my experience that will be more often with your appliances and fixtures than you are being led to believe.

The SFR is ultimately limited by your 3/4" plumbing and you can't get around that cost effectively. The control valve will limit the SFR but the SFR of a control valve far exceeds that of resin in a tank on a residential softener.

Sizing the softener up to what the average water use for your size family should(?) be and the SFR your appliances and fixtures expect (demand?) will IMO get you channeling.

All things considered... the most sensible thing is to get a 1.5 cube softener and moderate your use of high water use appliances and live with the compromise until the kids grow up to average water use and then get a new correctly sized softener.

I know that's hard to take but remember, there are millions and millions and millions of water softener owners out there living with grossly undersized softeners cause they and their softener sellers don't know any better.

RE: Water Softener Sizing - SFR vs capacity

Thanks. I just wanted to make sure I was thinking about this correctly. You've been very helpful.

RE: Water Softener Sizing - SFR vs capacity

You have a good understanding of the right way to size a softener and know enough to tell when you are being advised incorrectly.

Let us know what you decide and how it works out for you.

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