SFR through the softener is dependent on resin volume AND tank size... not just the tank size.

Instead of putting the cart before the horse let's start at the beginning.

Water system or well?

What is the SFR of the plumbing and fixtures/appliances?

Any high water use appliances like a Jacuzzi?

hardness = 15
iron = ? (negligible is not a number)
manganese = ?
copper = ?
sodium = ?
arsenic = ?
pH = ?
TDS = ?
If on a well then test for nitrates and bacteria
people = 2
bathrooms = ?
size of the service plumbing?
size of the service plumbing at softener location if different?

With that information we can speak intelligently regarding water treatment and flow rates.

It's a well. 2 bathrooms, 3/4 inch plumbing both sides.

Two showers, washer, DW, - just the usual stuff. There is a hottub, but it fills off of a hose bib that won't be softened.

iron = 0.3 ppm
manganese = not detectable
copper = not detectable
sodium = 20 ppm
arsenic = not dectable
pH = 8.0
TDS = ? I have a 20 gph filter in line

Thanks

And again... "What is the SFR of the plumbing and fixtures/appliances?"

"TDS = ? I have a 20 gph filter in line"

You've got me on that one... TDS = Total Dissolved Solids

Sorry, I didn't have the TDS handy and apparently don't know when to put in a carriage return. TDS is 450 mg/l and sodium is 136.

And again... "What is the SFR of the plumbing and fixtures/appliances?"

2 showers = 10
2 toilets = 6
2 lav sinks = 4
kitchen sink = 2
dishwasher = 2
washer = 4

That's 28, but realistically, I doubt that it would ever be more than 15.

Since your original question was about SFR perhaps you should test it and see... IIRC the maximum flow rate for 3/4" plumbing is around 13 gpm velocities not exceeding 8 feet per second so 28 gpm is not realistic.

I have tested it at a hose bib, and I am getting about 15 gpm versus about 25 gpm at the well, so what you say sounds correct. My point is that I would rather over-buy on the flow rate rather than regret it later. The issue is the longer regeneration times.

The optimum sized softener for your situation is 1 cubic ft. However, a 1.5 cubic ft softener will work with some adjustments. You need to be aware that sizing it this way will not be as water efficient. The optimum point of water and salt efficiency for a softener regenerates with about 6 lb of salt per cubic ft of resin. A 1.5 cubic ft softener is oversized in your situation and would have to be regenerated with only 3 lb of salt per cubic ft of resin in order to be salt-efficient. If you are willing to make that trade-off, the larger softener will work.

Agree with Alice... to a point.

A 1.5 cube softener will get you 12 gpm SFR rather than the 9 gpm a 1 cube softener will get you and 3 lbs/ cu ft salt dose is a compromise, but the larger softener runs the risk of channeling.

Even with your negligible iron of .3 ppm more days between regeneration is a risk. The iron will accumulate in (on) the resin and that can ultimately be a problem.

I do not agree with those who advocate 10-14 days or more between regenerations and ESPECIALLY do not agree when iron or manganese is in the water, but those people are in the business and replacing resin is a profitable service call.

JMO

Regenerate after 7 days maximum - I have never advocated otherwise.

Channeling is a function of flow volume through a specified area and water temperature. Unless your typical flow rates through the softener are going to be very low, you are not going to experience channeling. If hardness were 30 gpg we would recommend a 2 cu ft softener without blinking.

Softener sizing will always be a compromise of some sort, weighing flow rate, hardness, money, space, etc. In this case, we are trading water efficiency for flow rate. Alternately, we could trade money, space and simplicity for flow rate and install two softeners that would alternate, but with the capability to run in parallel when a higher flow rate is desired.

Alice,

"Regenerate after 7 days maximum - I have never advocated otherwise"

You and I have always agreed on that but the OP has been told otherwise on another forum.

Ah, I see - I was only aware of the conversation on this forum.

The SFR of the system is a function of the volume of the resin and whatever physical constraints are introduced by the valve and plumbing. You can overrun the ability of the resin to soften water, but you can't overcome the physics of the valve. I found two papers that addressed this. One said that 40 times the base volume of the resin per hour is a reasonable maximum and the other said that 80 times the base volume over short periods is fine. What it actually noted is that 80 BV is typically what happens in a home. That puts the max SFR at (80 * BV):
1.0 cf 1.5 cf 2.0 cf So as justalurker noted, 3/4 plumbing is going to limit you to 15 anyway.

So, I need to accept the limitations on the resin volumes and the chemistry of the softener and get a valve that doesn't limit volume any farther. Thanks for the help.

Control valves generally offer far greater SFR than the vessels and resin volume they sit on.

The real world limiting factor on a residential softener's SFR is the volume of resin in a correctly sized tank. The generally accepted spec for that comes from the resin manufacturers.

In general, a 1 cube softener has an SFR of 9gpm and a 1.5 cube softener has an SFR of 12 gpm. When the SFR is exceeded hardness leaks through... simple as that. If you're paying for soft water then you ought to be getting it.

gjh - If you know which resin you are using, the manufacturer of the resin knows the precise characteristics of the resin and you can find out exactly how much water will pass through the resin bed. If you are dealing with a softener seller who will not tell you what resin you are buying - RUN.