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Softener Sizing Chart

IMPORTANT: There are two parts to correctly sizing a softener; capacity, which everyone has heard of somewhat but the critical part of corrrectly sizing a softener is the constant service flow rate (SFR) gpm.

Look at it like a balance beam scale. On one side is the constant SFR gpm and on the other is the K of capacity. The K of capacity is adjustable by changing the salt dose lbs. in a given volume and type of resin.

The constant SFR gpm is based on the cubic foot volume of resin and the volume of resin dictates the size of the resin tank. The control valve must be capable of servicing that size tank and they have a SFR also. It tells the dealer what size of tank can be used for a softener or filter but the SFR of the control valve has nothing to do with the SFR of the softener's volume of resin. A softener, or filter, has to be sized by the constant SFR gpm of the resin or mineral to be able to successfully treat the peak demand gpm that your plumbing can deliver; or actually, your peak demand gpm water use as a family. If the constant SFR gpm of the softener's volume of resin or mineral is exceeded, the softener or filter will not remove all the hardness, iron or H2S etc. that it is supposed to. Then there's little sense in having it.

To repeat myself... The constant SFR gpm of a softener is a function of the volume of resin, not the control valve (as some mistakenly believe). The SFR gpm of a control valve @ 15 psi (loss) is used only to determine the size of tank it can be used on. As an example, a Fleck 5600 control valve has a SFR of 21 gpm @ 15 psi (loss) and can be used for a softener with 6" - 12" dia tanks; a Clack WS-1 has a 27 gpm SFR and can be used on 6" - 21" dia tanks.

The constant SFR gpm of most softeners is: 1.0' cuft = 9, 1.25' = 10, 1.5' = 12, 2.0' = 13, 2.5' = 18, 3.0' = 20, 3.5' = 22 gpm, 4.0 = 25 etc..

FYI, the flow rate from a 100' of 3/4" pipe at 50 psi is 17.5 gpm, for 1", it is 37 gpm. At 30 psi, 3/4" is 14 gpm and 1" is 28 gpm. Of course you will not get that much flow if you have the same ID pipe because your plumbing includes tees and elbows and valves which cause pressure losses and the fixture risers are much smaller ID than the pipe feeding them.

Some people mistakenly think their well pump can't deliver more than the gpm rating of the pump. Any pump chart will prove that to be incorrect in most cases. It depends on the total dynamic 'head' of the system; mostly the static water level in the well.

Again, everytime your peak demand gpm flow rate exceeds the constant SFR gpm of the volume of resin in the softener, all of the hardness and iron etc. in your water will not be removed.

Click here for information on plumbing code constant SFR gpm sizing requirements. The gpm needed varies widely from one code to another (there are 4 codes) and, it is calculated as if every faucet in the house is running at once.

Unless you use the fixture count method to get an idea of the constant SFR you need, your bathtub or an outside faucet will probably have the highest individual fixture flow rate and is an excellent place to calculate **part** of your constant SFR gpm flow rate.

That only will give you a snapshot of the condition of your plumbing and pressure. Using a bucket or other container at a tub, with the hot and cold water running full flow, collect water for exactly 6 or 10 seconds etc. and then physically and accurately measure the amount of water. Then multiply the volume by 10 or 6 etc. to calculate the volume for 60 seconds which is the gpm. This is only the gpm of that tub and doesn't include other water that may be used normally while the tub is filling; usually with only hot water being used for most of the time to fill the tub. If the tub has an anti scald mixing valve or low (=< say 5 gpm) then use an outside faucet instead of a tub; or try another tub. Once you have the gpm figure, you have a snapshot of the condition of your plumbing and water pressure BUT... that gpm is not the constant SFR gpm you need. Call me (570-490-3201 M-F 11 AM-6:30 PM ET) and in a few minutes I will tell you what constant SFR gpm you need.

Once you know the constant SFR gpm, you need to find the capacity you need using compensated hardness.

Formula for determining compensated hardness: Hardness in gpg + ((iron ppm or mg/l * 4) + (manganese ppm or mg/l *2)) = compensated hardness round to next higher gpg. Example: 14 gpg + Fe .8 ppm * 4 = 3.2 gpg + Mn .6 ppm *2 = 1.2 gpg = 14+3.2+1.2 = 18.4 rounded to 19 gpg = compenstated hardness. To convert mg/l or ppm to grains per gallon (gpg) you divide them by 17.1; one gpg equals 17.1 ppm or mg/l.

Capacity calculation: # of People times 60 gals/person/day (or whatever gallons you think but 60 is usually right on) + all extra softened water use in gallons/day; IE horses etc. multiplied by the compensated hardness = total grains/day. Grains/day * 8 (days) for a once every 7-9 days regeneration (if less than 5 ppm iron; 4 days if you have over 5 ppm of iron) plus a day of reserve capacity (not needed for twin tank models or the Clack WS-1 CS version) = the total grains of capacity you need. Then from the chart below find the capacity at the best salt dose that covers your total grains needed. With twin tank softners you must subtract the total gallons used per each reganeration because they use softened water for the regeneration.

Example: 3 people *60 = 180 + 10 gals/day for a horse = total 190 gals/day, * 19 gpg compensated hardness = 3610 total grains/day. Then 3610 * 8 days = 28880 total grains needed which should be rounded up to 29 or 30k. Using the salt dose capacity chart below, you find the volume of resin that produces 29,000 or 30,000 grains of capacity using 6 lb/cuft of resin. That gives you great salt efficiency and covers your capacity requirement. As long as the constant SFR gpm of that size softener is correct, you have a properly sized softener with great salt efficiency. If your peak demand gpm is greater than the constant SFR gpm for that size softener, you would use the size the constant SFR dictates and then adjust the salt dose and thereby the capacity.

The 6 lb/cuft salt dose gives you 3333 grains of capacity per lb of salt used rather than the maximum salt dose per cuft of 15 lb which generates only 2000 grains of capacity per lb of salt. If you use potassium chloride instead of regular softener salt (sodium chloride) and you are using low salt doses (say 8 lb/cuft) below, you have to increase the salt dose by roughly 27-30%.

Sizing Capacity of various salt doses PER cuft of regular resin. Fine mesh and SST-60 resins are said to be more efficient but not necessarily and SST-60 is an excellent choice if you have more than 2 ppm of iron; fine mesh resin has a higher pressure loss than regular mesh or SST-60 resins.

EX: 1.5cuft using a 6 lb PER cuft (9lb) salt dose generates 30000 grains (3333 grains/lb); 10 lb PER cuft (15lb) = 40500 (2700 grains/lb) etc.

6lb PER ft
8 lb PER ft
10lb PER ft
15lb PER ft
1.0 20,000 24,000 27,000 30,000
1.5 30,000 36,000 40,500 45,000
2.0 40,000 48,000 54,000 60,000
2.5 50,000 60,000
3.0 60,000 72,000 81,000 90,000
4.0 80,000 96,000 108,000 120,000
5.0 100,000 120,000 135,000

To check your math go here Cubic Foot and Capacity Calculator.