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.
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.
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.
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
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..
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.
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.
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.
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.
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.
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.
you know the constant SFR gpm, you need to find the capacity
you need using compensated hardness.
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.
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
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)
6lb PER ft
8 lb PER ft
10lb PER ft
15lb PER ft
To check your math go here Cubic
Foot and Capacity Calculator.