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
service flow rate (SFR).
Look
at it like a balance beam scale. On one side is the SFR
and on the other is the capacity. The capacity is adjustable
by changing the salt dose in a given volume and type of
resin. The SFR 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. A softener, or filter, has to be sized
by the SFR 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 water use as a family. If
the SFR of the softener or filter 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 SFR of a softener is a function of
the resin, not the control valve (as some mistakenly believe).
The SFR of a control valve 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 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
SFR gpm of most softeners in gpm 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.
If
your peak demand flow rate exceeds the SFR gpm of the volume
of resin in the softener, all of the hardness and iron etc.
in your water can not be removed.
Click
here
for information on plumbing code SFR sizing requirements.
Unless
you use the fixture count method to get an idea of the 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 flow rate. 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 and then physically and accurately
measure the amount of water. Then multiply the volume by
10 or 6 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 was
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 SFR you need. Call me for assistance.
Once
you know the 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 per week regeneration (if
less than 2 ppm iron; 3 days if you have over 2 ppm of iron)
plus a day of reserve capacity (not needed for twin tank
models) = the total grains of capacity you need. Then from
the chart below find the capacity at the best salt dose
that just covers your total grains needed.
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 SFR 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 SFR gpm for that
size softener, you would use the size the 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%.
Sizing Capacity of various salt doses PER
cuft of regular resin. Fine mesh and SST-60 resins are more
efficient and are a good choice if you have more than 2
ppm of iron, although fine mesh 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.
cuft |
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 |
67,500 |
75,000 |
| 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 |
150,000 |
To check your math go here Cubic
Foot and Capacity Calculator.
