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Water Softening/Conditioning

How to size a water softener

October 13, 2010
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The following must be considered when sizing a water softening system:

  • Flow rate (gallons per minute)
  • Influent water hardness (grains per gallons)
  • Usage (gallons per day)
  • Hours of operation
  • Economics
  • Physics
  • Flow rate

In general, it is best to base the size of the water softening system on continuous flow rather than peak flow rates. During peak flow rates the jeopardy of hardness breakthrough is present.

If flow rates are not known, Table 2 and Table 3 show approximate flow rates.

Estimating flow rates

The following information has been prepared as a guide for estimating maximum flow rates for private and public buildings. The numbers assigned to the various fixtures are based on a combination of flow rate and probability of use.

1. Count and total the number of each type of fixture to be serviced by the water softening system.

2. Multiply the number of each type of fixture by the unit count given in Table 1: Fixture unit table.

Private: Motels, Apartment Buildings, Trailer Parks, Group Homes

Public: Office Buildings, Hospitals, Country Clubs, Schools

3. Find the total fixture count by adding up the values found in Step 2.

4. Using the correct table, find the fixture count closest to the calculated value. The figure given in the right hand column is the approximate maximum GPM required.


Type of Fixture Qty. Unit Count Total
Water Closet (FV) 8 X 10 = 80
Shower 10 X 4 = 40
Lavatory 15 X 2 = 30
Total fixture unit count = 150
Estimated Flow Rate = 80 GPM

Another way to determine your approximate flow rate is by the size of your plumbing feeding the water softening system (see Table 2 and Table 3.)

Water hardness, quality and daily use

Hardness is present in the water supply as calcium and magnesium bicarbonate, CaCO3. Other water quality factors will influence the way the water softening system works.

Dissolved iron, if present, must be taken into account when sizing the water softening system. Maximum allowable iron is 2 ppm.

To calculate the required capacity of a water softening system take a water sample and have it analyzed for hardness and iron content, or call the local municipal water treatment facility.

Hardness, as CaCO3, if expressed in parts per million (ppm) or milligrams per liter (mg/l) is converted to grains per gallon (gpg) by dividing ppm or mg/l by 17.1.

If iron is present, multiply the amount of dissolved iron (ppm) by 4 and add it to the total grains of hardness. At this point, your total grains per gallon has been determined.

Multiplying the gallons of usage per day by grains per gallon will determine the capacity per day.

Water softening systems are typically rated at 30,000 grains of removal per cubic foot of resin. However, the systems are typically operated at 20,000 grains of removal per cubic foot of resin for economy salt dosing.


Hardness 250 ppm
Convert to grains per gallon (divide by) 17.1
Hardness in grains per gallon 14.6
Iron 1 ppm
Convert to grains per gallon 4
Iron in grains per gallon 4
Total grains per gallon (Add iron + hardness) 18.6 grains
Daily Usage 1000 grains
Daily Capacity (Multiply usage by total grains per gallon) 18,600

Hours of operation

One of the most important factors in determining the size and configuration of a water softening system is the hours of service the system will be called upon to produce soft water.

If the system will be required to produce soft water 24 hours per day and no down time can be determined, a dual system will be required.

If the system is only required to produce soft water part of the day and down time can be determined, a single-tank water softening system may be used.

However, in certain circumstances when capacity greatly exceeds flow rate requirements, a dual system can provide a cost savings.


Once the minimum size and type of water softener needed have been determined, there may be several water softener options.

Your customer may only want to spend a minimum amount of money for the water softener or they may want a more expensive meter initiated system in order to spend less money on salt in the future.

If the capacity required greatly exceeds the flow rate requirements, a multiple-tank system could be more economical than a single-tank water softener.


A water softener may be the right type and may be properly sized for the application it will serve, but there are other factors to be considered before quoting the system:

  • The softener must fit in the space allowed;
  • The brine tank must fit through the door;
  • A drain must be within the specified range and must be capable of handling the drain flow;
  • The temperature of the water must meet specifications;
  • The drain line should not be susceptible to freezing, etc.

Dayton Water Systems, Dayton, OH, provides sales and service of residential, commercial and industrial water purification equipment throughout the Ohio/Indiana/Kentucky area.

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