Once a water softener is plumbed in, someone has to tell it three things: how hard the incoming water is, how large the resin tank is, and when it should regenerate. Those settings control how much salt the unit uses and whether hard water gets through to your taps. Some homeowners set them alone. Many have the installer do it. Either route can work. The real question is who has the hardness number, and who will be able to show later what was entered.
What the settings actually control
Most softeners ask for hardness in grains per gallon, often labeled gpg, or in milligrams per liter. The display units vary by model, so check the manual for the exact unit you own before you change anything. The capacity setting tells the unit how much treated water it can produce between cycles. The regeneration trigger, whether by volume of water used or by days, and the time of day the cycle starts also live in the menu. If the hardness value is too low, the unit regenerates too rarely and hard water starts to show. If it is too high, the unit regenerates too often and wastes salt and water. Both errors are easy to make and easy to miss.
Start with a hardness number you can name the source of
Before you touch a menu, find out where the hardness figure came from. Acceptable sources include a test reading taken at the tap before the softener, a lab report, or a water utility's annual water quality report. Ask the installer which one they used. If the installer tested your water on site, ask for the result in writing. If nobody can say where the number came from, treat it as a guess and get a proper test before you accept the settings.
Set it yourself when
Doing it yourself is reasonable when all of these are true: you have a hardness reading from a source you can name, you have the manual for the exact model, the menu is clear to you, and you can read the display back against the manual after each change. Keep a note of every setting you change, along with the old value it replaced. If the unit has a manual regeneration option, run one and confirm the display shows the cycle the way the manual describes.
Have the installer set it when
Ask the installer to set it, and to stay until you see the display confirmed, if the hardness number is unknown, if your water source has been changing, or if the model uses a setup sequence you have never used before. Read the warranty terms as well, since some tie coverage to the setup record. A few minutes of the installer's time at this step costs far less than correcting a wrong setting after the first service call.
What to write down before the installer leaves
Whoever sets the unit, get a short written record that lists the hardness number and where it came from, the capacity value entered, the regeneration trigger and start time, the salt setting, and the bypass position. If the installer allows it, photograph the display. Store the sheet with the manual, not in a drawer you will forget. When the unit is serviced later, the technician will ask for these values first, and having them ready shortens the visit.
Check the settings again after the first few weeks
Settings made on day one assume the water stays the same. Compare the display with your record after the first few regenerations, then again whenever the water seems harder or softer than usual, or when salt drops faster than expected. A change in your water source or a shift in what your supplier delivers can move the hardness figure. If you find a mismatch, check it against your record and the original test before you change anything.
For the full sequence from choosing a unit through setup and follow-up, the Missouri Water Softener Buyer's Guide walks through each step in order. Our methodology page explains how we weigh setup detail when we rank providers.