A water treatment install often feels finished when the crew starts packing the van. The tank is in place, the lines are connected, and water is running. But the close-out is where problems show up that were not visible during the work, and it is where you learn how to run the system yourself. Treat the last visit as a checklist, not a formality.
Run the system with the crew before they leave
Ask the installer to walk through a full cycle while you watch. For a softener, that means a manual regeneration so you can see the unit move through its stages and confirm the drain line is discharging. For a filter or reverse osmosis unit, have them open a faucet and let it run until the air clears and the flow is steady. Check every fitting for drips, especially the ones they just made. A small weep at a compression fitting is far easier to fix while the crew is still in the house.
Confirm the bypass is set the way you were told
Ask them to show you the bypass valve and which position means treated water and which means untreated. Have them turn it to bypass and back, then state the position they left it in. Write that down. The bypass is a common source of confusion later, so the close-out is the moment to make sure you and the installer agree on the normal setting.
Get the settings on paper
The settings that matter depend on the equipment. For a softener, that includes the hardness setting, regeneration time, and regeneration frequency. For a filter, it includes the flush cycle. For a reverse osmosis unit, it includes the pressure reading recorded at startup. Ask the installer to write these on the sheet you keep, or to record them where you can see them. If the controls are electronic, ask how to reset them after a power outage, since a reset that goes unnoticed can leave the unit on the wrong program.
Find the shutoffs and the drain
Have the installer point out the shutoff valves for the whole system and for each treatment unit separately. Ask which valve to close if a line leaks and which one isolates a single unit. Confirm where the drain discharge goes and whether it uses an air gap or a standpipe. You should be able to show any household member or helper these locations without searching.
Ask about the first test and who runs it
Ask whether a water sample will be taken after the system has run for a while, and who is responsible for arranging it. If you had a pre-install test, ask for a post-install comparison using the same parameters, so the results can be read side by side. Ask what the report will measure and what format it will come in. Get the expected turnaround in writing. Whether a result is acceptable depends on what you were testing for and how you plan to use the water, so ask the installer to explain what the numbers mean for your situation before you accept the work as complete.
Leave with the paperwork in hand
Before the crew drives away, get the installation invoice showing what was done, the equipment model and serial numbers, the warranty terms, a name and phone number for service calls, and the schedule for any follow-up visits. Ask who to call if something looks wrong in the first few weeks and whether that call is covered. Keep these together in one folder. Anything left open at the end of the visit, such as a leak to recheck or a test result still pending, should be written on the sheet along with a scheduled follow-up visit.
If you are still choosing an installer, the winners listed on the main page of this site, Best Water Companies STL, are a reasonable place to start your comparison.