An installation is not finished when the last fitting goes on. It is finished when someone has walked through how the system runs, confirmed it matches what you agreed to, and recorded it. That walk is commissioning, and it is the last point where the people who did the work are standing in your home. Once the truck leaves, every question becomes a phone call, and phone calls take longer to resolve than a five-minute check on site.
Get the paperwork before the first valve turns
Ask for the model and serial number of each unit, the settings the tech programmed, and the name of the person who will sign the completion record. If the job needed an inspection, ask whether it has already been done and whether you will receive a copy of the sign-off. Keep these in one folder with your quote so you can compare them later.
Check the settings against the written quote
Compare what is on the controller or the spec sheet with what the quote promised. For a softener, that means the hardness setting, the salt type noted, and the regeneration schedule. For a filter, it means the cartridge type and the change interval, plus any change date entered in the controller. If a number on the screen does not match a number on paper, stop and ask which one is correct and why. Do not accept an answer like 'it will adjust itself.'
Walk the water path with the tech
Have the tech show you the main shutoff, the bypass handle, and which position sends treated water to the house and which sends untreated water. Ask them to move the bypass to bypass and back while you watch, so you know the valve works and where it stops. Then open a cold tap and a hot tap, and the drinking tap at the sink if there is one. Look at the flow, not only whether water comes out. Note any sputtering, air, or pressure drop, and ask the tech to explain each one before they leave.
Look for leaks while the system is under pressure
Dry each connection on the treatment unit and along the new lines with a paper towel, then watch them while the system runs. A joint that looks sound when the water is first turned on can weep once pressure has built up for a while. Ask the tech to stay until the lines are at full pressure and the joints have been looked at once more. Then schedule your own look under the sink and around the unit after the system has run through a normal day. Write down the time you checked so you can tell a new drip from an old one.
Ask about the first automatic cycle
If the unit regenerates, flushes, or refills on its own, ask when the first automatic cycle is set to run and what you should expect to hear, see, or notice. Ask whether it is safe to be in the room while it runs, and whether the tech will be available by phone if something looks wrong during that cycle. If the timing allows, watch the first cycle yourself rather than hearing about it later.
Leave with the contacts and the startup record
Get the direct phone number and name of the person who handles calls after the install, and ask whether that line goes to the installer or to a general service desk. Get the startup record in writing, showing the settings as left, the bypass position at hand-off, the readings or checks the tech made, and the time the system was started. File it with your warranty papers. If a problem shows up later, that record is the first thing anyone will ask for.
Before the crew packs up
Run through this list one last time: the paperwork is in hand, the settings match the quote, you have seen the bypass work, the taps run normally, the connections are dry, the first automatic cycle is scheduled, and you know who answers the phone. If any item is open, say so now and get it written on the record. An open item at hand-off is easy to resolve while the tech is still in the house.