The installer packs up, the tools go in the van, and the system is either running or it is not. Most homeowners treat that moment as the end of the job. It is really the start of the first real test. Many problems show up in the first few hours and days, and they are much easier to sort out while the installer is still in the driveway or on the phone.
Set the valves before the water goes on
Before anyone opens the main supply, confirm where the bypass is set and which handle does what. Ask the installer to show you the isolation valves and the bypass, and to say in plain words which position sends water through the treatment and which sends it around. Write that down. If the bypass is left open, the water you paid to treat goes straight to the taps. If a valve is closed by mistake, the house can lose water entirely until someone finds it.
Pressurize slowly and purge the air
Air trapped in the lines and vessels causes sputtering at faucets, false readings, and sometimes disturbed media. Ask the installer to open the supply slowly, then run each cold tap one at a time until the stream runs steady without spitting. Do the same for hot taps if the system treats hot water. For tanks and filter housings, the installer should confirm that air has been bled from the top of the vessel rather than assuming it has.
Check the drain and any brine path
A system that discharges water needs a drain path that actually moves water. Watch the drain line during the first regeneration or flush. It should flow without backing up, and the hose should be secured so it cannot pull out of its fitting. If the system has a brine tank, confirm there is salt in it at the level the manual calls for and that brine is forming as expected. A drain that works on day one but backs up later is a common complaint, so ask who checks the discharge point and how often.
Program the settings and write them down
Settings are where a clean install often goes wrong without anyone noticing. Before the installer leaves, ask for the following in writing. First, the hardness or capacity figure the controller was set to, and where that figure came from, such as a water test result or the installer's own reading. Second, the regeneration schedule and the time of day it runs. Pick a time when the house is quiet, because a regeneration in the middle of a shower is an easy complaint to avoid. Third, any flow rate or service setting the controller uses. Fourth, the starting point for the next media change or filter replacement, so you know what to expect and when.
Look for leaks after the first hour and the next morning
Slow leaks at compression fittings often appear hours after the work is done, once pressure settles. Walk the equipment with a dry paper towel and check every joint, the valve stems, and the base of the tank or housing. Come back the next morning and look again, including the floor under the unit. A small weep at a connector should be fixed before the crew leaves, not after they have gone.
Run a test draw and compare it with the goal
Run a cold tap and a hot tap for a few minutes, then taste or look at the water. Water coming through a softener should not taste salty. A newly installed carbon filter may give off a faint odor for a short time, and the installer should be able to explain that and tell you how long it should last. If the system was bought to solve a specific problem, such as staining, odor, or scale on fixtures, note what you see now so you can compare it later. You will not see the full result in one day, but you should see the first signs of change or the first sign that something is wrong.
Ask the questions before the crew leaves
Ask who you call if something goes wrong on the second day, what the first service visit covers, and what the installer expects you to do yourself, such as adding salt or reading a gauge. Keep the startup record with the quote and the warranty so the settings are not lost when you need them. A careful startup walk costs little, and it is the step most often skipped.