The install crew's work is not finished when the last fitting is tight. A new treatment system starts working on your water immediately, and whatever is left in the lines, the media, or the joints goes to your taps first. The minutes before the crew leaves are the cheapest time to catch a problem, so use them.
Ask what flush or startup step they run
Ask the installer to walk you through the exact startup sequence they follow. For a filter or softener, that usually means rinsing or backwashing new media and running water through the system until the discharge runs clear. For an under-sink unit, it usually means flushing the storage tank and the new faucet line before the first drink. Ask how long the flush runs, whether they check the outflow with their own eyes or a meter, and whether the step is written on your install paperwork. If the answer is vague, ask them to show you.
Check the leak points while the water is running
Pressure changes when the system comes online, and a joint that held during the test can weep once the water is moving. Before the crew leaves, look at every fitting, valve, and drain connection with the system running, then look again after ten minutes. Place a dry paper towel under the connections if you can. Ask what they would do if a joint seeps, and who returns to fix it.
Confirm the bypass and the settings
Make sure the bypass position matches what the installer tells you is correct for normal use. Ask them to point to the valve, name the position for treated water, and tell you what it looks like when the system is bypassed. For a softener or any unit with adjustable settings, ask for the values they entered, such as hardness setting, regeneration timing, or flow rate, and where that record is kept. Write the settings in your own file so the next technician does not have to guess.
Draw a sample and keep a record
Run a cold tap for a few minutes, then fill a clean glass from the treated side and compare it to an untreated draw. Note the time, the taste, the odor, and whether the water looks clear. Some cloudiness or a brief burst of sediment early on is common after a flush, and it often clears within a short period. Persistent cloudiness, a strong or unusual odor, or grit that keeps appearing is different, and you should write it down and raise it with the company rather than assume it will fade.
Know when to call back
Before the crew leaves, ask for the phone number to call during the first few days, not just the office line. Ask what counts as a callback they will come back for at no charge and what they will bill you for. Ask whether a second visit to check the system after it has run for a while is included in the install. The answers to these questions tell you more about the first week than the brochure does.