A whole-house filter is installed once, but it is only useful if it is commissioned properly. Commissioning is the stretch between the first cut in the pipe and the moment the installer hands you a written startup record. Knowing the sequence helps you see whether each step is being done, and it gives you a clear point to ask questions.
Before the first cut
Ask how the house will be isolated, where the shutoff will be, how long the water will be off, and whether you should store water or plan for a dry period. Confirm the installer will check the inlet pressure before connecting anything. If the work involves changes to the supply line, ask whether the installer holds the local license that the job requires, and whether a permit or inspection is needed under your local rules.
Check the site before mounting
Look at the mounting spot, the wall or framing, the clearance needed to change the cartridge or media, the floor drain or discharge route if one applies, and the power outlet if the unit has controls. The filter housing should be supported so that the pipes do not carry its weight. Ask the installer to show you where the flow direction arrow points, and confirm it matches the direction of supply.
Connect and pressurize slowly
Open the supply gradually rather than all at once, so that fittings are not stressed by a sudden surge. Watch every joint as the system fills. Check the connections again after the system has held pressure for a while, since a slow seep can take time to show.
Flush before you judge the water
Run water from an outlet for the length of time the installer states, and watch the discharge for fine particles, air, or color. A new filter can release fine material at first, so do not judge the result by the first few seconds of flow. Ask the installer what a normal flush looks like for the equipment you bought.
Verify the settings and record them
Ask the installer to confirm the flow rating in service, the media or cartridge type installed, any control settings, and the service interval that applies. Check that these match the scope sheet and the quote. If something differs, ask why before you sign off.
Sign off on a written startup sheet
Before the installer leaves, you should have a written startup record that lists the outlet tested, the settings, the serial number or model, the first service interval, and the phone number to call. Keep this sheet with the warranty paperwork. If the installer will not provide a written record at startup, that is an important point to raise before you accept the work as finished.