A bypass valve lets water skip the treatment unit so the house still has water while the unit is serviced or out of service. On a new install, it is the part most likely to be set wrong, and the mistake often shows up only after the crew has left. The safest way to handle it is to know the order of the hookup and to watch each step.
Before the crew connects anything
Ask the installer to confirm which valve is the main shut-off, which is the bypass, and which handle position means treated water, bypass, or off. Labels on the valves help, but ask the installer to point to each one and say what it does. If the labels are unclear, write the positions down on a piece of paper and keep it with your other paperwork.
Ask where the drain line will run and where it will end. A drain that empties into a low spot, a floor drain without an air gap, or a line that could back up is a problem to raise before the unit is connected, not after.
The order the work should follow
A careful installer will isolate the unit from the house water supply, drain pressure, and dry-fit the bypass and the unit connections before any joint is made or tightened. You should see the unit taken off line and the supply shut off at the main before any cutting or fitting starts.
Once the connections are made, the water should come back on slowly. Watch each joint for drips while pressure builds, and check the unit's own fittings separately from the pipe joints. A joint that holds at low pressure can still weep when the house is running at full flow.
The test that matters
Before the installer leaves, have them run water at a kitchen or utility tap with the unit in service and again with the bypass in use. Flow should be steady at both settings. If the bypass position delivers water while the unit is in service, or the reverse, the valves are wrong and the unit is not ready.
Ask the installer to show you how to return to bypass mode and how to get back to treated water. You should be able to do this yourself without calling anyone, because the day you need to isolate the unit in an emergency is not the day to be reading a manual.
What to leave the visit with
You should leave with a written list of valve positions, the drain location, the warranty and service contact details, and a plain note of the date the test was run and what was observed. Keep the list with the unit's documents so the next person who needs to isolate the system knows what to do.
If you want a broader way to judge who is doing this work, the Missouri Water Experts directory at missouriwaterexperts.com lists providers, and our methodology page at missouriwaterexperts.com/methodology.html explains how we review them.