Before any pipe gets cut, read the valve. The control valve on most softeners has several ports that look alike from across the room. Getting one of them backward is an easy mistake on install day and a hard one to see once the tubing is buried behind a wall or inside a cabinet. The checks below take a few minutes at the valve and can save a return trip.
Find the flow arrow on the valve body
Look for a flow arrow cast or printed on the valve body. It shows the direction water must travel through the unit. Trace it from the supply side toward the house side. Untreated water should enter the inlet port, and treated water should leave through the outlet. If the arrow and the port labels do not agree, stop and ask which one the manufacturer intends. Do not go by what looks natural in the space.
Ask the installer to point to the inlet and the outlet with a finger before the first fitting is measured. A capable installer can do this in seconds. If the answer is vague, treat that as a reason to slow down and check the manual that came with the unit, which the Missouri Water Softener Buyer's Guide explains how to read.
Match the drain and brine connections
Most softeners have a separate drain fitting for the regeneration discharge and a brine line that feeds the salt tank. These are not interchangeable. Each line goes to its own matching port. Check that the brine line is not run to the drain fitting and the drain line is not run to the brine connection. Compare the tubing size against the fitting it goes into, and do not force a line onto a fitting that is a different size or shape.
Check the bypass before water flows
Look at the bypass position and its handle or indicator before the supply is opened. Know which position sends water around the softener and which sends it through. Ask the installer to show you both positions and to say which one is normal service. Write the answer on the install paperwork so the next person who works on the unit does not have to guess.
What to ask before you sign off
Ask the installer to show you each port label, the flow arrow, the drain line route, and the brine line route. Ask where the untreated line is shut off or capped for service. Ask who will confirm the first regeneration and when. Compare the answers with the install sheet, and make sure the sheet names the ports and the flow direction, not only the total price and the install date. If a point cannot be checked on the valve in front of you, ask for it in writing before you accept the job as finished.