A whole-house unit sits on the main line where water enters the house, so the first questions are not about the filter. They are about where the unit can legally and physically go, and who is responsible for the pipe on each side of it. Settle these before you pay a deposit or sign a contract.

Ask the supplier what sits in front of your unit

Find out where your property line, meter, and shutoff valve are. Ask whether the supplier maintains anything upstream of your shutoff, and whether it allows equipment to be installed on the customer side of the meter. Some suppliers have written rules about pipe materials, pressure, or backflow protection. Get those rules in writing or as a link to the policy.

Ask whether a backflow device or permit is required

Ask your local building or plumbing office whether a whole-house treatment unit needs a permit, an inspection, or a backflow prevention device. Ask whether the installer will pull the permit or whether you must. If a backflow device is required, confirm whether it is included in the quote or billed separately.

Confirm the bypass and the shutoff plan

Every whole-house unit should be installed with a bypass so the house still has water when the unit is being serviced. Ask the installer to show you where the bypass valve sits, how to operate it, and where the main shutoff is. Ask who will be responsible for closing the line if a filter change or leak happens when no one from the company is on site.

Ask about pressure and the existing plumbing

Ask how the installer will check your household pressure before the unit goes in, and whether a pressure reducing valve or a pressure gauge is part of the plan. Ask whether any of your current pipes, fittings, or water heater connections will be altered. Get the answer written into the scope, because a change to the main line is the point where a small job becomes a larger one.

What to bring back to the table

Keep a short list: the supplier's rules on the service line, the permit status, the backflow requirement, the bypass location, the shutoff plan, the pressure check, and the list of pipes that will be touched. If an installer or supplier cannot answer one of these clearly, that is a reason to slow down and get the answer before any equipment is ordered.