Before you compare any equipment, confirm where your water comes from. A home on a municipal supply and a home on a private well face different questions, different responsibilities, and different tests. If you skip this step, you may end up comparing systems that do not match your water at all.
Find out which supply you have
Look at your water bill or your utility account. A municipal supply shows a utility name and a service address. A private well has no utility water bill, and the home usually has a pump, a pressure tank, and a wellhead, either outside or in a basement. If you cannot tell, ask the seller, the previous owner, or the local utility. Some homes have both, such as a city connection for the house and an older well used for outdoor water. Confirm which one feeds the kitchen tap before you test anything.
What the utility report gives you on city water
Most utilities publish a water quality report, usually once a year. It lists results from routine testing and describes the treatment the utility already applies. Read it before you test your own water. It shows what the utility is responsible for and what may remain in the water after its treatment. The report can also reveal past issues that make a home test more useful. Ask each installer whether they will use the utility report, their own test, or both.
What changes if you have a well
On a private well, you are responsible for the water and for the equipment that delivers it. The pump, the pressure tank, and the wellhead are your concern, along with the water test. Ask a testing lab or installer how they will sample the well, which items they will check, and how they will report results. A test that covers only a few items does not describe the whole supply. Keep the report with your treatment paperwork, because any installer will need it to size equipment.
Questions to ask before you commit
Ask each provider which supply they assume you have, what test they are using to size the system, who services the pump or pressure tank if you have a well, and what happens to the design if the utility changes its treatment. Write the answers down and compare them side by side. A provider who does not ask which supply you have has not started the job yet. For a clear way to judge what each provider has actually shown you, see the methodology page on this site.