Before anyone quotes you a price, you should already have a document that says what is in your water. For most homes that is either the water quality report from the utility that serves you, or the lab result from your private well. A treatment quote built without it rests on a guess about what needs removing, and the unit you end up with may not address the problem you actually have.
Find the report that matches your supply
If you are on a city or district supply, look for the consumer water quality report the utility publishes for its customers. Check the year printed on it, then read the table of detected substances. Note which items were reported as detected, which were listed as not detected, and which show a measured level next to a standard. If you are on a private well, the document you need is a lab test of your own water, run by a certified lab and covering the substances you care about. A well that has never been tested has no report yet, and that gap is the first thing to fix.
Match each detected item to the unit's job
Write down every substance the report shows as detected. Then ask each provider, in writing, which of those items their proposed unit is designed to reduce and what standard that reduction is certified against. A unit built for taste and odor will not address a metal or a bacteria result, and a sales line that claims it removes everything is not an answer. If a listed item is not covered by the proposed unit, ask what would be needed to cover it and whether that changes the quote.
Ask what the test will confirm
Ask how the provider will confirm the unit works on your water after it is installed. Ask what they will test, where the sample is taken, which lab runs it, and who pays for a retest if the result misses. Keep those answers with the quote. Ask for the test method by name rather than accepting a general promise that the water will be better.
Bring the same packet to every quote
Put the report or lab result, a copy of a recent water bill if you have one, a short list of any taste, odor, staining, or scale problems you notice, and the number of people who use the water each day into one folder. Give the same folder to every provider. When each quote starts from the same facts, the numbers become comparable. If someone in the home has a health condition that may be connected to the water, take the report to your doctor or your local health office, since those questions belong to them, not to the seller.
Red flags in the quote
Be wary of a quote that never mentions your report, names no specific contaminant, or asks you to decide before the provider has seen a result. A provider who asks for the report and reads its findings back to you is doing the job properly. For a sense of how providers are assessed before you compare them, see our methodology page.