Many treatment products carry a certification mark, and it is easy to read that mark as a general promise that the product works. It is not. A certification is a claim about specific contaminants, tested under specific conditions, for a specific product. Before you commit to a purchase, you need to know what the claim covers and whether it covers the problem in your own water.

Start with your own water data

Begin with your water test results or a lab report, not with the product sheet. Write down the contaminants that were found above the level you care about, and note whether the report came from your tap or from a source you do not control. A certification that covers a contaminant you do not have is not useful, and a contaminant you do have but that is not on the certificate is a gap you need to close another way. If you have no test data, say so, and treat any treatment claim as unconfirmed until you have some.

Read the listing, not the label

Look up the certificate in the certifying organization's public listing. The listing usually names the standard, the contaminant reduction claims, and the model or product numbers covered. Compare the claim to the product you plan to buy. Check whether the claim applies to the specific model, the specific cartridge or media, and the specific installation type, such as point-of-use or whole-house. A product line can carry several certifications, and only some models may be listed for each one.

Check the conditions behind the claim

Certification testing runs under set conditions, such as a stated water chemistry, flow rate, pressure, and a specified test concentration. Ask whether those conditions match your house. If your supply pressure is low or your flow is high, the reduction result may not match the claim. Ask the seller to show you the test conditions in writing, and ask whether the replacement media is covered by the same listing. A unit can be listed while its replacement parts are not.

Questions to put to the seller

Ask for the certificate number and the listing page for the exact model you are buying. Ask which contaminants are listed for that model, and whether any of them match your report. Ask whether replacement cartridges or media carry the same listing, and how you can confirm that on the packaging or the certifying body's listing. Ask what the claim does not cover, because a clear answer to that question tells you more than the mark does.

How to compare two products

When two products have similar claims, compare the listings line by line. Look at the standard, the contaminant, the model, the replacement media, and the test conditions. The product whose claim matches your own data most closely, and whose replacement parts stay covered, is usually the better fit. For a broader view of how this site weighs claims, see the How Grades Work page.