How we rate systems
Overview
Every one of the 56 systems in our database gets a letter grade from A to F on five measures, and an overall grade weighted from them. The rules are fixed and applied by code to the same data you see on each data sheet. Nobody at Well & Tap can move a grade by hand, and no brand can pay for one.
We do not lab test systems ourselves. The grades measure what can be verified: independent certification listings, published prices and replacement part costs, rated flow, and written warranty and return terms.
1. Certification evidence (30% of the overall grade)
| Grade | Rule |
|---|---|
| A | The whole system is listed by NSF, IAPMO, WQA or CSA to a health-effects standard (NSF/ANSI 53, 58, 55 Class A, 401, P473, 44 for softeners, or 62, 177). |
| B | The whole system is listed only for taste and odor (NSF/ANSI 42), or there is no listing but the maker publishes a full third-party lab report for the contaminants it claims. |
| C | Only parts or media are certified (for example the tank or valve material), not the system's performance. |
| D | Maker claims only, including "tested to NSF standards" with no listing and no published report. |
2. 10-year cost (25%)
Purchase price, plus every replaceable part the maker lists multiplied by the number of replacements due in ten years, plus installation ($500 for whole-house systems, $150 for under-sink units, nothing for countertop units). Systems are ranked against others in the same category: the cheapest third earn A, the middle third B, the most expensive third C.
3. Flow (15%, whole-house systems only)
Rated flow in gallons per minute: 12 or more A, 9 to 11.9 B, 7 to 8.9 C, 5 to 6.9 D, under 5 F. Where a maker advertises a higher flow than its certification was granted at, we use the certified flow. Single-tap systems are not graded on flow.
4. Warranty (15%)
Coverage on the main parts (tank, valve, housing): lifetime A, 10 years or more B, 5 years C, 2 to 4 years D, under 2 years F.
5. Returns (15%)
90 days or more with no restocking fee A; at least 30 days with a fee of 10% or less B; a fee up to 20% C; anything harsher D.
Overall grade
Each letter converts to points (A 4, B 3, C 2, D 1, F 0), weighted as above. Measures that do not apply, such as flow for a countertop unit, are left out and the other weights scaled up. 3.5 or more is an A, 2.75 a B, 2.0 a C, 1.25 a D, below that an F.
Where the data comes from
- Certifications: the NSF, IAPMO, WQA and CSA public listings, linked from each data sheet.
- Prices and parts: each maker's own store, checked September 2026.
- Specs, warranty and returns: product pages, manuals and published policies, linked in each data sheet's sources.
- Water quality: EPA's final UCMR 5 results for our PFAS lookup, plus EPA, CDC and USGS guidance on our contaminant pages.
Money and independence
We earn commissions from some sellers when readers buy. Commission rates are not an input to any grade, and systems without an affiliate program are graded the same way. See our affiliate disclosure. Spotted an error? Tell us and we will correct it and date the change.