Our methodology

How Home Water Compass decides what to recommend — and what it refuses to guess.

Diagnose the decision before any recommendation

Every recommendation starts from three questions: where does your water come from, what have you actually noticed, and what evidence exists? A homeowner on municipal water with white scale on fixtures is making a different decision than a renter with a chlorine taste at one tap, or a well owner whose hot water smells of sulfur — even if a store would sell all three the same filter.

Three levels of evidence, never blended

• Utility-reported data — what your water system publishes about the water it delivers. Real, sourced, but area-level: it describes the system, not your tap.
• Your observations — smells, stains, scale, pressure. Genuine clues with several possible causes each; treated as clues, never as proof.
• Laboratory results — the only evidence that establishes what is in the water at your address. Home strips screen; accredited labs establish.

These stay visibly separate on every report, each with its own provenance badge. We never collapse them into a composite "safety score" — a single number would have to hide exactly the distinctions that make the decision sound.

The precedence order

Routes are decided by rules applied in a strict order. Each rule is versioned, stored with its reasoning, and testable — the same inputs always produce the same route, and every decision records which rule version produced it.

• 1. Official guidance outranks everything. An active advisory from your utility, or flooding at a well, suppresses every commercial suggestion until the issuing authority's instructions are satisfied.
• 2. Eligibility and authority. A renter is not routed to whole-home installation they can't authorize; point-of-use filtration is never presented as a fix for whole-home hardness.
• 3. Evidence sufficiency. A named contaminant without a qualifying lab result routes to testing — anxiety plus area data cannot justify an exact treatment claim.
• 4. Route fit. Source, scope (one tap vs. every tap), symptom pattern and your stated preference shape which honest path fits: learn, test, product or professional.
• 5. Availability, verified. A professional route requires a verified provider actually covering your ZIP, offering that service, open now and under capacity — checked before we ask for any contact detail. Coverage in a market is not the same as availability today.
• 6. Economics last. Only among options that already passed every test above does anything commercial get considered — and the disclosure sits next to the button.

What products must prove

A product appears only with an exact, model-level certified claim matching your concern — the specific unit, certified by a recognized body under the relevant NSF/ANSI standard, with the certification linked and its verification date shown. "The brand has certifications" is not enough; the claim must belong to the model. Costs are shown as five-year cost, not just the sticker price, because filters and salt are where the real money goes.

Honest failure states

A recommendation system is defined by what it does when the answer is no. No coverage in your ZIP, a provider at capacity or closed for the evening, no product with a verified claim, missing utility data — each produces a plain statement and a real alternative. We never render a fake success, and we never collect your phone number "just in case" when no one can actually serve you.

What we will not do

• No safety scores.
• No diagnosing a cause from a symptom alone — "one possible cause" is the strongest language a symptom earns.
• No presenting area-level data as a test of your tap.
• No health questions, ever — we do not ask about pregnancy, illness or medical vulnerability, and we do not build audiences on health traits.
• No fake urgency: no countdown timers, no "3 people are looking at this," no scare copy.

Accountability

The data pipeline behind this system is documented in data sources, the writing rules in the editorial policy, and the rules providers agree to in partner standards. If a recommendation ever looks like it broke the order above, tell us — that's a defect, and defects get fixed.