Methodology: how tankless heater ratings are built

What a rating on this site measures

A rating is an index of two things: how completely and how strongly the exact Amazon listing documents the unit, and how the public Amazon customer record for that same ASIN looks in volume and average. It is not a measured flow rate, not an efficiency test, not a verdict on safety and not a substitute for the sizing your installer will do. Two units with the same rating may suit very different houses.

Step 1: attributes are read from the exact listing only

For gas units we look for a stated maximum flow in GPM, a stated input in BTU per hour, the word condensing in the title and an indoor or outdoor designation. For electric units we look for a stated heating power in kW, a stated 240 V requirement and, where present, a stated GPM. Each match is stored with its verbatim phrase and its location (title or feature bullets). Unit families are never mixed: a BTU figure on an electric listing is ignored, and a kW figure on a gas listing is ignored. Model-number suffixes are used only where the manufacturer’s convention is unambiguous, such as Rinnai’s i for indoor and e for exterior, and that use is recorded as the provenance.

Step 2: two documentation formulas

Documented spec fit for gas units = 100 × [0.45 × min(stated maximum flow in GPM / 11, 1) + 0.30 × min(stated input in BTU/h / 199,900, 1) + 0.15 × (the title states a condensing design ? 1 : 0) + 0.10 × (indoor or outdoor placement is stated ? 1 : 0)], rounded to a whole number. For electric units it is 100 × [0.55 × min(stated heating power in kW / 36, 1) + 0.25 × (240 V is stated ? 1 : 0) + 0.20 × (a maximum flow in GPM is stated ? 1 : 0)]. A gas unit needs a stated GPM and an electric unit a stated kW in the exact Amazon listing text to use this index; BTU/h and kW are never converted into each other. This index summarizes how completely a listing documents capacity, fuel input, design and placement — it is not a flow measurement, an efficiency test, a safety judgment or an installation recommendation.

Documentation completeness = 100 × [0.5 × min(feature bullets / 8, 1) + 0.3 × min(listing images / 5, 1) + 0.2 × (brand stated ? 1 : 0)], rounded to a whole number, computed from the stored Amazon listing record when the listing text states no maximum flow (gas) or no heating power (electric). It measures how completely a seller documents a listing — nothing about how the heater performs.

A gas listing that states no GPM, or an electric listing that states no kW, cannot use the spec-fit index and is scored on documentation completeness instead. The label under each score says which one applied.

Step 3: public reviews are blended in

Where Amazon publicly displays customer reviews for a listing (average star rating and review count captured from the public product page), the raw editorial score is 100 × [0.45 × (Amazon average / 5) × min(review count / 50, 1) + 0.35 × min(log10(1 + review count) / log10(1 + 5000), 1) + 0.20 × documentation score / 100]. The confidence factor min(review count / 50, 1) damps averages based on very few reviews, and the logarithmic volume term rewards large public review volumes, so widely and highly reviewed listings rank ahead of thinly reviewed ones. Listings with no captured public review data use the documentation score alone. This combined index reflects the public Amazon reputation and the listing documentation — it is not a physical test and not our own product testing.

Review counts and averages are read from the public product page for the exact ASIN, with the URL, page title and raw selector text kept as evidence. A listing whose page hides its reviews or shows none is left without review data rather than estimated. Displayed counts are rounded down to the hundred at 1,000 and above; the exact figure still drives the score.

Step 4: display

The displayed editorial rating is the raw blended score divided by 10, shown to one decimal on a 0-10 scale. There is no catalog normalization, rank interpolation, forced winner, forced floor or artificial gap. Equal scores remain equal: the available data does not justify a distinction. Ranking uses the unmodified raw score descending, then listing slug for exact ties; slug is a stable identifier, not a quality signal. The same raw score always has the same display rating in every shortlist. This is not a physical test result or an Amazon customer rating.

Who is eligible for a comparator

We include complete tankless units with a captured Amazon average and positive review count for the exact ASIN, matching the comparator’s stated rule: indoor natural gas, outdoor propane, or 240 V electric at 18 kW and above. Bundles that pair a heater with a valve or vent kit, portable camping showers, point-of-use units below 18 kW, mini-tank heaters and accessories are kept in the catalog but never ranked against a whole-house unit. Listings with conflicting fuel, placement or capacity evidence, and explicit portable/camping or point-of-use applications, are withheld from whole-house shortlists pending clarification. Each comparator ranks the full eligible pool by raw score before selecting five entries, retaining only the highest-ranked listing when exact listing titles identify the same model. Different documented model codes remain distinct; reviews and specifications are never transferred between ASINs.

Why sizing beats rating

The most common tankless complaint is not a bad unit but a unit sized for a warm-climate brochure and installed where groundwater arrives at 40 °F. Our calculator prints the inlet temperature it assumes for each region, the fixture flows it adds up and the arithmetic that turns them into a required output; treat its result as a starting point for the conversation with your installer, not a specification.

What never appears here

No prices baked into a page (offer pills load live and hide when stale), no invented review counts, no efficiency figures we did not read in the listing, and no instruction on gas piping, venting, combustion air, breakers or conductor sizes. Where a guide reaches an installation decision, it names the licensed trade that makes it and stops.