Best Tankless Water Heater: 39 Amazon Bestsellers Checked Against the AHRI Directory

Four are computed from the decode table lower on the page, one is the certified directory itself, four are measured, specified or ruled by someone else and link to the document.

1. 12 of 39

   12 of the 39 bestselling tankless water heaters on Amazon are on the AHRI directory, all of them Rheem or Rinnai. The other 27 are not, and 17 of those burn gas or propane: APUS, Baococo, Camplux, GASLAND, MIZUDO, NOVIEEX, Ranein, VEVOR, Westinghouse, thermomate. A listing on AHRI is voluntary; the DOE certification report a maker must file before selling a water heater is not, and none of these listings says whether one exists. ^[1]

   The top results for six Amazon searches on 2026-09-19, deduplicated to 39 ASINs and matched by brand and model code against the 993 Instantaneous rows of the AHRI directory in the site corpus (663 gas and propane, 330 electric).

2. 1.7× to 2.8×

   Every AHRI-listed unit that prints a flow prints more than it certifies: 11 of 11, by 1.7 to 2.8 times, median 2.1. Rheem's RTG-84 says 8.4 GPM and certifies 4.4; the RTG-70s say 7.0 and certify 4.0; the RTEX-24 electric says 5.9 and certifies 2.4; Rinnai's RE199 says 9.8 and certifies 4.6. None of the numbers is false. Rheem states its gas figures at a 35°F rise in the bullets; the certified figure is at 67°F. ^[1]

   Box figure from each listing's title or first bullet, certified figure from the directory. 8 of the 30 listings that print a flow state the rise it was measured at; 22 print a maximum or a bare number.

3. 500 × GPM × °F

   The arithmetic behind every flow claim: gallons per minute × 500 × degrees of rise = BTU per hour delivered. Rheem's 180,000 BTU/hr burner, at the recovery efficiency the directory lists for it, comes to 8.7 GPM at 35°F, 4.6 at 67°F and 4.4 at the 70°F rise a 45°F winter inlet asks for, against 8.4 on the box and 4.4 certified. An 18 kW electric unit delivers 61,416 BTU/hr: 3.5 GPM at 35°F, 1.8 at 67°F, against 4.4 on the box and 1.7 certified. The decode table computes this for the 29 listings that print both a flow and a power; the box figure runs 1.4 to 2.4 times the arithmetic at the test rise. ^[14]

   Bob Vila prints the same formula; Stiebel Eltron and Rinnai print the resulting flow-vs-rise curve on their spec sheets, which is the only way to publish a GPM that tells a buyer their own flow.

4. 100 A · 3 × 40 A · 200 A

   What a 24 kW electric unit asks of the house, from the makers' sheets (Rheem's as retailers reproduce it; Stiebel's and EcoSmart's as published): Rheem's RTEX-24 draws 100 A and wants three 40 A double-pole breakers and a 200 A service; Stiebel Eltron's Tempra 24 wants two 50 A breakers on 6 AWG and a 150 A minimum service, and derates to 18 kW on 208 V; EcoSmart's ECO 27 draws 112.5 A on three 40 A breakers and 8 AWG. A 100 A panel cannot run any of them. ^[18]

   13 of the 17 electric listings state an amperage or breaker requirement at all. The WINTEMP 18 kW listing states its own draw as 75 A and specifies 80 A of breaker, 107% of the draw; Stiebel sizes at 100% and footnotes that it treats a tankless heater as a non-continuous load; the 125% margin an electrician may apply would want 94 A.

5. 24 of 39

   24 of the 39 listings state no UL, ETL or CSA safety listing anywhere in the title or bullets, Rinnai and Stiebel Eltron among them. 11 are indoor or outdoor combustion appliances with neither a safety mark stated nor an AHRI listing: APUS, NOVIEEX, Ranein, VEVOR, Westinghouse. On a gas unit a CSA or ETL mark is a safety listing to ANSI Z21.10.3; it says nothing about efficiency, and an AHRI listing says nothing about safety. A gas appliance sold with neither stated has told you nothing at all. ^[8]

   9 state CSA, 5 ETL, 1 UL. 0 of 39 claim ENERGY STAR, which is correct: the program requires UEF 0.87 and 2.9 GPM at 67°F, every Rheem RTG here is 0.82, and electric resistance units are outside its scope.

6. 37%, 20–40 years

   The one independent multi-site field study, 24 heaters rotated through 10 Minnesota homes for two years, measured 37% less water-heating energy on tankless gas units than on natural-draft storage tanks, about 6,000 kBTU a home a year, and a simple payback of 20 to 40 years at 2010 installed costs and gas prices. Measured efficiency averaged 16% less than the DOE energy factor across all units, and less far below it on the tankless units than on the storage tanks. ^[12]

   Bohac, Schoenbauer and Hewett of the Center for Energy and Environment with Lobenstein (Lobenstein Consulting) and Butcher (Brookhaven National Laboratory), 2010. Average draw was 1.2 gallons, 32 draws a day, and only 3% of draws exceeded ten gallons, which is where a tankless unit reaches its rated efficiency (DOE Building America). DOE's consumer page puts the saving at 24 to 34% for homes using 41 gallons a day or less, 8 to 14% at 86 gallons.

7. UEF 0.81–0.98

   The directory holds 663 gas and propane tankless models, certified from UEF 0.81 to 0.98, median 0.93; 459 clear ENERGY STAR's UEF threshold of 0.87 and 204 do not (the mark also needs 2.9 GPM, a warranty and a safety listing; the corpus flags 313 as ENERGY STAR). The federal floor is 0.81. Certified maximum flow among them runs to 6.0 GPM at the test rise, median 4.8; 624 certify at 4.0 or more. The 330 electric models certify a median UEF 0.95 and a median 0.75 GPM: heat loss is not their problem, flow is. ^[2]

   Counted from the AHRI Instantaneous rows in the site corpus. A high UEF on an electric unit is a statement about heat loss, not about running cost: Consumer Reports' engineer notes electric units run more efficiently and cost more to run, because of the price of the kilowatt-hour. The site's formula puts the RTEX-24 at $779 a year at 16¢ and the RTG-84 at $269 at $1.60 a therm, on the same draw pattern.

8. $1,400–$4,000

   Installed, a like-for-like tankless replacement runs $1,400 to $4,000 in the cost guides the site reconciles (one guide's blended average is about $2,500, a directional figure); converting from a storage tank runs $2,100 to $8,000, most national guides capping near $5,600 and one Houston contractor setting the ceiling, the difference being the gas-line upsizing (Rinnai's manual works a 245,000 BTU/hr load on 3/4-inch pipe), the listed stainless or polypropylene venting, and the mounting move. Consumer Reports puts installed cost at $800 to $1,500 for a gas tankless unit against $600 to $800 for a tank. ^[30]

   data/installed_prices_water_heaters.json, reconciled across four or more 2026 cost guides with ranges widened to the outer bounds where they disagreed; the field study's Minnesota contractors quoted $2,000 to $5,000 in 2010 for the same reasons.

9. 4 recalls

   Four CPSC recalls of tankless units: 42,200 Paloma-made Rheem, Ruud and Richmond units in 2007 for a carbon-monoxide hazard; 3,400 Navien units in 2018 for carbon monoxide; about 36,700 VST-brand units in 2025 for carbon monoxide after 33 reports of cracked exhaust ducts; and 42,000 Camplux portable propane units in 2024 for fire. Three of the four are carbon monoxide. A gas tankless unit is a combustion appliance on your wall; the exhaust path is where the danger is. ^[28]

   The four CPSC notices, linked; the site's water_heater_cpsc.json holds the per-brand record. 2 of the 39 bestsellers are portable propane showers of the Camplux kind; the Camplux listing scopes itself to outdoor use, the Baococo listing does not say.

Left, the gallons per minute each listing leads with. Right, the maximum flow AHRI certifies for the same unit under the DOE test at a 67°F rise. Same order, same units.

The word tankless covers a 3 kW plug-in sink heater and a 199,000 BTU wall-hung burner. Sorted by the certified flow and the utility they draw on, the 39 bestsellers are six things.

The question is what the house can feed. Gas needs a line that carries 199,000 BTU/hr and a place to vent; electric needs a panel with 60 to 120 A to spare. Neither is a hot-water question until that is settled.

• Works A gas house replacing a gas tankless — The easy case: the line, the vent and the mounting already exist, and the installed range is $1,400 to $4,000. Pick on certified flow at your winter rise and on UEF; a condensing unit at 0.95 earns the rebates (PSE&G $500 at UEF 0.87; SoCalGas $1,000 at 0.95). ^[30]
• partly A gas house converting from a tank — The line is usually 1/2-inch and the vent is B-vent; a 199,000 BTU unit wants 3/4-inch and its own listed vent. That is the $2,100 to $8,000 range, and it is where the field study's 20-to-40-year payback comes from. Worth it for endless hot water or a small utility closet; not for the gas bill. ^[12]
• partly An all-electric house in the South — A 24 kW unit at a 35°F rise is 4.7 GPM, two showers. It needs three 40 A breakers and a 200 A service; if the panel has them, it works. Running cost is the catch: the site's formula puts the RTEX-24 at about $779 a year at 16¢ against $269 for the gas RTG-84 at $1.60 a therm, on the same draw pattern. ^[2]
• Wrong tool An all-electric house in the North — A 70°F winter rise takes a 24 kW unit to 2.3 GPM, one shower with nothing else running, and the 18 kW units to about 1.8. Consumer Reports' testers watched two electric units fail to hold 120°F at a 58°F inlet. A heat-pump water heater is the electric answer here; the site's page on them is linked below. ^[13]
• Works One far-away sink — A point-of-use unit at 0.3 to 0.5 GPM certified, on its own circuit, ends the wait for hot water at a distant lavatory. It does not do a shower. ^[1]
• Wrong tool A cabin, an RV, a garden shower — The portable propane units are outdoor appliances; the Camplux recall was for fire. Use them outside, in the open, as the makers' own manuals say. ^[29]

A tankless heater adds heat to water passing through it at a fixed maximum rate, so the flow it can supply falls as the rise you ask for grows. The DOE test fixes the rise and reports the flow; a listing fixes the flow it wants to print and leaves you to find the rise.

DOE test condition, maximum GPM

58°F in, 125°F outa 67°F rise, 10 CFR 430 Appendix E; the legacy energy-factor test used 135°F out, a 77°F rise ^[5]

The arithmetic

500 × GPM × °FBTU per hour delivered; divide the burner's output by 500 × your rise to get your flow ^[14]

ENERGY STAR gas tankless

UEF ≥ 0.87 · ≥ 2.9 GPM at 67°Fplus a 6-year heat-exchanger warranty; electric resistance units are outside the program ^[8]

Federal minimum UEF

0.81 gas · 0.91–0.92 electric10 CFR 430.32(d), instantaneous units under 2 gallons of storage; 0.80 for the very-small draw pattern ^[6]

• The rule Before a maker distributes any water heater model it "shall submit a certification report to DOE certifying that each basic model meets the applicable energy conservation standard". That filing is mandatory; the AHRI listing is a separate, voluntary verification of UEF and maximum GPM. ^[7]
• The makers who show the curve Stiebel Eltron's sheet gives the Tempra 24 a 92°F rise at 1.5 GPM, 73°F at 2.25 and 54°F at 3.0; Rinnai's V65i curve holds 6.5 GPM to about a 25°F rise and falls to 2 GPM at 125°F. Rheem states one point, "8.4 GPM (35°F rise)", and the retailer copy of its sheet gives 4.5 at 67°F. ^[17]
• The field Households in the Minnesota study drew 1.2 gallons at a time, 32 times a day, and only 3% of draws exceeded ten gallons; Building America's monitoring found a tankless unit reaches its rated efficiency only above about 10 gallons a draw. That is why measured efficiency ran 10 points under the energy factor on the tankless units. ^[12]

The sequence the makers' manuals, the field study and the trade sources agree on, in the order the decisions bind.

1. Before you buy Look the model up on the AHRI directory and read the certified GPM and UEF. 27 of the 39 bestsellers are on no directory; a unit that is not there has no certified flow to compare. Then find your winter inlet temperature and read the flow at your rise from the maker's chart, or compute it.
2. Before you buy, gas Size the gas line and the vent, or get the quote first. A 199,000 BTU unit needs a line sized for it; Rinnai's manual works the pipe-size table and, for its non-condensing units, bans PVC, CPVC, ABS and galvanized vent material; condensing units vent per their own manuals, some in polypropylene. Use the components the manual lists. This is most of the conversion cost.
3. Before you buy, electric Count the breaker spaces and read the service size on the panel. Three free double-pole spaces and a 200 A service for 24 kW; two and 150 A for Stiebel's 24; one 50 A for 11 kW. If the panel is 100 A, stop here.
4. Install day Isolation valves and a drain port, so it can be flushed. Hard water scales the heat exchanger; the owner threads' maintenance answer is a yearly vinegar flush through service valves, which are cheap to fit now and a plumber's visit later.
5. Week 1 Set the outlet to 120°F and leave it. A higher setpoint costs flow at the rise you have; a lower one lets the cold-water sandwich show. The recirculation question comes after you have lived with the wait.
6. Each year Flush it, and check the vent terminal. Three of the four tankless recalls were carbon-monoxide hazards, one of them 33 reports of cracked exhaust ducts. A CO alarm in the same room as an indoor unit is not optional.

The pairings are the things the heater assumes exist: fuel it can draw on, a vent that stays intact, and a way to keep it clean.

One rule, applied to all 39: on the AHRI directory, sold by Amazon itself, orderable on 2026-09-19. Eight of the 39 pass it; these six are the ones with a job, and the other two are in the cut list with the reason. Each is here for its certified flow at the certified rise; where the box prints a bigger number, the pick says at what rise. The 27 unlisted bestsellers and the 25 third-party-sold listings are not eligible.

Every "Check price" link below is an affiliate link: if you buy, we earn a commission at no extra cost to you. It does not change which units are here — the picks are chosen on the certified data and the listing text, and no brand paid for a place.

1. 

   Whole house on natural gas: the certified 4.4 GPM

   Rheem RTG-84XLN-3 (outdoor)

   $964.10 · 4.4 GPM certified at a 67°F rise (box says 8.4) · UEF 0.82 · 180,000 BTU/hr · sold by Amazon.com

   The largest certified flow among the units Amazon itself sells: 4.4 GPM at the 67°F test rise (only the third-party Rinnai certifies more, at 4.6), 180,000 BTU/hr, UEF 0.82, on the AHRI directory. Outdoor mount, so no vent kit and no vent-material bill. The "8.4 GPM" in the title is Rheem's own figure at a 35°F rise, which it states in the bullets; a northern winter is a 70°F rise, and the retailer copy of Rheem's sheet gives 4.3 GPM there.

   Outdoor only; freeze protection is electric, so it needs power in winter. The indoor natural-gas version (RTG-70DVLN-3, 4.0 GPM certified) was on a ship-lead time on the build date. Low stock on 2026-09-19.

   Check price on Amazon

2. 

   Whole house on propane, indoors: direct vent

   Rheem RTG-70DVLP-3 (indoor)

   $785.13 · 4.0 GPM certified at a 67°F rise (box says 7) · UEF 0.82 · 160,000 BTU/hr · sold by Amazon.com

   4.0 GPM certified, 160,000 BTU/hr, UEF 0.82, direct-vent for an indoor wall, 15-year heat-exchanger warranty stated, sold by Amazon. The same certified capacity as its natural-gas sibling.

   Direct vent means a listed vent kit and a wall penetration; budget for the vent kit Rheem's manual lists (this page read Rinnai's manual, not Rheem's). Its safety bullet still says "Natural Gas" although this is the propane SKU. Low stock on 2026-09-19.

   Check price on Amazon

3. 

   Whole house on propane, outdoors, in stock

   Rheem RTG-70XLP-3 (outdoor)

   $770.17 · 4.0 GPM certified at a 67°F rise (box says 7) · UEF 0.82 · 160,000 BTU/hr · sold by Amazon.com

   The same 4.0 GPM certified unit as above without the vent, in stock and sold by Amazon on 2026-09-19. The one to buy if the wall you have is an outside wall.

   Same 7.0-at-35°F headline. The listing states built-in freeze protection, which is electric, so the unit needs power all winter, and a sheltered spot.

   Check price on Amazon

4. 

   Whole house on electricity, in a mild climate with a 200 A panel

   Rheem RTEX-24

   $549.00 · 2.4 GPM certified at a 67°F rise (box says 5.9) · UEF 0.93 · 24 kW, 100 A at 240 V · sold by Amazon.com

   The most capable electric unit on the directory in this set: 2.4 GPM certified at the 67°F rise, 4.7 GPM at a 35°F rise by the arithmetic, UEF 0.93, sold by Amazon. Its listing states the wiring honestly: three 40-amp double-pole breakers and a recommended minimum 200-amp service, and the retailer copies of Rheem's sheet say 100 A draw.

   The box says "up to 5.9 GPM"; that is a 28°F rise by the arithmetic. In a northern winter (a 70°F rise) it is 2.3 GPM, one shower. If your panel is 100 or 150 A this unit does not fit, whatever the listing implies; that is the complaint the electricians in the harvested threads make.

   Check price on Amazon

5. 

   One fixture on electricity

   Rheem RTEX-11

   $249.00 · 1.2 GPM certified at a 67°F rise (box says 2.68) · UEF 0.92 · 11 kW, 46 A at 240 V · sold by Amazon.com

   1.2 GPM certified, UEF 0.92, one 50-amp double-pole breaker per the listing, $249, sold by Amazon. A shower in the South or a sink anywhere; the certified figure is the honest size.

   "Up to 2.68 GPM" on the box is a 28°F rise by the arithmetic. 11 kW at 240 V is 46 A; the listing's single 50 A breaker is 109% of that; Stiebel sizes its own units at 100% of load and footnotes that it treats a tankless heater as a non-continuous load. An electrician applying the 125% continuous-load margin would want 60 A.

   Check price on Amazon

6. 

   Under a sink: point of use

   Rheem RTEX-06T

   $189.00 · 0.5 GPM certified at a 67°F rise · UEF 0.94 · 6.5 kW, 27 A at 240 V · sold by Amazon.com

   0.5 GPM certified, UEF 0.94, thermostatic, $189, sold by Amazon. A hand-washing sink at the far end of the house, not a shower.

   The listing states no GPM and no breaker size at all; 6.5 kW at 240 V is 27 A, so plan a 30 or 40 A double-pole circuit. The RTEX-04T ($186, 0.3 GPM) is the 120 V alternative for a single lavatory, hardwired on its own 30 A single-pole breaker.

   Check price on Amazon

The two Amazon-sold directory units not picked: Rheem RTEX-08 ($239, 0.7 GPM certified: the RTEX-11 certifies 1.2 for $10 more); Rheem RTEX-04T ($186, 0.3 GPM, 120 V: the RTEX-06T does 0.5 for $3.49 more, and the 04T is named in the point-of-use pick as the 120 V alternative). Fails the rule: Rheem RTG-70DVLN-3 (indoor natural gas, 4.0 GPM certified, on a ship-lead time on 2026-09-19). Not eligible: Rinnai RE199 (4.6 GPM certified, the most in the set, sold by eFaucets), Rheem RTEX-13 and RTEX-18 (sold by Miami Water Heater), Stiebel Eltron Tempra 24 (not in the AHRI corpus and prints no GPM at all), and the 27 units on no directory, 11 of them indoor or outdoor gas units that also state no safety listing. Amazon prices on 2026-09-19; they change. Affiliate links; we earn a commission at no cost to you.

Three phrases on the listings do most of the selling. Each has a document or a formula behind it; here is what it says.

"8.4 GPM" (Rheem RTG-84)

4.4 certifiedthe listing states the 35°F rise; the directory certifies at 67°F ^[1]

"up to 5.9 GPM" (Rheem RTEX-24)

2.4 certifieda 28°F rise by the arithmetic; the listing does not state one ^[1]

"No venting required" (the electric listings)

60–120 Atrue, and the wiring is the vent: three 40 A breakers and a 200 A service for 24 kW ^[18]

"Whole house"

4.6 GPM maxthe largest certified flow in the set at 67°F; two winter showers is about 4 ^[1]

• Maker "8.4 GPM at 35°F temperature rise provides 180,000 BTU/hr output", from the Rheem RTG-84 listing. Honest as far as it goes; the retailer copy of Rheem's own sheet continues the table down to 4.5 GPM at a 67°F rise. ^[22]
• Maker "24kW model provides up to 5.9 GPM", from the RTEX-24 listing, beside "three 40-amp double-pole breakers". The second half is the useful half. ^[1]
• Testers "Whole-house tankless water heaters are designed to produce a target amount of hot water—typically 3 to 4 gallons a minute", and two electric units in the test could not hold 120°F from a 58°F inlet. ^[13]
• Program ENERGY STAR asks 2.9 GPM at a 67°F rise and UEF 0.87 of a gas tankless unit; 0 of the 39 listings claim the mark, which is correct. ^[8]

We took the top results for six Amazon searches, removed duplicates, and looked each brand and model up in the 993 Instantaneous rows of the AHRI directory held in the site corpus. The table is what the listing claims beside what the directory certifies, with the physics of the box's own power figure beside both, and the verbatim fragment behind every hand-read cell.

AHRI listed

31%12 of 39 products

Wi-Fi

15%6 of 39 products

• 12 of 39 are on the AHRI directory (Rheem, Rinnai); 27 are not, 17 of them gas or propane units.
• 11 of the 11 listed units that print a flow print 1.7 to 2.8 times the certified figure (median 2.1). 8 listings state a rise for their figure; 22 print a maximum or a bare number; the rest print no flow.
• Against the box's own power figure at the 67°F test rise, the printed flow runs 1.4 to 2.4 times the arithmetic (median 1.8) across the 29 listings that print both.
• 13 of the 17 electric units state an amperage or breaker; 4 of 39 state a minimum flow; 8 state a UEF; 0 claim ENERGY STAR.
• 24 of 39 state no safety listing; 9 state CSA, 5 ETL, 1 UL. 11 indoor or outdoor gas units state neither a safety mark nor appear on the directory.
• 14 of 39 are sold by Amazon itself, 25 by third parties; 6 were on a lead time, scarce or unavailable on 2026-09-19. Prices run $55.98 to $1,280.79.

Certified figures are the AHRI directory's. GPM on the box = the figure the listing leads with; "rise on the box" = the condition it attaches, if any. Box ÷ certified = the ratio. GPM at 67°F (arithmetic) = the box's own power ÷ (500 × 67). Amps at 240 V = kW × 1000 ÷ 240. Electric units are taken at 100% conversion, unlisted gas units at 82%. Verbatim fragments, venting, warranty, minimum flow and the 35°F and 70°F arithmetic are in the CSV download. Prices, sellers and stock are Amazon's on the build date.

The test procedure and the standards it feeds, DOE and ENERGY STAR, AHRI, the field study and a Building America profile, Consumer Reports, three editorial guides with named authors, one trade article, four maker spec sheets, one maker's electrical chart and one install manual, two utility rebates, four recall notices and the site's own installed-cost and recall records, in one table. The sizing stance each takes is plotted in the findings above.

Harvested from the subreddits where people ask a plumber, an electrician or each other whether to go tankless and what went wrong, then filtered to the comments that themselves name a tankless or on-demand heater or a tankless brand. The shares below are of those comments, not of the whole harvest. Every quote is verbatim and links to the comment.

Reddit search across 8 communities for 14 queries, then each thread's top comments read, up to the harvest's thread cap; the threads that made the cut came from r/HomeImprovement, r/Plumbing. 370 threads and 4,720 comments were harvested, of which 1,126 tankless-naming comments are the denominator here. Themes are keyword matches over that set, each one's quote being a high-scoring comment that carries both the tankless term and the theme term, verbatim and linked. Harvested 2026-09-19.

• Mary H.J. Farrell writer, Consumer Reports
  Whole-house tankless water heaters are designed to produce a target amount of hot water—typically 3 to 4 gallons a minute. Consumer Reports, "Tankless Water Heaters vs. Storage Tank Water Heaters" · 2019-01-25 ↗
• John Banta engineer, Consumer Reports (quoted within Mary H.J. Farrell's article)
  Electric models actually run more efficiently, but the higher cost of electricity makes them more costly to operate. Consumer Reports, "Tankless Water Heaters vs. Storage Tank Water Heaters" · 2019-01-25 ↗
• Geno Caccia third-generation plumber (quoted within Mary H.J. Farrell's Consumer Reports article)
  When you make a change like that, the house operates a little differently. Consumer Reports, "Tankless Water Heaters vs. Storage Tank Water Heaters" · 2019-01-25 ↗
• Timothy Dale writer, Bob Vila
  500 x GPM x Temperature Rise = Required Power Input in BTU per Hour Bob Vila, "The Best Tankless Water Heaters of 2026" · 2025-12-30 ↗
• Mary Henn writer, Family Handyman (updated by Emily Way)
  Generating that much heat requires a significant amount of space in the electrical breaker box—you'll need to dedicate four 40-amp double-pole breakers to this heater. Family Handyman, "7 Best Water Heater Brands of 2026, Our Expert Picks & Reviews" · 2026-03-13 ↗
• Alexis Carthan writer, This Old House
  Gas-powered tankless water heaters require proper ventilation to operate effectively. This Old House, "The 5 Best Tankless Water Heaters (2026)" · 2026-06-15 ↗
• Dave Bohac researcher, Center for Energy and Environment (lead author, with Ben Schoenbauer and Martha Hewett of CEE, Mary Sue Lobenstein of Lobenstein Consulting, and Tom Butcher of Brookhaven National Laboratory)
  A 37% savings of water heating energy per household was found for replacing a typical natural draft storage water heater with a tankless one. Center for Energy and Environment, "Actual Savings and Performance of Natural Gas Tankless Water Heaters" (prepared for the Minnesota Office of Energy Security) · 2010-08-30 ↗
• Liam Cope founder, Engineer Fix (electrical and mechanical engineer)
  three dedicated 40-amp double-pole circuit breakers in the main electrical panel Engineer Fix, "EcoSmart ECO 27: Sizing, Electrical, and Maintenance" · 2026 ↗

Cited without a named author: the DOE energy.gov consumer page and Building America profile, the eCFR text of 10 CFR 430 Appendix E / 430.32(d) / 429.12(a), all three ENERGY STAR pages, the AHRI RWH certification page, Rheem (RTEX-24, RTG-84), Stiebel Eltron (Tempra), Rinnai (V65i spec sheet and Value Series install manual), EcoSmart's own electrical-requirements page, PSE&G's and SoCalGas's rebate pages, and all four CPSC recall notices; their pages are in the sources list. Angi's tankless-cost and installation articles were checked for a byline (fetch returned HTTP 403; no author name surfaced in search results either), so no Angi byline is listed.

What size tankless water heater do I need?

Size on the certified flow at your winter rise. Find your February inlet temperature (the Minnesota field study's mains ran 37°F to 72°F across the year; yours is on your utility's site or a thermometer under the cold tap), subtract it from about 105°F, and read the unit's flow at that rise from the maker's chart, or divide its delivered BTU/hr by 500 times the rise. Two showers at once is about 4 GPM; in this set only the gas units certify that at the 67°F test rise (4.0 to 4.6), and the largest electric certifies 2.4. The household rules of thumb ("8 to 9 GPM for four people") assume a summer rise no unit here delivers in a northern winter. ^[5][14][1]

Why does the box say 8.4 GPM when the certified number is 4.4?

Because the box figure is at a 35°F rise, which Rheem states in the bullets, and the certified figure is at the DOE test's 67°F rise (58°F in, 125°F out). Flow times rise is fixed by the burner, so 8.4 × 35 and 4.4 × 67 are the same amount of heat. Of the 11 listed units that print a flow, all 11 print 1.7 to 2.8 times the certified figure; 8 of the 30 listings with a flow state the rise at all. ^[5][1][22]

Can an electric tankless water heater run a whole house?

In a mild climate with the panel for it. A 24 kW unit certifies 2.4 GPM at the 67°F test rise and does about 4.7 at a 35°F rise; it draws 100 A on three 40 A double-pole breakers and Rheem's listing recommends a minimum 200 A service (Stiebel's Tempra 24: two 50 A breakers, 150 A minimum; EcoSmart's ECO 27: 112.5 A). In a northern winter it is one shower. Consumer Reports watched two electric units fail to hold 120°F from a 58°F inlet, and its engineer notes they cost more to run because of the kilowatt-hour. A heat-pump water heater is the electric answer in a cold climate. ^[18][17][13]

How much electricity does a tankless water heater use, in amps?

Kilowatts × 1000 ÷ 240 V: 8 kW is 33 A, 11 kW 46 A, 14 kW 58 A, 18 kW 75 A, 24 kW 100 A, 27 kW 112.5 A. The makers size the breakers differently around that draw: Rheem puts 24 kW on three 40 A breakers (120 A), Stiebel puts its 24 kW on two 50 A (100 A) and footnotes that it treats a tankless heater as a non-continuous load, and an electrician applying a 125% continuous-load margin would want more than either. 13 of the 17 electric listings state a figure; the WINTEMP 18 kW listing states its own 75 A draw and specifies 80 A of breaker. ^[19][1]

Does a tankless water heater save money?

Energy, yes; money, slowly. The one independent field study measured 37% less water-heating energy than natural-draft storage tanks in ten Minnesota homes over two years, about 6,000 kBTU a year, and a simple payback of 20 to 40 years at 2010 installed costs. DOE's page says 24 to 34% for a home using 41 gallons a day or less and 8 to 14% at 86. Consumer Reports puts it at $195 against $245 a year. The install is where the money goes: $1,400 to $4,000 to replace a tankless, $2,100 to $8,000 to convert from a tank. ^{[12][3][13][30]}

What is the difference between ENERGY STAR, AHRI and a CSA or UL mark?

Three different things. A CSA, ETL or UL mark is a safety listing (to ANSI Z21.10.3 for gas units): the appliance was tested not to burn the house down. An AHRI listing is a voluntary third-party verification of the efficiency (UEF) and maximum flow the maker certified to DOE, which every maker must do before selling. ENERGY STAR is a higher voluntary bar on top: UEF 0.87 and 2.9 GPM at 67°F for gas tankless, with electric resistance units outside the program. 24 of the 39 listings state no safety mark, 27 are on no directory, and 0 claim ENERGY STAR. ^{[7][11][8][1]}

Are the unlisted gas tankless heaters on Amazon safe?

The listings do not let you tell. 11 of the 39 bestsellers are indoor or outdoor gas units (the camp showers aside) that state no UL, ETL or CSA listing and appear on no directory: APUS, NOVIEEX, Ranein, VEVOR, Westinghouse. Some of the other unlisted units state a CSA mark, which is the safety half. Three of the four tankless recalls, including a 2025 recall of 36,700 VST-brand units after cracked exhaust ducts, were carbon-monoxide hazards. An indoor gas unit with no stated safety listing is a purchase we would not make, and a CO alarm in the room is the minimum with any of them. ^[1][28][8]

What is the cold-water sandwich, and does a tankless heater run out of hot water?

It does not run out; it runs slow. A tankless unit fires when flow passes a threshold (0.4 GPM on Rinnai's V65i sheet, 0.5 on Stiebel's Tempra) and heats only what passes through, so a trickle stays cold, a second fixture halves the rise, and the slug of cooled water sitting in the pipe between uses arrives as a cold sandwich. The Minnesota study measured 32 draws a day averaging 1.2 gallons and 54 seconds, against the federal test's 3.5-minute draws. A recirculation loop or a small buffer tank fixes the wait and costs efficiency: the buffered unit in the field study saved 12 to 21% against 22 to 53% for the rest. ^[12][21][17]

Do I need a bigger gas line for a tankless water heater?

Usually, when converting from a tank. A 199,000 BTU/hr unit on a 1/2-inch line will starve; Rinnai's manual works a 245,000 BTU/hr example on 3/4-inch pipe at 10 feet and gives the full pipe-size table; its non-condensing units may not be vented in PVC, CPVC, ABS or galvanized, and every unit must use the vent components its own manual lists. That work is the gap between the $1,400–$4,000 replacement range and the $2,100–$8,000 conversion range. ^[23][30]

Is a tankless water heater worth it?

For endless hot water, a utility closet, or a gas house replacing a gas tankless, yes. For the gas bill, no: the measured saving is about 37% of a bill that is already small, against a conversion that costs thousands, and the field study's payback is 20 to 40 years. For an all-electric house in the North, no: the certified flow at a winter rise is one shower and the running cost is the highest of any option here; a heat-pump water heater is the answer. ^[12][13][2]

• Best heat-pump water heaters — the electric answer in a cold climate
• Every certified water heater — the AHRI corpus, sortable by UEF, flow and fuel
• Best dehumidifier — the same decode method on the ENERGY STAR list
• Best smart thermostat — the same method on the connected-thermostat list
• Operating cost calculator — what the run time costs at your rates

1. 1

   Decode of 39 Amazon tankless water heater bestsellers against the AHRI directory (this page's table)

   Verified HVAC Data · verifiedhvacdata.com · 2026

2. 2

   AHRI Directory of Certified Product Performance, residential water heaters (Instantaneous), as pulled into the site corpus

   Air-Conditioning, Heating, and Refrigeration Institute · ahridirectory.org · 2026

3. 3

   Tankless or Demand-Type Water Heaters

   U.S. Department of Energy · energy.gov/energysaver · 2026

4. 4

   Tankless Gas Water Heater Performance (Building America Top Innovations Hall of Fame Profile, citing NREL 2006 and Davis Energy Group 2007)

   U.S. Department of Energy Building America / National Renewable Energy Laboratory · energy.gov/cmei (PNNL-SA-90582) · 2013

5. 5

   10 CFR Part 430, Subpart B, Appendix E — Uniform Test Method for Measuring the Energy Consumption of Water Heaters

   U.S. Department of Energy / eCFR (Cornell LII mirror) · law.cornell.edu (eCFR) · 2026

6. 6

   10 CFR § 430.32(d) — Energy and water conservation standards and their compliance dates: Water heaters

   U.S. Department of Energy / eCFR (Cornell LII mirror) · law.cornell.edu (eCFR) · 2026

7. 7

   10 CFR § 429.12(a) — Certification of compliance

   U.S. Department of Energy / eCFR (Cornell LII mirror) · law.cornell.edu (eCFR) · 2026

8. 8

   ENERGY STAR Program Requirements Product Specification for Residential Water Heaters, Eligibility Criteria (Version 3.3)

   U.S. EPA / ENERGY STAR · energystar.gov · 2015

9. 9

   Water Heaters Key Product Criteria

   U.S. EPA / ENERGY STAR · energystar.gov · 2026

10. 10

    Whole Home Tankless Gas Water Heaters

    U.S. EPA / ENERGY STAR · energystar.gov · 2026

11. 11

    Residential Water Heaters (RWH) certification program

    Air-Conditioning, Heating, and Refrigeration Institute · ahrinet.org · 2026

12. 12

    Actual Savings and Performance of Natural Gas Tankless Water Heaters

    Dave Bohac, Ben Schoenbauer, Martha Hewett (Center for Energy and Environment), Mary Sue Lobenstein (Lobenstein Consulting), Tom Butcher (Brookhaven National Laboratory) · Minnesota Office of Energy Security / Center for Energy and Environment · 2010

13. 13

    Tankless Water Heaters vs. Storage Tank Water Heaters

    Mary H.J. Farrell · Consumer Reports · 2019

14. 14

    The Best Tankless Water Heaters of 2026

    Timothy Dale · Bob Vila · 2025

15. 15

    7 Best Water Heater Brands of 2026, Our Expert Picks & Reviews

    Mary Henn (updated by Emily Way) · Family Handyman · 2026

16. 16

    The 5 Best Tankless Water Heaters (2026)

    Alexis Carthan · This Old House · 2026

17. 17

    Tempra & Tempra Plus Technical Specifications

    Stiebel Eltron · stiebel-eltron-usa.com · 2026

18. 18

    Rheem Professional Classic RTEX Tankless Electric Water Heater spec sheet (TKE-101)

    Rheem Manufacturing Company · files.myrheem.com · 2024

19. 19

    ECO 27 Electrical Requirements

    EcoSmart · ecosmartus.com · 2026

20. 20

    EcoSmart ECO 27: Sizing, Electrical, and Maintenance

    Liam Cope · Engineer Fix · 2026

21. 21

    V65i (VC2025FFU-US) Spec Sheet and Flow Table

    Rinnai America Corporation · media.rinnai.us · 2018

22. 22

    Rheem RTG Series High Efficiency Tankless Gas Water Heaters spec sheet (TK-RTG SERIES)

    Rheem Manufacturing Company · media.rheem.com · 2023

23. 23

    Direct Vent Tankless Water Heater Installation and Operation Manual (Value Series V65i to V94e)

    Rinnai America Corporation · media.rinnai.us · 2019

24. 24

    Tankless Water Heaters rebate program

    PSE&G (Public Service Enterprise Group) · myhomeenergy.pseg.com · 2026

25. 25

    ENERGY STAR Certified Natural Gas Water Heaters rebate flyer

    Southern California Gas Company · socalgas.com (hosted PDF) · 2023

26. 26

    Tankless Water Heaters Recalled for Repair Due to Carbon Monoxide Poisoning Hazard

    U.S. Consumer Product Safety Commission · cpsc.gov · 2007

27. 27

    Navien Recalls Tankless Water Heaters and Boilers Due to Risk of Carbon Monoxide Poisoning

    U.S. Consumer Product Safety Commission · cpsc.gov · 2018

28. 28

    VESTA.DS Recalls VST Brand Tankless Water Heaters Due to Carbon Monoxide (CO) Poisoning Hazard and Risk of Serious Injury and Death

    U.S. Consumer Product Safety Commission · cpsc.gov · 2025

29. 29

    The Home Appliances Recalls Camplux Brand Portable Tankless Water Heaters Due to Fire Hazard

    U.S. Consumer Product Safety Commission · cpsc.gov · 2024

30. 30

    installed_prices_water_heaters.json (VHD internal installed-cost compilation)

    VerifiedHVACData research corpus · data/installed_prices_water_heaters.json · 2026

31. 31

    water_heater_cpsc.json (VHD internal SaferProducts.gov recall pull, brand-matched to corpus)

    VerifiedHVACData research corpus · data/water_heater_cpsc.json · 2026