# Best tankless gas water heater: the maker spec sheets sort them by BTU input, UEF, venting class and the gas line each manual allows, and the gas line decides before the GPM does > Maker sheets compared: Rinnai RX199iN 199,000 BTU, UEF 0.98, PVC vent, ENERGY STAR; Rheem RTG-70XLN 160,000 BTU, UEF 0.82, outdoor; Rinnai V53DeN 120,000 BTU budget. | | | |---|---| | Source | https://ownerspec.com/plumbing/reviews/best-tankless-gas-water-heater/ | | Publisher | OwnerSpec (https://ownerspec.com/) | | Published | 2026-10-09 | | Updated | 2026-10-09 | | Facts verified | 2026-10-09 | | Disclosure | Contains affiliate links. OwnerSpec may earn a commission at no cost to the reader. | --- ## Short answer For a home with an outdoor wall, a checked gas line and no room for a vent, the mid-price pick is the Rheem RTG-70XLN-3: Rheem's RTG spec sheet prints 160,000 BTU/h, 7.0 GPM at a 35 F rise, UEF 0.82, no vent run and 1/2 in gas line compatibility subject to local code. The premium pick is the Rinnai RX199iN, on ENERGY STAR's certified list with its RXP199iN pump twin: Rinnai's spec sheet 100000883 prints 199,000 BTU/h, 11.0 GPM, UEF 0.98 and Schedule 40 PVC venting to 150 ft at 3 in. The budget pick is the Rinnai V53DeN at 120,000 BTU/h and 5.3 GPM, outdoor only. Skip all three until a gas fitter has measured the line: Rinnai's RX manual carries 199,000 BTU/h on 1/2 in pipe only to 40 ft at a 3.0 in wc pressure drop, and every maker on this page requires a trained, qualified installer or the warranty may be void. ## Fact sheet - **Answer:** For an outdoor wall and a checked gas line the mid-price pick is the Rheem RTG-70XLN-3: 160,000 BTU/h, 7.0 GPM at a 35 F rise, UEF 0.82, no vent run. The premium pick is the Rinnai RX199iN, on ENERGY STAR's list with its RXP199iN pump twin: 199,000 BTU/h, UEF 0.98, PVC vent to 150 ft at 3 in. Rinnai's manual carries 199,000 BTU/h on 1/2 in pipe only to 40 ft. - **Comparison basis:** Manufacturer spec sheets and installation manuals for the Rinnai RX (100000883 and 100000839), Rinnai REi, Rinnai V53De (100000624 and 100000561), Rheem RTG (TK-RTG Rev 1 and the AP22009 use and care manual), Rheem RTGH-RH11DVLN (TK-RH110DV) and the ENERGY STAR certified water heater dataset, compared on input BTU/h, maximum GPM at the maker's stated rise, UEF, condensing or not, vent material and length, the gas line each document allows, indoor or outdoor rating and warranty years. - **Assumptions:** Gas pipe capacities are quoted at the pressure-drop basis each maker's table states, 3.0 in wc with 8.0 in wc or more supply unless the sentence says 0.5 in wc, for Schedule 40 metallic pipe on natural gas; One cubic foot of natural gas is taken as 1,000 BTU where Rinnai's worked example does, and 1,024 BTU where Rheem's manual does; Maker maximum GPM figures are quoted with the temperature rise the maker states; ENERGY STAR records a lower maximum at its own test rise, and both are printed; Local permit and code rules vary and no number for them is printed here; only the makers' qualified-installer and warranty statements are cited - **Method:** OwnerSpec compared manufacturer-published specifications read on 2026-10-09 and confirmed every listing in stock the same day; it did not install, fire or run any heater, derived the cubic-feet-per-hour figures by the makers' own stated division, and reports only the figures the cited document states. - **Primary source:** [Rinnai, RX199iN product page with the Sensei RX spec sheet 100000883 and installation manual 100000839](https://www.rinnai.us/residential/product-detail/rx199in) (manufacturer or vendor document) - **Facts verified:** 2026-10-09 - **Last updated:** 2026-10-09 - **Canonical URL (cite this):** https://ownerspec.com/plumbing/reviews/best-tankless-gas-water-heater/ If you do not know your gas meter capacity, the pipe size and length to the heater, and whether the wall it hangs on faces outdoors, no unit below can be chosen yet: the gas line and the vent decide more than the GPM on the box. ## How was this shortlist built? Every figure here is a manufacturer-published specification read on 2026-10-09 from the sheet or manual named beside it. Nothing was installed, fired or run by this site, and every listing was confirmed in stock the same day. Units were shortlisted from current Amazon listings, then kept or rejected on the maker's document: input BTU/h, maximum GPM at the rise the maker states, UEF, condensing or not, vent material and length, the gas line the manual allows, indoor or outdoor rating and warranty years. ENERGY STAR status comes from ENERGY STAR's certified dataset, not a listing bullet. Uniform energy factor (UEF) is the efficiency figure on the yellow Energy Guide label; higher is better. Cubic feet an hour (cfh) is the gas flow a burner's BTU/h input needs. Navien, Takagi, Noritz and Eccotemp are absent on purpose: no Navien unit appeared in the Amazon search, Takagi and Noritz listings were scarce, and Eccotemp's page and listing name different models. ## How do the units compare on paper? | Model | Input BTU/h (min to max) | Max GPM, maker basis | UEF | Condensing | Vent | Gas line, as the maker prints it | Indoor / outdoor | ENERGY STAR list | | --- | --- | --- | --- | --- | --- | --- | --- | --- | | Rinnai V53DeN | 15,500 to 120,000 | 5.3 | 0.81 | No | None (outdoor) | "1/2 in. gas line compatible" | Outdoor only | Not listed | | Rheem RTG-70XLN-3 | 11,000 to 160,000 | 7.0 at 35 F rise, 4.5 at 67 F | 0.82 | No | None (outdoor) | "1/2 in subject to local code"; manual: 8.0 in wc or more and Table 4 | Outdoor | Not listed | | Rheem RTG-70XLP-3 (propane) | 11,000 to 160,000 | 7.0 at 35 F rise | 0.82 | No | None (outdoor) | Manual Table 5 for propane | Outdoor | Not listed | | Rinnai RE180iN | 10,400 to 180,000 | 8.5 | 0.82 | No | Listed stainless only, 3 in / 5 in concentric; no PVC, CPVC, ABS, galvanized | Manual tables start at 3/4 in | Indoor | Not listed | | Rinnai RX199iN | 15,000 to 199,000 | 11.0 (sheet); 5.8 (ENERGY STAR) | 0.98 | Yes | Schedule 40 PVC/CPVC, 150 ft at 3 in, 75 ft at 2 in | "1/2 in. gas line compatible"; Table 22 to 40 ft | Indoor, or outdoor with RXOVC cap | Listed | | Rinnai RXP199iN | 15,000 to 199,000 | 11.1 (product page); 5.8 (ENERGY STAR) | 0.98 | Yes | As RX199iN | As RX199iN | Indoor or outdoor | Listed | | Rheem RTGH-RH11DVLN | 18,000 to 199,000 | 11.0; 8.5 at 45 F rise | 0.94 | Yes | PVC/CPVC/PP, 100 ft at 3 in, 50 ft at 2 in | "Compatible with 1/2 in gas line" | Indoor | Not listed | The 11.0 GPM on Rinnai's sheet and the 5.8 GPM on ENERGY STAR's dataset describe the same RX199iN at different temperature rises. Size by rise, not by the headline: the [tankless sizing calculator](/plumbing/calculators/tankless-water-heater-size-gpm-temperature-rise/) returns the GPM your groundwater temperature needs. ## Will your gas line carry it? Rinnai's RX manual, Table 22, sizes Schedule 40 metallic pipe on natural gas at a 3.0 in wc pressure drop with 8.0 in wc or more supply; Rheem's RTG manual Table 4 prints the same 1/2 in column. | Pipe length | 1/2 in pipe carries (cfh, 3.0 in wc drop) | 1/2 in pipe carries (cfh, 0.5 in wc drop, Rinnai Table 23) | | --- | --- | --- | | 10 ft | 454 | 172 | | 20 ft | 312 | 118 | | 30 ft | 250 | not read | | 40 ft | 214 | not read | | 50 ft | 190 | not read | | 60 ft | 172 | not read | Rinnai's worked example takes one cubic foot of gas as 1,000 BTU, so a 199,000 BTU/h unit needs 199 cfh, and places that unit at 35 ft (read as 40 ft) on 1/2 in pipe. At 50 ft the column carries 190 cfh and fails. Rheem's manual divides by 1,024 for natural gas, which puts the 160,000 BTU/h RTG-70 at about 156 cfh by that arithmetic. The pressure-drop basis is the catch. Rinnai's Table 23, at a 0.5 in wc drop, carries only 172 cfh at 10 ft on 1/2 in pipe; its 3/4 in column carries 360 at 10 ft and 170 at 40 ft. Which table applies is a code and utility question, not a brand one. The Rinnai RX manual adds that the supply must carry the entire gas load at the location, so a furnace on the same line counts. ## Rinnai V53DeN: buy it or skip it? Buy it if the house is small and the wall faces outdoors. Rinnai's spec sheet 100000624 prints 15,500 to 120,000 BTU/h, 0.4 to 5.3 GPM, UEF 0.81, 3/4 in connections, a 1/2 in gas line compatible bullet and AHRI, ANSI Z21.10.3 and CSA 4.3 certification. The flaw is the warranty: the manual prints a 10-year heat exchanger (12 years with an isolation valve kit) against 15 on every other unit here. Skip it if the only wall is indoors: the manual says "Do not install the V53De indoors". ## Rheem RTG 7.0 outdoor: buy it or skip it? Buy it if an outdoor wall exists and you want no vent to buy. Rheem's TK-RTG sheet prints 11,000 to 160,000 BTU/h, 7.0 GPM at a 35 F rise and 4.5 at 67 F, UEF 0.82, a 15-year heat exchanger warranty with 5 years on parts and 1 on labor, and N/A in the vent rows for the XLN outdoor models. The propane version is the same sheet with P in place of N, the same 160,000 BTU/h and 7.0 GPM; the manual sizes its pipe from Table 5 at 2,546 BTU per cubic foot. The flaw is the gas rule: Rheem's manual permits half-inch lines only when system pressure is 8.0 in wc or greater and Table 4 is followed. The Amazon title does not print the model number, so confirm RTG-70XLN-3 on the listing. Skip it above 4.5 GPM at a 67 F rise; the [first-hour rating calculator](/plumbing/calculators/water-heater-size-first-hour-rating/) shows what a tank delivers instead. ## Rinnai RE180iN: buy it or skip it? Buy it if you are replacing an indoor non-condensing unit that already has a listed stainless vent. Rinnai's REi spec sheet prints 10,400 to 180,000 BTU/h, 0.13 to 8.5 GPM, UEF 0.82, a 3 in / 5 in concentric vent connection, 3/4 in gas and a 15-year heat exchanger warranty with 1 year on labor. The flaw is the vent: the RE manual states "DO NOT use PVC, CPVC, ABS or galvanized material to vent this appliance" and lists Ubbink, Heat-Fab and Metal Fab as approved vent makers. Its gas sizing tables start at the 3/4 in column. Skip it for a new installation with no vent in place: a stainless vent plus a 3/4 in gas run can cost more than the price gap to the condensing unit below. ## Rinnai RX199iN: buy it or skip it? Buy it if a PVC vent path, a condensate drain and a checked gas line are available. Rinnai's spec sheet 100000883 prints 15,000 to 199,000 BTU/h, 0.13 to 11.0 GPM, UEF 0.98, Schedule 40 PVC or CPVC venting to 150 ft at 3 in or 75 ft at 2 in, 3/4 in gas and water and a 1/2 in NPT condensate drain. ENERGY STAR's dataset lists the RX199I at UEF 0.98 and 199,000 BTU/h. The warranty is 15 years or 12,000 hours on the heat exchanger, 5 years on parts and 5 years on labor only if registered within 90 days, otherwise 1 year. Outdoors it needs the RXOVC Versa-Vent cap. The flaw is the installer line: the RX manual states a trained and qualified professional must install, inspect and leak test the heater, and improper installation may void the warranty. Skip it if the line is 1/2 in and longer than 40 ft at the 3.0 in wc basis, or no condensate drain can be run. ## Rinnai RXP199iN: buy it or skip it? Buy it if the far bathroom waits for hot water and a return line or crossover valve exists. Rinnai's product page prints 15,000 to 199,000 BTU/h, 11.1 GPM, UEF 0.98 and Residential ENERGY STAR Yes, and the RX manual's model key reads "P: Pump (Internal Pump Included in Model)". ENERGY STAR's dataset lists the RXP199I at UEF 0.98. The flaw is that the pump solves a pipe-length problem a cheaper fix may already solve: read [why hot water takes too long](/plumbing/guides/hot-water-takes-too-long-recirculation-options/) and the [recirculation pump review](/plumbing/reviews/best-hot-water-recirculation-pump/) before paying for the P. The named alternative is the Rheem RTGH-RH11DVLN: its sheet prints 18,000 to 199,000 BTU/h, UEF 0.94, 11.0 GPM, a built-in pump, PVC to 100 ft at 3 in and a 12-year heat exchanger, but the sheet is a 2017 form and the unit is not on ENERGY STAR's list. **Before you buy any gas tankless heater** Three numbers decide between these picks, and none is on the box: the pipe size and length from the meter, which the tables above turn into a yes or a no; the temperature rise at your address, which the [sizing calculator](/plumbing/calculators/tankless-water-heater-size-gpm-temperature-rise/) turns into the GPM you need; and the hardness in grains per gallon, which the [descale interval calculator](/plumbing/calculators/tankless-water-heater-descale-interval-by-hardness/) turns into a flush schedule. ## Who should skip gas tankless altogether? **A 1/2 in line longer than the maker's table allows.** Above 40 ft at the 3.0 in wc basis a 199,000 BTU/h unit needs 3/4 in pipe. The [electric tankless review](/plumbing/reviews/best-electric-tankless-water-heater/) sorts the alternative by breaker slots instead. **A house where the hardness is unknown.** Rinnai's RX manual Table 3 recommends a softener or its Scale Cutter from 61 mg/L (4 gpg) with a yearly flush and requires one above 200 mg/L (12 gpg); Rheem's RTG manual lists hardness up to 200 mg/L, iron up to 0.3 mg/L and manganese up to 0.05 mg/L, and neither warranty covers water outside its chart. The [white scale guide](/water/guides/hard-water-white-scale/) shows the deposit and the [softener sizing calculator](/water/calculators/water-softener-size-calculator/) sizes the fix. **Anyone counting on a self-install.** Every manual here names a trained, qualified installer and ties the warranty to it; permit rules are local and this page prints no number for them. ## What goes wrong? **The vent is the wrong class.** PVC on a non-condensing unit is forbidden by Rinnai's RE manual and Rheem's RTG manual alike; a listed stainless vent is what turns a cheap unit into an expensive one. **The old heater was replaced on suspicion.** The [serial number date code guide](/plumbing/guides/water-heater-age-serial-number-date-code/) tells you whether it is actually old, and the [no hot water guide](/plumbing/guides/water-heater-no-hot-water-electric-gas/) whether it is actually dead. Any indoor gas appliance also earns a [CO alarm](/air/reviews/best-carbon-monoxide-detector/) nearby. ## The picks - **Rinnai V53DeN (outdoor, natural gas, 5.3 GPM)**: Small home, outdoor wall, the lowest input on the page (Rinnai spec sheet 100000624: 15,500 to 120,000 BTU/h, 0.4 to 5.3 GPM, UEF 0.81, non-condensing, outdoor only, 1/2 in gas line compatible, 3/4 in connections; manual: 10-year heat exchanger, 12 with an isolation valve kit) Amazon: https://www.amazon.com/dp/B07MH3C76M/?tag=ownerspec-20 - **Rheem RTG Series 7.0 GPM outdoor, natural gas (RTG-70XLN-3)**: Outdoor wall, wants no vent to buy, natural gas (Rheem TK-RTG sheet: 11,000 to 160,000 BTU/h, 7.0 GPM at 35 F rise and 4.5 at 67 F, UEF 0.82, non-condensing, outdoor, vent run N/A, 1/2 in gas line compatibility subject to local code, 15-year heat exchanger, 5-year parts, 1-year labor) Amazon: https://www.amazon.com/dp/B0C827FQPG/?tag=ownerspec-20 - **Rheem RTG Series 7.0 GPM outdoor, liquid propane (RTG-70XLP-3)**: Same outdoor unit for a propane home (Rheem TK-RTG sheet: the propane version carries the same 160,000 BTU/h input, 7.0 GPM at 35 F rise and UEF 0.82, model number with P in place of N; the RTG manual sizes propane pipe from its own Table 5 at 2,546 BTU per cubic foot) Amazon: https://www.amazon.com/dp/B0C828MJBK/?tag=ownerspec-20 - **Rinnai RE180iN (indoor, non-condensing, 8.5 GPM)**: Indoor replacement onto an existing listed stainless vent (Rinnai REi spec sheet: 10,400 to 180,000 BTU/h, 0.13 to 8.5 GPM, UEF 0.82, non-condensing, indoor, 3 in / 5 in concentric vent, 3/4 in gas; manual: no PVC, CPVC, ABS or galvanized vent, gas sizing tables start at 3/4 in, 15-year heat exchanger, 1-year labor) Amazon: https://www.amazon.com/dp/B0B57PQYLR/?tag=ownerspec-20 - **Rinnai RX199iN (condensing, 11.0 GPM, ENERGY STAR)**: Indoor or outdoor, PVC vent path and a condensate drain available (Rinnai spec sheet 100000883: 15,000 to 199,000 BTU/h, 0.13 to 11.0 GPM, UEF 0.98, condensing, Schedule 40 PVC/CPVC vent to 150 ft at 3 in, 1/2 in gas line compatible, 3/4 in gas and water, 1/2 in condensate; ENERGY STAR list: UEF 0.98 at a 5.8 GPM maximum; 15-year heat exchanger, 5-year labor if registered) Amazon: https://www.amazon.com/dp/B0CM4L8VKC/?tag=ownerspec-20 - **Rinnai RXP199iN (RX199iN with a built-in recirculation pump)**: Far fixture waits for hot water and a return path or crossover exists (Rinnai product page: 15,000 to 199,000 BTU/h, 11.1 GPM, UEF 0.98, indoor or outdoor, Residential ENERGY STAR Yes; RX manual model key: P means an internal pump is included; ENERGY STAR list: RXP199I UEF 0.98, 199,000 BTU/h) Amazon: https://www.amazon.com/dp/B0CNQHT7CL/?tag=ownerspec-20 ## Frequently asked questions ### Can a 199,000 BTU tankless water heater run on a 1/2 inch gas line? Only by the maker's table. Rinnai's RX manual Table 22, at a 3.0 in wc pressure drop and 8.0 in wc or more supply, carries 214 cfh on 1/2 in pipe at 40 ft and 190 at 50 ft; its worked example puts a 199,000 BTU/h unit at 35 ft on 1/2 in. At a 0.5 in wc drop the same pipe carries 172 cfh at 10 ft and fails. ### What is the difference between a condensing and a non-condensing tankless water heater? A condensing unit pulls more heat from the exhaust, so the flue gas is cool enough for plastic pipe: Rinnai's RX sheet prints UEF 0.98 and Schedule 40 PVC or CPVC venting. A non-condensing unit exhausts hot: Rheem's RTG manual allows Category III stainless steel vent only, and Rinnai's RE manual forbids PVC, CPVC, ABS and galvanized. The condensing unit also needs a condensate drain. ### Which gas tankless water heaters are ENERGY STAR certified? ENERGY STAR's criteria page for gas instantaneous water heaters requires a UEF of 0.95 or higher, at least 2.8 GPM over a 67 F rise and a heat exchanger warranty of at least 6 years. On its certified dataset the Rinnai RX199I and RXP199I are listed at UEF 0.98; the Rinnai RE180iN and V53DeN, the Rheem RTG-70XLN and the Rheem RTGH-RH11DVLN are not on that list. ### Does a gas tankless water heater need a professional install? The makers say so and tie the warranty to it. Rinnai's RX manual states that a trained and qualified professional must install, inspect and leak test the heater, and that improper installation may void the warranty; Rinnai's warranty document repeats it. Rheem's RTG manual permits 1/2 in gas only when system pressure is 8.0 in wc or more and its pipe table is followed. Permit rules are local; check yours. ### How hard can the water be before a tankless gas heater needs a softener? Rinnai's RX manual Table 3 recommends a softener or its Scale Cutter from 61 mg/L (4 gpg) with a yearly flush, and requires one above 200 mg/L (12 gpg). Rheem's RTG manual lists total hardness up to 200 mg/L in its water quality chart and does not cover failures from water outside it. Both makers list iron up to 0.3 mg/L and manganese up to 0.05 mg/L. ## Sources - [Rinnai, RX199iN product page with the Sensei RX spec sheet 100000883 and installation manual 100000839](https://www.rinnai.us/residential/product-detail/rx199in) (manufacturer or vendor document) - [Rinnai, RXP199iN product page](https://www.rinnai.us/professional/product-detail/rxp199in) (manufacturer or vendor document) - [Rinnai, RE199iN and RE180iN product page with the REi spec sheet and installation manual](https://www.rinnai.us/residential/product-detail/re199in) (manufacturer or vendor document) - [Rinnai, V53DeN product page with spec sheet 100000624 and installation manual 100000561](https://www.rinnai.us/residential/product-detail/v53den) (manufacturer or vendor document) - [Rheem, RTG Series non-condensing tankless spec sheet TK-RTG Series Rev 1 (07/23)](https://media.rheem.com/media/uploads/iat/sites/36/2023/07/TK-RTG-Series-Rev-1-Non-Condensing-Tankless-SS-Color_0719.pdf) (manufacturer or vendor document) - [Rheem, RTG Series use and care manual with installation instructions AP22009 Rev 04](https://files.rheem.com/blobazrheem/wp-content/uploads/sites/2/RTG-Use-and-Care.pdf) (manufacturer or vendor document) - [Rheem, Prestige RTGH-RH11DVLN spec sheet TK-RH110DV Rev 1 (10/17), distributor catalogue copy](https://cdn.spsmsys.com/shared/dbi/product/pdf/RTGH_RH11DVLN.MAIN.PDF) (manufacturer or vendor document) - [ENERGY STAR, residential water heaters key product criteria](https://www.energystar.gov/products/water_heaters/residential_water_heaters_key_product_criteria) (government or regulatory source) - [ENERGY STAR, certified residential water heaters dataset (data.energystar.gov pbpq-swnu)](https://data.energystar.gov/resource/pbpq-swnu.json) (government or regulatory source) --- Citation: OwnerSpec, "Best tankless gas water heater: the maker spec sheets sort them by BTU input, UEF, venting class and the gas line each manual allows, and the gas line decides before the GPM does", https://ownerspec.com/plumbing/reviews/best-tankless-gas-water-heater/, facts verified 9 October 2026. 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