Best Gas Tankless Water Heaters (2026): 6 Picks Compared

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Choosing the best gas tankless water heater is mostly about installation, not the box. Natural gas or propane, a condensing or non-condensing heat exchanger, direct-vent or power-vent or outdoor mounting, whether your gas line can feed a burner rated near 200,000 BTU, and, in Colorado, whether the unit is rated to run at your elevation: those decide whether a heater works in your house. This page compares gas-fired whole-home units only (electric models are in our main tankless roundup) and flags what each listing says, and does not say, about those points.

Part of our Plumbing & Water guide — start there for the full picture →

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How we compared: we compared 14 current Amazon listings for gas tankless water heaters on what each listing states: fuel type, BTU input, flow rate and the temperature rise it is quoted at, uniform energy factor (UEF), condensing design, venting method, stated altitude limits, safety certification and warranty. Four listings say they are not recommended above 2,000 or 3,000 ft, so we left them out for Front Range buyers. We did not lab-test or install any heater. Prices and availability change often, so check the current listing before buying.

Quick picks: the best gas tankless water heaters by use case

Product	Best for	Key features	Price	Buy
Rheem RTGH-84DVLN	Best condensing indoor pick	Condensing, 0.93 UEF 157,000 BTU, 8.4 GPM at 35 F rise Direct vent, ultra-low NOx	$$$$	Buy now →Check current price
Rheem RTG-70DVLN	Best non-condensing indoor pick	Direct vent indoor install 160,000 BTU, 7.0 GPM at 35 F rise LeakGuard leak detection	$$$$	Buy now →Check current price
Rinnai RX199iN	Best for propane or dual fuel	Natural gas or propane, one model Condensing, 0.98 UEF, 11.1 GPM Indoor or outdoor venting	$$$$	Buy now →Check current price
Rheem RTG-70XLN Outdoor	Best outdoor, no vent pipe	Outdoor mount, no venting 160,000 BTU, 7.0 GPM 15-yr heat exchanger warranty	$$$$	Buy now →Check current price
MIZUDO FDG-CS120AW	Best mid-size with app control	120,000 BTU, 5.1 GPM Indoor, outdoor kit available Wi-Fi app monitoring	$$$	Buy now →Check current price
ThermoMate CZ422NG	Best compact power-vent unit	113,000 BTU, up to 5 GPM 3 in vent, 1/2 in gas inlet stated 120V cord, 22 lb	$$$	Buy now →Check current price

Price key: $ under $50 · $$ $50–150 · $$$ $150–500 · $$$$ $500+. Prices change often; check Amazon for today’s price. As an Amazon Associate we earn from qualifying purchases.

The 6 picks, compared listing by listing

1. Rheem RTGH-84DVLN-3 — Best Condensing Indoor Pick

Best condensing indoor pick

Why we like it

• Condensing heat exchanger with 0.93 UEF
• Direct vent for indoor closets and basements
• Ultra-low NOx compliant per listing
• 15-year heat exchanger, 5-year parts warranty

Rheem RTGH-84DVLN-3 — NG, condensing, 157k BTU, 8.4 GPM @ 35 F rise, 0.93 UEF, direct vent, ultra-low NOx, 15/5/1-yr warranty

The Most Efficient Natural-Gas-Only Unit on the Shelf

Seven of the 14 listings state a UEF. Five sit at 0.81 or 0.82, which is typical for non-condensing units that send hot exhaust straight out the vent. Two are condensing: the Rinnai at 0.98 and this Rheem at 0.93. A condensing unit runs the exhaust through a second heat exchanger to pull out more heat, which lowers flue temperatures, typically allows plastic vent pipe, and produces acidic condensate that needs a drain. For an indoor natural-gas install where you want the efficiency gain without a dual-fuel premium, this is the pick.

Be clear on what 0.93 means. ENERGY STAR’s criteria for gas-fired instantaneous water heaters require a UEF of at least 0.95, a maximum flow of at least 2.8 GPM over a 67 F rise, and warranties of at least 6 years on the heat exchanger and 5 on parts. This unit clears the warranty and flow bars but falls just short on UEF, and the listing does not claim ENERGY STAR certification. If a utility rebate depends on that label, confirm before you buy.

It is the heaviest unit here at about 60 lb, and the listing does not state the vent material, the vent kit, the gas connection size or any altitude rating. Those come from the installation manual, and your installer should read it before quoting.

Check Rheem RTGH-84DVLN price on Amazon

2. Rheem RTG-70DVLN-3 — Best Non-Condensing Indoor Pick

Best non-condensing indoor pick

Why we like it

• Indoor direct-vent install stated
• LeakGuard internal leak detection
• 160,000 BTU for two to three showers
• Freeze and flame-failure protection

Rheem RTG-70DVLN-3 — NG, non-condensing, 160k BTU, 7.0 GPM @ 35 F rise, 0.82 UEF, indoor direct vent, LeakGuard, CSA

The Only Indoor Pick With Built-In Leak Detection

A tankless heater mounted in a finished basement or a closet above a hardwood floor is a water-damage risk if a fitting or heat exchanger weeps. This listing is the only one on the shelf that describes internal leak detection: Rheem’s LeakGuard, which it says detects large or small internal leaks. That is a meaningful feature for the indoor installs most Front Range homes need, since outdoor mounting is a poor fit for many cold-climate houses.

As a non-condensing unit at 0.82 UEF, it is less efficient than the condensing picks, but it is also simpler: no condensate drain and no condensate neutralizer. The trade-off is that non-condensing exhaust runs hot, so venting is typically stainless or other high-temperature category vent, which can cost more to run than plastic.

Gaps to note: this listing gives only the one-year labor term and does not repeat the heat exchanger and parts warranty that Rheem’s sibling listings state. It does not give the gas inlet size, vent diameter or an altitude rating.

Check Rheem RTG-70DVLN price on Amazon

3. Rinnai RX199iN — Best for Propane or Dual Fuel

Best for propane or dual fuel

Why we like it

• One model runs on natural gas or propane
• Highest stated UEF on the shelf at 0.98
• Indoor or outdoor via Versa Vent
• Quick Flush feature simplifies descaling

Rinnai RX199iN — NG or LP, condensing, 199k BTU, 11.1 GPM, 0.98 UEF, Versa Vent indoor/outdoor, 15/5/5-yr warranty

The Only Unit That Switches Between Natural Gas and Propane

Every other listing on the shelf is built for one fuel. The Rinnai’s Smart Sense gas control lets the same model run on natural gas or propane, which matters for rural Elbert or El Paso County homes on propane today that may get a gas main later, and for buyers who are not sure which fuel the next house will have. It is the top choice in our parent roundup; here it earns its spot on fuel flexibility rather than as the default pick.

It is also the most efficient listing, at 0.98 UEF, and the only one whose stated UEF and warranty both clear ENERGY STAR’s thresholds for gas-fired instantaneous heaters. The listing itself does not state ENERGY STAR certification, so check the product finder for the exact model. Rinnai’s listing is unusually frank about installation, recommending a qualified technician and pointing buyers to a groundwater temperature map for sizing.

At 199,000 BTU it has the largest burner of our picks, so it is also the most likely to need a gas line upgrade. The labor warranty drops to one year if the unit is not registered within 90 days. No altitude rating is stated.

Check Rinnai RX199iN price on Amazon

4. Rheem RTG-70XLN-3 Outdoor — Best Outdoor Install With No Vent Pipe

Best outdoor, no vent pipe

Why we like it

• Outdoor mount eliminates indoor venting
• Durable metal cabinet for exterior walls
• 15-year heat exchanger, 5-year parts warranty
• Built-in freeze protection

Rheem RTG-70XLN-3 — NG outdoor, 160k BTU, 7.0 GPM @ 35 F rise, 0.82 UEF, no vent pipe, freeze protection, 15/5-yr warranty

When There Is No Good Path for a Vent

Some houses make venting hard: a mechanical room in the middle of a slab-on-grade ranch, a finished basement with no clear route out, or a vent termination that would land under a deck or next to a window. An outdoor unit avoids the vent run entirely, since it exhausts directly from its cabinet. Two of the four Rheem listings on the shelf are outdoor models; this 7.0 GPM version is lighter than most picks at about 39 lb and carries the full 15-year heat exchanger and 5-year parts terms in its listing.

Colorado is the catch. The unit has freeze protection, but that protection needs electrical power, and the water and gas lines running to it still need insulation and heat tape. A long power outage during a cold snap can freeze an outdoor unit. Wind can also be an issue on exposed Front Range walls.

The listing does not state an altitude rating, the minimum operating temperature, or the gas connection size. Clearances to windows, doors and the property line still apply to the exhaust outlet.

Check Rheem RTG-70XLN Outdoor price on Amazon

5. MIZUDO FDG-CS120AW-NG — Best Mid-Size With App Control

Best mid-size with app control

Why we like it

• Moderate 120,000 BTU burner eases gas-line demands
• Indoor unit convertible to outdoor with a kit
• App shows status and maintenance alerts
• Exhaust blockage and flame failure protection

MIZUDO FDG-CS120AW-NG — NG, 120k BTU, 5.1 GPM, indoor/outdoor-kit, Wi-Fi app, CSA, copper HX, stainless chamber

A Smaller Burner That Is Easier to Feed

Big tankless burners need big gas supply. At 120,000 BTU, this MIZUDO asks for well under the 157,000 to 199,000 BTU of the Rheem and Rinnai picks, which can make the difference between using an existing gas line and paying to run a larger one. In a two-bathroom house where only one shower and a sink typically run at once, a 5 GPM class unit can be enough. The listing also notes freeze protection and exhaust blockage detection, and an app that sends maintenance notifications.

Note that MIZUDO sells two units on this shelf. The larger 199,000 BTU model states it is not recommended above 3,000 ft, which rules it out for most of the Front Range. This 120,000 BTU listing makes no altitude statement at all, so confirm with the manufacturer that it can be set up for your elevation before buying.

The listing gives no UEF, no temperature rise for its 5.1 GPM figure and no warranty term in its feature bullets, which makes it harder to compare than the Rheem and Rinnai units.

Check MIZUDO FDG-CS120AW price on Amazon

6. ThermoMate CZ422NG — Best Compact Power-Vent Unit

Best compact power-vent unit

Why we like it

• Lists vent, gas and water connection sizes
• Strong DC fan for powered exhaust
• Small at about 21 x 13 x 7 in, 22 lb
• Detailed diagnostic display with error codes

ThermoMate CZ422NG — NG indoor, 113k BTU, 5 GPM, 0.81 UEF, 3 in vent, 1/2 in gas, 3/4 in water, 120V, CSA, 22 lb

The Most Complete Installation Spec in Any Listing

Installers need connection sizes before they can quote, and most listings leave them out. This ThermoMate lists 3/4 in NPT water connections, a 1/2 in NPT gas inlet, a 3 in OD vent connection and a 120V power cord. It also gives its flow claim with a stated inlet and output temperature (two showers at 105 F from 65 F water), which is more honest than a bare GPM number. Its DC fan is rated at 360 Pa static pressure, which the listing ties to resisting backdraft in high wind.

A 1/2 in gas inlet does not mean a 1/2 in supply line is enough; the line size depends on the total demand on the gas system and the length of the run. That is a sizing calculation for the installer.

At 0.81 UEF it is the least efficient of our picks, and the listing does not state an altitude rating or a warranty term. It is one of the parent gas water heater guide’s picks; we include it here for buyers with a small mechanical space and modest demand.

Check ThermoMate CZ422NG price on Amazon

Natural gas vs propane, and how to read BTU and GPM

Gas tankless heaters are built for one fuel unless the listing says otherwise. Natural gas and propane burn differently and need different orifices and gas pressures, so a natural gas unit cannot simply be connected to a propane tank. Among our picks, only the Rinnai RX199iN runs on either fuel. If you are on propane, buy a propane-specific model or a dual-fuel unit, and have your propane supplier confirm the tank and regulator can deliver the heater’s full BTU demand in January, when tank pressure drops in the cold.

BTU input and GPM output are linked by temperature rise, which is where Colorado buyers often get caught. Rheem quotes its flow at a 35 F rise; Front Range groundwater in winter is much colder than that implies for a 120 F shower. A unit rated 7 GPM at a 35 F rise will deliver far less at a 70 F rise. ENERGY STAR’s criteria use a 67 F rise for that reason. To size:

• Add up the fixtures likely to run at the same time (a showerhead is typically rated 2.0 to 2.5 GPM).
• Estimate your coldest incoming water temperature, and the rise needed to reach about 120 F.
• Check the manufacturer’s flow chart at that rise, not the headline GPM.

Our water heater sizing guide walks through the math, and if you are still deciding whether to go tankless at all, see tankless vs tank water heaters.

Venting: direct vent, power vent, outdoor and condensing

Every gas water heater produces combustion gases, including carbon monoxide, and they must leave the house. The CDC advises making sure gas appliances are vented properly, with horizontal vent pipes rising slightly toward the outdoors, and having water heaters serviced by a qualified technician every year. With tankless units, the main options are:

• Direct vent. A sealed system that brings combustion air in from outside and sends exhaust out, often through a concentric pipe. Best for basements and closets because it does not draw on indoor air.
• Power vent. A fan pushes exhaust out, and combustion air comes from the room. The room needs enough air supply, and the fan needs a 120V outlet.
• Outdoor. No vent pipe, but exposure to cold and wind. Exhaust clearances to windows and doors still apply.
• Condensing. Cooler, acidic exhaust that usually allows plastic vent pipe, plus a condensate drain and often a neutralizer.

A tankless unit almost never reuses the old tank heater’s metal B-vent or a shared masonry chimney. Plan for a new vent run, and make sure there is a working CO alarm near the heater and outside sleeping areas. Our guides to direct-vent water heaters and water heater vent pipes cover what inspectors look for.

Gas line sizing and altitude derating on the Front Range

Two installation issues decide more Colorado tankless jobs than any feature on a listing.

Gas supply. The tankless picks here run 113,000 to 199,000 BTU, several times the input of a typical tank heater. ENERGY STAR advises that your installer should confirm the right GPM size, whether your existing natural gas line is sufficient, whether electricity is available, and how the combustion gases will be vented. Expect the installer to size the line from the meter based on the total load of every gas appliance and the length of the run. Undersized lines cause flame-outs and error codes when the furnace and the heater fire at the same time, and in some homes the meter or regulator itself needs an upgrade from the utility.

Altitude. Thinner air at Front Range elevations changes how a burner mixes gas and air, so gas appliances are commonly derated or set up differently at altitude, following the manufacturer’s high-altitude instructions. Four listings on this shelf state outright that they are not recommended above 2,000 or 3,000 ft, which excludes nearly every Front Range home. None of our six picks states an altitude limit in its listing, but none states a high-altitude rating either. Before you buy:

• Find the installation manual for the exact model and check its maximum elevation and any required settings or kits.
• Ask the installer how the unit’s output changes at your elevation, and size with that reduced output.
• Confirm the local permit office’s requirements; most Front Range jurisdictions require a permit and inspection for a new gas water heater.

Efficiency, ENERGY STAR and long-term upkeep

UEF is the standardized efficiency figure. Non-condensing units on the shelf sit at 0.81 to 0.82; the two condensing units reach 0.93 and 0.98. Only units at 0.95 or above, with the required flow and warranty, can carry the ENERGY STAR label for gas tankless heaters. A label matters if you want a utility rebate, so check the ENERGY STAR product finder for the exact model number rather than relying on listing wording.

Hard water is the other long-term cost. Scale builds inside tankless heat exchangers just as it does in tank heaters, which is why the Rinnai’s Quick Flush feature and service valves on any install are worth having. EPA WaterSense suggests flushing water heaters once a year to reduce sediment buildup that can decrease efficiency. Our tankless water heater maintenance guide covers descaling step by step. For electric tankless models and a wider comparison, see our parent roundup of the best tankless water heaters overall.

When to bring in a pro or an inspector

A gas tankless heater is not a DIY appliance. Gas piping, venting, combustion air and permits all need a licensed installer, and several listings here recommend or require professional installation. It is also worth an independent look when:

• You are buying a home with a tankless heater and do not know who installed it or whether it was permitted.
• The vent terminates near a window, door, deck or gas meter, or the vent run looks improvised.
• You see error codes, flame-outs or cold-water sandwiches after the install.
• There is no CO alarm near the mechanical room.

A home inspection checks the water heater’s venting, gas connection, relief valve, and the condition of the space around it. See our plumbing and water inspection guides, or get in touch to connect with a vetted Front Range inspector.

Frequently asked questions

References

• ENERGY STAR — Residential water heaters key product criteria: https://www.energystar.gov/products/water_heaters/residential_water_heaters_key_product_criteria
• ENERGY STAR — Whole home tankless gas water heaters: https://www.energystar.gov/products/whole_home_tankless_gas_water_heaters
• CDC — Carbon monoxide poisoning basics: https://www.cdc.gov/carbon-monoxide/about/index.html
• U.S. EPA WaterSense — Home maintenance: https://www.epa.gov/watersense/home-maintenance