Best Gas Tankless Water Heaters in the USA

This makes gas tankless systems particularly attractive for households that want high hot-water output, continuous operation, compact wall-mounted equipment and strong efficiency without maintaining a large storage tank.

This guide focuses specifically on gas tankless water heaters. It explains condensing and non-condensing designs, BTU input, GPM, temperature rise, gas-line sizing, venting, recirculation, sizing for multiple bathrooms, cold-water conditions, maintenance, installation and the most important differences between current models.

If you want to compare gas tankless systems with heat-pump, electric storage, gas storage and solar water heaters, see our Best Water Heaters in the USA guide.

Quick Answer: Best Gas Tankless Water Heaters in the USA

Our overall choice: The Navien NPE-240S2 is an excellent all-around gas tankless water heater because it combines a 199,900 BTU/h maximum input, 0.96 UEF, high flow at moderate temperature rise, dual stainless-steel heat exchangers and flexible venting.

For smart-home integration: The Rheem RTGH-95DVELN-3 combines high output with built-in EcoNet Wi-Fi and leak detection.

For a compact indoor installation: The Rinnai RU199iN is a strong option, particularly where a compact wall-mounted unit and high maximum flow are priorities.

What Is a Gas Tankless Water Heater?

A gas tankless water heater is an instantaneous water heater that uses natural gas or propane to heat water as it flows through the unit.

There is no large storage tank holding hundreds of gallons of heated water. Instead, the appliance detects water flow and modulates the burner according to the required heating load.

The basic relationship is:

Required heating power ≈ water flow × temperature rise

This relationship explains why a tankless water heater can deliver a large amount of hot water but may deliver considerably less flow when the incoming water is very cold.

ENERGY STAR currently requires certified gas-fired instantaneous water heaters to achieve at least 0.95 UEF, at least 2.8 GPM at a 67°F temperature rise, a minimum six-year heat-exchanger warranty and five-year parts warranty, and compliance with ANSI Z21.10.3/CSA 4.3.

How a Gas Tankless Water Heater Works

1. Water flow begins: A fixture is opened and water starts moving through the heater.
2. Flow sensor detects demand: The control board determines that hot-water production is required.
3. Ignition occurs: The burner starts electronically.
4. Gas valve modulates: The appliance adjusts gas input according to water flow and required temperature rise.
5. Heat exchanger transfers heat: Water passes through the heat exchanger while combustion gases transfer energy to it.
6. Controls regulate temperature: Sensors continuously monitor inlet and outlet temperatures.
7. Burner shuts down: When water demand stops, the burner turns off.

This modulation is one of the most important differences from a conventional storage water heater. The burner does not simply operate at one fixed output throughout the entire demand period.

Condensing vs Non-Condensing Gas Tankless Water Heaters

The first major decision is whether to choose a condensing or non-condensing tankless water heater.

Condensing models recover additional heat from combustion exhaust. That lowers exhaust temperature and allows the system to extract more useful energy from the fuel.

The tradeoff is that a condensing system generates acidic condensate and therefore needs appropriate drainage.

Why the 0.95 UEF Threshold Matters

ENERGY STAR’s current criteria place gas-fired instantaneous water heaters at a minimum UEF of 0.95 for certification.

This makes efficiency comparisons easier, but UEF still should not be the only specification used to choose a tankless water heater.

A heater with excellent UEF may still be a poor choice if its output is insufficient for the home’s peak demand.

For tankless systems, the following specifications must be considered together:

• Maximum BTU input
• Minimum BTU input
• GPM at a specified temperature rise
• UEF
• Minimum activation flow
• Gas-line capacity
• Venting configuration
• Recirculation capability
• Incoming-water temperature

GPM Is Not a Single Number

One of the most common mistakes when comparing tankless water heaters is looking at the largest GPM number printed in a specification table.

A tankless heater may advertise 10 or 11 GPM maximum flow, but that does not mean it can produce 10 or 11 GPM of shower-temperature water when the incoming water is cold.

Always ask:

At what temperature rise is that GPM measured?

For example, the Navien NPE-240S2 is rated at 11.2 GPM at a 35°F temperature rise, 8.7 GPM at a 45°F rise, 5.8 GPM at a 67°F rise and 3.9 GPM at a 100°F rise.

The same appliance therefore has dramatically different practical flow capacity depending on incoming-water temperature and desired outlet temperature.

Understanding Temperature Rise

Temperature rise is:

Temperature Rise = Desired Hot-Water Temperature − Incoming Cold-Water Temperature

Suppose the target water temperature is 120°F.

This is why a tankless water heater installed in a warm climate can deliver substantially more usable flow than the same model installed where groundwater is very cold.

How to Size a Gas Tankless Water Heater

Step 1: List Simultaneous Fixtures

Determine which fixtures may operate simultaneously.

Actual flow depends on the fixture and local plumbing conditions, so use measured or manufacturer-rated values whenever possible.

Step 2: Add the Peak Flows

For example, two showers at 2 GPM each plus a kitchen faucet at 1.5 GPM creates approximately:

2 + 2 + 1.5 = 5.5 GPM

The selected water heater must be able to deliver approximately 5.5 GPM at the required temperature rise.

Step 3: Determine Incoming Water Temperature

This is particularly important in cold climates.

A system that comfortably provides 6 GPM at a 60°F rise may not provide 6 GPM at an 80°F rise.

Step 4: Check the Manufacturer’s Performance Table

Do not use the headline maximum GPM. Use the GPM corresponding to your expected temperature rise.

Example Tankless Sizing Calculation

Assume a household has:

• Two showers at 2 GPM each
• One faucet at 1 GPM
• Desired hot-water temperature of 120°F
• Incoming water temperature of 50°F

Total demand:

2 + 2 + 1 = 5 GPM

Temperature rise:

120°F − 50°F = 70°F

Therefore, the water heater should be capable of approximately 5 GPM at a 70°F temperature rise.

This is very different from selecting a unit simply because its brochure says “11 GPM maximum.”

BTU Input: The Real Power Behind a Tankless Heater

BTU/h is a measure of thermal input capacity.

A larger BTU input generally allows the heater to raise more water to the desired temperature at a given flow rate.

Current large residential gas tankless systems commonly approach 200,000 BTU/h.

For example, the Navien NPE-240S2 has a maximum input of 199,900 BTU/h and a minimum input of 13,300 BTU/h.

The minimum input is also important because a heater that cannot modulate low enough may cycle more often when only a small amount of hot water is being used.

Why Turndown Ratio Matters

The turndown ratio describes how far the burner can reduce its input relative to its maximum input.

For example, a heater capable of approximately 200,000 BTU/h maximum input that can modulate down toward approximately 13,000 BTU/h has a very wide operating range.

A wide modulation range helps the appliance match small and large hot-water loads without unnecessarily operating at maximum fire.

Gas Tankless Water Heaters and Gas-Line Sizing

This is one of the most important installation issues.

A 199,000 BTU/h tankless water heater can require substantially more gas flow than a conventional 40- or 50-gallon storage water heater.

That means an existing gas line may not automatically be adequate.

Before installation, the contractor should evaluate:

• Gas meter capacity
• Total connected BTU load
• Gas-line diameter
• Gas-line length
• Natural gas or propane
• Gas pressure
• Local fuel-gas code
• Other gas appliances operating simultaneously

Some modern appliances are designed to simplify retrofit installations. For example, Navien states that selected NPE-S2 configurations can use 1/2-inch gas pipe for up to 24 feet, subject to local codes and installation requirements.

This does not mean that every installation can simply use a 1/2-inch line. The actual pipe sizing must be determined according to the installation manual and applicable fuel-gas code.

Venting Requirements

Venting is another major difference between gas tankless and electric water heaters.

Condensing Tankless Venting

Because condensing units extract more heat from combustion gases, exhaust temperatures are substantially lower than those of many non-condensing appliances.

This allows approved plastic venting systems on many condensing models.

The Navien NPE-240S2, for example, supports approved 2-inch or 3-inch PVC, CPVC, polypropylene and stainless-steel vent configurations, with maximum venting dependent on the configuration. Navien specifies up to 75 feet for its 2-inch PVC arrangement.

Non-Condensing Tankless Venting

Non-condensing units have hotter exhaust gases and therefore require vent materials rated for the applicable exhaust temperature.

This can make vent replacement more expensive in some retrofit situations.

Direct Vent vs Outdoor Installation

Gas tankless heaters are available in different installation configurations.

Outdoor installation can simplify venting but introduces freeze-protection considerations in cold climates.

Cold Climate Considerations

Gas tankless heaters are not affected by cold incoming water in exactly the same way as heat pump water heaters are affected by cold installation air, but cold groundwater still reduces hot-water flow capacity.

If groundwater falls from 60°F to 40°F, the required temperature rise to produce 120°F water increases by 20°F.

That additional temperature rise consumes more heating capacity.

This is why homes in colder regions may need a larger BTU-class tankless heater than homes with the same number of occupants in warmer regions.

See Best Water Heaters for Cold Climates for the broader comparison.

Gas Tankless Water Heaters for Large Families

Large families often benefit from tankless technology because the system is not limited by a fixed storage volume.

However, the phrase “endless hot water” can be misleading.

The supply may be effectively continuous only if the heater has enough heating capacity to maintain the desired outlet temperature at the required flow rate.

For example, four simultaneous showers can exceed the capacity of a single residential unit if the incoming water is cold enough.

In extremely high-demand applications, multiple tankless units can be cascaded.

Navien’s NPE-S2 platform supports cascading of multiple units and common venting configurations for larger applications.

See Best Water Heaters for Large Families.

Multiple Bathrooms

Multiple bathrooms are a classic tankless application, but bathroom count alone does not determine heater size.

The important question is how many fixtures can operate simultaneously.

A three-bathroom house where only one shower operates at a time may require less capacity than a two-bathroom house where two showers and a bathtub are routinely used together.

For high simultaneous demand, compare the manufacturer’s flow tables at the actual temperature rise rather than using the maximum GPM number.

See Best Water Heaters for Multiple Bathrooms.

Recirculation Systems

One common complaint about tankless water heaters is not the amount of hot water available but the time it takes for hot water to reach a distant fixture.

A recirculation system can address this problem.

There are several possible configurations:

• Dedicated return line
• On-demand recirculation
• Integrated pump
• External pump
• Thermal bypass systems

The correct configuration depends on the home’s plumbing layout and the particular water heater.

Recirculation can improve convenience but also increases energy consumption because heated water is circulated through the plumbing system.

Gas Tankless Water Heaters and Cold-Water Sandwiches

A “cold-water sandwich” refers to a burst of cooler water that can occur between periods of hot-water use in some tankless installations.

Modern systems use several strategies to reduce this effect.

For example, Rheem’s RTGH series includes Hot-Start Programming designed to reduce cold-water bursts.

Proper recirculation configuration and plumbing design can also influence perceived temperature stability.

Minimum Flow Rate Matters

A tankless heater does not necessarily operate at every tiny flow rate.

The flow sensor must detect sufficient water movement to activate the burner.

If a fixture draws less than the activation threshold, the heater may not fire.

This can matter with:

• Low-flow faucets
• Bathroom sinks
• Small utility fixtures
• Very low-flow showerheads

The Navien NPE-240S2, for example, lists a minimum flow rate of 0.5 GPM.

Gas Tankless Water Heater Maintenance

1. Flush the Heat Exchanger

Mineral deposits can accumulate inside the heat exchanger, particularly in hard-water regions.

Periodic descaling may therefore be required.

2. Inspect the Inlet Filter

The water inlet screen can collect debris and should be checked according to the manufacturer’s maintenance schedule.

3. Check the Vent System

Indoor units should have their combustion-air and exhaust systems inspected when required.

4. Inspect the Condensate System

Condensing models produce condensate that must drain correctly.

5. Check the Burner and Combustion System

Professional service may be required to inspect combustion performance.

6. Check Water Quality

Hard water can increase scale formation and maintenance requirements.

See Best Water Heaters for Hard Water.

Hard Water and Gas Tankless Water Heaters

Scale is especially important for tankless systems because the heat exchanger contains relatively narrow water passages and transfers large amounts of heat through its surfaces.

When hard-water deposits accumulate, they can:

• Reduce heat-transfer efficiency
• Restrict water flow
• Increase burner cycling
• Increase outlet-temperature instability
• Increase maintenance requirements

Homes with hard water should therefore pay particular attention to water treatment and periodic descaling.

Natural Gas vs Propane Tankless Water Heaters

Never assume a natural-gas model can simply be connected to propane. Some models are field-convertible, while others require a specific fuel version.

The Navien NPE-S2 family, for example, is field convertible between natural gas and LP configurations according to Navien.

Gas Tankless Water Heater Safety

Gas appliances require careful installation because they involve combustion, fuel gas and exhaust gases.

Important safety provisions include:

• Correct gas pressure
• Correct gas-line sizing
• Proper combustion air
• Correct venting
• Carbon-monoxide protection
• Proper condensate drainage where applicable
• Flame detection
• High-temperature protection
• Water-temperature control
• Qualified installation and service

Gas tankless water heaters should be installed according to the manufacturer’s instructions and applicable local fuel-gas, plumbing, mechanical and building codes.

Gas Tankless vs Gas Storage Water Heaters

Best Gas Tankless Water Heaters: Our Recommended Models

1. Navien NPE-240S2 — Best Overall Gas Tankless Water Heater

The Navien NPE-240S2 is our best overall choice because it combines very high efficiency with a large maximum burner input and strong flow performance across different temperature rises.

Why We Selected It

• 199,900 BTU/h maximum input
• 13,300 BTU/h minimum input
• 0.96 UEF
• 5.8 GPM at 67°F rise
• 11.2 GPM at 35°F rise
• Dual stainless-steel heat exchangers
• Field convertible between natural gas and LP
• 2-inch PVC venting capability up to 75 feet in the specified configuration
• Indoor or outdoor wall-hung installation
• Cascading capability
• Low-NOx emissions
• Built-in EZNav control
• High-altitude capability listed to 10,100 feet

Navien lists CSA, NSF Standard 5, low-lead, SCAQMD and ENERGY STAR certifications for the NPE-240S2.

Relevant Certification / Standards

The model is ENERGY STAR certified and listed with ANSI/CSA-related approvals. Exact installation must follow the manufacturer’s current installation manual and local codes.

Important Limitations

• Requires professional gas, water and venting installation.
• Condensing design requires condensate management.
• Maximum GPM falls as temperature rise increases.
• A 199,900 BTU/h gas appliance can require significant gas-supply capacity.
• Recirculation configuration can increase energy consumption.

Who Should Buy It

It is particularly suitable for medium-to-large homes with multiple bathrooms and substantial simultaneous hot-water demand.

Who Shouldn’t Buy It

It may be unnecessarily large for a small household with one bathroom and low simultaneous demand.

Maintenance Considerations

Plan periodic heat-exchanger flushing, inlet-filter inspection, condensate-drain inspection and combustion/venting checks according to Navien’s service requirements.

Retailer / Product Information

Purchase through Navien distributors and qualified installers.

For multiple-bathroom applications, see Best Water Heaters for Multiple Bathrooms.

2. Rheem RTGH-95DVELN-3 — Best Smart Gas Tankless Water Heater

The Rheem RTGH-95DVELN-3 combines a high-output condensing platform with built-in EcoNet Wi-Fi and LeakGuard leak detection.

Why We Selected It

• 199,900 BTU/h class
• 9.5 GPM maximum rated flow
• 0.93 UEF
• Natural gas model
• Built-in EcoNet Wi-Fi
• LeakGuard leak detection and prevention
• Hot-Start Programming
• 15-year heat-exchanger warranty
• 5-year parts warranty
• 1-year labor warranty
• Indoor installation
• Outdoor conversion kit available

Rheem lists the model at 18 inches wide, 23 inches high and 9 inches deep, with a 9.5 GPM rating and 0.93 UEF.

Relevant Certification / Standards

Rheem lists the model with applicable product certifications and low-emission compliance. Confirm the exact certification label and installation configuration for the model purchased.

Important Limitations

• Requires professional gas and vent installation.
• Condensing operation requires condensate management.
• 9.5 GPM is a maximum flow figure rather than guaranteed shower-temperature flow.
• Gas-line capacity must be verified before installation.

Who Should Buy It

It is a strong choice for homeowners who want high-output tankless heating combined with connected monitoring and leak-protection features.

Who Shouldn’t Buy It

Do not choose it solely for Wi-Fi if the home has low hot-water demand or if another model provides a better installation configuration.

Maintenance Considerations

Maintain the heat exchanger, inlet filter, condensate drain, venting and water-treatment system according to Rheem’s instructions.

Retailer / Product Information

Use Rheem’s product and professional-installation network to locate current availability.

For existing-home replacement, see Best Water Heaters for Existing Homes.

3. Rinnai RU199iN — Best Compact High-Output Indoor Gas Tankless Water Heater

The Rinnai RU199iN is a compact indoor natural-gas condensing tankless water heater designed for homeowners who want high output in a relatively small wall-mounted package.

Why We Selected It

• 199,000 BTU/h maximum input
• 15,000 BTU/h minimum input
• 0.95 UEF
• Maximum rated flow of 11 GPM
• Indoor installation
• Wi-Fi capable
• Recirculation capable
• Natural gas
• Compact 18.5-inch-wide housing
• 3/4-inch gas, cold-water and hot-water connections
• 1/2-inch NPT condensate connection

Rinnai lists the RU199iN at 18.5 inches wide, 31.3 inches high and 11.45 inches deep, with 0.95 UEF and an 11 GPM maximum flow specification.

Relevant Certification / Standards

Rinnai lists the RU199iN as ENERGY STAR qualified and SCAQMD ultra-low-NOx approved.

Important Limitations

• The 11 GPM maximum does not apply at every temperature rise.
• Requires condensate drainage.
• Indoor installation requires correct venting and combustion-air provisions.
• Exact recirculation configuration should be matched to the home’s plumbing system.

Who Should Buy It

This model is attractive for homeowners who want high output while keeping the tankless appliance relatively compact.

Who Shouldn’t Buy It

It may not be the best fit if an outdoor installation is strongly preferred or if the home’s gas infrastructure cannot support its input requirement.

Maintenance Considerations

Follow Rinnai’s flushing, filter, condensate, venting and combustion-maintenance procedures.

Retailer / Product Information

Rinnai recommends professional installation and provides a contractor-finder system for the RU199iN.

For limited-space installations, see Best Water Heaters for Limited Space.

4. Navien NPE-210S2 — Best for Moderate-to-High Demand

The Navien NPE-210S2 provides the same general NPE-S2 condensing architecture as the larger NPE-240S2 but with a lower maximum input.

Why We Selected It

• 180,000 BTU/h maximum input
• 12,000 BTU/h minimum input
• 0.96 UEF
• 5.3 GPM at 67°F rise
• 10.1 GPM at 35°F rise
• Dual stainless-steel heat exchangers
• Field convertible between natural gas and LP
• Indoor or outdoor wall-hung installation
• 2-inch or 3-inch approved venting configurations
• 120V electrical supply
• High-altitude capability listed to 10,100 feet

Navien lists the NPE-210S2 with 0.96 UEF, 5.3 GPM at a 67°F rise and 10.1 GPM at a 35°F rise.

Relevant Certification / Standards

Navien lists the model as ENERGY STAR certified, with CSA, NSF Standard 5, low-lead and SCAQMD approvals.

Important Limitations

• Lower maximum input than the NPE-240S2.
• Flow capacity falls as temperature rise increases.
• Still requires condensing-system condensate management.
• Gas-line sizing must be verified.

Who Should Buy It

It is a strong option for homes that need substantial tankless capacity but do not require the maximum output of a roughly 200,000 BTU/h model.

Who Shouldn’t Buy It

Homes with several simultaneous showers in a cold-water climate may need the larger NPE-240S2 or multiple units.

Maintenance Considerations

Use the manufacturer’s flushing and inspection schedule and maintain the inlet filter, condensate drain and vent system.

Retailer / Product Information

Purchase through Navien’s authorized distributor and professional installer network.

For four-person homes, see Best Water Heaters for 4 People.

5. Noritz NRC111-DV-NG — Best Noritz Condensing Alternative

The Noritz NRC111 platform provides another established condensing tankless option for homeowners and installers who prefer Noritz equipment.

Why We Selected It

• Natural-gas condensing design
• Maximum input in the approximately 199,900 BTU/h class
• Flow range beginning around 0.5 GPM
• Indoor direct-vent configuration
• Electronic ignition
• ENERGY STAR qualified platform
• Low-NOx configuration
• Stainless-steel and commercial-grade heat-transfer components

Noritz’s NRC111 documentation identifies a 0.92 energy factor for the platform and flow rates up to 11.1 GPM depending on model/configuration.

Relevant Certification / Standards

Noritz documentation identifies ANSI Z21.10.3/CSA 4.3 certification, ENERGY STAR qualification and low-NOx compliance for the NRC111 platform.

Important Limitations

• Verify current model availability because Noritz has changed product generations over time.
• Exact performance varies by configuration.
• Requires condensate management.
• Professional installation is strongly recommended.

Who Should Buy It

It is a sensible alternative for homeowners whose installer prefers Noritz or who want another established tankless platform to compare against Navien, Rheem and Rinnai.

Who Shouldn’t Buy It

Do not choose an older model merely because it has a high advertised GPM. Confirm that the exact model is current and that its performance matches the home’s demand.

Maintenance Considerations

Periodic flushing and inspection of the heat exchanger, inlet filter, condensate drain and venting are important, especially in hard-water areas.

Retailer / Product Information

Verify the exact current Noritz model through Noritz’s support and professional distribution network before purchase.

For hard-water installations, see Best Water Heaters for Hard Water.