12 Best Gas Tankless Water Heaters (June 2026) Expert Reviews

Switching to a gas tankless water heater changed how our team thinks about hot water. No more running out during back-to-back showers. No more paying to keep 50 gallons hot all day and night. After testing and researching dozens of units, we put together this guide to the best gas tankless water heaters you can buy in 2026.

Gas tankless water heaters (also called on-demand water heaters) heat water only when you turn on a faucet. They use powerful burners to warm water as it flows through a heat exchanger, giving you continuous hot water without the standby energy losses of a traditional tank. The result is lower energy bills, unlimited hot water, and a unit that takes up a fraction of the space.

In this guide, we review 12 gas tankless water heaters across every budget and household size. Whether you need to supply a 5-bedroom home with 4 bathrooms or just want endless hot water for a small apartment, we found options that deliver. We cover everything from flow rates and efficiency ratings to installation costs and maintenance, so you can pick the right unit with confidence.

Top 3 Picks for Best Gas Tankless Water Heaters

Best Gas Tankless Water Heaters in 2026

1. Rinnai RX199iN – Best Overall Performance

The Rinnai RX199iN is the unit I would install in my own home without hesitation. With 11.1 GPM and 199,000 BTU, it handles a large household running multiple showers, a dishwasher, and a washing machine all at the same time. During our research, we found homeowners who regularly run 4 to 5 fixtures simultaneously with zero temperature drops.

The condensing design with a UEF of 0.98 puts this unit near the top of the efficiency chart. That means almost all the gas you pay for actually goes into heating your water rather than escaping out the vent. Over a year, the energy savings compared to a tank heater or even a non-condensing tankless unit are substantial.

What sets the RX199iN apart is flexibility. The Versa Vent system gives you more options for venting placement, and Rinnai designed this one model to work for both indoor and outdoor installation. That is a big deal if you are still deciding where to mount it. The Smart Sense gas control system also lets you switch between natural gas and propane without changing the unit.

WiFi capability is available through the Controlr module (sold separately). While that is an extra cost, the app lets you adjust temperature, monitor usage, and get maintenance alerts from your phone. For a unit at this level, the connected features are worth adding.

The biggest tradeoff is the wait time for hot water at distant faucets. Several owners reported 25 to 30 seconds before hot water reaches fixtures far from the unit. This is not unique to Rinnai but is noticeable with any tankless system in a large home. A recirculation pump solves this if it bothers you.

Who Should Buy This Unit

This Rinnai is ideal for large families in homes with 3 or more bathrooms. If you have a household where 3 people shower at the same time while the dishwasher runs, the 11.1 GPM capacity handles it without breaking a sweat. It is also a strong choice if you want top-tier efficiency and plan to stay in your home for 10-plus years.

Installation and Maintenance Considerations

Professional installation is strongly recommended. The unit weighs 55 pounds and requires proper gas line sizing, venting through the Versa Vent system, and code-compliant water connections. The good news is that Rinnai has one of the largest networks of certified installers in the US, so finding a qualified technician is easier than with many smaller brands. Annual flushing with white vinegar keeps the heat exchanger running efficiently, especially in hard water areas.

2. A. O. Smith ATI-310CX3 – Best Scale Prevention

The A. O. Smith ATI-310CX3 solves the single biggest headache tankless owners face: scale buildup. Its X3 Scale Prevention Technology actively prevents mineral deposits from forming on the heat exchanger, which means you can skip the annual descaling routine that other units require. For anyone living in a hard water region, this feature alone makes the unit worth serious consideration.

With 8.0 GPM and 190,000 BTU, this unit has enough power for a busy 3-bathroom household. The commercial-grade copper heat exchanger delivers 25 times better heat transfer than standard steel, and A. O. Smith backs it with a 15-year limited warranty. That is one of the longest heat exchanger warranties we found across any brand.

The 4.9-star average rating from verified buyers is remarkable. Out of 22 reviews, 93 percent gave it 5 stars. Owners consistently praise the stable temperature output, even when running multiple fixtures at once. The unit handles back-to-back showers without the temperature fluctuations that plague cheaper models.

A. O. Smith includes a propane conversion kit in the box, so you can switch fuel types without buying extra parts. The concentric vent system (APNC35) simplifies installation by using a single pipe for both exhaust and combustion air intake. This reduces the number of wall penetrations needed.

The main drawback is cost. This is a premium unit with a price to match. You also need to use A. O. Smith’s specific concentric venting, which limits your options compared to units with standard vent compatibility. If your installer is not familiar with the concentric system, there can be a learning curve.

Who Should Buy This Unit

This is the pick for homeowners in hard water areas who want a low-maintenance tankless solution. If you know you will not keep up with annual descaling, the X3 technology protects your investment automatically. It is also a strong choice for anyone who values long-term reliability, given A. O. Smith’s commercial water heater pedigree and the 15-year heat exchanger warranty.

Understanding the Concentric Vent System

Unlike traditional dual-pipe venting that requires separate exhaust and intake runs, the concentric vent uses a pipe-within-a-pipe design. The inner pipe exhausts combustion gases outside while the outer pipe draws fresh air in. This single-vent-pipe approach makes for a cleaner installation with fewer wall or roof penetrations. The vent kit (APNC35) is sold separately, so factor that into your total installation budget. Most professional installers appreciate the simpler venting route once they are familiar with the system.

3. Rheem RTG-70DVLN-3 – Best Value Indoor Unit

The Rheem RTG-70DVLN-3 delivers strong performance at a competitive price point, making it our pick for the best value indoor gas tankless water heater. At 7.0 GPM and 160,000 BTU, it has enough flow for 2 to 3 bathroom homes where simultaneous use is common but not extreme.

The standout feature here is the hot-start programming. If you have ever experienced a “cold water sandwich” (that annoying blast of cold water between uses), Rheem’s hot-start technology virtually eliminates it. The unit keeps a small amount of hot water ready in the system so that subsequent draws start warm immediately.

Rheem built this unit with universal venting compatibility, which gives your installer more flexibility in choosing vent pipe materials. It also works with a 1/2-inch gas line in many installations, saving you the cost of upgrading your gas piping. These small details make a real difference in total installation cost.

Owners consistently praise the digital display for making temperature adjustments easy. The user-friendly interface lets you set your preferred temperature in seconds. Several reviewers mentioned significant energy savings after switching from a 40-gallon tank heater to this Rheem unit.

The tradeoff is that 7.0 GPM may feel tight in homes with 3 full bathrooms and high-flow showerheads. If your household regularly runs 3 showers plus a dishwasher at once, you might want to step up to a higher GPM unit. The warranty structure is also more standard than premium: 15 years on the heat exchanger, 5 years on parts, and 1 year on labor.

Who Should Buy This Unit

This Rheem is ideal for 2 to 3 bathroom homes where 2 people might shower at the same time. It hits the sweet spot between capacity and cost, giving you reliable hot water without overpaying for GPM you do not need. If you are replacing an old tank heater and want a straightforward upgrade, this unit makes the transition simple.

Gas Line Compatibility and Sizing

One advantage of the RTG-70DVLN-3 is that it can work with a 1/2-inch gas supply line in some installations, depending on the length of the run and the total gas demand in your home. Most tankless units require 3/4-inch lines. This can save you several hundred dollars in gas piping upgrades during installation. However, have your installer verify the gas pressure and line size before committing. An undersized gas line causes performance issues that no tankless unit can overcome.

4. FOGATTI Comfort 180S – Best Outdoor Performance

The FOGATTI Comfort 180S is designed specifically for outdoor installation, which eliminates the need for complex indoor venting. With 8.1 GPM and 180,000 BTU, it delivers serious heating power for homes in warmer climates where outdoor mounting is common. The bottom-mounted fan prevents smoke backflow, and the 304 stainless steel burner holds up well against weather exposure.

Temperature control is impressive, with sensors maintaining output within 1 degree Fahrenheit of your set point. The four-stage combustion system adjusts the flame intensity to match demand, which helps with efficiency and prevents the temperature swings that some cheaper units exhibit during low-flow situations.

The included remote control panel lets you adjust settings from inside your home, so you do not need to walk outside to change the temperature. Multiple reviewers noted this was more useful than expected, especially during winter months when you want to check on the unit without braving the cold.

The main frustration is that FOGATTI does not include valves, fittings, or connection accessories in the box. You will need to purchase these separately, which adds to the total cost and requires a trip to the hardware store on installation day. A few owners also reported occasional ignition issues, though most were resolved quickly.

Who Should Buy This Unit

The FOGATTI 180S is best for homeowners in mild to warm climates who prefer an outdoor installation to save indoor space. It works well for 3 to 5 outlet points, making it suitable for medium to large households. If you live in a region where freezing temperatures are rare and you want to skip the venting complexity of indoor units, this is a solid choice.

Outdoor Installation Requirements

Even though this unit is built for outdoor use, you still need to follow local codes for clearance from windows, doors, and air intakes. Most jurisdictions require at least 4 feet of clearance from any opening into the home. The unit should be mounted on an exterior wall with proper support for its 43-pound weight. Gas and water lines need to be run through the wall to the unit, and proper sealing around the penetrations is critical to prevent moisture and pest intrusion.

5. MIZUDO GEN 3 WiFi – Best Smart Features

The MIZUDO GEN 3 stands out because it comes with WiFi and app control built right in. No separate module to buy, no extra setup. You download the app, connect the unit to your home network, and you can adjust temperature, monitor energy usage, and get diagnostic alerts from anywhere. For tech-savvy homeowners, this is the connected experience you want.

With 5.1 GPM and 120,000 BTU, the GEN 3 handles 2 to 4 usage points comfortably. Owners report getting hot water in 35 to 40 seconds, which is faster than many competitors at this price range. The phosphorous-deoxidized copper heat exchanger is a step up from standard copper, offering better corrosion resistance over time.

The convertible indoor/outdoor design is a genuine advantage. If your installation plans change or you move to a different home, this unit adapts. You are not locked into one mounting location from day one.

Installation reports are overwhelmingly positive. One homeowner documented a complete swap from an old tank heater in just 3 hours, including removing the old unit. The compact 14.56 by 22.83-inch footprint fits in tight utility closets where tank heaters would never go.

The main downside is the gas fitting size. MIZUDO uses a 1/2-inch fitting rather than the 3/4-inch standard that most US plumbers expect. This can cause confusion during installation and may require an adapter. A few owners also noted that the unit draws combustion air from inside the home rather than having a dedicated fresh air intake, which is worth knowing for tightly sealed homes.

Who Should Buy This Unit

This MIZUDO is the right pick if you want smart home integration without paying a premium. The built-in WiFi and app control give you remote management capabilities that usually cost extra with brands like Rinnai. It works well for families with 2 to 3 bathrooms and moderate simultaneous hot water demand.

How the WiFi and App Control Work

The MIZUDO app connects to your unit over your home WiFi network. You can set precise water temperature, schedule heating modes, and view error codes if something needs attention. The app works on both iOS and Android. Several owners mentioned using the app to turn down the temperature when leaving for vacation and turning it back up before arriving home, saving gas while away. The temperature control within 1 degree Fahrenheit matches what you get from the physical controls on the unit itself.

6. GASLAND CE686NG – Best High BTU Value

The GASLAND CE686NG packs 199,000 BTU into a compact indoor unit, giving you 7.6 GPM of continuous hot water at a price that undercuts most competitors with similar specs. That combination of high BTU output and reasonable cost makes it one of the best values for whole-house heating in this roundup.

The 0.8 UEF rating means this unit is 34 percent more efficient than a standard 50-gallon tank water heater. While it falls short of the 0.82 threshold needed for Energy Star certification and the associated tax credit, the real-world gas savings are still significant compared to any tank heater.

Safety features are comprehensive. You get overheat protection, flame failure protection, dry combustion protection, freeze protection, and an integrated error code indicator for troubleshooting. The CSA certification confirms the unit meets US safety standards.

At 37.5 pounds and measuring just 14.25 by 24.69 inches, the GASLAND fits in tight basement corners and utility closets. Owners describe it as “suitcase-sized” and appreciate how much space they reclaim by ditching their old tank heater.

The most common complaint involves venting pipe selection. Some owners found that 4-inch vent pipe did not match the unit’s requirements, while others had trouble with the pressure relief valve installation instructions. These are not performance issues but they can slow down installation. Having a knowledgeable plumber on the job helps avoid these headaches.

Who Should Buy This Unit

The GASLAND CE686NG is a strong pick for homeowners who want high flow capacity without paying premium-brand prices. If you have a 2 to 3 bathroom home and need 7-plus GPM, this unit delivers at a fraction of what Rinnai or Rheem charge for similar output. It is best suited for budget-conscious buyers who still want reliable whole-house performance.

Why It Is Not Energy Star Certified

The GASLAND CE686NG has a UEF of 0.8, which falls just below the 0.82 minimum required for Energy Star certification on gas tankless water heaters. This means it does not qualify for federal tax credits. However, the difference in real-world efficiency between 0.80 and 0.82 UEF is minimal. If the tax credit is important to you, consider the Rinnai RX199iN with its 0.98 UEF instead. Otherwise, the GASLAND still delivers meaningful energy savings over any tank-style heater.

7. MIZUDO GEN 2 Outdoor – Best for Big Families

The MIZUDO GEN 2 Outdoor is built specifically for big families who need reliable hot water across multiple bathrooms. With 120,000 BTU and self-modulating technology, it supports up to 3 showers running at the same time. For households with teenagers who all seem to shower at 7 AM, that matters.

The self-modulating technology is key here. The unit automatically adjusts gas consumption based on real-time water demand. When one shower is running, it uses less gas. When three are going, it ramps up. This means you only burn the gas you actually need, and owners report saving up to 20 percent on their gas bills compared to their old tank heaters.

The outdoor design means zero indoor venting requirements. The unit is windproof and waterproof, built to handle exposure year-round. Both CSA and UL certifications confirm it meets US safety and electrical standards, which is something not every outdoor unit can claim.

Customer support gets high marks from reviewers. Multiple owners who had questions during installation received quick, helpful responses from the MIZUDO team. The 2-year whole unit warranty and 5-year heat exchanger coverage provide solid protection for your investment.

The main limitation is that this unit is outdoor only. You cannot convert it for indoor use. It also has an elevation restriction above 2,000 feet, where the thinner air reduces combustion efficiency. If you live in a mountain town, check with MIZUDO before purchasing. A few users in cold climates also noted that the freeze protection could be more robust.

Who Should Buy This Unit

This MIZUDO is the right choice for families in warm or temperate climates with 2 to 3 bathrooms where outdoor installation is practical. If your household regularly has multiple people showering at the same time and you want to save on installation costs by skipping indoor venting, this unit delivers excellent value.

Performance at Elevation

MIZUDO explicitly notes that this unit is not recommended for use above 2,000 feet elevation. At higher altitudes, the thinner air reduces the oxygen available for combustion, which lowers the unit’s effective BTU output and flow rate. If you live in Denver, Salt Lake City, or any mountain community above 2,000 feet, consider the Rinnai RX199iN instead, as Rinnai units are generally rated for higher elevations and can be derated for mountain installations.

8. Rheem RTG-70XLN-3 Outdoor – Best Outdoor Durability

The Rheem RTG-70XLN-3 is the outdoor counterpart to the indoor DVLN model we reviewed earlier. The key difference is the stainless steel cabinet designed to withstand weather exposure without rusting or corroding. For homes in coastal areas or regions with high humidity, this weather-resistant build is a meaningful advantage.

Like its indoor sibling, the 70XLN-3 features hot-start programming that prevents cold water bursts between uses. The self-diagnostic program displays error codes on the digital panel, making it easier for you or your technician to identify and fix issues without guessing.

At 7.0 GPM and 160,000 BTU, it is sized right for 2 to 3 bathroom homes. The low activation flow rate means it turns on even with low-flow fixtures, so you get hot water from water-saving showerheads and faucets without issues.

The freeze protection is built in, which is important for any outdoor unit. However, some owners in regions with extended sub-freezing temperatures reported that the built-in protection was not always sufficient on its own. Adding pipe insulation and a recirculation system provides extra protection in extreme cold.

The 4.1-star rating is the lowest in our roundup, pulled down by a small number of owners who received defective units and had frustrating customer service experiences. The majority of reviews are positive, with 67 percent giving 5 stars. But the customer service concerns are worth noting if something goes wrong.

Who Should Buy This Unit

This Rheem outdoor unit is best for homeowners in coastal or humid areas who need a durable, weather-resistant outdoor tankless heater. If you liked the features of the indoor DVLN model but need an outdoor installation, this is the same proven platform in a weather-resistant enclosure.

Maintenance for Outdoor Units

Outdoor tankless units face additional exposure that indoor units do not. Salt spray in coastal areas can accelerate corrosion on connections and fittings, even with a stainless steel cabinet. We recommend rinsing the exterior with fresh water every few months if you live near the coast. Annual inspection of the gas connections, water inlet filter, and vent outlet helps catch small issues before they become costly problems. The self-diagnostic program will alert you to most internal issues, but a visual inspection twice a year is still good practice.

9. ThermoMate CZ422NG – Best Compact Pick

At just 22 pounds, the ThermoMate CZ422NG is the lightest unit in our roundup by a wide margin. Most tankless gas heaters weigh 35 to 55 pounds. If you have limited wall space or need to mount the unit on a surface that cannot support heavy loads, this ThermoMate is your best option.

Despite its small size, the CZ422NG delivers 5.0 GPM and 113,000 BTU with 98 percent thermal efficiency. The advanced water flow sensor adjusts heating power 20,000 times per second, maintaining temperature stability within 1 degree Fahrenheit. Four ceramic heating cores with automatic frost protection activate at 41 degrees Fahrenheit to prevent freeze damage.

The 3.5-inch LED touchscreen is a standout feature at this price. Instead of tiny buttons or basic digital displays, you get a modern interface for setting temperature and viewing diagnostic information. It feels more like operating a smart appliance than a traditional water heater.

The DC high-speed fan generates 360Pa of static pressure at 3,000 RPM, which ThermoMate says resists winds up to Category 3 hurricane force. While most indoor installations will never test that claim, the powerful venting does help maintain consistent combustion and efficiency in drafty installations.

The biggest complaint is unclear venting documentation. Several owners struggled to determine whether they needed Category 3 venting or could use a Class B vent. The installation manual does not spell this out clearly, leading to frustration and extra research. Some users also reported minor temperature consistency issues during rapid demand changes.

Who Should Buy This Unit

The ThermoMate is ideal for small to medium households with 1 to 2 bathrooms where space is limited. Its light weight and compact 13.77 by 21.65-inch footprint make it easy to install in tight utility closets, apartments, or mobile homes. If you want a modern touchscreen interface and strong efficiency without paying for GPM capacity you will not use, this unit hits the mark.

Safety Features Breakdown

The ThermoMate includes 8 distinct safety protections: boil-dry prevention, freeze protection, flame-out detection, overheat cutoff, over-temperature lock, pipe blockage detection, high-temperature lock, and excessive air pressure relief. This is one of the most comprehensive safety packages in this price range. The integrated error code indicator on the touchscreen helps you diagnose issues quickly, and the auto-activating frost protection at 41 degrees Fahrenheit is a nice touch for homes in regions with occasional cold snaps.

10. Ranein RH10K – Best Budget Option

The Ranein RH10K is our top budget pick because it delivers real tankless performance at a price point that makes the switch from tank heaters accessible. With 100,000 BTU and 4.3 GPM, it handles 3 to 4 water points, which covers most small to medium households.

The 4 intelligent preset modes set this unit apart from other budget options. You get dedicated settings for 4-season use, kitchen, bathtub, and shower. Each mode optimizes the temperature and flow rate for that specific use case, taking the guesswork out of settings. The precision temperature control ranges from 95 to 155 degrees Fahrenheit with a high-temperature lock to prevent scalding.

The oxygen-free copper heat exchanger is a nice find at this price. This material resists corrosion better than standard copper and improves heat transfer efficiency. Ranein also includes the vent pipe in the box, which is one less thing to buy separately.

The LED display with temperature memory shows your last setting when you power on, so you do not have to re-adjust every time. Owners report the unit is quiet during operation and appreciate how much space they save compared to their old tank heaters.

Durability concerns are the main drawback. A subset of reviewers reported reliability issues cropping up around the 6-month mark, including intermittent heating failures. Ranein offers a 2-year manufacturer warranty, which covers these issues, but the process requires contacting customer support. This is a solid budget pick, but manage your expectations for long-term longevity compared to premium brands like Rinnai or A. O. Smith.

Who Should Buy This Unit

The Ranein RH10K is best for budget-conscious homeowners in moderate climates who want to switch to tankless without a large investment. It works well for small homes, apartments, or as a point-of-use heater for specific areas like a garage or guest house. If you live in an area with hard freezes, consider a unit with stronger cold-weather protection.

Understanding the Preset Modes

The 4 preset modes adjust both temperature and burner intensity for common use cases. The Kitchen mode targets a higher temperature for dishwashing and food prep. The Bathtub mode maximizes flow rate for filling tubs quickly. The Shower mode balances temperature and flow for comfortable showering. The 4-Season mode automatically adjusts output based on incoming water temperature, running hotter in winter when ground water is colder. These presets make the Ranein one of the most user-friendly budget units available.

11. NOVIEEX RS-16 – Best for Small Households

The NOVIEEX RS-16 is designed for smaller households that need dependable hot water without overpaying for capacity they will never use. At 4.3 GPM and 100,000 BTU, it handles 2 to 4 usage points, covering the typical demand of a 1 to 2 bathroom home or apartment.

The compact wall-mounted design saves over 50 percent of the space compared to a tank water heater. At just 19.84 pounds, it is one of the lightest gas tankless units available, making installation feasible on a wider range of wall types.

Temperature control ranges from 95 to 149 degrees Fahrenheit with precision within 1 degree. The LED touchscreen interface is responsive and easy to read, which is a welcome upgrade from the basic button controls found on many budget units. The multi-protection system guards against gas leaks, overheating, dry-firing, and freeze damage.

Users report consistent performance when running 1 to 2 fixtures at a time. The unit heats water on demand, so you only pay for gas when hot water is actually flowing. Energy savings compared to tank heaters are immediate and noticeable on your gas bill.

The limitation becomes apparent when you push beyond 2 simultaneous outlets. Several owners noted temperature drops and flow reductions when trying to run 3 or more fixtures at once. If your household rarely exceeds 2 simultaneous hot water draws, this is not an issue. But for larger families, the 4.3 GPM ceiling will feel restrictive.

Who Should Buy This Unit

The NOVIEEX RS-16 is perfect for small homes, apartments, and condos with 1 to 2 bathrooms. If you live alone or with a partner and want to switch from a tank heater to save space and energy, this unit provides exactly what you need without paying for extra capacity. It is also a good fit for guest houses, in-law suites, or accessory dwelling units.

Professional Installation Requirements

The NOVIEEX requires natural gas and should be installed by a licensed technician. The unit uses 1/2-inch NPT connections for gas inlet, cold water inlet, and hot water outlet. It operates on standard 110V to 120V AC power for the control board and ignition system. While the lightweight design makes physical mounting straightforward, proper gas line connections and code-compliant venting still require professional expertise. The included flue pipe covers basic venting needs, but your installer may recommend additional materials based on your home’s layout.

12. MIZUDO GEN 2 Indoor – Best for Apartments

The MIZUDO GEN 2 Indoor is our top pick for apartments and smaller homes that need a reliable, efficient indoor tankless unit. At 4.0 GPM and 90,000 BTU, it is sized right for 1 to 2 bathroom spaces where simultaneous demand rarely exceeds 2 fixtures.

The UEF rating above 0.81 exceeds the Department of Energy baseline, meaning this unit is officially classified as high efficiency. That matters not just for energy savings, but also for potential utility rebates and tax incentives in some states. The 3/4-inch NPT connectors are compatible with standard US plumbing, simplifying installation.

CSA certification confirms the unit meets all applicable US safety codes. The upgraded temperature control with self-modulating technology maintains consistent water temperature even when flow rates change, such as when someone turns on a second faucet. Multiple safety protections include anti-freeze, overheat, and dry combustion prevention.

Owners highlight the gas bill savings as the biggest win. Several reviewers reported cutting their gas bills by 15 to 20 percent compared to their old tank heaters. The compact 22.76 by 14.57 by 7.0-inch design fits in tight apartment utility closets where space is at a premium.

Like other MIZUDO units, the vent pipe and supply lines must be purchased separately. The warranty covers the whole unit for 2 years and the heat exchanger for 5 years, which is competitive for this price range. A few users reported temperature inconsistency during rapid demand changes, and the same 2,000-foot elevation restriction applies here as with other MIZUDO models.

Who Should Buy This Unit

This MIZUDO indoor unit is ideal for apartment dwellers and small homeowners who want a certified, efficient tankless heater at an accessible price. If you are replacing an old 30 or 40-gallon tank in a 1 to 2 bathroom home, this unit provides the right capacity without wasting gas on standby heating.

Apartment Installation Tips

Installing a gas tankless water heater in an apartment requires careful planning. First, check with your landlord or building management about gas appliance installations. Most apartments have shared gas lines, so your unit needs to work within the existing gas pressure available. The MIZUDO’s 90,000 BTU is low enough that it typically does not require gas line upgrades. Venting is the bigger challenge in apartments, as you need a route to the exterior for exhaust. If your current tank heater has an existing vent, your installer can often use the same path with proper sizing verification.

How to Choose the Best Gas Tankless Water Heater

Choosing the right gas tankless water heater comes down to a few key factors: your household size, how many fixtures run at the same time, your climate, and your budget. Here is what you need to know before making a decision.

Flow Rate and GPM: Getting the Right Size

Flow rate, measured in gallons per minute (GPM), is the most important spec to get right. Every fixture in your home has a flow rate: showers use about 1.5 to 2.5 GPM, kitchen faucets use 1.5 to 2.0 GPM, and washing machines use about 2.0 GPM. Add up the fixtures you expect to run simultaneously and buy a unit that exceeds that total by at least 10 percent.

For a 2-bathroom home, we recommend at least 5 to 7 GPM. For 3 or more bathrooms, look for 8 to 11 GPM. Buying too small is the number one complaint we see in forums, so err on the side of more capacity rather than less.

Condensing vs Non-Condensing

Condensing tankless water heaters capture heat from the exhaust gases and use it to pre-warm incoming water. This extra heat recovery pushes UEF ratings above 0.90, compared to 0.80 to 0.82 for non-condensing models. Condensing units cost more upfront but save more on gas over time.

Non-condensing units are simpler, lighter, and cheaper. They waste more heat through the vent, but the lower purchase price makes sense if you are on a tight budget or plan to move within a few years. The Rheem RTG-70DVLN-3 and GASLAND CE686NG are solid non-condensing options.

Efficiency Ratings: UEF Explained

The Uniform Energy Factor (UEF) measures how efficiently a water heater converts energy into hot water. Higher numbers mean less wasted gas. For context, standard tank heaters have UEF ratings around 0.58 to 0.65. Even the least efficient tankless unit in our roundup (0.80 UEF) beats every tank heater on the market.

Units with UEF above 0.82 qualify for Energy Star certification, which may qualify you for a federal tax credit of up to 30 percent of the unit cost, capped at $600. The Rinnai RX199iN leads our roundup with a UEF of 0.98.

Installation Costs and Considerations

Budget for installation costs ranging from $500 to $2,500 on top of the unit price. Factors that drive up installation costs include gas line upgrades (many homes need a larger gas line), venting installation (especially for indoor units), and any required electrical work for the control board and ignition system.

Outdoor installations generally cost less because there is no venting to run through your home. However, you need a suitable exterior wall and must meet local clearance codes. Indoor installations require proper venting to the exterior, which adds material and labor costs.

We strongly recommend professional installation. Gas appliances require code-compliant connections, proper venting, and pressure testing. A licensed installer also ensures your warranty remains valid, as most manufacturers require professional installation for warranty coverage.

Hard Water and Maintenance

Hard water is the biggest enemy of tankless water heaters. Minerals in the water build up on the heat exchanger over time, reducing efficiency and eventually causing failure. If you live in a hard water area (most of the southwestern and midwestern US), you need to flush your tankless unit with white vinegar every 12 months.

The A. O. Smith ATI-310CX3 is the only unit in our roundup with built-in scale prevention that eliminates the need for annual descaling. For every other unit, we recommend installing isolation valves for easy flushing and considering a whole-house water softener if your water hardness exceeds 7 grains per gallon.

Warranty Coverage

Pay attention to what each warranty covers and for how long. Most manufacturers offer separate warranty periods for the heat exchanger, parts, and labor. The heat exchanger warranty is the most important because it is the most expensive component to replace.

A. O. Smith leads with a 15-year heat exchanger warranty. Rheem offers 15 years on the heat exchanger for its non-condensing models. Most MIZUDO, FOGATTI, and other value brands offer 5 years on the heat exchanger. Rinnai covers parts for 5 years. Labor coverage varies from 1 to 5 years across all brands, and some manufacturers require registration within 30 days of installation for full warranty benefits.

FAQs

Final Verdict

After reviewing 12 gas tankless water heaters across every price range and household size, three units stand out. The Rinnai RX199iN is our Editor’s Choice for its unmatched 11.1 GPM capacity, 0.98 UEF efficiency, and proven long-term reliability. The A. O. Smith ATI-310CX3 earns our Premium Pick for its unique X3 Scale Prevention Technology that eliminates the most common maintenance headache. And the Rheem RTG-70DVLN-3 delivers the best overall value for 2 to 3 bathroom homes.

Switching to the best gas tankless water heater for your home is a decision that pays off every single month in lower energy bills and the comfort of never running out of hot water. Size your unit correctly, have it professionally installed, and stay on top of annual maintenance in hard water areas. Do those three things and your tankless unit will provide reliable hot water for 15 to 20 years or more.