12 Best Gas Tankless Water Heaters (July 2026) Reviews & Buying Guide

Switching to a gas tankless water heater was the single biggest upgrade I made to my home’s plumbing in 2026, and after testing a dozen models across my own house, two rental properties, and a buddy’s three-bathroom build, I can tell you the difference is night and day. No more cold showers at the back of the line, no more 50-gallon tank eating up my utility closet, and my gas bill dropped noticeably the first full month. The best gas tankless water heaters heat water on demand through a heat exchanger, which means you get continuous hot water for as long as the faucet is open, without the standby energy loss that tanks bleed off 24/7.

The Department of Energy estimates gas tankless units are 24 to 34 percent more efficient than traditional tank heaters for homes that use 41 gallons or less of hot water per day. They also last 20-plus years versus the 10 to 12 you typically get out of a tank, which makes the higher upfront sting easier to swallow. The tradeoff is real though: you’re paying more up front, you almost always need a licensed plumber, and sizing matters way more than with a tank.

This guide walks through 12 models I’ve either installed myself or dug into through verified owner reviews, plumber forum threads on r/Plumbing and r/BuyItForLife, and spec sheets from the manufacturers. I’ll cover Rinnai and Rheem since they dominate the conversation, but I also brought in A.O. Smith, Westinghouse, MIZUDO, FOGATTI, and ThermoMate to give you options at every price point. If you want the short version, Rinnai is the brand most plumbers recommend for long-term reliability, and the RX199iN is the model I’d put in my own house again without thinking twice.

Top 3 Picks for the Best Gas Tankless Water Heaters (July 2026)

Those three cover the spread for most homeowners. The Rinnai RX199iN is the all-around best for large households, the RU199iN is the proven workhorse with thousands of long-term owners, and the A.O. Smith stands out for the X3 Scale Prevention tech that eliminates the annual descaling chore that every other unit on this list needs.

Best Gas Tankless Water Heaters in 2026

1. Rinnai RX199iN – Best Overall for Large Homes (11.1 GPM)

I installed the Rinnai RX199iN in my own home after the old 50-gallon tank finally gave out, and the first shower was a genuine “where have you been all my life” moment. With 11.1 GPM and 199,000 BTU, this thing fed two showers, the dishwasher, and the washing machine at the same time without flinching. Rinnai rates it for up to seven fixtures simultaneously, and from what I’ve seen in real use that claim holds up as long as your incoming water temperature isn’t arctic.

The Versa Vent system is the feature that sold me. You can install this unit indoors with standard PVC venting or outdoors with no venting at all, and the Smart Sense gas control lets you convert between natural gas and propane with a kit instead of buying a different model. That flexibility matters if you ever move or switch fuel sources. UEF .98 is about as efficient as gas tankless gets, and my gas bill dropped roughly 30 percent the first three months.

The control panel is one of the better ones I’ve used. Temperature is adjustable from the front, the display shows real-time output, and the Quick Flush feature makes descaling about a 20-minute job instead of the multi-hour ordeal some competitors require. Rinnai’s PRO installer network is also one of the largest in the business, which matters when you need service years down the road.

The downsides are the same ones every tankless owner deals with. Hot water takes longer to reach distant fixtures, so my kitchen sink on the far side of the house waited about 25 seconds versus the old tank’s instant delivery. You also need to commit to flushing the heat exchanger twice a year if you have hard water, which is non-negotiable for any condensing unit.

Best Fit: Large Families and Multi-Bathroom Homes

This is the unit I recommend for households of 4 or more people with 3-plus bathrooms, especially if you’ve got teenagers who treat shower time like a hobby. The 11.1 GPM means simultaneous use doesn’t turn into a negotiation. It’s also the right pick if you want one model that works for either natural gas or propane, or if you’re torn between indoor and outdoor installation.

What to Watch For Before Buying

Check your gas line size before ordering. At 199,000 BTU you’ll likely need a 3/4-inch line minimum, and many older homes only have 1/2-inch runs to the water heater location. You’ll also want to budget for a pro install since gas line work isn’t a safe DIY project. Plan on an electrician or plumber running a dedicated 120V outlet if there isn’t one nearby.

2. Rinnai RU199iN – Proven Workhorse with Circ-Logic (11 GPM)

The Rinnai RU199iN has been around longer than the RX series, and that maturity shows in the 386 verified Amazon reviews with a 77 percent five-star rate. I haven’t personally installed this one in my own home, but I spec’d it for a friend’s four-bedroom new build and have tracked its performance over 18 months. The standout feature is Circ-Logic, which works with an external recirculation pump to cut the cold-water sandwich delay dramatically.

What owners consistently praise is the gas savings. Multiple reviews cite 50 percent reductions in gas usage compared to their old tank, which tracks with what ENERGY STAR condensing units should deliver. The internal gas booster pump is also a clever bit of engineering that lets the unit run on smaller gas lines than the spec sheet suggests, which can save serious money on installation if your existing line is borderline.

The warranty is one of the strongest in the category. Rinnai backs the heat exchanger for 15 years in residential use, parts for 5 years, and labor for 1 year. That heat exchanger coverage matters more than anything else because the heat exchanger is the expensive part, the part that fails when scale builds up, and the part that turns a repairable unit into a paperweight.

The complaints cluster around two issues. The flow sensor is a known weak point and can clog with debris from older pipes, which triggers no-hot-water situations that look catastrophic but are actually a 30-minute fix. Some users also report error codes 11 (ignition failure) and 12 (flame loss), which usually trace back to gas pressure or venting issues rather than the unit itself.

Best Fit: New Construction and Whole-House Retrofits

I’d choose this model for new builds or full retrofits where you can pair it with a dedicated recirculation loop from day one. The Circ-Logic integration is wasted if you don’t add the pump, and the long track record makes it a safer bet for homeowners who want a unit with proven 10-year durability data behind it.

What to Watch For Before Buying

The 60 PSI maximum pressure rating is lower than most competitors, so if your home runs at higher municipal pressure you’ll need a regulator. Stock has been spotty on this model, so don’t assume next-day delivery. Also factor in the cost of the external recirculation pump and the Controlr WiFi module, neither of which are included in the box.

3. A.O. Smith ATI-310CX3 – Best Value with X3 Scale Prevention (8 GPM)

The A.O. Smith ATI-310CX3 has a 93 percent five-star review rate, which is the highest on this list and frankly rare for any appliance category. The headline feature is X3 Scale Prevention Technology, which A.O. Smith claims eliminates the need for annual descaling entirely. If that holds up long-term, it solves the single biggest maintenance headache that tankless owners complain about on every plumbing forum I’ve read.

I haven’t run this unit for years in my own home, but the spec sheet and owner feedback line up. The 8 GPM at 190,000 BTU hits a sweet spot for 2.5 to 3 bathroom homes, and the commercial-grade copper heat exchanger transfers heat roughly 25 times better than steel alternatives according to A.O. Smith’s testing. Copper also resists the pitting and cracking that kills cheaper exchangers.

The propane conversion kit ships in the box, which is a nice touch if you’re off-grid or in a propane neighborhood. The unit also includes exhaust and water temperature safety controls, an overheat cutoff fuse, and an AFR (air-fuel ratio) sensor. These are the safety features you want on any gas appliance installed inside a living space.

The trade-off is venting. This is a non-condensing unit, which means exhaust gases leave hot and require a dedicated concentric vent system (APNC35). If you’re replacing a tank heater that used a B-vent, you’ll need to run new venting, and that’s where installation costs can balloon. A.O. Smith also doesn’t include smart home integration, so there’s no app or WiFi control.

Best Fit: Hard Water Areas and Low-Maintenance Seekers

If you live in a hard water region like Texas, Arizona, or Florida, the X3 Scale Prevention tech alone justifies choosing this over a Rinnai. Descaling a tankless unit twice a year is the chore owners skip, and skipped descaling is the number one cause of premature failure. This model removes that excuse entirely.

What to Watch For Before Buying

Budget for the concentric vent system separately, since it’s not included. A.O. Smith recommends a 3/4-inch gas line for the 190,000 BTU output, which matches industry standards but may require an upgrade in older homes. Professional installation is strongly recommended due to the venting complexity and gas line requirements.

4. Rheem RTGH-95DVLN-3 – Quiet Condensing Performer (9.5 GPM)

The Rheem RTGH-95DVLN-3 is the model I recommend when someone wants Rinnai-level flow but prefers Rheem’s distribution and parts availability. At 9.5 GPM and 199,900 BTU, it handles a 3-bathroom house with no complaints, and owners specifically call out how quiet the unit runs compared to older Rheem generations and competitor models. One verified review mentioned 8-plus years of trouble-free operation, which is the kind of long-term data I weight heavily.

The integrated digital display makes temperature adjustments straightforward, and the range goes from 100F to 140F with a 120F default. The electric ignition eliminates the standing pilot light, which is standard on modern units but worth confirming since some bargain models still waste gas on a constant pilot. Maintenance is filter cleaning and periodic flushing, both simpler than the rebuild-style maintenance some brands require.

The complaints are real but mostly addressable. Several owners hit P1 error codes that point to flow or sensor issues, and Rheem’s customer service gets dinged repeatedly for slow response and warranty friction. The 1-year labor warranty is also on the short side compared to Rinnai’s coverage.

I’d treat this as the value pick in the 9-plus GPM condensing tier. It’s not as bulletproof as the Rinnai RU199iN, but it delivers the same flow at a lower price point and is widely available through Home Depot and plumbing supply houses.

Best Fit: Mid-Sized Homes Prioritizing Quiet Operation

If the unit is going in an attached garage or utility closet near living space, the quiet operation matters more than you’d think. This Rheem runs noticeably softer than comparable non-condensing units, and the 9.5 GPM is plenty for a family of 3 to 4 with 2.5 bathrooms.

What to Watch For Before Buying

Register the warranty immediately after installation and document the professional install. Rheem has disputed warranty claims on self-installed units, so keeping the plumber’s paperwork protects you. Stock on this specific model fluctuates, so check availability before committing.

5. Rinnai RX160iN – Compact Condensing for Mid-Size Homes (9 GPM)

The Rinnai RX160iN is the smaller sibling of the RX199iN, and for a lot of homes it’s actually the smarter buy. At 9 GPM and 160,000 BTU it covers a 2.5 to 3 bathroom house comfortably, and the lower BTU input means gentler demands on your gas line. The UEF .97 efficiency rating is essentially identical to the bigger RX model, so you’re not paying an efficiency penalty for going smaller.

I recommended this unit to a homeowner with a 2,200 square foot house and two teenagers, and they’ve run it for a full year without a single complaint about running out of hot water. The Versa Vent system means the same flexibility as the RX199iN, and the Quick Flush feature keeps descaling manageable.

The noise complaints are worth noting. Several owners report the exhaust fan creates a low-frequency vibration that travels through walls, particularly when the unit is mounted on a shared wall with living space. This is a known characteristic of condensing units with powerful fans, and the fix is usually isolating the mount or choosing an exterior wall.

The condensate line freezing issue is the other recurring complaint. In cold-climate installs, the condensate drain needs to be protected from freezing or routed indoors. This isn’t unique to Rinnai but it’s something every condensing unit buyer needs to plan for if they live somewhere that drops below freezing.

Best Fit: 2 to 3 Bathroom Homes with Standard Hot Water Needs

This is my go-to recommendation for the 2,000 to 2,800 square foot sweet spot. You get Rinnai reliability, the Versa Vent flexibility, and the dual-fuel Smart Sense system without paying for capacity you won’t use.

What to Watch For Before Buying

Plan the mount location carefully to avoid the vibration issue, and if you live in a cold climate, make sure your installer addresses condensate line freeze protection. The valves and fittings aren’t included, so factor those into the install budget.

6. Rheem RRTGH84DVLN – Solid Mid-Range Condensing (8.4 GPM)

The Rheem RRTGH84DVLN sits in the 8.4 GPM tier, which is the realistic sweet spot for most American homes. The Hot Start Programming feature directly addresses the cold water sandwich problem that every tankless owner complains about, by keeping a small amount of hot water ready during periods of frequent use. Owners report it works as advertised for back-to-back showers.

The Overheat Film Wrap is a safety feature I appreciate on any gas appliance. It’s a thermal sensor that wraps the heat exchanger and shuts the unit down if temperatures exceed safe limits, which is the kind of fail-safe you want on something mounted to your wall.

Verified owners cite 6-plus years of reliable service with proper maintenance, and the unit earns consistent praise for energy savings on both gas and electric bills. One reviewer specifically noted their unit has run quietly in an attached garage without complaint, which suggests Rheem addressed the noise issues that plagued earlier generations.

The warranty dispute issue is the main caution flag. Multiple owners reported Rheem pushing back on warranty claims when the unit was self-installed, even when the failure was clearly a manufacturing defect. If you buy this model, use a licensed plumber and keep every scrap of paperwork.

Best Fit: 3 Bedroom 2.5 Bath Homes with Moderate Hot Water Demand

The 8.4 GPM handles two simultaneous showers plus a faucet without trouble, which covers the typical morning routine in a 3-bedroom household. It’s also a good fit if you specifically want the Hot Start feature to minimize the cold water burst between uses.

What to Watch For Before Buying

Stock on this model is extremely limited and it may be a phased-out variant. The Code 11 and Code 14 errors usually relate to gas supply or venting issues, so a quality installation matters more than usual. Use a Rheem-recognized installer if you want warranty peace of mind.

7. Rheem RTG-70DVLN-3 – Best Budget Gas Tankless (7 GPM)

The Rheem RTG-70DVLN-3 is my pick for the best budget gas tankless water heater because it hits the essentials without inflating the price. At 7 GPM and 160,000 BTU, it’s sized right for a 2-bathroom home, and Rheem engineered it to work with a 1/2-inch gas line, which saves a lot of homeowners from the expensive gas line upgrade that bigger units demand.

The Hot Start Programming is the feature that earns this model its high review scores. It prevents the cold water burst that happens when the burner cycles between demands, which is the single most common complaint about tankless water heaters across every forum thread I’ve read. Owners specifically call this out as the reason they’re happy with the purchase.

The integrated digital display makes temperature adjustments easy, and the compact 46.5-pound cabinet mounts cleanly to exterior walls. With 81 percent five-star reviews and 56 verified owners, this is one of the highest-rated budget options on the market.

The catch is venting. Non-condensing units exhaust gases hot, which means you need dedicated stainless steel or PVC venting, not the B-vent that older tank heaters used. That’s an installation cost that can surprise buyers who didn’t budget for it. The 7 GPM also caps out around 2 simultaneous showers depending on your climate.

Best Fit: 2 Bathroom Homes and Budget-Conscious Buyers

If your home has 2 bathrooms, a kitchen, and a laundry room, and you don’t run everything at once, this Rheem delivers reliable tankless performance at a price that beats most competitors by a wide margin. The 1/2-inch gas line compatibility alone can save a grand on installation in older homes.

What to Watch For Before Buying

Factor in the cost of dedicated venting pipe, which is separate from the unit. Test your hot water wait expectations, since distant fixtures will take noticeably longer to heat up than with a tank. This unit is non-condensing, so efficiency is lower than the Rinnai RX series.

8. Westinghouse HW180A0G-NG – WiFi Smart Control with Tight Temp Stability (7.9 GPM)

The Westinghouse HW180A0G-NG brings WiFi app control and ±1 degree Fahrenheit temperature stability to a price point that undercuts most premium brands. The app lets you monitor and adjust water temperature remotely, set schedules, and receive error alerts, which is genuinely useful for vacation homes or rental properties where you want to dial down the temp when nobody’s there.

The ±1F temperature stability is better than most competitors at this price. Many tankless units swing 3 to 5 degrees during use, which most people don’t notice but matters if you have temperature-sensitive applications like a soaking tub or a baby’s bath.

Installation gets consistent praise in reviews for the clear step-by-step instructions, which matters since finding a contractor familiar with newer brands like Westinghouse can be hit or miss outside major metros. The variable-speed combustion fan adjusts based on demand, which contributes to both the temperature stability and energy efficiency.

The control board reliability is the main concern. A handful of reviews report units arriving with defective boards or boards failing within the first few months. Westinghouse offers 2 to 5 years of after-sale support, but dealing with warranty service on a less-established brand is inherently more friction than going with Rinnai or Rheem.

Best Fit: Tech-Savvy Homeowners Who Want App Control

If you specifically want to control your water heater from your phone, set vacation schedules, or get alerts when something goes wrong, this is the most affordable way to get those features without jumping to a premium brand. The temperature stability also makes it a strong pick for anyone sensitive to hot water fluctuations.

What to Watch For Before Buying

Confirm a local installer is willing to work on Westinghouse before you buy, since some plumbers only service brands they sell. The 3 to 5 fixture limit means this isn’t the right choice for a large household with heavy simultaneous demand. Budget extra time for the initial hot water delivery to distant fixtures.

9. MIZUDO HW180A0W-NG – Best Value WiFi Gas Tankless (7.9 GPM)

The MIZUDO HW180A0W-NG brings sealed combustion, WiFi app control, and CSA certification to a price point that undercuts every comparable brand on this list. With 237 verified reviews and a 70 percent five-star rate, it’s one of the most reviewed budget-friendly options, which gives you more data to evaluate than most unknown brands offer.

The sealed combustion system is a feature I didn’t expect at this price. It draws combustion air from outside rather than using indoor air, which is safer and more efficient than open-combustion designs. The oxygen-free copper heat exchanger matches what premium brands use, and the minimum activation flow of 0.53 GPM means the burner kicks on even with low-flow fixtures.

The WiFi and app control work well according to most reviews, and the compact size saves roughly 50 percent of the space compared to a tank heater. Owners consistently praise the value proposition, with several noting their propane tanks last 2 to 3 weeks longer than with their old tank unit.

The freeze protection is the weak point. Several owners in cold climates report the anti-freeze feature failing in extreme conditions, and MIZUDO explicitly states the unit isn’t recommended for elevations above 2,000 feet. Burner alignment can also shift during shipping, which requires adjustment during installation.

Best Fit: Mild Climate Homes Seeking Maximum Value

If you live somewhere that rarely drops below freezing and you want the most features per dollar, the MIZUDO is hard to beat. The combination of WiFi control, sealed combustion, and CSA certification at this price is genuinely impressive.

What to Watch For Before Buying

Skip this unit if you live above 2,000 feet elevation or in a region with sustained sub-freezing temperatures. Plan to buy a separate vent pipe since it’s not included. Have your installer verify the burner alignment before firing the unit for the first time.

10. FOGATTI FOG-PM18OS – Best Outdoor Gas Tankless (8.1 GPM)

The FOGATTI FOG-PM18OS is my pick for the best outdoor gas tankless water heater because it’s purpose-built for exterior installation, which solves two problems at once: no indoor venting to deal with, and no utility closet giving up square footage. At 8.1 GPM and 180,000 BTU it’s got the flow for a 3-bathroom home, and the water servo technology maintains precise output temperature.

The four-stage combustion system is more sophisticated than I expected at this price, and the 304 stainless steel burner should resist the corrosion that kills outdoor units prematurely. FOGATTI rates it for 3 to 5 outlet points, which lines up with the 8.1 GPM spec.

The unit operates across a wide water pressure range of 15 to 150 PSI, which matters if you’re on a well system or have low municipal pressure. The active heating anti-freeze device is essential for outdoor placement and kicks on automatically when temperatures drop.

The temperature control issue at low flow is the recurring complaint. At very low flow rates, output temperature can spike to 170F, which is a scald risk. This isn’t unique to FOGATTI but it’s worth knowing if you have low-flow fixtures or occasional low-demand scenarios. Some owners also report ignition errors that typically trace back to gas pressure issues.

Best Fit: Warm Climate Homes and Outdoor Installation Preferences

If you live in a warm climate where freezing isn’t a concern, or you specifically want to reclaim indoor space by moving the water heater outside, this FOGATTI is the most capable outdoor-focused option in this price range. The low-pressure tolerance also makes it a fit for well water systems.

What to Watch For Before Buying

Install a tempering valve if you have low-flow fixtures to address the temperature spike risk. Confirm your climate is compatible with outdoor placement since even with anti-freeze protection, sustained deep freezes are risky. Plan for a weatherproof enclosure or covered installation location.

11. MIZUDO FDG-CS120AW-NG – Most Compact Dual Install (5.1 GPM)

The MIZUDO FDG-CS120AW-NG is the smallest unit on this list at 5.1 GPM and 120,000 BTU, but that’s exactly the point. For a 1 to 2 bathroom home, apartment, or small house, this is more than enough capacity, and the indoor-outdoor convertible design gives you installation flexibility that bigger units don’t offer.

Owners consistently praise how easy the installation is. One verified review mentioned hanging and hooking up the unit in minutes, which is unusual for any gas appliance. The phosphorous-deoxidized copper heat exchanger is the same material used in premium brands, and the stainless steel combustion chamber adds durability.

The ±1F temperature control matches what Westinghouse offers, and the WiFi app provides useful real-time information including temperature, flow rate, and error codes. The 2-year full unit warranty plus 5-year heat exchanger coverage is competitive at this price tier.

The non-standard gas fitting is the main annoyance. Several owners needed adapters because the unit uses a fitting that doesn’t match typical US gas plumbing. Plan for extra fittings and possibly a trip to the supply house. The unit also uses indoor air for combustion when installed indoors, which is less ideal than sealed combustion.

Best Fit: Small Homes, Apartments, and Accessory Dwelling Units

If you’re heating water for a 1-bathroom home, a guest house, an ADU, or a small apartment, this MIZUDO delivers the right amount of capacity without overpaying for flow you won’t use. The dual indoor-outdoor capability also helps if your install location is uncertain.

What to Watch For Before Buying

Buy gas fitting adapters in advance since the included fitting isn’t standard US plumbing size. Don’t expect instant hot water at fixtures more than 30 feet away, since owners report 35 to 40 second waits. Plan for additional venting if installing indoors, since the unit requires a 3-inch OD vent pipe system.

12. ThermoMate CZ422NG – Lightest Most Compact Option (5 GPM)

The ThermoMate CZ422NG is the lightest gas tankless water heater on this list at just 22 pounds, and at 5 GPM with 113,000 BTU it’s sized for a 1 to 2 bathroom home where space is at a premium. The 98 percent thermal efficiency rating is impressive for a non-condensing unit, and ThermoMate claims about 30 percent operating cost savings versus traditional tank systems.

The smart flow sensor with control chip adjusts 20,000 times per second, which is the kind of spec you usually see on units costing twice as much. The 3.5-inch LED display shows real-time diagnostics and error codes, which takes the guesswork out of troubleshooting.

Eight safety protections are included: boil-dry, freeze, flame-out, overheat, over-temperature, pipe blockage, high-temperature lock, and excessive air pressure. The DC high-speed fan with 360Pa static pressure makes the unit wind resistant up to Category 3 hurricane conditions, which is a unique selling point if you live in a coastal storm zone.

The freeze protection is the weak link. Multiple owners report the unit freezing even with protection enabled, particularly in sustained sub-freezing weather. ThermoMate’s customer service gets positive reviews for responsiveness, but you shouldn’t rely on this unit in genuinely cold climates without additional protection.

Best Fit: Small Homes and Storm-Prone Regions

The 22-pound weight makes this the easiest gas tankless to wall-mount, which matters if you’re working with a tight utility closet or a masonry wall. The hurricane wind resistance rating makes it worth considering for Gulf Coast and Atlantic seaboard homes where lesser units could be knocked offline by storm winds.

What to Watch For Before Buying

This is an indoor-only unit, so plan your installation location accordingly. Read the venting documentation carefully and don’t hesitate to contact ThermoMate support if anything is unclear, since several owners noted the instructions could be better. Add pipe insulation or a small enclosure if you live anywhere that drops below freezing for extended periods.

Gas Tankless Water Heater Buying Guide

Buying a gas tankless water heater comes down to four numbers and one decision. The numbers are GPM (flow rate), BTU (heat input), UEF (efficiency), and your incoming groundwater temperature. The decision is condensing versus non-condensing. Get those right and you’ll be happy with whatever brand you choose.

Sizing by Flow Rate (GPM)

GPM, or gallons per minute, is the single most important spec. A standard showerhead flows at 2.5 GPM, a bathroom faucet at 1.5 GPM, a kitchen faucet at 2.2 GPM, a dishwasher at 2 to 3 GPM, and a washing machine at 3 GPM. Add up the fixtures you might run at once to get your peak demand. For a family of four doing morning routines, that’s typically two showers (5 GPM) plus a sink (1.5 GPM) plus maybe the dishwasher (2.5 GPM), or about 9 GPM peak. The Rinnai RX199iN at 11.1 GPM gives you headroom, while the ThermoMate at 5 GPM means one shower and one sink at the same time.

Here’s the catch: rated GPM is at a specific temperature rise, usually 35F. If your incoming groundwater is 40F and you want 120F output, that’s an 80F rise, and your effective GPM drops significantly. Manufacturers publish temperature rise charts that show actual flow at various rises, and you should always check those before buying.

BTU Input and Gas Line Sizing

BTU is the heat input. More BTU means faster heating and higher GPM at greater temperature rises, but it also means bigger gas line requirements. A 199,000 BTU unit like the Rinnai RX199iN typically needs a 3/4-inch gas line. A 113,000 BTU unit like the ThermoMate can often work with a 1/2-inch line. If your home has an older 1/2-inch gas run to the water heater location, you’re either paying to upgrade the line or choosing a lower-BTU model like the Rheem RTG-70DVLN-3 that’s designed for 1/2-inch compatibility.

Condensing vs Non-Condensing

Condensing units capture waste heat from exhaust gases to pre-heat incoming water, pushing efficiency to 0.96 to 0.98 UEF. They use PVC venting because exhaust exits cool. Non-condensing units exhaust gases hot, which means dedicated stainless steel venting but lower equipment cost. Condensing units cost more up front but save on gas over time and use cheaper venting. For most homeowners running the math, condensing pays off within 4 to 6 years.

Venting Requirements

Indoor gas tankless units need dedicated venting, and the type depends on condensing versus non-condensing. Condensing units like the Rinnai RX series use PVC or ABS pipe because exhaust is cool enough to be safe in plastic. Non-condensing units like the Rheem RTG-70DVLN-3 require stainless steel category III venting, which is significantly more expensive. Outdoor units like the FOGATTI need no venting at all but require freeze protection for the climate.

Installation Cost Reality Check

Forum data from r/Plumbing and r/homeowners consistently shows installation running $500 to $2,000 on top of the unit price, with most installs landing around $1,200 to $1,500. Variables include gas line upgrades, venting runs, electrical work for the 120V outlet, plumbing repiping, and permits. Get three quotes and don’t go with the cheapest, since most tankless headaches trace back to sloppy installation rather than equipment failures.

Hard Water and Maintenance

If you have hard water, you need to descale every 6 to 12 months or your heat exchanger will fail early. Descaling involves circulating a vinegar or descaling solution through the unit with a small pump, which takes about 30 minutes. The A.O. Smith ATI-310CX3 is the only unit on this list that claims to eliminate this requirement with X3 Scale Prevention, which is a meaningful advantage if you know your water is hard and you know you’ll skip maintenance.

FAQs

Final Thoughts on the Best Gas Tankless Water Heaters

After living with a Rinnai RX199iN for over a year and researching every major competitor in the 2026 market, my bottom line is this: Rinnai owns the top of the reliability and warranty heap, A.O. Smith’s X3 Scale Prevention is the most meaningful innovation in this category for years, and Rheem gives you the best bang for the buck if your budget is the deciding factor. The best gas tankless water heaters all share the same core promise: endless hot water, lower energy bills, and a 20-year service life that makes tank heaters look disposable.

If you want one recommendation, the Rinnai RX199iN is what I’d put in my house again. If you live in hard water territory, the A.O. Smith ATI-310CX3 deserves a hard look for the scale prevention alone. Whatever you choose, budget for a licensed plumber, register the warranty the day it’s installed, and commit to the descaling schedule if your unit doesn’t have scale prevention built in. Do those three things and you’ll get the 20-year lifespan tankless heaters are capable of delivering.