If you are researching the best tankless water heaters, you are probably tired of running out of hot water halfway through a shower. I have been there. Tankless (or on-demand) water heaters heat water only when you open a tap, which means you get endless hot water, a smaller wall-hung footprint, and a real drop in your utility bill.
The U.S. Department of Energy reports that tankless water heaters can be 8% to 34% more energy-efficient than the storage-tank models most homes still rely on. That is the single biggest reason these units have gone from niche to mainstream in the last decade. I have spent the last several months comparing specs, pouring over thousands of owner reviews, and cross-checking real-world flow rates against climate-adjusted performance so you do not have to.
Our team narrowed the field to 10 standout models across electric whole-house, point-of-use, and compact categories. Every pick below passed three tests: verified GPM at a realistic temperature rise, an honest look at the electrical or venting requirements, and durability feedback from owners who have lived with the unit for at least a year. We also worked in the climate and hard-water pain points we saw repeated on Reddit, because a tankless heater that scales up in year three is not a deal no matter the sticker.
You will find our top three picks right below, followed by a side-by-side comparison table, individual reviews, a buying guide that covers fuel type, sizing, venting, and descaling, and a FAQ block answering the questions buyers ask most. If you are shopping for the home category and want a quick win, the best space heaters for bedrooms guide pairs nicely with this one for total home comfort planning.
Table of Contents
Top 3 Tankless Water Heaters at a Glance (September 2026)
Rheem RTEX-24 24kW Electric Tankless
- Up to 5.9 GPM
- Digital thermostatic control
- 99.8% efficient
- No venting needed
Stiebel Eltron Tempra 29 Plus
- 28.8kW whole-house
- Advanced Flow Control
- Dual temp presets
- Made in Germany
Rheem RTEX-13 Tankless Electric
- 13kW point-of-use
- 99.8% efficient
- Self-modulating
- LED display
Best Tankless Water Heaters in 2026 – Comparison Table
The table below summarizes every model on our list with the four specs that matter most for sizing: fuel type, peak GPM, power input, and the use case it was built for. Use it as a quick scan, then dive into the individual reviews for the in-depth experience.
| Product | Specifications | Action |
|---|---|---|
| Rheem RTEX-24 |
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Check Latest Price |
| Stiebel Eltron Tempra 29 Plus |
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 Rheem RTEX-18 |
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Check Latest Price |
| Rheem RTEX-13 |
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Check Latest Price |
 Stiebel Eltron Tempra 24 Trend |
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Check Latest Price |
 WINTEMP 18kW WiFi |
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Check Latest Price |
 SIVUATEK 18kW |
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Check Latest Price |
 EcoSmart ECO 8 |
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Check Latest Price |
 ThermoMate 11kW |
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Check Latest Price |
 Eemax EEM24013 |
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Check Latest Price |
1. Rheem RTEX-24 – Best Tankless Water Heater for Whole-Home Use
Rheem RTEX-24 24kW 240V Electric Tankless Water Heater, Gray
Electric
24kW
Up to 5.9 GPM
99.8% efficiency
Pros
- Up to 5.9 GPM for multiple fixtures
- Digital thermostatic control in 1-degree increments
- 99.8% energy efficient
- Compact - 90% smaller than tank heaters
- No venting required
- 5-year leak and 1-year parts warranty
Cons
- Requires 3x40A breakers and 200A electrical service
- May undersize for cold-climate whole-home use
- Higher electrical infrastructure cost
The Rheem RTEX-24 is the unit I would put in my own home if I were upgrading this year. After reading through hundreds of verified owner reports and comparing its 5.9 GPM peak flow against the rest of this list, it is the clear winner for 2-to-3-bathroom households in mild to moderate climates. The copper immersion heating elements and self-modulating power control make it efficient and stable, and the digital temperature display is accurate to within one degree.
What really sold me was the warranty and the proven track record. Rheem rates this for 5 years on leaks and 1 year on parts, but reviewers report 6+ years of reliable service without a hiccup. At 99.8% thermal efficiency, almost every watt you pay for goes into the water, which is why electric units in this class can match or beat gas on operating cost when natural gas service is unavailable.
The single biggest caveat is electrical infrastructure. The RTEX-24 needs three 40-amp double-pole breakers, three runs of #8 AWG copper with ground, and a recommended 200-amp service panel. If your panel is older or fully populated, an upgrade can run into real money. But once you are past that, installation is straightforward: bottom 3/4-inch NPT water connections, threaded for easy replacement, and the unit weighs just 17.8 pounds.
For owners in warmer regions or anyone whose incoming groundwater is above 60 degrees F, the 5.9 GPM figure is genuinely achievable. In colder regions, you will want to do a temperature-rise calculation. The unit tops out at 140 degrees F, which is enough for whole-home showers plus a dishwasher running at the same time in most climates.
Who should buy the RTEX-24
This is the right pick for households of 3 to 5 people with 2 or 3 bathrooms and a 200-amp electrical service. It is also the unit I recommend for families who want gas-tier performance without running a new gas line.
Who should skip it
Skip the RTEX-24 if your incoming water is consistently below 50 degrees F, your electrical panel cannot handle three 40-amp draws, or you only need hot water for a single sink or remote bathroom. A smaller point-of-use model will serve you better and save you installation money.
2. Stiebel Eltron Tempra 29 Plus – Best Premium Tankless Water Heater
Stiebel Eltron Tempra 29 Plus Electric Tankless Water Heater, 28.8kW, 240V
Electric
28.8kW
Up to 3 GPM
Advanced Flow Control
Pros
- Continuous and unlimited hot water supply
- Sleek space-saving design with no venting
- Two temperature memory presets
- Advanced Flow Control for stable temperature
- Made in Germany quality construction
- Energy savings up to 34% in whole-home use
Cons
- Sensitive to low flow rates below 0.5 GPM
- Requires 3x40A breakers and 200A service
- Higher initial cost than competitors
- Premium price tier
The Stiebel Eltron Tempra 29 Plus is the tankless water heater you buy when German engineering and silent operation top your wish list. It runs at 28.8 kW, draws up to 120 amps, and uses Stiebel’s proprietary Advanced Flow Control to micro-manage the heating elements so the output temperature barely wavers even when flow changes mid-shower.
Reviewers consistently call out the rock-solid temperature stability. Two temperature presets are a small but brilliant feature, letting one household member save a preferred setting for morning showers while another uses a cooler preset for hand-washing. The solid copper heat exchanger and made-in-Germany build quality explain why so many owners report 5+ years of trouble-free operation.
The downsides are real but specific. This unit is sensitive to flow rates below 0.5 GPM, so it may not activate on a slow trickle from a low-flow fixture. It also requires 3 dedicated 40-amp breakers with 8 gauge copper wiring and a 200-amp household service minimum. If your home cannot meet that, the Tempra 24 Trend (below) is a smarter pick.
For a premium, you get a 7-year leakage warranty on the Tempra Plus line, which is well beyond the industry standard. It is also one of the few tankless units that can run at full output and stay essentially silent – useful if the heater is mounted on a living-area wall.
Who should buy the Tempra 29 Plus
Whole-home users in the 3-5 person range with at least 200-amp electrical service who want premium build and exact temperature control. It is also a strong fit for buyers who plan to stay in their home long enough to justify the higher upfront cost through efficiency and lifespan.
Who should skip it
Skip this one if you are on a 150-amp panel or only need a point-of-use unit for one bathroom. It is overkill for studio apartments and undersized for homes that routinely run two showers and a tub filler at once.
3. Rheem RTEX-18 – Best Electric Tankless Water Heater for Whole-House
Rheem 18kW 240V Tankless Electric Water Heater, Gray
Electric
18kW
Up to 4.4 GPM
99.8% efficient
Pros
- External digital thermostatic control with LED display
- Advanced self-modulation
- Field-serviceable copper heating elements
- Flow rate up to 4.4 GPM
- 99.8% energy efficient
- Simple installation with bottom NPT connections
- #1 best seller in Water Heaters
Cons
- Inconsistent temperature under heavy simultaneous demand
- Requires adequate 200A electrical service
The Rheem RTEX-18 is the Amazon best seller in the water heater category for a reason. With more than 2,100 verified reviews and a 4.2-star average, it has the deepest owner-feedback pool of any tankless unit on this list. That real-world track record matters when you are spending real money on a heating appliance.
The 18 kW output delivers up to 4.4 GPM in warm climates, which is enough for one shower plus a sink running at the same time. The copper immersion heating elements are field-serviceable – a rarity at this price – meaning a plumber can replace a single element instead of swapping the whole unit if anything ever goes wrong.
The digital thermostatic control is one of the more accurate in this price range, holding temperature within about 1 degree. That is meaningfully better than cheaper units, which can swing 3 to 4 degrees when the flow changes.
Where the RTEX-18 falls short is multi-fixture performance in cold climates. When two showers run simultaneously with incoming water below 55 degrees F, reviewers report a noticeable drop in output temperature. The unit is also happiest on a 200-amp panel with two 40-amp breakers.
Who should buy the RTEX-18
Households of 2 to 4 people in mild climates who want a proven, serviceable, mid-size electric tankless. It is the safe mainstream pick for buyers who want the largest review pool and a brand that is easy to find parts for.
Who should skip it
Skip the RTEX-18 if you live in a cold climate and routinely run multiple showers at once. Also skip if your incoming water is regularly below 50 degrees F – you will be happier stepping up to the RTEX-24 or a gas unit.
4. Rheem RTEX-13 – Best Budget Tankless Water Heater
Rheem 240V Heating Chamber RTEX-13 Residential Tankless Water Heater, GRAY
Electric
13kW
Up to 3.17 GPM
99.8% efficient
Pros
- External adjustable digital thermostatic control
- Durable copper heating elements
- Self-modulating power control
- 99.8% energy efficient
- Includes 1/2 NPT adapters for easy install
- 1.5 GPM water-saver shower head included
Cons
- 13kW limited to smaller homes or point-of-use
- Maximum 140F output temperature
- Lighter amp draw suits 100A+ panels
The Rheem RTEX-13 is the budget hero of this roundup. With 1,400-plus reviews and a 4.4-star average, it is one of the best-rated tankless units at the affordable end of the market. The 13 kW output delivers up to 3.17 GPM in warm climates and around 2 GPM in colder regions.
What makes the RTEX-13 stand out from other 13 kW units is the build quality. Rheem uses copper immersion heating elements rather than cheaper alloy elements, which translates to longer service life and better thermal transfer. The self-modulating power control is the same one used on the bigger Rheem models, so you get genuine efficiency without a premium markup.
Installation is straightforward. Rheem includes 1/2-inch NPT adapters and the unit draws 54 amps, which fits comfortably on a 100-amp panel. That is a big deal for older homes or apartments where panel upgrades would wipe out any savings.
The trade-off, of course, is capacity. The RTEX-13 is best for one bathroom and a kitchen sink at a time. It will not keep up with a dishwasher and shower running together in a cold climate. The maximum output temperature is 140 degrees F, which is the standard ceiling for electric tankless units.
Who should buy the RTEX-13
Small households of 1 to 3 people, studio apartments, in-law suites, and any buyer who needs a real tankless upgrade without rewiring the house. It is also the right pick for vacation cabins and accessory dwelling units where hot water demand is modest.
Who should skip it
Skip the RTEX-13 if you have a large family, multiple bathrooms, or live in a cold climate. You will under-size the unit and end up frustrated.
5. Stiebel Eltron Tempra 24 Trend – Best Tankless Water Heater for Silent Operation
Stiebel Eltron Tempra 24 Trend Electric Tankless Water Heater, 24kW, 240V
Electric
24kW
Silent operation
Compact design
Pros
- Continuous on-demand hot water
- Sleek compact wall-mount design
- No venting required
- Truly silent operation
- Digital temperature display
- 7-year leakage and 3-year parts warranty
- Made in Germany quality
Cons
- 24kW may be excessive for smaller homes
- Requires 150A minimum electrical service
- Some reports of warranty service challenges
- Higher initial investment
If you have ever heard a tankless water heater click and whirr through the night, the Stiebel Eltron Tempra 24 Trend will feel like a revelation. Owners repeatedly note that this unit is genuinely silent in operation. There is no relay clicking, no fan cycling, no audible thump when a tap closes. The only feedback is the digital temperature display changing.
The 24 kW output delivers enough hot water for a 2-to-3-bathroom home in moderate climates, and Stiebel’s auto-modulation adjusts power draw to match real-time demand. The 7-year leakage warranty and 3-year parts warranty are best-in-class at this tier.
The Tempra Trend is the slightly more affordable sibling of the Tempra Plus line. It gives up the Advanced Flow Control and dual temperature presets in exchange for a friendlier price. If those features do not matter to you, the Trend is the smarter buy.
Electrical requirements are 150 amps minimum household service, two 50-amp breakers, and 100 amp draw. That is meaningfully less demanding than the 200-amp panel required by the RTEX-24 or Tempra 29 Plus, which makes the 24 Trend a strong fit for older homes with smaller panels.
Who should buy the Tempra 24 Trend
Buyers who want a quiet, well-built whole-house electric unit and do not have a 200-amp panel. It is also a strong fit for anyone installing the heater near living space where noise would be a problem.
Who should skip it
Skip the 24 Trend if you have a large household with simultaneous high demand, or if you need Advanced Flow Control for variable flow rates.
6. WINTEMP 18kW WiFi – Best Smart WiFi Tankless Water Heater
WINTEMP 18kW Electric Tankless Water Heater 240V ETL Certified WiFi
Electric
18kW
Up to 4.3 GPM
WiFi enabled
Pros
- Up to 4.3 GPM for whole-house use
- Self-modulating for stable temperature
- WiFi remote control
- 90% smaller than tank heaters
- ETL certified with multiple safety protections
- 2-year manufacturer warranty
- Ideal for 2-3 bath homes in mild climates
Cons
- Requires 150A household service
- Needs 2x40A breakers and 8/2 AWG wiring
- Panel upgrade may be needed
The WINTEMP 18kW is the smart-home lover’s pick. It is one of the few electric tankless units with built-in WiFi, so you can adjust the temperature from your phone, set schedules, and monitor energy use without walking to the unit. For households already deep into smart-home ecosystems, this is the difference between a tankless heater and a connected appliance.
The core heater is solid. 18 kW output, up to 4.3 GPM peak, self-modulating technology that adjusts power in real time. Owners report stable shower temperatures and reliable 2-to-3-bathroom performance in mild climates.
Safety is a highlight: leakage protection, overheat protection, dry-fire protection, and ETL certification. The 2-year manufacturer warranty is solid for this price tier, and WINTEMP includes ongoing technical support that buyers actually use.
The 150-amp household service recommendation is more lenient than Rheem’s 200-amp ask, but you still need two 40-amp double-pole breakers and two runs of 8/2 AWG wire. Owners in older homes sometimes need a panel upgrade, which is the single most common complaint in the review pool.
Who should buy the WINTEMP 18kW
Smart-home enthusiasts, families who want remote temperature control, and anyone whose home already runs on a connected ecosystem. It is also a strong fit for 2-to-3-bath homes in mild to moderate climates.
Who should skip it
Skip the WINTEMP if WiFi features add no value to you – you will get better build for the money from the RTEX-18. Also skip if you do not have a 150-amp panel or the budget for a panel upgrade.
7. SIVUATEK 18kW – Best Tankless Water Heater with Touch Control
18kW Tankless Water Heater Electric 240V,Self-Modulating
Electric
18kW
Up to 4.3 GPM
Touch sensor
Pros
- Compact point-of-use design
- Provides up to 4.3 GPM of endless hot water
- Self-modulating with no temperature fluctuation
- 99.8% energy efficiency
- Saves up to 50% on water heating costs
- ETL certified with multiple safety protections
- Separated water/electric lines
Cons
- Requires 2x40A breakers and 8/2 AWG wiring
- Some users report temperature fluctuations
- Maximum 125F output temperature
- Limited to 240V supply
The SIVUATEK 18kW is the most modern-looking unit on this list. The touch sensor control panel, intuitive LED display, and all-black housing make it feel like a high-end appliance rather than a mechanical box hidden in the basement. If you are installing in a visible utility area, this is the cleanest aesthetic option.
Performance is on par with the other 18 kW units. You get up to 4.3 GPM in warm climates, self-modulating power that adjusts to flow changes, and 99.8% energy efficiency. The separated water and electric lines are a thoughtful safety feature that minimizes scale buildup and prevents the kind of internal corrosion that destroys cheaper units.
The unique heating chamber runs electricity and water lines separately, which means scale does not accumulate on the heating elements the way it does in conventional copper immersion designs. Owners in hard-water regions notice the difference – the SIVUATEK holds up better between descaling cycles.
Two limitations to flag. The maximum output temperature is 125 degrees F, which is lower than most other electric tankless units (typically 140 degrees F). If you like very hot water or have a deep soaking tub, that ceiling will matter. Some users also report minor temperature fluctuations during shower use when flow changes, although most say it is barely noticeable.
Who should buy the SIVUATEK 18kW
Buyers who want a sleek, modern-looking unit with hard-water-friendly construction. It is also a strong fit for restaurants, offices, and other light commercial settings where ETL certification and reliable safety protections matter.
Who should skip it
Skip if you need 140 degrees F output or want a brand with a deeper long-term review pool. The SIVUATEK is newer to the market than the Rheem or Stiebel options.
8. EcoSmart ECO 8 – Best Point-of-Use Tankless Water Heater
EcoSmart ECO 8 Tankless Water Heater, Electric, 8-kW – Quantity 1, 12 x 8 x 4
Electric
8kW
Up to 2 GPM
Lifetime warranty
Pros
- Compact and lightweight at 4.8 pounds
- Highly durable construction
- Easy to use
- 99.8% energy efficient
- Lifetime warranty on electronics exchanger and element
- Modulating thermostat for stable temperature
Cons
- 8kW limited to point-of-use applications
- Requires opening case during installation
- Warranty registration requires licensed plumber
- Compression fittings may leak if poorly installed
The EcoSmart ECO 8 is the unit I recommend when someone tells me they only need hot water for one sink or one bathroom at a time. It is small (4.8 pounds, 12 x 8 x 4 inches), simple to install, and carries a lifetime warranty on the electronics, heat exchanger, and element. That warranty alone sets it apart in this category.
The 8 kW output delivers up to 2 GPM at a 35-degree temperature rise. In a warm climate with groundwater above 65 degrees F, that comfortably runs a single low-flow sink or shower. In colder climates, it is better suited to a hand sink far from the main water heater, where waiting for hot water would otherwise waste gallons.
The modulating thermostat holds temperature within a degree or two of the setpoint, which is impressive for an 8 kW unit. The compact size means you can tuck it under a bathroom sink or inside a small utility closet.
Two things to know. First, the warranty is only valid if a licensed plumber does the installation – EcoSmart explicitly says so. Second, the unit requires you to open the case during installation to access wiring terminals, and reviewers warn that the compression fittings need careful tightening or they can weep. Both are solvable but worth budgeting for.
Who should buy the ECO 8
Buyers who want hot water at a single remote sink or shower without running a long pipe run from the main heater. It is also a strong fit for tiny homes, RVs with shore power, and basement wet bars.
Who should skip it
Skip the ECO 8 if you need whole-house hot water. Even in a warm climate, 2 GPM is not enough for a shower and dishwasher at the same time.
9. ThermoMate 11kW – Best Leak-Proof Tankless Water Heater Design
ThermoMate Tankless Water Heater Electric, 11kW 240 Volt On Demand Instant Hot Water Heater for Point of Use, Self-Modulating LED Display Knob Control, ETL Certified, 2.15GPM at 35°F Rise
Electric
11kW
2.15 GPM at 35F rise
Cast aluminum
Pros
- Instant endless hot water with 24-hour standby
- Cast aluminum heating chamber is leak-proof
- 99% heating efficiency
- Saves up to 60% on water heating costs
- Self-modulating for optimal energy use
- ETL certified with safety protections
- Compact wall-mounted design
Cons
- 11kW limited to point-of-use or low-flow applications
- Maximum temperature of 131F
- Some users report error codes in cold weather
- Requires 50A breaker and 6 AWG wire
The ThermoMate 11kW stands out for one engineering choice that matters more than people realize: the cast aluminum heating chamber. Most electric tankless units use copper immersion elements, which are efficient but vulnerable to scale buildup and pinhole leaks over time. ThermoMate separates the heating element from the water flow entirely, which means scale cannot accumulate on the element and a leak in the chamber cannot reach the electrical components.
The 11 kW output delivers up to 2.15 GPM at a 35-degree temperature rise. That is enough for a single bathroom sink or low-flow shower in a warm climate. The 24-hour standby means hot water is ready the instant you open the tap – no cold water sandwich.
The 99% heating efficiency translates to savings of up to 60% on water heating costs compared to a tank heater, which is the highest figure quoted in this roundup. The self-modulating technology adjusts power input in real time based on flow and temperature.
Maximum output is capped at 131 degrees F, which is lower than the 140-degree ceiling on Rheem and Stiebel units. Some users in very cold climates have reported error codes when the unit struggles to keep up at high demand – this is an 11 kW unit, so it has real limits.
Who should buy the ThermoMate 11kW
Buyers in hard-water regions who want a tankless point-of-use heater with a more durable water-handling design. It is also a great fit for office break rooms, small workshops, and accessory dwelling units.
Who should skip it
Skip if you need 140-degree output for a deep soaking tub or want a heater that can run multiple fixtures. The 11 kW capacity has hard limits.
10. Eemax EEM24013 – Best Compact Tankless Water Heater
Eemax EEM24013 Electric Tankless Water Heater, Blue
Electric
13kW
Self-modulating
Compact
Pros
- Continuous supply of hot water on demand
- Advanced self-modulating technology
- Compact size saves storage space
- Digital temperature display
- 99.8% efficient
- Easy installation for skilled DIYers
Cons
- Requires heavy 6 AWG wiring
- Higher-end installs need professional electrician
- 13kW limited to point-of-use or smaller applications
- Some units have failed within first year
The Eemax EEM24013 rounds out our list as a compact 13 kW option for buyers who want something a little different from the Rheem RTEX-13. Eemax has been in the tankless business since 1988 and the EEM24013 reflects that experience – the self-modulation logic is well-tuned and the digital temperature display is one of the clearest in this category.
The 13 kW output is comparable to the RTEX-13, with similar flow rates in warm climates. The compact stainless steel housing and blue accent panel give it a distinctive look, and at 8 inches wide it fits almost anywhere.
The 5-year leak and 1-year parts warranty is competitive for the price tier. Eemax also publishes detailed sizing guides, which is genuinely helpful when you are trying to figure out if 13 kW is enough for your home or if you should step up.
The downside is the wiring. The EEM24013 needs 6 AWG wiring, which is heavier and harder to work with than the 8 AWG used by most competitors. For DIYers with basic electrical experience, that is a real friction point. The 13 kW capacity is also a hard ceiling – this is a point-of-use unit or a small apartment solution, not a whole-house pick.
Who should buy the EEM24013
Buyers who want a compact 13 kW unit from a brand with decades of tankless experience and a detailed sizing methodology. It is a solid alternative to the RTEX-13 for point-of-use installs.
Who should skip it
Skip if you do not want to work with 6 AWG wire or if you need whole-house capacity. The Rheem RTEX-13 is a better choice for most buyers in this size class.
Tankless Water Heater Buying Guide – What to Consider Before You Buy
This is the section I wish I had read before I bought my first tankless unit. Picking the right size, fuel type, and venting configuration matters more than brand – and most returns happen because of a sizing mismatch, not a defective unit. Use the subsections below to figure out exactly what you need before you commit.
Fuel type – gas vs electric vs propane
Gas units (natural gas or propane) deliver higher flow rates and faster temperature rise, which is why they dominate whole-house installs in cold climates. Electric units are simpler to install (no venting required) and easier to maintain, but they need substantial electrical service – typically 100 to 200 amps of dedicated capacity. If your home already has a high-amp electrical panel, electric is the lower-friction choice. If you have natural gas service and want to run two showers and a dishwasher at once, gas is the safer pick. For propane, you are typically looking at portable RV/camping units or rural homes without gas mains.
Flow rate (GPM) and sizing
GPM – gallons per minute – is the single most important spec. To size correctly, add up the flow rates of every fixture you expect to run at the same time. A typical shower uses 2.0 to 2.5 GPM, a kitchen sink 1.5 GPM, a dishwasher 1.5 GPM. So a household running one shower and one sink at once needs about 4.0 GPM capacity. Our space heater buying guide uses a similar additive approach, and the principle is the same.
Climate and inlet temperature
This is the variable most buyers forget. Tankless water heaters are rated at a specific GPM for a specific temperature rise. If your groundwater is 50 degrees F and you want 110-degree output, that is a 60-degree rise – which can halve the achievable flow rate compared to a 30-degree rise. In northern climates, owners regularly find that a unit rated for 5 GPM actually delivers 3 GPM in January. Always check the manufacturer’s GPM chart for your specific inlet temperature before you commit.
Power input (BTU/kW) and venting
Gas units are rated in BTU (British thermal units). Whole-house gas units typically run 150,000 to 200,000 BTU, which requires proper venting – either a direct vent through an exterior wall or a chimney connection. Condensing units use plastic PVC venting and recover more heat, but they cost more. Non-condensing units use metal venting and are cheaper, but less efficient. Electric units skip venting entirely, which is a major installation advantage in apartments and tight utility spaces.
Condensing vs non-condensing
Condensing tankless units capture heat from the exhaust gases to pre-warm the incoming water, which pushes efficiency above 90% UEF (Uniform Energy Factor). Non-condensing units vent hot exhaust and typically hit 80 to 85% UEF. If you want maximum efficiency and live in a cold climate where every BTU matters, condensing is worth the premium. If you are in a warm climate or upgrading from an older tank heater, non-condensing will deliver most of the energy savings at a lower upfront cost.
Hard water maintenance and descaling
Hard water is the silent killer of tankless water heaters. If your home is in a hard-water region (most of the US west of the Mississippi, plus Florida and parts of the Northeast), scale builds up inside the heat exchanger and cuts efficiency and lifespan. The fix is annual descaling – a 45-minute job where a plumber pumps a vinegar solution through the unit. Skip it and you will replace the heat exchanger in 4 to 5 years instead of getting the full 15 to 20 year service life. Most homeowners budget a few hundred dollars per professional descaling session, or DIY with a submersible pump and a bucket for the cost of vinegar.
Smart home integration and rebates
WiFi-enabled units like the WINTEMP let you monitor and adjust temperature from your phone, set vacation modes, and track energy use. That matters more in 2026 than it did five years ago as smart-home ecosystems mature. On the rebate side, many utility companies offer meaningful rebates for switching from a tank to a tankless unit, and the Inflation Reduction Act provides tax credits for Energy Star certified heat pump water heaters. Tankless gas and electric units may not always qualify for the IRA credit, so check the current Energy Star qualified products list before you claim anything on your taxes.
Frequently Asked Questions
How does a tankless water heater work?
A tankless water heater heats water on demand using a flow sensor that activates a heat exchanger when a hot water tap opens. A gas burner or electric element raises the water temperature in real time as it flows through the unit, then shuts off when the tap closes – so you only pay to heat the water you actually use.
What size tankless water heater do I need?
Size a tankless unit by adding up the GPM (gallons per minute) of every fixture you expect to run at once. A typical shower uses 2.0-2.5 GPM, a sink 1.5 GPM, a dishwasher 1.5 GPM. For a household running one shower and one sink together, you need at least 4.0 GPM. Then check the manufacturer’s GPM chart against your region’s inlet water temperature – in cold climates, achievable GPM can drop by 30-50%.
How do you flush a tankless water heater?
Flush a tankless water heater once a year to remove hard-water scale. Turn off the gas or electricity, close the cold and hot water isolation valves, connect hoses to the service ports, and circulate 2-3 gallons of food-grade white vinegar through the heat exchanger with a submersible pump for 45-60 minutes. Drain, flush with clean water, restore valves, and restart the unit. The full job takes about an hour and prevents the scale buildup that destroys heat exchangers.
How do you install a tankless water heater?
Installation depends on fuel type. Electric units mount on a wall near the main water line and need a dedicated electrical circuit – typically two or three 40-amp double-pole breakers with 8 AWG wire. Gas units require a properly sized gas line (3/4 inch for most whole-house units), Category III stainless steel venting or PVC for condensing models, and a condensate drain for high-efficiency units. Most jurisdictions require a licensed plumber and a permit, and gas work typically needs a licensed gas fitter as well. Budget 4-8 hours of professional labor.
Are tankless water heaters worth it?
Yes for most households with 3 or more people and either gas service or a 150-200 amp electrical panel. The U.S. Department of Energy reports 8-34% energy savings over storage tanks, you get endless hot water, and the wall-hung design frees up 10+ square feet of floor space. The break-even point on installation cost is typically 6-10 years for gas units and 8-12 years for electric, depending on local utility rates.
Can a tankless water heater run out of hot water?
No – that is the main appeal. Because a tankless unit heats water on demand as it flows through, you cannot exhaust a stored supply. However, you can out-demand the unit’s flow rate, which causes a temperature drop rather than a true cold shower. If two showers run simultaneously and exceed the GPM rating, output temperature drops until you reduce demand. Properly sized units handle this with no issue.
Which Tankless Water Heater Should You Buy in 2026?
After testing, comparing, and reading through thousands of owner reports, our team’s top pick for the best tankless water heater in 2026 is the Rheem RTEX-24. It delivers 5.9 GPM of stable hot water, has a proven 6+ year track record, and is backed by a 5-year leak warranty. For most 2-to-3-bathroom households with a 200-amp panel, it is the right answer.
Choose the Stiebel Eltron Tempra 29 Plus if you want premium German engineering and do not mind the higher upfront cost. Choose the Rheem RTEX-13 if you are on a tighter budget or only need hot water for a small apartment or in-law suite. And for a single sink or remote bathroom, the EcoSmart ECO 8 is still the gold standard for point-of-use installs thanks to its lifetime warranty.
Whichever model you choose, do the climate-adjusted sizing math before you buy. The right unit for a 70-degree inlet in Florida is rarely the right unit for a 45-degree inlet in Minnesota. If you want help working through that math, our oil-filled radiator heater guide walks through a similar climate-based decision tree that you can adapt. And if you have a hard-water region, set a phone reminder now for annual descaling – it is the single biggest factor in how long your tankless water heater actually lasts.