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Last updated on January 2, 2018

The Best Tankless Water Heater

Powerful, efficient, and reliable
The 30-Second Review

Your tankless water heater should be powerful enough to pump out the gallons per minute you need. To find the best units for a range of home sizes and fuel types, we consulted plumbers, combed through user reviews, and compared the specs of every gas and electric series we could find.

Best Gas Series for
Mid-Sized Homes

The K4 series is perfect for a two-bathroom home that often runs two showers at once. Plus, high-tech features that ensure reliable water temperature come standard.

Best Gas Series for
Large Homes

Rheem Professional Prestige
The most efficient tankless unit on the market is one of the mightiest. It can simultaneously power a washing machine, dishwasher, and shower. We recommend it for homes with three or more bathrooms.

Best Electric Series for
Mid-Sized Homes

Electric units aren’t as powerful as gas, but Stiebel Eltron models come the closest. The Tempra Plus series offers top power and efficiency, plus flow control so your water stays warm no matter what. The top model is capable of producing enough hot water for a two-bathroom home.

The Best Tankless Water Heaters

Tankless water heaters supply hot water to your pipes every time you turn on a faucet or start a load of laundry. The more powerful the water heater is, the quicker it can heat and supply. Because they produce hot water on demand, it’s critical to choose one that’s sized accordingly. This is where the gallons per minute (GPM) measurement comes into play. The larger your household and the colder your incoming water, the higher GPM you’ll need.

Gas or Electric? Every home’s heating system runs on either natural gas, propane, or electric. Whatever your current water heater is hooked up to is very likely the easiest and most cost-effective fuel option for your next one.

We like the Takagi K4 for small- to medium-sized households running gas. Even in cold climates it can supply hot water to as much as two showers and a sink. It also offers safety features that keep water temperature within safe levels, like anti-freeze protection and a hi-limit switch, which turns off the unit if a malfunction leads to overly hot water. Together these prevent sudden temperature changes common in other gas water heaters.

Representing the next level in gas water heating, the Professional Prestige series from Rheem can keep up with three showers plus a sink in the chilliest climates. And at nearly 9 GPM, it approaches the upper limits of tankless power. Its condensing technology makes this unit incredibly energy efficient, as it ensures every drop of heat is squeezed out of emitted gases rather than allowing it to dissipate as vapor.

If you don’t have a gas hookup, Stiebel Eltron is your best bet for electric. The company specializes in electric models, and has led the industry forward with new energy-saving tech that automatically adjusts heating intensity to use only as much as incoming water requires. Its top models can supply well over 5 GPM, ample water for a couple of simultaneous showers — and even more in warm southern climates.

Choosing the right-sized model within those series depends on your climate and water usage: How cold is the water coming into your pipes and how much of it needs to get hot? If you need help sizing your home’s hot water needs, follow our sizing guide down below. We’ll walk you through each step in order to find the model that matches your temperature rise, ideal flow rate, and fuel type.

Our Picks for the Best Tankless Water Heater

Best Gas Series for Mid-Sized Homes

Takagi K4 Tankless Water Heater A solid option for most gas-fueled households, offering a mid-high GPM and a plenitude of customer support.

Takagi crafts well-equipped tankless gas units for just about every flow rate, and was the only manufacturer other than Stiebel Eltron that could confirm its models use flow control technology. We like the K4 series for a mid-sized household, but Takagi also produces gas units capable of supplying more than 8 GPM, even with extremely cold incoming water. When we calculated price per GPM, we found that the K4 offers the most hot water bang-for-your-buck, around $50 less per GPM than its peers. The K4’s lower price is in part due to having a lower Energy Factor than some of its peers. But at 0.79, it still sits squarely at the midrange of gas efficiency. The K4 offers a lot of power, plus nice-to-have features, at a reasonable price.

Pay attention to fuel type. Natural gas and propane models are generally identical in terms of cost, EF, and GPM, but are not interchangeable. Be sure you're buying a unit that's compatible with your home's gas type, as well as the correct model for your desired install location — indoor or outdoor.

Standard features include anti-freeze technology, so its outdoor units run smoothly through all seasons, and a Hi-Limit Switch that ensures water temperature stays within safe levels. This is important for gas units, as inaccurately sized gas lines or venting may produce what’s known as “cold water sandwiches” — unexpected bursts of cold water.

We’re happy to report that calling Takagi was quick and painless every time. Within minutes of dialing, a knowledgeable support rep answered all of our questions — without having to look them up or transfer us. We also appreciated the library of resources on Takagi’s website. It’s chock-full of product specifications, manuals, troubleshooting guides, and even a rebate finder. Whether you prefer to call for assistance or want to DIY the solution yourself, it’s clear that Takagi excels in customer support.

Best Gas Series for Larger Homes

Rheem Professional Prestige Series Virtually endless hot water via one of the most efficient condensing gas units on the market.

The Professional Prestige series can put you in a lot of hot water. Even more impressively, it generates that hot water with maximum efficiency, boasting one of the highest Energy Factors of all gas models. A large part of its efficiency is due to condensing technology which prevents hot gas from escaping, reusing it to heat more water instead. This feature does cost more upfront, but if your household size demands a high flow rate, condensing ensures that every drop of energy is going toward hot water. The more efficient your water heater, the lower your gas bill.

The top model in this powerful series is the Professional Prestige 96, and its dimensions match its stats. While other manufacturers brag about their tankless units’ portable dimensions, the 96 is not a unit you would want to haul around. At approximately 28 x 19 x 10, it resembles a piece of checked luggage. And what it has on the other models in size, it also has on them in GPM capacity. The machine boasts a powerful flow rate (8.5 GPM) and an impressive 0.96 energy factor, well above Energy Star’s expectations for gas efficiency.

It’s also compatible with EcoNet Smart Home Technology. EcoNet-compatible appliances can be remotely managed via app, meaning that you can turn off your water heater even when you’re far from home. While vacationing in tropical waters, a few phone taps ensures your water heater isn’t producing any of its own.

The Rheem website isn’t as user-friendly as Takagi’s — it requires a fair amount of snooping to dig up the same information. You have to hunt down individual models to find their specifications and troubleshooting documents, but with enough poking around, you can find what you need to make a selection, install your unit of choice, and keep up with maintenance later on.

The Professional Prestige line is available in both natural gas and propane, and for both indoor and outdoor installation. Again, double check that you are buying the right model for your specifications. Most homes run on natural gas.

Best Electric Series for Mid-Sized Homes

Stiebel Eltron Tempra Plus The reigning champ of electric water heating, offering stellar technology, sleek design, and reliable service.

It’s only right that Stiebel Eltron should excel in the tankless market: It was the first to manufacture them back in 1927. Of course, tankless water heating has developed substantially since then, but the company maintains its position by continuing to develop new technology.

Few other manufacturers of electric models can come close to Stiebel Eltron in terms of GPM and energy efficiency — all its models boast an energy factor of 0.99, the highest rating among electric tankless. Owning the best of the best comes with a price, however, as Stiebel Eltron surpasses the competition in terms of capacity and cost. Its models typically ring in at $100 to $200 more than units from other brands with equivalent or slightly lower GPM.

So is it worth the upcharge? We say yes, based on power, reliability, cutting-edge technology, and customer support resources.

Stiebel Eltron’s two series, Tempra and Tempra Plus, both come in the same six models: 12, 15, 20, 24, 29, and 36. Like climbing the steps of a staircase, each one increases in power. If you want one to supply your whole house, stick with the 29 and 36 models, which can provide over 4 GPM, but check the chart below to find the model that can deliver your ideal flow rate at your current temperature rise.

All its models come in the exact same dimensions. If the Rheem Professional Prestige sizes to luggage, the Stiebel Eltron is a briefcase, at about 17 x 15 x 5 inches. Installing one in a basement or closet would free up a significant swathe of space. The Tempra and Tempra Plus are sleek too. They feature brushed gray metal covers and detailed control knobs. According to the Stiebel Eltron rep we spoke to, the main difference between the two is that the Plus model uses Advanced Flow Technology. We preferred this option because it means your water will stay warm no matter what, but it also comes with $50 to $200 price jumps.

We appreciated that getting information from Stiebel Eltron was always a straightforward affair. Its customer service phone number actually brings you to customer service, not impenetrable webs of reconnections and dialing 0 for an operator.

Electric Runner-Up

EcoSmart ECO 27 Tankless Water Heater If your water needs are small, a budget-friendly option from EcoSmart may do the trick.

While moving down in price means sacrificing some power and reliability, we were impressed with the EcoSmart ECO 27. The ECO 27 costs less than $500, yet its 3 GPM in a 45° F rise is enough to power two low-flow showers simultaneously.

Be aware that as price reduces, so does GPM, and so does your shower duration. Many cheap units ($100 to $200) are really only good for point-of-use installation at a sink.

How to Choose the Right-Sized Model for Your Home

As mighty as these mini machines are, manufacturers invariably exaggerate their capabilities. They crow that you will “never run out of hot water,” but the truth is you can run out if you overtax the unit’s flow rate — measured in gallons-per-minute (GPM). Purchasing a unit with the right GPM for your household is key.

So, how much hot water do you need? A tankless unit has to quickly heat incoming cold water and supply it to your pipes every time you turn on a faucet or start a load of laundry. The more powerful the water heater is, the quicker it can heat and supply. But a big house in Minneapolis is going to need a lot more power than a condo in San Diego. Using the steps below, we’ll help you choose the unit that’s best for your home.

First, figure out your temperature rise.

Water temperature varies by region and season. The colder the water coming into your home is, the more your unit will have to work to bring it up to temperature, so it is best to size your unit based on winter conditions.

That's 120° F minus your local groundwater temperature.

Average Groundwater Temperature
Temperature Rise
Northernmost States
40° F
80° F
Midwestern and Southeastern States
50° F
70° F
Southern California, the Southwest, and Gulf States
60° F
60° F
Hawaii and Puerto Rico
75° F
45° F

To find the exact water temperature of your area, check with your local utility company.

Tankless units sense incoming water temperature and heat it to match your water heater’s thermostat setting. The standard is 120° F, unless you’re running a sanitizing cycle in your washing machine. The greater the difference between groundwater temperature and 120° F, the greater the demand placed on your unit.

Next, find your flow rate.

This is a measurement of how much hot water your household uses. Sites like Compact Appliance say to add up the GPM of only the fixtures you plan on using during a time of peak demand — like when your entire family is getting ready in the morning. Buying a tankless unit with a capacity lower than that means you’ll quickly exceed its limits and feel the icy effects.

However, the Bosch representative we spoke to insists you should add up the GPM of every fixture in your house. This way, it’s impossible to run out of hot water. But how often do you run every single fixture at the same time? We recommend a moderate approach:

Add the GPM of the fixtures you simultaneously use, then round up generously.



Bathroom sink






Kitchen sink




Washing machine


*Older fixtures tend to have higher GPM than modern, water-saving designs.
Adapted from The Engineering Toolbox.

Your water heater should deliver the flow rate you need at your local temperature rise.

In all the customer reviews we read, the number one complaint was clear: The purchased unit was not as powerful as manufacturers led them to believe. Many manufacturers list the max flow rate — how many gallons of hot water the unit would produce if it was, for example, working with the lukewarm groundwater of the Gulf states. This optimistic GPM will not apply to the vast majority of homeowners.

Robert Dishman, General Manager of Alliance Plumbing in Portland, Oregon, reports that homeowners need to do their research and choose a unit that is “sized and designed correctly for their household hot water demand. Then they’ll be a happy customer.” While it may be tempting to go with a cheaper, lower-GPM model, we garnered from reviews and expert input that these will likely fail to supply the hot water your household needs.

For each of our preferred series, we have provided their GPM capacities at a range of temperature rises, allowing you to see their power as it applies to your climate. If you opt for a different unit, find the same information in spec PDFs on the company website or farther down the page on sites like The Home Depot. It may take a few extra clicks, but finding a unit’s flow rate for real temperatures will give you much more accurate expectations.

Takagi Models

80° F
70° F
60° F
45° F

Unlike our other two top picks, Takagi’s series only include the two standard gas types (natural gas or propane), not a range of different GPM. To show you more options, we have pulled in the stats of a few neighboring Takagi series. We chose the K4 because it offers a high GPM at a good price, but if you live in a one-bedroom, one-bath in the Arizona desert, you can get away with a less powerful piece of equipment.

Rheem Professional Prestige Models

80° F
70° F
60° F
45° F

We were blown away by the high power of the Professional Prestige 96, but if you live in a warm climate, you can easily get away with one of the lesser models.

Stiebel Eltron Tempra and Tempra Plus Models

80° F
70° F
60° F
45° F

Each model in the series reaches a different GPM based on the temperature rise. If you’re looking for a whole-house water heater, stick to the upper models — 29 or 36. If you only want a point-of-use water heater or live somewhere tropical, check out your options on the smaller end of the GPM scale.

What about warranty coverage?

There’s a lot of hairsplitting when it comes to warranty coverage; companies accept responsibility for parts, labor, and replacements for different amounts of time — around five years, three years, and one year, respectively. Often, the aspect of the water heater that comes with the longest warranty is the heating chamber (electric) or the heat exchanger (gas); after that: parts.

Many warranties only apply if the unit is professionally installed and, in some cases, regularly serviced.

Manufacturers should make all this information readily available to you as you go through the purchasing process — another reason we prioritized good customer resources in our selection process.

The most common issue affecting tankless water heaters is also one that manufacturers wash their hands of: hard water damage.

And this is unfortunate because, according to Ryan Gardener, a tech support representative with HomePlus Products, “By far, the most common thing that can kill a tankless water heater is excessive hardness.” He is referring to hard water’s tendency to deposit scale over time, and tankless’ vulnerability to it. As the minerals in hard water calcify, scale begins to insulate the heating element, forcing it to work harder and harder to get its job done.

You can run at-home tests to check the mineral composition of your water supply, or you can make an approximation using geological maps. Experts recommend performing active treatment on your tankless unit before calcification becomes a problem. Every six months to a year, flush it with store-bought descaler or clean white vinegar.

The Best Tankless Water Heater: Summed Up

Tankless Water Heater
The Best
Takagi K4
Best Gas Series for Average Households
Stiebel Eltron Tempra Plus
Best Electric Series for Average Households
Rheem Professional Prestige
Best Gas Series for Larger Households