The Best Tankless Water Heaters

The best 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.

The 3 Best Tankless Water Heater Series

Best Gas Series
For Mid-Sized Homes
Takagi K4 Tankless Water Heater
Takagi K4 Tankless Water Heater
A solid option for most gas-fueled households, offering a mid-high GPM and a plenitude of customer support.
Pros
High flow rate
Temperature regulation
Customer support
Cons
Lower energy factor

Why we chose it

High flow rate

Even in cold climates, the Takagi K4 can supply eight gallons of hot water per minute — enough for two showers and a sink to run at the same time. When we calculated price per GPM, we found that it offers the most hot water bang-for-your-buck, around $50 less per GPM than its peers. 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.

Temperature regulation

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.

Customer support

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.

Points to consider

Lower energy factor

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. For smaller and mid-sized homes, it’s unlikely that the energy savings from a more efficient model will have enough impact to offset the significantly higher price tag.

Best Gas Series
For Larger Homes
Rheem Professional Prestige Series
Rheem Professional Prestige Series
Virtually endless hot water via one of the most efficient condensing gas units on the market.
Pros
High energy factor
Condensing technology
Smart home compatible
Cons
Bulkier dimensions
Bare bones website

Why we chose it

High energy factor

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 at 0.96 — well above Energy Star’s expectations for gas efficiency. The machine also boasts an impressive flow rate of 8.5 GPM, meaning you’ll be able to run two hot showers and a sink at the same time.

Condensing technology

A large part of the Professional Prestige series’ efficiency is due to condensing technology which prevents hot gas from escaping, reusing it to heat more water instead. This feature costs 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.

Smart home compatible

The Professional Prestige series is 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.

Points to consider

Bulkier dimensions

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 inches and 82 lbs, it resembles a heavy piece of checked luggage.

Bare bones website

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.

Best Electric Series
For Mid-Sized Homes
Stiebel Eltron Tempra Plus
Stiebel Eltron Tempra Plus
The reigning champ of electric water heating, offering stellar technology, sleek design, and reliable service.
Pros
Ultra efficient
Variety of models
Small size
Customer support
Cons
Price
Low flow rate

Why we chose it

Ultra efficient

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 water heaters. While that comes at a higher upfront price, it can also save you a ton in utility bills over the life of the heater.

Variety of models

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 in our guide to find the model that can deliver your ideal flow rate at your current temperature rise.

According to the Stiebel Eltron rep we spoke to, the main difference between the the Tempra and Tempra Plus 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.

Small size

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 and 19 lbs. 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.

Customer support

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.

Points to consider

Price

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.

Low flow rate

Electric models generally have a much lower flow rate than gas, and Stiebel is no exception. 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.

How to Find the Right Tankless Water Heater 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.

Calculate 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.

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

Note: 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.

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.

Fixture GPM
Bathroom sink
1.5–2.2*
Bathtub
4
Dishwasher
1.5
Kitchen sink
1.5–2.5*
Shower
2–3*
Washing machine
2

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

Choose a model that meets 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, you can find the same information in specification 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

KJr2 H3M K4 H3S
80℉
2.9
3.8
3.9
4.3
70℉
3.3
4.3
4.4
4.9
60℉
3.8
5.1
5.2
5.7
45℉
5.1
6.6
6.9
7.6

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 in table below. 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

84 90 96
80℉
3.8
4.3
4.8
70℉
4.3
4.9
5.5
60℉
5.0
5.8
6.4
45℉
6.7
7.7
8.5

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

20 24 29 36
80℉
1.65
2.0
2.5
3
70℉
1.9
2.25
2.75
3.5
60℉
2.25
2.75
3.25
4
45℉
2.75
3.64
4.37
5.46

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.

Pay attention to fuel type

Natural gas and propane models are generally identical in terms of cost, Energy Factor, and GPM, but they 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.

Tankless Water Heater FAQ

Should I choose a gas or electric tankless water heater?

Smaller and mid-sized homes can choose between gas and electric, but larger homes should stick to gas.

Electric units are by nature less robust than gas, with flow rates maxing out around 5.5 gallons per minute — enough to simultaneously power a a shower and a washing machine in most climates.

Gas units often have GPMs soaring toward the double digits, but there’s a drawback to all that power: You pay for it. Top-of-the-line gas models crest $1,000, but those options use condensing tech, so they’re significantly more energy-efficient.

That said, 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.

What do tankless water heater warranties cover?

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.

Unfortunately, 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.

Can hard water damage a tankless water heater?

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 Heaters: Summed Up

Takagi K4
Stiebel Eltron Tempra Plus
Rheem Professional Prestige
The Best
Gas Series for Mid-Sized Homes
Electric Series for Mid-Sized Homes
Gas Series for Large Homes
Price
$770
$805
$1400
Energy factor
0.79
0.99
0.96
Energy Star certification
Warranty
15 years on heat exchanger, 5 years on parts
7 years against leakage, 3 years against defects in workmanship and materials
12 years on heat exchanger, 5 years on parts, 1 year on labor