Tankless water heaters provide hot water only as it is needed. They don’t produce the standby energy losses associated with storage water heaters, which can save you money. (EPA on Tankless Water Heaters)
Tankless water heaters offer on-demand hot water, last for 20 years (vs. 13 years for heaters with tank), and come with 2 distinct characteristics:
- Incredibly high energy-efficiency. You only turn the heater on when you need water. No standby or cycling energy losses from the tank and the superb coefficient of performance (COP) results in an unbeatable energy factor (EF). This alone can result in a saving of at least $100 per year.
- No tank. Having a giant 70-gallon tank is a waste of space and looks untasteful.
But, above all else, the best tankless water heater gives you one priceless thing: Peace of mind.
In essence, the modern tankless heaters are a wonder of technology. Cold water runs through powerful heating elements (powered by electricity or gas), which heats up to 140˚F in less than 5 seconds.
You turn on a faucet or a shower, and you get hot water almost immediately. It will run for as long as you want, and you will spend a minimal amount of energy to get it.
Here is how a tankless water heater can distribute hot water throughout the house:
It’s no wonder that everybody building a house or replacing an existing water heater system is looking to buy a tankless hot water heater. The convenience of having on-demand instantaneous hot water alone is enough.
Add the low maintenance costs, superb energy-efficiency, and the absence of an awkward rusty tank, and you can see why the tankless on-demand water heaters are so popular.
Once you buy and install one, you don’t have to worry about repairs, lack of hot water, having an ugly big tank in your house. Buying one, however, can be a bit confusing. The specs include:
- GPM over a certain temperature interval.
- Electrical metrics (wattage (W), amps (A), voltage (V)).
- EF rating.
- Associated warranties.
To simplify things, we have created a list of the best tankless water heaters you can buy in 2021 based on thermodynamic and electrical specifications. But before we look at the best on-demand water heaters, let’s learn a bit about what to look for when in the market for a tankless heater.
Here’s are the 3 key things to ask yourself before buying a tankless hot water heater:
3 Key Questions To Answer Before Comparing Tankless Water Heaters
The main idea of a tankless water heater is to instantaneously heat water when we need it. This process is governed by the laws of thermodynamics; the specification sheet provides us with all the numbers we need.
Based on the specs, here are 3 specs we need to check before buying a tankless water heater:
- What size tankless water heater do I need? It all comes down to power (in kilowatts) and your household needs. Most households need a 5-10 GPM or about 25-35 kW sized heater.
- How to evaluate energy-efficiency? Obviously, we want the most energy-efficient tankless water heater. We’ll look at the EF rating; the #1 Rinnai RU Sensei Super even has an Energy Star certificate (extremely hard to get, uses 10% less energy, according to Energy Star factsheet).
- Which kind of tankless water heater to get; electric, propane, or natural gas. Electric ones are cheaper, easier to install, more compact, and energy-efficient. Gas ones are usually more powerful and much cheaper in the long term (they may cost $1,500, but you can save more than $2,000 in 10 years on water costs because you’re burning cheap gas instead of expensive electricity).
All in all, most often, the best tankless water heaters (be it electric or gas) have an EF rating above 0.8 and can deliver 5-10 GMP maximal hot water flow.
HVAC installers recommend gas tankless water heaters over electric due to much lower long-term costs of natural gas and propane vs. electricity. Here are both the best gas and electric heaters combined:
List Of Best Tankless Water Heaters With Reviews (Based On Specifications)
#1 Rinnai RU Sensei Super (Best Gas Tankless Hot Water Heater With Energy Star Certificate)
Gas tankless heaters are known to be powerful and extremely cost-efficient. None is better than the Japanese-made Rinnai RU Sensei Super, the best tankless water heater overall.
The Japanese company Rinnai is traditionally considered the best tankless water heater brand. The Japanese heat pump makes Rinnai produce a gas tankless water heater RU160iP Sensei Super which even earned an Energy Star label for energy efficiency. The Rinnai RU160ip heater, for example, has an EF factor of 0.95. That alone makes the Rinnai heater the most energy-efficient gas tankless hot water heater you can buy.
The RU Sensei Super uses 40% less energy than traditional water heaters with tanks. What is more, you have hot water on-demand with substantial capacity. The Rinnai gas tankless heaters can produce as much as 160,000 BTU energy. This translates in anywhere between 9 GPM and 11 GPM maximum hot water flow at 32˚F temperature step. That is something even the most powerful electric tankless heaters struggle to achieve.
On top of that, the Rinnai has also implemented its Circ-Logic technology to make the hot water more readily available.
Rinnai RU Sensei Super is, obviously, heavier and bigger than electric tankless water heaters. The 9 GPM propane version, for example, weighs 62 lbs and has 18.5 x 11.45 x 31.3 inches dimensions. On the other hand, the gas Rinnai tankless water heater is more cost-efficient due to the lower cost of gas.
You can choose between natural and propane-fueled heaters. Most of the specs are the same; the fuel is the only significant difference. Rinnai offers models of different power – from 7 GPM to 11 GPM. The GPM specs were created for the US market; in Japan, they use the maximal BTU output.
Here’s how you navigate the Rinnai Sensei Super tankless water heaters: You will see models like RU160ip, RU180in, and so on. The ‘RU’ stands for ‘Rinnai’. The ‘160’, ‘180’ numbers are the maximal BTU output. For example, RU160ip has an output of 160.000 BTU, which equals about 9 GPM. The last letter ‘p’ or ‘n’ designates ‘propane’ or ‘natural gas’, respectively.
On top of being Energy Star certified, the Rinnai is also confident enough in their engineering to offer a 10-year warranty on the heat exchanger and a 5-year warranty for parts. In short, you have pretty much everything covered. The only thing you have to decide is how big of a tankless water heater you need and if you’ll opt for natural gas or propane.
#2 Stiebel Eltron 239222 Tempra Plus (Best Electric Tankless Water Heater)
When it comes to efficient engineering, Germany usually leads the way. That’s certainly right in the case of tankless water heaters. Stiebel Eltron is the most well-renowned German producer of heating pumps. They have several eco electric tankless water heaters in their portfolio, with the Stiebel Eltron 239222 Tempra being their crowning achievement.
In the 239222 Tempra Plus, several technological advancements are implemented as far as the heating exchanger and the overall energy-efficiency of on-demand hot water are concerned. These include:
- Advanced Flow Control. This is basically the smartest way to handle the hot water output of a tankless water heater.
- Incredible 0.99 Energy Factor (EF). This makes the Stiebel Eltron Tempra the most energy-efficient tankless water heater on the market.
- Easy-to-handle everything. Germans made the sometimes complex installation and handling of tankless heaters easy.
The 239222 Tempra Plus model comes with everything most people need from a tankless water heater. The patented Advanced Flow Control keeps a constant flow of hot water much more stable than any other tankless water heater. You can heat the water anywhere from 86˚F to 140˚F. The temperature range here is the widest, giving you an option to shower with mildly hot water to burning hot 140˚F water.
Given the advanced tech that goes into making Stiebel Eltron Tempra electric tankless water heater so energy-efficient, they have decided to create a whole capacity range. The smallest one is powered by 12 kW; if you live in the south of the USA and don’t have big hot water needs, the 12 kW might even be enough.
The most popular ones are the 20 kW Plus, 24 kW Plus, and 29 kW Plus models. These can deliver at least around 5 GPM water flow given the 32˚F temperature step. For big houses, the most powerful 36 kW Plus model should be considered.
Hint: Stiebel Eltron offers Tempra Plus (new model) and Tempra Trend (old model) heaters. Always go for the Tempra Plus models because they have the eco-friendly Advanced Flow Control technology. That extra energy efficiency can lower your hot water costs considerably.
Overall, the Stiebel Eltron 239222 Tempra Plus is considered one of the best tankless water heaters in 2021. It’s small; you can put it anywhere. It’s powerful, and you can choose the size you need. It achieves the best continuous flow at the highest energy-efficiency possible. Being electric also makes every much simpler and safer.
#3 EcoSmart ECO 27 kW (Best-Selling Eco-Friendly Tankless Water Heater)
EcoSmart ECO tankless water heaters (they have several with different wattage but basically the same engineering) are designed to be very eco-friendly. First of all, the EcoSmart ECO 27 weighs less than 11 pounds, making it the most lightweight tankless water heater on the list.
One unique advantage is the very low 0.3 GPM activation flow. When hot water is needed, most of the tankless water heaters turn on at 0.5 GPM hot water flow. EcoSmart ECO series heater with 0.3 GPM activation flow responds faster to low flow hot water demand.
Obviously, for a 27 kW power on 240 V, you will need a 112.5 A grid. What is more, a 200 A grid is recommended to handle the EcoSmart ECO needs when operating at full power. For the best operational results at high (above 4 GPM) hot water flow, 3 double poles breakers are recommended; 40 A each.
Once the EcoSmart ECO is installed, you will have on-demand hot water with high energy efficiency. In fact, you can save up to 50% on hot water as with a comparable storage water heater:
What is more, it has a self-modulating technology that balances the electric power output and the hot water output, adding to the heater’s energy-efficiency.
The EcoSmart ECO is the best selling tankless water heater on Amazon. You can opt for a standard 18 kW heater or the more powerful 27 kW one. Of course, both devices come with a limited lifetime warranty.
#4 Rinnai V Series HE (Classic Gas Tankless Hot Water Heater)
Rinnai V Series HE is a line of classic gas tankless water heaters. This is the type of gas heater most people who have tankless heaters have.
It comes in several sizes, up to 9.4 GPM. Rinnai offers both the natural gas and propane version of their V Series HE heaters. It can achieve a maximum water temperature of 140˚F. The one thing you need to be careful about is that the Rinnai V Series HE has to be installed outdoor.
In many ways, having a gas tankless heater with an outdoor installation is a good idea. Yes, having a tankless heater installed as close to the water faucets and showers does make sense; the time needed for the hot water to reach that particular faucet is shorter. However, gas heaters contain gas, an explosive and smelly substance, and having Rinnai V Series HE installed outdoor makes sense from this viewpoint.
The V Series HE has a standard energy-efficiency for a gas-powered tankless water heater; the EF rating for the natural gas 6.5 GPM unit, for example, is 0.82. Electric heaters can reach an EF rating of above 0.9. However, the price of natural gas and propane is much lower than the price of electricity. This makes the V Series HE heaters more cost-efficient than even the most energy-efficient electric tankless heater.
Rinnai offers 8 different models of their V Series heaters. You have 5.3, 6.5, 7.5, 9.4 GPM devices; you can choose between natural gas or propane. The Rinnai designations for these heaters are quite easy to understand. For example, a V65eN tankless water heater translated to ‘V Series’, ‘6.5 GPM’, and ‘Natural gas’.
All in all, the V Series is as reliable a line of the gas tankless water heater as you can get. On top of that, you get the best-in-industry warranty from Rinnai – with 10 years warranty on the heat exchanger.
#5 Rheem 240V RTEX-18 (Best Small Electric Tankless Heater)
Not everybody has enormous on-demand hot water needs. For some, having the luxury of a hot shower without simultaneously using 2 faucets and running a dishwasher is enough.
The Rheem 240V RTEX-18 is the best small tankless heater that costs less than $500. If you have a small household and not many simultaneous hot water needs (faucets + dishwasher + shower all at once), you don’t really need to spend more than $1,000 for a big tankless heater. A small electric one with simple enough installation should do just fine.
The Rheem 240V RTEX-18 can be hooked up to a 240 V, 75 A grid (no need to upgrade your electricity grid to 200 A). It produces 18 kW of power, which is enough for the maximal hot water flow of 4.4 GPM. In reality, that’s what most of the smaller households need.
Being small also means it’s ultra-lightweight. The Rheem 240V RTEX-1 weighs only 8.5 lbs but can wield 18 kW power thanks to the 2 heating chambers (most heaters only use the 1).
The trio of being easy to use, simple to install, and has a straightforward digital thermometer display makes it very user-friendly. This shows that electric tankless heaters don’t need to be complex devices only HVAC engineers can install using 100+ A power grids.
You will find several models of Rheem tankless hot water heaters with up to 4 heating chambers, reaching the full power of 36 kW. However, the 18 kW is the best one for most small households, produces a sufficient amount of on-demand hot water, and costs less than $500.
#6 Sio Green SIO18 V2 (Infrared Electric Tankless Heater)
Sio Green V2 is a unique tankless heater; you might even say it’s a next-gen device. While both electric and gas heaters heat metallic parts of the heat exchanger to produce on-demand hot water, the Sio Green V2 heater uses Quartz elements and far infrared technology to do the same.
The combination of Quartz and the infrared spectrum is definitely interesting. For decades, NASA has been developing infrared heating technology. When applied to Quartz tubes (instead of metallic tubes), you can generate a high-temperature gradient across the Sio Green heating chamber. That, in turn, heaters the water very quickly and is environmentally friendly.
What is more, you have no corrosion and no calcium deposits without the contact between water and metallic parts. This doesn’t only prolong the lifespan of the infrared tankless water heater, but it also saves you many maintenance problems.
Here is how the Quartz tubes look (they are located inside the Sio Green heater):
The far-infrared light that powers up the Quartz tubes is, in turn, powered by electricity. If we simplify things, the Sio Green is an electric tankless heater with NASA tech energy transfer from electrical power to water heating.
You have two choices of Sio Green V2 heaters for residential use: the smaller 9 kW model and the bigger 18 kW model. Most households do need the bigger 18 kW model, which can achieve the maximum hot water flow of 6.6 GPM.
#7 Noritz NRC66DVNG (Long-Term Cheap Natural Gas Heater)
Noritz NRC66DVNG is a compact and reliable gas tankless water heater. Of course, the big problem gas heater producers always want to resolve is the lower energy-efficiency.
Noritz has managed to get the EF (a rating that determines energy-efficiency) beyond 0.90. In fact, the NRC66DVNG has an EF rating of 0.91. That is, broadly speaking, the same energy efficiency range as average-efficiency electric tankers.
On top of that, the Noritz NRC66DVNG is capable of heating the water to 140˚F. This is the maximum temperature most households require. The maximum hot water flow is 6.6 GPM, a little bit on the lower side.
For some installers, having the ability to use the Schedule 40 PVC piping and fitting is advantageous. The Noritz NRC66DVNG tankless water heater offers Schedule 40 piping, but that’s not an essential thing a good tankless heater installer would require.
The best thing about natural gas and high energy-efficiency are the costs. The Noritz NRC66DVNG costs less than $1,000, but during the lifespan of a tankless water heater, the price of hot water can reach up to $10,000.
The price of hot water is based on the energy price and the efficiency of the tankless water heater. The lowest price of fuel for a tankless heater is, and will likely be in 10 years, and so on, natural gas. The 0.91 EF rating of the Noritz will take that natural gas and use it very effectively.
In the long-term, the Noritz is one of the cheapest ways how to get on-demand hot water.
Do You Agree With The Gas And Electric Tankless Heat Picks?
Some people have a great experience with a tankless water heater while others have a horrible one. If you own one of the heaters above, please do give us a short review in the comments.
If you have some other tankless heaters to add to the list, especially those with high EF ranking, please let us know below.