Let’s start with the technology. They all use the same technology, pretty much same efficiency, no secrets there. Noritz advertises the "Dual Burner" technology. Most all others have it too, that allows for better flame regulation. They all modulate their burners to adjust for the desired heat and therefore desired water temperature.
I like simple elegant solutions to problems. Less parts, less things to go bad and longer lasting, usually.
Noritz, Rinnai and Takagi are made in Japan and share some components and have very comparable warranties. Rinnai has a different venting system than the others. Better? Not really. One thing I like about the Rinnai is that it has the condensation trap built in, but it takes special venting.
Rinnai has a very nice look to it. I sometimes refer to it as the Stainless Steel refrigerator and the other as the regular. Rinnai is a bit more expensive than the others and has spent more money on branding and marketing.
Navien is made in South Korea and fairly new to the US. It has a better presence in Canada. It has some innovative features, like some with a built in small tank that lets you trickle hot water. That little tank acts like a buffer and has a built-in pump. It allows you to circulate the hot water throughout the plumbing of the house. The upside is that you can get hot water instantly at most faucets, the downside is of course that the heater runs constantly, although not full power, and wastes energy.
Now is it better or does it heat the water better? Not in my opinion. As said Navien doesn't have much history here but in Canada they had product recalls and many warranty calls. It has 2 microprocessors, just in case one fails. In my book it means there are 2 microprocessors to fail.
Navien claims 98% efficiency, most others have about 94%. Is the 4% difference real? I don't know. Does it matter? I don't think so.
Bosch is a large German company, also in the appliance business. They had some initial problems with heaters acquired from the company Aquastar they purchased but all those initial problems are behind them and their products are now top the of the line, in performance and price.
Quietside, manufactured in South Korea and pretty much in line with the others in performance. Simple elegant design, one of our favorites. They offer the newest tankless designs in condensing units with high efficiency and great performance and price. The company has some affiliation with Samsung, also a big appliance brand.
Eternal is the latest brand manufactured in Taiwan. It took a different approach with its heat exchanger and has a small built-in water tank that allows trickling hot water, just like the regular water heater. Most tankless units need a minimum flow, typically 0.5 gallon per minute. That is not a problem unless you need to be able to trickle hot water. As a reference 0.5 GPM is like having the kitchen faucet half open. The unit comes on periodically to heat the water in the small tank as it cools down over time. It is very well built unit. The term hybrid refers to the fact that it acts like a storage water heater but is more like a tankless.
So at the end all these brands, Bosch, Navien, Noritz, Rinnai, Takagi, Quietside and Eternal Hybrid are technically very much the same. They deliver all very much the same amount of hot water using the same amount of heating energy. They also have pretty much the same warranties. It comes down to reliability and support in case something goes wrong.
I personally like Eternal Hybrid, Bosch and Quietside. They are well engineered and offer good service and are very responsive. They all are the newest type of condensing high efficient units with solid reliability and price performance.
Noritz, Takagi and Rinnai are also solid performers with many years of experience in manufacturing tankless water heaters.
Proper sizing and installation is key to trouble free operation, like many things.
Proper sizing means choosing the right size unit for the expected hot water demand. With tankless water heater it is the FLOWRATE, like how many gallons of hot water do I need per minute. All tankless manufacture greatly sugar coat their performance.
Just as an example, Noritz changed their product designations beginning 2010. A unit that was called 751, implying delivery of 7.5 gallon per minute to NR98, implying 9.8 gallon per minute, very deceptive. 9.8 gallon per minute at about 80 degree is not hot, it is warm at best. Now 120 degree is hot but then the unit can deliver only 5 gallon per minute then.
As a reference a regular shower head is about 2.5 gallon per minute if the flow restricter hasn't been taken out. Tank water heaters are sized by CAPACITY meaning how long they can deliver the hot water before they run out, i.e. 40 gallon heater. Tankless doesn't run out of hot water but need to deliver the right amount continuously.
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