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A relatively new type of water heater combines features of both tankless and storage water heaters.

A.O. Smith's NEXT Hybrid water heater.Click to enlarge.
Photo Credit: A.O. Smith

In last week’s blog I compared tankless and storage water heaters and explained why tankless water heaters often don’t make that much sense.

This week I’ll describe a newer type of water heater that has some features of both storage and tankless designs and solves several problems that are common with tankless models. While these are referred to as hybrid water heaters, they are quite different from heat-pump water heaters, which are also often referred to as hybrid. I’ll cover heat-pump water heaters next week.

Some storage but also continuous hot water

As far as I can tell, the hybrid water heater was invented in 2006 by a relatively small company, Grand Hall USA of Garland, Texas, a company that also makes barbeque grills. Grand Hall’s Eternal Hybrid water heater defined this product type.

In 2010, the nation’s largest water heater manufacturer, A.O. Smith followed suit with their NEXT Hybrid, which the company has been promoting fairly actively.

Both are gas-fired tankless water heaters that have a small buffer tank, which is kept hot. The Eternal Hybrid has a two-gallon tank; the A.O. Smith NEXT Hybrid tank is probably about the same size, though the company doesn’t divulge the specifics.

There are two advantages of the buffer tank: first, it eliminates the so-called “cold water sandwich” problem in which someone taking a shower may suddenly get a shot of cold water from a standard tankless water heater; and second, it allows hot water to be delivered with even tiny loads, as might be delivered in a low-flow bathroom faucet. (With most tankless water heaters the burner isn’t activated unless the hot water flow exceeds 0.5 or 0.6 gallons per minute.) Like other tankless water heaters, its small size is another big benefit.

High efficiency, condensing technology

Both the A.O. Smith NEXT Hybrid and Eternal Hybrid use condensing combustion technology to exceed 90% efficiency. Grand Hall claims up to 98% efficiency with the Eternal Hybrid. The flue gases are cool enough that they are vented through a side wall using PVC or ABS plastic pipe. In fact, due to the acidic condensate, these water heaters should not be vented into a masonry chimney.

Like other advanced, state-of-the-art tankless water heaters, both products have electronic ignition rather than a standing pilot light.

Various sizes

The A.O. Smith NEXT Hybrid has a maximum gas input of 100,000 Btu/hour, which the company claims is enough to provide a “first-hour rating” of 189 gallons. (See last week’s blog for more on water heating ratings.)

A cut-away of Grand Hall's Eternal Hybrid water heater. Click to enlarge.
Photo Credit: Grand Hall

The Eternal Hybrid is available in three sizes with maximum gas inputs of 100,000, 145,000, or 195,000 Btu/hour. The minimum gas input is 16,000 Btu/hour for the smallest model and 26,000 Btu/hour for the other two. The largest of these models, the GU195, is available for modulating installations in which up to eight units are installed together for commercial applications.

The A.O. Smith NEXT Hybrid and the smallest Eternal Hybrid can be supplied with the 1/2-inch gas line; the larger Eternal Hybrid models require 3/4-inch gas lines.

Hot water output

The hot water delivery from these and all tankless water heaters depends on the temperature rise. The smaller, 100,000 Btu/hour models provide about 3.8 gpm at a 50°F temperature rise, but only 2.1 gpm at a 90°F rise. The largest Eternal Hybrid provides 7.6 gpm at a 50°F temperature rise and 4.2 gpm at a 90°F temperature rise—which should be plenty for two or three simultaneous showers.

Cost

As with most other tankless water heaters, cost is the Achilles heel. Prices of the A.O. Smith NEXT Hybrid are typically in the $1,800 to $2,000 range (not including installation), and I think the Eternal Hybrids are even more expensive. For the larger Eternal Hybrid models, there may also be the added cost of running 3/4” gas lines, instead of more typical 1/2”.

Yes, they appear to have some performance advantages over conventional tankless water heaters, but whether they will make economic sense over conventional gas storage water heaters will depend on the situation and usage habits.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. He also recently created the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.

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Comments

1 Apples and Oranges? posted by Robert Riversong on 09/14/2012 at 12:04 pm

It's a shame that AO Smith is not willing to reveal much about their hybrid technology, but from the description it appears that they combine a conventional tankless coil with a tiny tank that condenses the flue gasses and maintains a hot water buffer.

The Eternal Hybrid, on the other hand, seems to use an entirely different technology, eliminating the typical restrictive (and prone to scaling) coils and using instead a counterflow flue gas design which directs the hot gasses up through a mini-tank and down around it to condense the exhaust and transfer virtually all the heat to the water without restricting water flow and also creating sufficient water turbulence at the cold water entry point to eliminate or reduce sedimentation.

This is similar to the design of the Triangle Tube SmartTank indirect water tanks (for use with a boiler), which employs a corrugated stainless steel water tank with the boiler water jacketed completely around it rather than constrained in heat transfer coils, thereby eliminating pressure drop and allowing a smaller and more efficient circulator.

 

2 dual purpose - space and water heateing posted by Helena Meryman on 09/20/2012 at 03:17 pm

We are in the process of installing a modulating hybrid heater for both hot water and space heating (radiators). The dual function helps justify the extra cost.  Here is a link to product we are using - its rated at 95% efficiency.  http://www.cadetboiler.com/Welcome. We are months away from being fully operational - but I rember I will report back.

 

 

 


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