Last week 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.
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: 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 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
Both the A.O. Smith NEXT Hybrid and Eternal Hybrid use condensing combustion technology to exceed 90-percent efficiency. Grand Hall claims up to 98-percent 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.
Both products have electronic ignition rather than a standing pilot light.
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 column for more on water heating ratings.)
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-degree temperature rise, but only 2.1 gpm at a 90-degree rise. The largest Eternal Hybrid provides 7.6 gpm at a 50-degree temperature rise and 4.2 gpm at a 90degree temperature rise.
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 is also the added cost of running 3/4-inch gas lines, instead of more typical 1/2 inch.
Yes, they appear to have some performance advantages over conventional tankless water heaters, but its hard to imagine (for most applications) that the economics will be better than for a conventional gas storage water heater.
Alex Wilson is the founder of BuildingGreen, Inc. in Brattleboro. Send comments or suggestions for future columns to firstname.lastname@example.org.