The bottomless tank: How hybrid water heaters put a stop to shower scheduling

Hybrid water heaters bridge the performance gaps between tank and tankless units.

By Cheryl Weber

It's easy to see why tankless water heaters appeal to homeowners. The units deliver a steady stream of hot water on demand — no waiting for a depleted storage tank to refill — and eliminate the standby energy losses associated with tanks. If there's a downside, it's that their heat output has a limit, which in turn affects the water flow. Propane-fueled heaters kick out a higher flow rate than electric ones do, simply because they generate more Btu. Yet even the most robust models may not be able to keep up with the demands of a large household when, say, the clothes washer and showers are running at the same time.

Hybrid water heaters fill that gap by combining the higher hot water output of a tank heater with the endless heating capacity of a tankless. And in recent years, manufacturers have introduced different variations on the theme. What the hybrids have in common, though, is that unlike tankless heaters, they use the same gas and venting hookups as a conventional tank. Last October, for example, Rinnai announced the RH180 Hybrid Tank-Tankless Water Heater, a 91,500-Btu model that produces more than 180 gallons of hot water in the first hour. "It installs like a storage tank model, requiring a ½-inch gas line, and is compatible with a 4-inch B-vent common on furnaces," says Joe Holliday, business development manager at Rinnai. "The hot and cold water connections are on the top, so you can do a drop-in replacement."

The gas-fired RH180 features a small tankless unit attached to the side of a 40-gallon tank. When activated, cold water is drawn into the bottom of the tank, flash-heated, and piped out at the top. "It's ideal for large households, for filling a big spa tub, and for light commercial applications," Holliday says.



Rinnai's new RH180 Hybrid Tank-Tankless Water Heater combines an efficient storage tank with on-demand tankless technology to deliver up to 180 gallons of hot water in the first hour.
Click to enlarge

The GU120 Eternal hybrid water heater is a tankless unit with a 
two-gallon reserve tank inside. It's designed to provide endless hot water for 
up to two applications simultaneously, such as a shower and a 
dishwasher.
Click to enlarge



Eternal recently introduced a similar solution for homeowners looking to increase their hot-water supply without reconfiguring their existing setup. The GU120 — a tankless with a two-gallon reserve tank inside — is designed to provide endless hot water for up to two applications simultaneously, such as a shower and a dishwasher.

Dippel Plumbing, in St. Louis, installed its first hybrid water heater in 2010, in a house with eight bathrooms. "When family came for the holidays, they had to make a shower schedule," says general manager Chris McNulty. "We put one in right before Christmas, and they said they never ran out of hot water all week." Dippel Plumbing began selling the units to restaurants and hotels, often exchanging a conventional 300-gallon water heater for two hybrid heaters.

Hybrid units overcome a common problem when demand is high — the "cold water sandwich," McNulty says. "A tankless heater does supply endless hot water at a certain flow rate. The problem is the water has to reach the right temperature before it will leave the heater. Most tankless [units] will only deliver four gallons of hot water per minute, and then cold water comes out before it goes back to hot. That's where a hybrid works great."


"When family came for the holidays, they had to make a shower schedule. We put a hybrid water heater in right before Christmas, and the family said they never ran out of hot water all week."


A.O. Smith's propane-fueled Vertex GDHE-50 is another version of the technology. It combines a 50-gallon tank with a plug-in power-assist for the heat exchanger, a 30-foot coil that disperses hot combustion gases with 96 percent efficiency. "It avoids the maximum flow rate restrictions on tankless heaters while using 100,000 Btu, which overcomes the gas line challenge," says David Chisolm, director of marketing. "Anything above 150,000 Btu, you'd have to upgrade the gas line to handle the demand load." While propane-fueled tank water heaters recover faster than electric heaters in general, hybrids go a step further by cutting that recovery time in half, Chisolm says: 8 to 10 minutes, compared to 20 minutes or more for a traditional tank.

To learn more about high-efficiency water heating technology, check out these Propane Energy Update articles:


Oil-to-propane conversion will save thousands a year
 
How hybrid water heaters put a stop to shower scheduling
 
The best measurement apps for builders and contractors
 

 

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