Efficient Water Heating Options: Page 5 of 6

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General Electric’s Geospring heat-pump water heater.
General Electric’s Geospring heat-pump water heater.
Rheem's Marathon electric tank-type water heater
Rheem’s Marathon electric tank-type water heater has an EF of up to 0.94.
A Geyser add-on heat pump for a tank-type water heater.
A Geyser add-on heat pump for a tank-type water heater.
American Water Heaters’ high-efficiency, tank-type gas water heater.
American Water Heaters’ high-efficiency, tank-type gas water heater.
A Rinnai tankless gas water heater.
A Rinnai tankless gas water heater.
Stiebel Eltron’s Tempra tankless water heater
Stiebel Eltron’s Tempra whole-house electric tankless water heater, with three heaters (exterior).
Stiebel Eltron’s Tempra whole-house electric tankless water heater
Stiebel Eltron’s Tempra whole-house electric tankless water heater, with three heaters (interior).
A. O. Smith’s hybrid water heater.
A. O. Smith’s hybrid water heater.
The SuperStor Contender Indirect Water Heater
The SuperStor Contender Indirect Water Heater draws energy from a boiler. Hot boiler water flows through an internal heat exchanger in the tank, heating the domestic water.
American Water Heaters’ Polaris combination water heater.
American Water Heaters’ Polaris combination water heater.
General Electric’s Geospring heat-pump water heater.
Rheem's Marathon electric tank-type water heater
A Geyser add-on heat pump for a tank-type water heater.
American Water Heaters’ high-efficiency, tank-type gas water heater.
A Rinnai tankless gas water heater.
Stiebel Eltron’s Tempra tankless water heater
Stiebel Eltron’s Tempra whole-house electric tankless water heater
A. O. Smith’s hybrid water heater.
The SuperStor Contender Indirect Water Heater
American Water Heaters’ Polaris combination water heater.

Always follow manufacturer recommendations on water testing, installation, and maintenance to ensure a long life for your tankless heater. The bottom line is that electric-tankless water heaters do not make sense in most whole-house applications. However, in applications where small models can be installed at a point of use, doing so may be a reasonable option.

“Hybrid” Water Heaters

A relatively new type of water heater includes features of both storage and tankless models. These gas-fired models have small buffer tanks that eliminate the cold-water sandwich problem and deliver hot water even at very low flow. (Note that “hybrid” is also often used to describe heat-pump water heaters, which are quite different.) There are at least three manufacturers of hybrid water heaters: Grand Hall, which produces the Eternal Hybrid water heater; A.O. Smith, which produces the NEXT Hybrid water heater; and Navien, with its CR-240A. Hybrids use condensing technology to achieve very high efficiencies (greater than 90%) and high flow rates that should satisfy almost any residential situation.

Drawbacks include high cost and standby heat loss—which is the most common reason to choose a tankless model in the first place. These units have small tanks with little or no insulation. In a recent study of tankless water heaters in Minnesota, the Navien hybrid model performed abysmally, which was attributed to the uninsulated buffer tank.

Indirect Water Heaters

Indirect water heaters generally operate as one zone of a boiler system that is used for space heating. They have an insulated tank and a heat exchanger to transfer heat from the boiler into the stored water. Because combustion does not occur at the storage tank, the tank can be better insulated than a conventional gas- or oil-fired storage water heater.

An indirect water heater is usually preferable to a tankless coil (a common feature in gas- and oil-fired boilers) because heat from the boiler is not called for every time hot water is drawn. A tankless coil installed in a boiler functions much as a tankless water heater, with the boiler having to fire every time hot water is needed. The difference is that the boiler has a lot more mass than a tankless water heater, so there is significantly more energy used to heat it up and it takes longer to cool down. Tankless coils in boilers can make sense during the winter in cold climates, when the boiler is hot much of the time, but they are not usually recommended for year-round use.

Coupled with a high-efficiency boiler, indirect water heating is often one of the most cost-effective water heating options. There are some inherent efficiency losses from the heat exchanger, and electricity is required for pumping, but this can still be a very efficient system. It can also be adapted to SHW preheating.

Combination Space & Water Heating

A step beyond indirect water heaters are integrated space- and water-heating appliances. Several approaches can be taken with these “combo” systems. There are a number of high-efficiency boilers with integrated storage tanks—quite different from the tankless coil systems that can be added to standard boilers, but some such products have disappeared.

While there is a lot of appeal to a space-saving system that serves multiple functions, combined systems are more complex. If one component fails, you may lose both heat and hot water until it is fixed.

Comments (4)

Mike Taylor_5's picture

I installed a heat pump hot water system that uses a 200 gal tank with (100' 3/8") copper coil to deliver hot water at 115F. In the warmer months, it takes 1.5 kWh to provide all our hot water ($0.45). In the winter, when our unheated basement drops to about 50F, it will take about 8 kWh to heat the water reservoir. This winter I want to add a heating loop from my pellet stove (Harmon P38) to heat the water reservoir. Has anyone done this?

Fred Golden's picture

You can use a 50' long coil of 1/2" or 3/4" copper tubing, and a standard solar hot water heating controller. Only difference is the controller will sense the hot coil of tubing at night, when the stove is lit, and turn on the pump then. Mount the tubing about 1/2" from the stove, or try direct contact for better heat transfer. You probably only need about 25' to be in direct contact with the stove, and can use the remaining copper roll of tubing to run the water lines.

If you later decide to install a evacuated tube solar collector, it will have the potential to reduce your energy needs further, and provide hot water all year long. Once you have a excess of hot water, running some PEX tubing secured to the ceiling of the basement roof will transfer the excess heat to the living space above.

Fred Golden.

Fred Golden's picture

One option not talked about (or at least I have never heard of it happening) is using a solar water heater controller and for a collector use a coil of 3/4" copper tubing mounted behind a wood burning stove. When the coil is warmer than the tank, then the pump would come on and heat the water in the water heater. When the stove is cold, it would not come on, but would save a lot of kilowatts while the stove is hot.

Michael Welch's picture

Too extend your mentioned option, it does not even need a pump, if the setup is such that it will thermosiphon to the hot water tank.

Also, there are heat exchangers that can be retrofit to the inside of the wood heater, obtaining even more hot water. Of course, the usual cautions are that it can get too hot and flash into steam if not properly designed; and also the exchanger might take enough heat out of the firebox to cool the gases enough to deposit creosote on the heater walls and chimney -- a potential fire hazard.

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