Efficient Water Heating Options: Page 6 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.

A less expensive approach for compact, energy-efficient homes is to use a water heater both for water heating and for space heating through a fan coil—a hydronic coil installed in the air handler that distributes conditioned air throughout the house. Different types of water heaters can be used for this application, but it will take some careful engineering—or experience—by the heating contractor to be sure that hot enough water is produced and that the hot water for domestic uses is tempered so that it isn’t dangerously hot (see “Renewable Hydronic Heating” in this issue).

Purchase Considerations

Water heating is lot like space heating, cooling, or even electrical energy production—it makes sense to focus first on reducing demand. With water heating, we can do this by installing water-conserving plumbing fixtures and appliances. Next, we should look at how hot water is distributed and try to improve efficiency there (see “Reduce Demand—Improve Efficiency” in this issue). Then we can optimize the system’s efficiency by choosing the most appropriate heating source.

With natural gas prices as low as they are, it’s hard to compete with the economics of simple gas-fired water heaters today. The most environmentally attractive options are solar thermal or electric water heating, powered by a PV system. Beyond that, the options can become quite complex and dependent on other factors, such as climate, the type of space heating system, and usage patterns. If you are in a cold climate and have hydronic heat with a high-efficiency boiler, an indirect water heater may be the best bet. If off-peak electric rates are available, a simple, inexpensive electric-resistance water heater can be an attractive option, especially since the tank can be well-insulated.

In other situations, a high-efficiency tankless water heater with electronic ignition, an integrated space-and-water-heating system with a condensing boiler, a heat-pump water heater, or a gas water heater with electronic ignition and condensing technology may be the best choice. All of these options are more expensive than a basic storage water heater, so the life-cycle costs should be considered.

With whatever type of water heater is selected, make sure it is installed properly. Hire an experienced installer, and ask for references if you are not familiar with the company. Proper installation involves safely venting flue gases with gas-fired water heaters, making sure there’s plenty of insulation on a storage-type water heater (adding more than came with the water heater may make sense), installing “heat traps” on the supply and discharge piping where it leaves the water heater to prevent thermosiphoning, and insulating all hot water pipes.

Access

Alex Wilson is the founder of BuildingGreen, in Brattleboro, Vermont, and executive editor of Environmental Building News. He also recently founded the Resilient Design Institute (resilientdesign.org).

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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