Efficient Water Heating Options

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Inside this Article

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.

In the world of ultra-low-energy homes, water heating is often the largest energy user. It’s time to pay attention to this significant energy load.

Selecting a water heater involves decisions about the type of fuel used, how and where the water is heated (especially the issue of storage versus tankless water heaters), and whether water heating can be combined with space heating. Here’s a review of the most efficient products on the market to suit your particular water-heating needs.

Storage Water Heaters

The vast majority of water heaters in North America are storage-type—an insulated tank with either an integrated gas burner, or one or two electric-resistance elements. The advantage of storage water heaters is that the water can be heated with a relatively small burner or heating element, while still providing ample hot water. Even when much of the hot water is used up, the tank remains stratified, with remaining hot water staying at the top where it is drawn off. 

Recovery time depends on the capacity of the gas burner or electric elements. Hot water recovery is an important property of a storage water heater; it is generally presented as the “first-hour rating” in gallons. This is the amount of hot water that can be delivered by a water heater during one hour. With storage water heaters, this assumes the tank starts fully charged with hot water, so the first-hour rating is the volume of the tank, plus the volume of water that can be heated in an hour (assuming standard assumptions for temperature rise).

To calculate hot water demand for a family, you should figure out the peak usage period, then use rules for hot water use for those needs. The U.S. Department of Energy (DOE) assumes 10 gallons of hot water for a shower, 2 gallons for shaving, 4 gallons for hand dish-washing and food prep, 6 gallons for a dishwasher, and 7 gallons for a clothes washer. So for the morning period, one-hour water use might total three showers, one shave, and one hand dish-washing, for a total of 36 gallons. The first-hour rating would have to meet that.

Disadvantages of storage-type water heaters include the potential to run out of hot water and the standby heat loss from the large temperature difference between the outside and inside (often 60°F or more). Inexpensive storage water heaters may have just an inch of polyurethane insulation, providing as little as R-6. The best have R-12 to R-15 in foam insulation. With gas storage water heaters that have pilot lights, the pilot itself throws out a lot of heat (typically about 300 Btu/hour), which can contribute some heat to the tank, though most of the pilot light heat actually escapes up the central flue. Depending on water heater location in a building, the climate, and the time of year, standby losses may either reduce a building’s heating load or increase its cooling load, though the impact of this is fairly modest.

There can also be losses from storage water heaters via the supply and discharge pipes. Storage water heaters should be installed with “heat traps” that prevent the thermosiphoning (natural circulation from hot water rising) that can occur if the water heater is located below a home’s plumbing. Adequate pipe insulation should also be a part of water heater installation (see “Reduce Demand—Improve Efficiency” article in this issue).

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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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