Efficient Water Heating Options: Page 2 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.

Electric Storage Water Heaters

Electric resistance. Most electric water heaters rely on electric-resistance heating elements. The conversion of electric current into usable heat is extremely efficient—100%—though the true efficiency drops by about two-thirds when one considers energy generation and transmission by the utility. Water heater efficiencies are generally listed as the energy factor (EF). This is the ratio of the useful energy output of the water heater to the total amount of energy delivered to the water heater and it accounts for heat loss from the tank. The higher the EF, the more efficient the water heater. U.S. law requires a minimum EF of 0.90 for new electric storage-type water heaters. The minimum EF for electric storage water heaters today is volume-dependent, and ranges from about 0.86 for an 80-gallon model to about 0.92 for a 40-gallon model. The highest EFs today are about 0.94.

Electric-resistance water heaters become more economically attractive when off-peak electric rates are offered or if special utility smart meters can turn them off when the utility is operating at or near peak generation capacity—allowing a utility to operate fewer power plants. Because no vent is required for electric water heaters, they can be insulated extremely well, reducing standby heat loss to well below that of the best gas-fired water heaters.

Heat pumps. To exceed an EF of 1.0 (100% efficient), heat-pump technology is required. Heat-pump water heaters are gaining popularity, and will get a boost from federal water heater standards that become effective on April 16, 2015. As of then, any electric water heater larger than 55 gallons has to be a heat-pump water heater. The federal standard for these larger water heaters is based on a formula that accounts for storage capacity, but an EF close to 2.0 will be required. For smaller water heaters, the EF requirement will increase somewhat, but will remain below 1.0—so can be achieved with electric-resistance models.

With heat pumps and heat-pump water heaters, the performance is often reported as the coefficient of performance (COP), which is similar to the EF, but not based on the same test standard. A COP of 2.4 means that for every kWh of electricity input, an average of 2.4 kWh of heat output in the hot water is attained—which is like being 240% efficient.

A heat pump uses electricity not to produce heat directly but rather to move heat from one place to another (using the same principle as a refrigerator). It extracts heat from the air and delivers that heat into the water tank—even though the water tank is at a higher temperature than the surrounding air. It does this by alternately condensing and evaporating a refrigerant; the electricity powers only the pumps and fans—though some models offer an option for more rapid recovery, in which case one or two electric-resistance elements may be included. The most efficient heat-pump water heaters have EF ratings as high as 2.5, and all exceed 2.0.

Because heat-pump water heaters cool the surrounding air as they extract heat from it, they can increase space-heating loads during the heating season. A recent utility-funded study of heat-pump water heaters in Massachusetts and Rhode Island found that a heat-pump water heater that delivers a COP of 2.35 with a room temperature of 68°F will drop to a COP of 1.8 at a 50°F room temperature. If the water heater is installed in an unheated or semi-conditioned space, such as a basement, garage, or attic, the impact on heating costs will be lower, but the heat pump performance may drop. Just as heat-pump water heaters rob heat from the house in winter, they provide free cooling and dehumidification in the summer.

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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