Energy Services Bulletin - Technology Spotlight: Residential heat-pump water heaters

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Technology Spotlight: Residential heat-pump water heaters

Heat-pump water heaters (HPWH) use about half as much electricity as conventional electric water heaters. Since water heating accounts for 12 percent of all of the energy used in buildings and buildings account for one-third of all energy used in the nation, the potential for total savings is large. HPWHs use the air of whatever space you install it in (house, garage, or basement) as a heat source and the water as a heat sink, transferring energy from the air to the water. This is very similar to the operating principle of a refrigerator, which transfers energy from inside the refrigerator to outside.

Types of HPWHs

Add-on HPWHs: An add-on HPWH is installed in conjunction with an existing storage water heater and may be a good option as a retrofit. It converts the conventional water heater into a HPWH by replacing the function of the tank’s lower element. The add-on unit is intended to provide most or all of the water heating needs, while the standard water heater provides storage and can also serve as a back up heater to provide additional capacity as needed during periods of high use. When the tank needs to be heated, a small pump circulates water through piping installed between the existing storage water heater and the HPWH.

Manufacturers and models of add-on HPWHs include Nyle International’s Nyletherm, Applied Energy Recovery Systems E-Tech, Trevor Martin’s Hot Water Generator, and Beyond Pollution’s Air Tap.

Drop-in HPWHs: In a "drop-in" or "integrated" HPWH, the heat pump portion is integral to the tank, which has the same footprint and connections as a conventional water heater. Installation procedures are essentially the same, except for the requirement to provide for drainage of condensate.

General Electric has developed a drop-in HPWH that is expected to be available by late 2009. The drop-in Watter$aver, manufactured by ECR, was introduced in 2002. However, production has been suspended, according to a 2006 market research study by the Northwest Energy Efficiency Alliance, "while they review their marketing approach."

Desuperheaters: Heat-pump water heating is also available as the desuperheater feature on some central air conditioners and heat pumps. A desuperheater is a small, auxiliary heat exchanger that uses superheated gases from the heat pump's compressor to heat water. As a caution to avoid confusion, the term "add-on HPWH" may also be used to describe desuperheater water heaters.

Performance

Two measures of efficiency often used with HPWHs are the Coefficient of Performance (COP) and Energy Factor (EF). The COP of a HPWH typically does not include the standby loss of the tank, while the EF does. EFs that are certified by the Gas Appliance Manufacturers Association (GAMA), are useful in comparing different models. COPs are most useful for examining how the performance of a water heater changes with operating conditions.

Average COPs have been found to vary widely, from 1.0 (i.e. no savings compared with a conventional electric water heater) to 2.44, with an average of 2.0 in one field study. Actual performance depends on many site-specific conditions, including the temperature of the space in which it is installed, the water heater’s set point, water use, manufacturing quality and whether the heat pump is installed properly. The most common reason for low COPs is the resistive heating elements operating frequently because of heavy household water use. This can generally be avoided by ensuring the HPWH is properly sized. Another factor that can reduce performance of add-on units over time is the build up of hard water scale on their tube-in-tube condensers.

Energy Star criteria for water heaters

In April 2008, the U.S. Department of Energy announced new Energy Star criteria for water heaters, effective Jan. 1, 2009. Residential drop-in HPWHs require a minimum EF of 2.0 and a minimum First-Hour Rating of 50 gallons-per-hour. Add-on HPWHs will not qualify.

Air conditioning and dehumidification

A HPWH will provide air conditioning and dehumidification for either the space in which it is located or a space to which it is ducted. This will result in additional energy savings, to the extent that it avoids the use of central air conditioning and/or dehumidification. In the heating season, the cool air can be ducted outdoors to avoid simultaneous heating and cooling. The air conditioning benefit and the potential for increased space heating use, if the unit is not vented, should be included in an energy analysis. Other non-energy benefits should be considered, as well. For example, damp basements will benefit from a HPWH’s dehumidification.

Location

Since heat-pump efficiency drops significantly at cold temperatures, HPWHs are best installed only in spaces where the temperature does not fall below about 40 degrees F, such as indoor utility rooms, enclosed garages and basements. Warm spaces, such as a furnace room, can be used to take advantage of increased efficiency at higher temperatures, although the temperature typically should not rise above 90 degrees F. Be sure to check minimum and maximum temperatures recommended by the manufacturer.

HPWHs should not be installed in isolated, tight spaces due to the need for air flow across their evaporator coil. Ducts and dampers may be installed to improve air flow. The compressor motor on an HPWH system produces some noise, as with refrigerators, so pick a location where it won’t be bothersome.