What's New in Water Heating?

A version of this article appears in the September/October 2012 issue of Home Energy Magazine.

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Over the past year, you may have seen advertisements or promotions for new “hybrid electric” water heaters that use less than half as much energy as traditional electric-resistance water heaters. What are these products, and do they really save so much energy? These hybrid water heaters, also known as heat pump water heaters (HPWHs), are not entirely new, but they have not sold widely in the past. Earlier products, produced by small niche-market manufacturers, have proven unreliable, and the market for them has been limited. Recently, several manufacturers—including General Electric, Rheem, A.O. Smith, Stiebel-Eltron, and AirGenerate—have entered the HPWH market.

HPWHs have the potential to save a lot of energy. While traditional electric-resistance water heaters perform with energy factors (EF) of around 0.9, new HPWHs boast EF of over 2. In the United States, water heating is the third-largest contributor to residential energy consumption after space heating and cooling. For the 36% of homes that use electric resistance water heaters (Table 1), changing to HPWHs could result in roughly 50% reduction in water-heating energy consumption.

How Does It Work?

A heat pump is a device that moves heat from one place to another (see Figure 1). In the case of a HPWH, heat is moved from the air surrounding the unit into the hot-water tank. HPWHs are designed primarily as replacements for standard electric-resistance water heaters, but they can also be an excellent alternative to oil or propane water heating.

Most heat pumps operate as hybrid devices—that is, they use the heat pump whenever possible, but built-in controls switch to conventional resistance heating when the demand for hot water is high. The heat pumps in these hybrid water heaters can heat water at high efficiencies, but their heating capacity is lower than that of a traditional electric-resistance heater. A typical 4.5 kW electric-resistance element can reliably heat over 20 gallons of water per hour. The heat pump has a lower heating rate; General Electric, for example, publishes a rate of 8 gallons per hour at 68°F air temperature. This gap illustrates the benefit of a hybrid configuration—heat pumps are used whenever possible, for highest efficiency, but resistance heat is used when there is a high demand for hot water.

Most HPWHs have several operating modes from which to choose. The names differ with the manufacturer, but most models include some combination of the following operating modes:

• Hybrid mode is typically the default; it uses both the electric-resistance element and the heat pump to meet demand, but the heat pump is used whenever possible to maximize efficiency.
• Heat pump mode uses only the heat pump. This can improve efficiency, but it dramatically reduces the recovery capacity of the water heater. If occupants use a large amount of hot water in a short period of time, they are more likely to run out of hot water in this mode.
• Electric-resistance mode works like a traditional electric-resistance water heater. This mode can be used when the ambient temperature of the space is too cold for the heat pump to operate properly, or if the heat pump is not working.

Getting the Most from a HPWH

Although HPWHs have the potential to save a lot of energy, it is important to ensure that a HPWH is installed correctly to maximize the efficiency of the unit. If the unit is not installed under ideal conditions, the true savings may be considerably smaller. HPWHs should be installed according to manufacturer guidelines, particularly with regard to clearance and maintenance. The specifications of some HPWHs currently on the market are shown in Table 2.

Over the past year, Steven Winter Associates, Incorporated (SWA) has been monitoring the performance of 14 HPWH units installed in Massachusetts and Rhode Island homes with the support of DOE’s Building America program and several New England utility companies, including National Grid, NSTAR, and Cape Light Compact. This field monitoring is an important step in gauging the real-world performance of HPWHs.

The efficiency of residential water heaters in the United States is measured using the EF value. The EF represents the efficiency of the system under a specific, 24-hour test procedure. For a unit that operates under real-world conditions, the coefficient of performance (COP) is the term used here to describe the efficiency of the unit. Results from the monitored sites showed a wide range of performance in HPWH units; measured COP ranged from 1 to 2.6.

Based on SWA’s monitoring evaluation and other literature, the primary variables that affect HPWH performance are hot-water usage, ambient temperature, and available ambient space. Special consideration must be given to these variables when installing HPWHs in homes, to optimize the performance of the unit.

Hot-Water Usage

The primary driver of energy used by a HPWH is hot-water consumption. Electricity consumption increases—and HPWH efficiency decreases—with overall water consumption (that is, average gallons used per day) and with intensity of water consumption (that is, gallons consumed over a short time period). Though this is true for any water heater, hot water consumption has larger effects with HPWHs due to the lower recovery rate of the heat pump (typically around a third of an electric resistance water heater).

Overall Water Consumption

SWA found that HPWH efficiency for the 50-gallon tanks peaked when consumption averaged 20 to 30 gallons per day. The average household consumption in this evaluation was 50 gallons of hot water per day, and higher consumption resulted in increased electric-resistance heating. The COP for 50-gallon units ranged from 1 to 2.2. Units with a capacity of 60 to 80 gallons were better able to meet the hot-water demand with minimal auxiliary heating. The COP for these larger units ranged from 2 to 2.6. When selecting a water heater, pay attention to the first-hour rating—not just the tank size.

Intensity of Water Consumption

During intense, high-volume hot-water draws, a HPWH will often operate in electric- resistance mode in order to meet the demand. Electric-resistance mode can provide more hot water faster, but it also consumes significantly more electricity than heat pump mode. Figures 2 and 3 show nearly identical overall water consumption (70 gallons) over the course of a day, but very different consumption patterns. Figure 2 shows consumption distributed fairly evenly throughout the day; Figure 3 shows larger, concentrated draws during the afternoon and evening. Each data point in these charts is the totalized consumption over a 15-minute period. In Figure 2, the heat pump is able to satisfy all hot-water demand, as the unit has time to recover in the slower but more efficient mode. In Figure 3, the same HPWH must rely extensively on electric-resistance mode to meet demand. Therefore, if people in your household take a lot of high-volume or back-to-back showers, anticipate the electric-resistance mode kicking in to keep up with this demand.

Ambient Temperature

The efficiency of a HPWH increases substantially as the ambient temperature rises. (Ambient temperature is the air temperature of the space where the HPWH is located.) Heat pumps become much more efficient as the heat source (in this case, the ambient air) becomes warmer. Warmer air also reduces standby losses.

Higher ambient temperatures result in considerably higher heat pump COP. At 62 gallons per day—the national average daily hot-water consumption of U.S. households—an increase in ambient temperature from 50°F to 80°F can result in an increase in COP from 2.2 to 3.2 (assuming that the HPWH is running in heat pump mode only). This results in a 30% drop in electricity consumption.

Most HPWHs will operate in heat pump mode only if the temperature of the air entering the water heater is above 45°F or thereabouts. When the temperature of the incoming air drops below the minimum temperature, most HPWHs will switch into electric-resistance mode, reducing the efficiency of the unit. In practice, the temperature of the space where the HPWH is located must start several degrees above the minimum temperature, because the heat pump will cool the space. At one test site, heat pump operation dropped the ambient temperature of the basement mechanical room by over 4°F—from 49°F to less than 45°F, which is the minimum allowable ambient temperature for this unit. Colder air temperatures at this site year-round increased the use of electric-resistance mode to provide hot water.

Conversely, at another of the test sites, the HPWH unit was placed in a basement mechanical room next to the boiler. The waste heat from the boiler kept the room at 70°F or above throughout the winter. In fact, the ambient temperature was higher in the winter than it was in the summer. These higher temperatures allowed the HPWH to operate much more efficiently.

Inadequate Space

A HPWH must be able to extract sufficient energy from the surrounding air to function, and the energy available in the air is primarily a function of the size of the space. Therefore, HPWHs must be installed in rooms big enough to ensure efficient operation. (See manufacturer literature for space requirements.) Adequate clearances must be provided to allow for proper airflow and maintenance. If a unit is installed in an area with insufficient space, the ambient temperature can drop dramatically during HPWH operation.

At one test site, a HPWH was installed in a small, unconditioned basement mechanical room with a door that was kept closed. The volume of the mechanical room was approximately 440 ft3, significantly less than the required 750 ft³. The overall COP of the unit was 30% lower than the expected COP of a HPWH installed with adequate space.

Have Realistic Expectations

When a HPWH replaces an electric-resistance water heater, the replacement usually results in substantial savings. Savings may also be possible when replacing a fuel-fired water heater, though this is far from certain. Potential savings depend on consumption, fuel prices, climate, and other factors. Table 3 shows expected water-heating costs for standard tank electric-resistance water heaters versus HPWHs. While replacing an electric-resistance heater with a HPWH always results in some savings, these savings may be very modest if the HPWH is installed in a small or cold space, or if water consumption is very intense.

Other Considerations

When selecting an appropriate location for a HPWH, consider how it will interact with the home’s heating system. Because a HPWH extracts heat from—and therefore cools— the air where it is installed, the home’s heating system may need to run more frequently. Also consider comfort; if a HPWH is installed in or near living space, that space will likely be cooled by the HPWH year-round.

Bear in mind that a HPWH can be noisy. Manufacturers list sound pressure levels of 58–65 decibels (dBa). For reference, a refrigerator may generate around 50 dBa, and a vacuum cleaner may generate 70–80 dBa. Make sure the HPWH is located where this noise will not be disruptive.

Robb Aldrich and Carl Shapiro are engineers with Steven Winter Associates (SWA) in Norwalk, Connecticut.

The authors gratefully acknowledge the contributions of SWA’s Srikanth Puttagunta, National Grid, Cape Light Compact, NSTAR, DOE’s Building America program, and the homeowners who participated in the study upon which this article is based.

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