Heat Pump Water Heaters

What Are the Options?

How to Make the Best Choice

What's on the Horizon?

Heat pump water heater (HPWH) systems mine the energy content of air to produce hot water very efficiently (Figure 1). Depending on cold-water and ambient-air temperatures and on patterns of hot water use, heat pump water heaters do the same job as standard electric water heaters using two to three times less electric energy.

Figure 1: The heat pump cycle Air is cooled as it passes through the evaporator's fins, and water is heated as it passes through the condenser's heat exchange surfaces. Source: E Source

Heat pump water heaters use a motor to run a compressor. The compressor draws a gaseous refrigerant through an evaporator, raising its pressure until it liquefies in the condenser. This familiar process heats the condenser and cools the evaporator. In wringing the heat from air, HPWHs both cool and dehumidify the air that passes through them, thus helping to meet space conditioning needs during cooling seasons. Under most scenarios, the extra costs of heat pump water heaters over standard electric water heaters are paid back in two to three years. In areas like Hawaii—where natural gas in unavailable, electric energy costs are high, and the need for dehumidification is virtually constant—paybacks of well less than two years are routine.

What Are the Options?

HPWH systems are available in the U.S. in a variety of capacities, from small residential to large commercial, producing more than 350 gallons per hour of hot water and 6 tons of air conditioning. Options to consider include system configurations and efficiencies.

Integrated versus add-on systems. With integrated systems, the heat pump apparatus is physically connected to the hot water tank, and the condenser is typically wrapped around the tank, surrounded by insulation. Integrated systems are also called "drop-in" systems for good reason. They are designed to require no special expertise in HVAC installation or wiring; ordinary plumbers can install them without the aid of other tradespeople. More compact than add-on systems, they tend to have the same footprint as ordinary electric hot water systems of the same water capacity. Accordingly, integrated systems tend to be easier to retrofit. On the other hand, the largest model currently sold in the U.S. has a capacity of 120 gallons. This is adequate for many light commercial applications, but not for installations where the average demand for hot water exceeds 30 gallons per hour.

With add-on systems, the heat pump apparatus stands alone (Figure 2). Heat is transferred from the condenser to the water tank via a heat exchanger and a small pump, using the tank's water as a heat exchange medium.

Figure 2: Configurations of heat pump water heaters Integrated HPWH systems come in a single package and may be installed quickly. Add-on systems use an existing hot water tank (and its electric resistance heaters as backup), but the heat pump may be placed at some distance from the tank to accomodate space or air handling needs. Source: E Source

An add-on HPWH system has the virtue of making use of the existing water heater that heats by electrical resistance. Thus, the capital expense is lower and, in many cases, installation can be more flexible, since there can be some distance between the HPWH apparatus and the water tank.

HPWH system efficiencies. The instantaneous energy efficiency of a heat pump water heater system depends on incoming water temperature, intake air temperature, the heat transfer characteristics of the heat pump, and various conductive and convective losses throughout the system. Further, although in most circumstances the hot water output is useful throughout the year, the cold air output may not be. Accordingly, there is no simple index that accounts for both outputs and describes overall HPWH efficiency. Instead, the HPWH industry relies on two indexes of energy efficiency—coefficient of performance (COP), which is favored by manufacturers of larger HPWH systems, and energy factor (EF), routinely used by manufacturers of domestic HPWH systems.

COP is a measure of the instantaneous energy output of a system in comparison with its instantaneous energy input. Standby losses and the interaction of changing water and air temperatures are not reflected in measurements of COP. Accordingly, the COP of a standard hot water system is close to 1, and the COP of a typical HPWH heater may be 3.

The EF is a more useful measure, for it reflects more realistic circumstances likely to occur in the field. The test to determine EF is conducted over a 24-hour period with temperatures of incoming water and input air held constant. A measured amount of water is pulled from the system every other hour for the first 12 hours, and no water is drawn for the final 12 hours. Because this test reflects standby losses, the EF of a typical hot water system is 0.86, and the EF of a typical HPWH heater may be 2.2. This represents an efficiency improvement of more than 250 percent, even ignoring the cooling benefit.

Buyers of commercial systems should be aware that COPs quoted by manufacturers sometimes reflect the combination of the production of cold air and hot water in relation to energy input. This is helpful if full use is made of the cold air, but not otherwise.

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How to Make the Best Choice

Pick the right size. Picking the right size HPWH system requires estimating daily hot water needs in gallons—just like sizing any other water heating systems. Chapter 48 of the 1999 ASHRAE Applications Handbook has a useful section on "Hot Water Requirements and Storage Equipment Sizing" covering residential, commercial, and institutional hot water system sizing. However, for HPWH systems, an allowance must be made for high peak hot water demands. HPWH systems are quite efficient, but they are slow and steady. A key factor to consider is the rate of hot water production, listed in product literature as "recovery rate" and measured in gallons per hour. Recovery rates are typically half as much as those of traditional water heaters, but the instantaneous power consumption (demand) is eight times lower. Accordingly, electric demand savings with HPWH systems can be substantial, but only if the use of backup electric resistance heat is quite low.

If you'll be using HPWH systems in applications that require considerable hot water over a short time, choose either a larger tank than a traditional hot water system has or an HPWH system of higher recovery rate. Either choice will help smooth over peak hot water loads.

If the installation has only modest hot water needs (say 300 gallons a day or so, with no large peak hot water loads), an integrated system may be the easiest to install and the most cost-effective over its lifetime. Otherwise, an add-on system is the only practical choice.

Perform a quick cost/benefit estimate. Having determined the size of the HPWH system, examine the product literature and talk with potential suppliers. Check out cost and efficiency numbers. If energy factor information is available on a system of the right size for your application, so much the better. If only COP information is available, multiply the COP by 0.86 if the unit is well insulated, 0.80 if it isn't. Then use Table 1 to estimate what the savings will be for your application.

Match the technology to the application. Because HPWHs produce cool, dry air as a by-product of heating water, the best applications are those that take advantage of both outputs. Accordingly, HPWHs are especially well fitted for commercial sector applications where demand for hot water is relatively constant and the need for cooling or dehumidification is continuous. Commercial laundries fit this description, as do many commercial kitchens and even fast-food restaurants, particularly in climates where space cooling is essential. (A tale is told of a kitchen worker who, realizing the connection between hot water and cool air with HPWH systems, ran extra hot water just to stay cool!)

For buildings that use rooftop cooling towers or large refrigerators, it may be worthwhile to harvest waste heat from these units, using the HPWH system both to produce hot water and to help meet the air conditioning load (Figure 3).

Figure 3: HVAC heat recovery via a heat pump water heater Using the return water to a chiller as input to a HPWH system enhances the system efficiency of both the HVAC system and the hot water system. The result is double-dip savings. Source: E Source

Pick a good location. All HPWH systems should be installed with careful attention to the flow of air across their evaporators. First, because air flow is a necessity (several hundred cubic feet per minute even for smaller systems), do not place systems in isolated, tight areas. Second, because they produce dry, cool air, put them where their output air will be useful, such as damp basements or spaces that need cooling most of the year. Of course, ducts and dampers may be employed to achieve the needs of source and output air, thus allowing flexibility in choosing a location. Finally, as with refrigerators, the compressor motor on a HPWH system produces some noise. Although most users report that it is unobjectionable, it may be wise to pick a location where the noise can never be a nuisance.

Perform regular maintenance. Heat exchange surfaces perform better when clean, and HPWH systems are no exception to the rule. To maintain good energy performance, keep the filter that protects the evaporator's heat exchange surfaces clean. This is particularly important in kitchens and other areas that contain airborne pollutants.

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What's on the Horizon?

If heat pump water heaters are so efficient, why aren't more of them being purchased? This key question was addressed for the U. S. Department of Energy (DOE) by researchers at Arthur D. Little. They concluded that high front-end costs, spotty reliability of early models, complicated installation procedures, and inadequate consumer knowledge combined to keep HPWHs from being successful in the marketplace. Accordingly, the Arthur D. Little research team, working with a heat pump manufacturer (EnviroMaster International) and researchers at Oak Ridge National Laboratory, set out to solve all four problems.

The team designed a "drop-in" HPWH (Figure 4) that resembles ordinary hot water heaters but uses 2.5 times less energy and costs only $300 to $400 more. Under most installation scenarios, this translates into paybacks of from one to three years. A 50-gallon version of the EnviroMaster system is slated for market entry early in 2001, to be followed soon by an 80-gallon version suitable for light commercial applications.

Figure 4: The EnviroMaster "drop-in" HPWH This model is undergoing laboratory and field tests to verify energy performance and identify maintenance problems. To make the system extra-easy to install, one version of the EnviroMaster needs no condensate drain, a feature that diminishes overall energy performance by several percent. Source: Arthur D. Little

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Copyright © 2006 E Source Companies LLC

Last Modified: January 31, 2007