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Air-to-Water Heat Pumps for the Home

Home Energy Magazine Online July/August 1997

Air-to-Water Heat Pumps for the Home

by Steven Bodzin

Steven Bodzin is Home Energy's associate editor.

Heat pump water heaters may be on the rise again. Retrofitters have shied away from this form of water heating due to concerns about cost, noise, efficiency, and maintenance. Recent advances have overcome some of these problems and are helping the technology find a niche in both hot and cold climates.
Over a third of America's domestic hot water is heated by electric resistance elements. For many residences, electric resistance is a more expensive method of water-heating than fossil fuels or solar, but there are buildings where these latter alternatives aren't feasible. For homes where electricity is the best energy source, air-to-water heat pump water heaters offer an energy-saving alternative. Watt for watt, a heat pump water heater can heat two to three times as much water as an electric resistance heater. If installed properly, it can also provide useful byproducts--space cooling, dehumidification, or heat-recovery ventilation.

Figure 1. A heat pump water heater can be used for air-conditioning or for heat-recovery ventilation, but either way it will rely on a vapor-compression refrigeration cycle. In a vapor-compression cycle, heat from warm air evaporates liquid refrigerant in the evaporator coil. This gaseous refrigerant is then pressurized, raising the temperature of the condenser coils. This heat is transferred to the domestic hot water, turning the refrigerant back into a liquid. After the refrigerant releases its heat, it passes through an expansion valve, causing its pressure to drop and thus completing the cycle.

In the early 1980s, over 10,000 of these water heaters were sold every year. The early machines suffered from high purchase prices, high maintenance costs, excessive noise, poor longevity, and limited installation options. This led the market to collapse. As of 1995, the two remaining manufacturers were only selling about 2,000 residential units per year. The Therma-Stor division of Dairy Equipment Corporation (DEC) produces a high-end integrated heat pump/storage tank. And E-Tech, a subsidiary of Crispaire, makes a compact stand-alone heat pump that retrofits onto an electric resistance storage water heater.

Today, heat pump water heating may be on the comeback, thanks in part to utility-sponsored programs. For example, Connecticut-based Northeast Utilities (NU) is currently sponsoring a demand-side management (DSM) program to install 2,600 E-Tech units in the basements of single-family homes. Installing heat pump water heaters in chilly unconditioned basements would have been technically foolhardy a couple of years ago. But the addition of a defrost cycle, along with various new configurations, make such installations possible today.

How They Work

A heat pump water heater moves heat from the air into water using a vapor compression refrigeration cycle (see Figure 1). It uses a small amount of energy to move and upgrade the temperature of a larger amount of energy, so it is able to provide enough hot water for a household with much lower electric demand. Typical residential heat pump water heaters use 500 to 1,200 watts at peak load, compared with 4,500 watts for most electric resistance units. The heat pump gets about two thirds of its heat energy from the air and one third as leftover heat from the compressor motor.

Pictured here is a heat pump water heater with exhaust air heat recovery.

Heat pump water heaters can heat water at 40%-100% the rate of electric resistance units, and 30%-50% of the rate of gas units. To provide quick recovery, a household must have either a large heat pump, an unusually large storage tank, or a control system that turns on electric resistance backup heat. However, large tanks are more expensive, take up more space, and use more energy to maintain a setpoint. A higher-capacity heat pump is undesirable as it costs more and will often short-cycle, resulting in lower efficiency. Some designs let the small heat pump supply most of the hot water load, but include controls to activate the tank's electric resistance element when demand is high. However, this increases peak electrical demand and reduces energy efficiency. An option for some households is to reduce peak demand by spreading out their hot water use over the day.

Air from Where, to Where?

All air-source heat pump water heaters cool air to heat water, but installers choose where to release the cool air. There are three common options. One is called an exhaust air system or heat-recovery ventilation system. In cool and cold climates, house air is typically drawn into a unit, cooled, and then vented outdoors, in what is called an exhaust air configuration. Northeast Utilities is placing units in unconditioned basements, where they act as dehumidifiers, exhausting drier, cool air within the basement. In hot climates, an air conditioning set-up known as an ambient air configuration provides space cooling. These systems draw in house air, capture the heat, and release
the cooled air within conditioned space.

Air Conditioning

Ambient air units are most economical in homes with high space-cooling loads and minimal space-heating loads. They get their heat from indoor air, transmit the heat to the water, and vent cool, dry air into the indoor space. If the house needs cooling, this is a major benefit. But when the house is being heated, the heat pump can become a load on the space-heating system. Water heating efficiency then becomes limited by house space-heating efficiency.

Ambient air units also need plenty of air circulation. The capacity and efficiency of a heat pump both decline at low temperatures, so the cooled ambient air must be able to circulate away. In recent years, ambient air units have had defrost cycles added, and prototypes have been designed that can run at 40°F. However, E-Tech still specifies that inlet air must be above 55°F. Controls have been developed to permit the units to switch to resistance heat if the ambient air becomes colder than the design temperature. These advances do not mean that a heat pump water heater should be put in a closet, but it does make installation easier than it was in the 1980s.

Heat Recovery Ventilation

Exhaust air heat pumps are best for residences where regular mechanical ventilation is necessary, especially in cold climates. A potential drawback of this design is that house ventilation becomes tied to hot water demand; when demand is low, a unit will not necessarily ventilate the house. Current models from DEC include a timer, allowing residents to schedule ventilation hours. In Sweden, heat recovery water heaters are used in about 75% of new homes, because Swedish building codes effectively require heat recovery ventilation in new single-family residences.

This E-Tech heat pump is the result of several years of collaboration between the manufacturer and the Electric Power Research Institute. It features a defrost cycle, low temperature operation, easy installation, and low cost.

Hybrid Systems

Creative ducting can help supply consistently warm air to a heat pump while increasing the home's energy efficiency. For example, in a mild climate, a water heater can draw air from a ventilated attic and exhaust it outdoors. This will recover heat lost through the upper-floor ceiling in winter, provide very hot supply air in summer, and reduce attic heat gain. However, some compressors will suffer if the supply air is too hot; E-Tech units, for example, are restricted to supply air under 110°F. Anyone installing such a design should make sure that the inlet air won't stress the heat pump or void any warranties. A creative installation in Connecticut draws moist supply air from an unheated basement and vents the dry exhaust air into an unconditioned crawlspace where there had been a moisture problem. Manufacturers and local utilities may be able to help configure systems for individual climates and homes.

Nuisances

Heat pump water heaters have a reputation for being noisy and drafty. An Army Corps of Engineers study recorded average noise levels of 67 average decibels (dBA) at a distance of 1 ft from the heater (comparable to a window air conditioner), 52 dBA 10 ft away, and 45 dBA in the residence bedrooms (comparable to a quiet radio playing). The report said that occupants generally tolerated the noise, while housing personnel at federal facilities suggest that the additional noise is only a minor irritation to the occupants. All the same, the current NU DSM progam has found that the units are louder than a refrigerator, and that they do bother residents if installed in the main living area. Location has a major effect on noise; installers should be cautious not to place the units near hollow walls or on floors that will amplify and project the compressor noise.

Another nuisance is the breeze from the evaporator fan. To keep chilled air from being reingested into the evaporator, ambient-air heat pumps use a fan with considerable throw. This fan can feel like a draft when a unit is installed in conditioned space.

Maintenance

Heat pump water heaters should be maintained only by trained, experienced technicians. Unfortunately, training requires knowledge from traditionally unrelated disciplines--plumbing and HVAC. For example, not only must technicians know about water heater service, they must have the knowledge, skill, and equipment to comply with refrigerant handling regulations. For this reason, large installations, such as military bases and DSM programs, are likely to continue to be the best outlet for heat pump water heaters.

To Install or Not to Install

Heat pump water heaters should be considered

Where electric-resistance water heaters are currently used
Where electricity rates are high and where other alternatives (natural gas, propane, or oil) are expensive or unavailable
Where hot-water use is high, typically in households of four or more persons
Where adequate space for a large storage tank is available
In warm climates where space cooling is important and space heating needs are low (ambient air type)
In mild to cool climates where there is a demand for cooling, and heat is provided by a heat pump (ambient air type) or oil.
In mild to cool climates where there is a need for mechanical ventilation (exhaust air type)
Where electrical peak loads coincide with residential water heating peak loads, typically at 7 am and 6 to 7 pm
In mild to cool climates where the water heater can extract heat from a large, unconditioned basement or crawlspace
Where the exhaust and inlet airstreams can be easily ducted to minimize negative impacts on household space-conditioning energy use

They should not be installed

Where hot-water consumption is low, such as in small residential units or in units that are often vacant
In conditioned space where electric resistance space heat is used, in mild to cold climates
Where regular maintenance cannot or will not be provided
In unventilated closets or small rooms unless equipped with either a ducted air supply or ducted exhaust air (ambient air type)
Where they may be exposed to freezing temperatures

There has never been a strong service insfrastructure for heat pump water heaters. However, the units do require regular maintenance. A 1995 report written by Pacific Northwest National Laboratory (PNNL) for the Federal Energy Management Program stated that mainenance needs were on the order of two hours per year. Warranties typically cover parts for one to four years and labor for only a short time after purchase. PNNL claims that compressors, water pumps, fan motors, and expansion devices all frequently need repair or replacement during the design life of a unit, but this is based on military housing complexes with older units. Based on this same experience, PNNL recommends service checks twice a year to

Inspect for water leaks that can corrode components
Remove scale buildup from the water side of the condenser coil, especially in areas of hard, alkaline water
Clean air filters and condensate drains (this can be done by a resident)
Check the refrigerant charge and electric draw of the compressor.

Accepting the lack of qualified field maintenance workers, Crispaire's new strategy is to have all units sent to a central service center for repairs. The new units are under 70 lbs, so problem units can be changed out and shipped easily.

So How About the Money?

Suggested retail prices for E-Tech heat pumps start above $700, not including the hot water tank, compared to a low end of about $200 for electric resistance water heaters, which include the tank. These high first costs would have to be recouped through low operating costs to make heat pump water heating cost-effective outside of DSM programs.

Cost-effectiveness is most likely where air conditioning and hot-water energy use are relatively high. According to a recent report by Arthur D. Little for the Department of Energy, a premium of $500 will be paid back in an average of 3.5 years, depending on location. However, where hot-water energy use is low, the water heater may not pay for itself before the end of its 12- to 15-year expected life.

A breakdown of where heat pumps are likely to be cost-effective is included in the PNNL report (see "To Install or Not to Install"). The report focused its examination on military bases, where units are installed in poorly maintained units where residents do little routine service. Thus, the recommendations are probably conservative.

PNNL also produced Figure 2, showing possible break even electricity cost curves for air conditioning heat pump water heaters. Each curve shows the average electricity cost (including demand costs) necessary for a heat pump water heater to be cost-effective compared with an existing electric resistance water heater. The horizontal axis defines the climate by cooling degree-days at a 65°F base temperature (CDD65). The upper two curves are based on a home with space heating provided by electric resistance heat, and the lower two are based on air-source heat pump space heating. For electric costs higher than the chosen curve at a specific CDD65 value, the heat pump water heater is likely to prove cost-effective.

The DSM program from Northeast Utilities (NU) aims for maximum electric savings. It requires that residents have an electric resistance water heater of 50 to 80 gallons outside the conditioned space. The existing tank keeps its upper resistance element, while power to the lower element is diverted to the heat pump. As of late May, NU reports that, without advertising, they have installed 50 units and have a waiting list of over 100 names. They report high customer satisfaction.

Figure 2. To develop the curves in this graph, 32 locations in the United States were analyzed using federal Building Life-Cycle Cost software. The calculations assumed that homes were being retrofitted with a 6,000 Btu/hr air-conditioning heat pump water heater with an average daily COP of 2.5, a 65°F baseline temperature for cooling degree days, and installed costs of $985. All residences were assumed to have air-conditioning with a nominal SEER of 9.0. The areas above each curve represent when a heat pump water heater is likely to be cost-effective compared with an existing electric resistance water heater for a given space heating system. For example, for a three-person residence with electric resistance space heat, a climate with 2,000 cooling degree days, and an electricity cost of 9¢ per kWh, a heat pump water heater is probably cost-effective.

Proof in the Putting

Field tests report water heater energy savings of 40%-70% under a variety of test conditions. Payback estimates, however, vary widely--from 3 to 20+ years in residential applications. Heat pump water heaters have an expected life of about 12 years. However, local conditions can help to determine whether the unit will live to pay for itself. Environmental factors, such as salt spray in the air, very hard water, or extreme temperatures, may reduce a unit's life, while routine maintenance is likely to prolong it.

The Bonneville Power Administration (BPA) examined the performance of exhaust air heat pump water heaters in the Pacific Northwest. It monitored 31 homes over 13 months in 1992 and 1993, and continued to monitor 10 of the units from September 1994 to April 1995. Investigators found daily average coefficients of performance (COPs) of 2.0, not including any resistance heat backup. The units had rated Energy Factors (EFs) of 2.5 (see "Performance: A Trick to Quantify"). Several units performed at or above this level, several were slightly below, and two units were well below this level. One unit was installed in an unheated basement, but did not have a defrost cycle. Its evaporator iced up, resulting in an overall COP of 1.3.

On average, between 2% and 4% of the energy use in heat pump water heaters in the Bonneville study went to electric resistance backup heat. Energy savings averaged about 2,200 kWh per year for the exhaust air units. The average installed cost was $1,800, and the simple payback period was 16.4 years at 5¢/kWh--longer than the unit's expected life, but at lower-than-average electric rates. Average hot-water energy use in this study was approximately 13.5 million Btu per year--the DOE estimates that households typically use 15 million Btu per year.

Another military study found higher average COPs with ostensibly higher savings. In military studies from 1988, ambient air units installed at two bases in Hawaii had average COPs of 2.7 during testing, 94% of the manufacturer's rating for the actual operating conditions.

Resources

Market Disposition of High-Efficiency Water Heating Equipment, by Arthur D. Little, Incorporated, 1997.

Much of this article is adapted from the U.S. Department of Energy Federal Energy Management Program Technology Alert, Residential Heat Pump Water Heaters by David Winiarski. Available from the FEMP Help Desk at (800)363-3732.

Performance: A Trick to Quantify
Two terms are used to describe the performance of heat pump water heaters--the heating coefficient of performance (COP) and the Energy Factor (EF). The EF is more common, as it is also used for other residential water heaters.
EF is based on a particular 24-hour test procedure. The test is the same for all residential water heaters, regardless of energy source, so it offers an easy comparison within a type of water heater and also among different types. It measures the ratio of energy that goes in (in electricity or fuel) to the amount of hot water that comes out of a water heater. It tries to account for both recovery efficiency and standby losses from the water heater and storage tank. In theory, a heat pump water heater with an EF of 2.0 will provide the residence with hot water containing energy equal to twice the water heater's electrical energy input.
To measure EF, testers remove 64.3 gallons of 135°F water from the tank in six equal draws at the beginning of each of the first six hours of the testing period. No other draws are made for the rest of the 24-hour test period. The test fixes ambient air temperature at 67.5°F, inlet water temperature at 58°F, and relative humidity between 49% and 51%.
COP is the ratio of heat energy output to electrical energy input when both are measured in consistent units. A heating COP of 2.0 means that the heat energy output of the water heater is twice the electrical energy input. However, heat pump water heater COP ratings, unlike those for air conditioners, are not standardized in the industry. They do not necessarily include standby losses, and are not typically used for other types of water heaters. Thus, manufacturer-reported COPs are not useful for comparison shopping. The EF rating is more appropriate for this purpose. COPs are most useful for examining how the performance of a water heater changes with operating conditions.
Regardless of COP or EF, heat pump performance depends on operating conditions. Hot water temperature controls the condensing temperature and inlet air temperature controls the evaporator temperature. In general, colder supply air and hotter water both lower a heat pump's efficiency. Both efficiency and capacity improve with humid inlet air. Humidity provides latent heat, so high humidity allows the heat pump to extract more energy from the air at a given evaporator temperature. One manufacturer claims to have seen a 20% improvement in performance when a system is operated at a relative humidity of 70% compared to a relative humidity of 30%.
The EF test has been criticized as potentially biased in favor of heat pump water heaters. In many households, hot water demand is often very high in the morning for about an hour, moderately high in the evening hours, and very low during much of the day. Especially in smaller tanks, high morning draws can cause the backup resistance elements to operate, reducing efficiency below the reported EF.
On the other hand, heat pump water heaters can get more efficient with higher use. Once the water temperature in the tank has been reduced substantially, the condenser temperature is lower and the heat pump water heater will perform more efficiently. COP improves with higher hot water use and with big water draws. In a Bonneville Power Adminstration study, daily COP varied between 1.5 and 2.5 as the hot-water usage varied between 500 and 3,500 gallons per month. The variation of performance under different use scenarios is substantially less with other water-heating technologies.

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