Drop-in replacement models use the same footprint as a conventional electric tank. And they can be installed by any plumber.

Enviromaster International in Rome, New York, recently introduced a heat pump water heater with the same footprint, and the same electrical and plumbing requirements, as a conventional 50- or 80-gallon tank unit. Called the WattSaver, it was developed in partnership with Oak Ridge National Laboratory as a "drop-in" replacement for conventional units. That's good news for builders who want to market their homes as energy-efficient: water heating consumes about 17 percent of the energy for a typical home, and about 28 percent of the total energy used by the 40 million all-electric homes in the United States. The new drop-in uses only 30 percent as much energy as an electric resistance water heater.

How It Works

A heat pump water heater, or HPWH, is like a conventional heat pump in that it extracts heat from the surrounding air, and then concentrates that heat. But while the conventional heat pump uses this concentrated heat to warm the living space, the HPWH uses it to heat a tank of water.

Of course, an electric resistance unit also heats a tank of water. The reason the HPWH does it for so much less has to do with the efficiency with which the two units do their work. An electric resistance heater is actually a model of efficiency, converting 100 percent of the incoming electricity to heat, which is in turn used to heat the water. After factoring in standing heat loss through the tank walls, it is still 95 percent efficient at delivering hot water to the house.

That statistic only seems impressive until you compare it with the 200 percent to 300 percent efficiency of an HPWH. But how can anything create more Btu's than it uses? The answer is that it can't. Remember, the HPWH doesn't create heat, like an electric resistance unit; instead, it moves existing heat from one place to another.

The heat transfer process has a potential side benefit in hot climates: space cooling. A heat pump is essentially a refrigerator in reverse. While refrigerators remove heat from inside a box to create a cold space, the HPWH removes heat from its surroundings and uses that heat to warm water. In air-source units, this creates a stream of cool exhaust air, which means the unit can also provide air conditioning.

The combination of heating and cooling makes swimming pools and spas particularly good applications for this technology. The HPWH can be used to warm the pool's water, while the cool exhaust cools and dehumidifies the pool room. If there's no need for cooling or dehumidification, the exhaust can be directed outside.

Builder Resistance

The drop-in unit isn't the first water-heating heat pump. But despite their efficiency benefits, very few homebuilders or homebuyers have shown interest. A big drawback has been the high initial cost, which can range from $600 to over $2,000, compared with the $150 to $350 for an electric water heater. These high initial prices give HPWHs long payback periods, even with the substantial energy savings they provide each month.

Another HPWH drawback is that it does not heat water as fast as a conventional model. Homeowners who use only moderate amounts of hot water at one time will likely not notice the difference, but those who exhaust the supply of hot water stored in the tank may be dissatisfied with how long the HPWH takes to replace it. The builder may have to install an electric-fired tank water heater or a tankless, on-demand heater as a backup. Since the extra costs can defeat the purpose of installing the HPWH in the first place, this technology probably isn't the best solution for someone who uses a lot of hot water.

Many earlier HPWH models also experienced failures with major components, including compressors or pump impellers. And because they required combined knowledge of traditionally separate disciplines, such as plumbing and refrigeration/HVAC, improper installation was commonplace. These problems gave HPWH a reputation as unreliable.

The Drop-In

The drop-in unit doesn't heat water any faster than its predecessors, but it does address the other drawbacks. It's both more reliable and easier to install in new construction and retrofits. It can be installed by your plumber, and will fit anywhere a conventional water heater will, whether in a garage, a basement, a utility room, or a closet. If all you intend to do with the unit is heat domestic water, you need not install a floor drain, but if you want space cooling and dehumidification, there will need to be a drain to carry away condensate.

The drop-in is also more affordable. At a few hundred dollars above the installed cost of a conventional 50-gallon electric water heater, and with an efficiency of over 200 percent, energy savings will pay for the extra cost in three years. (That assumes the national average water use of 64.3 gallons per day of hot water per day, and the national average electric rate of about 9 cents per kWh for electricity.) It's most cost-effective where energy rates are high, and in hot climates that can make best use of the cooling effect. It is less cost-effective when low-cost natural gas is available.

On the horizon, you can expect more innovation in this emerging technology. Researchers at Oak Ridge are developing an HPWH that can be retrofitted to a standard electric storage tank. The small heat pump will be placed on top of the tank, and a rod inserted in the tank through a threaded fitting. The heat pump will heat the rod, and the rod will heat the water.

Lower costs and more product choices should make the technology more appealing to consumers. In fact, they could provide the drive that the technology needs to gain popularity in the United States.

Executive Summary

The latest models are more affordable and simpler to install than older ones, and they pay for themselves in less than three years. But before this super-efficient technology enters the mainstream, consumers will have to understand its value.

The PATH Inventory

This is one of a series of articles on innovative home building technologies, many of which are included in the Partnership for Advancing Technology in Housing's (PATH) Technology Inventory. The PATH Program is a consortium of homebuilders, manufacturers, and government agencies that seek to improve the affordability, quality, energy efficiency, environmental performance, durability, disaster resistance and/or safety of American housing. The inventory focuses on "emerging" technologies (with a market share of 5% or less) in a range of categories, from new materials, components, and systems to complete houses. For the complete Inventory, go to www.toolbase.org/techinventory.

Craig Drumheller is a Senior Energy Engineer at the National Association of HomeBuilders Research Center.