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In hot water.

A net zero energy house may not be far off. Oak Ridge National Laboratory has three next-generation houses up and running as part of the Department of Energy's Building America Zero Energy Habitat for Humanity project. And it costs a family of four just 82 cents a day in energy costs to live in one.

A key feature of the houses, which are located in Lenoir City, Tenn., is a second-generation heat pump water heater that helps control the interior environment.

The heat pump water heater, from EnviroMaster International of Rome, N.Y., uses a refrigeration compressor to generate heat. EnviroMaster said that it worked with the Oak Ridge lab and the utility industry to test the design, which is a more efficient alternative to electric resistance water heating. The company calls it WatterSaver.

The condenser coils are wrapped around the tank to heat the water.

The arrangement in the Zero Energy House uses the water heater's evaporator coil as an air conditioner during the cooling season. At all other times, a separate stream flows past the coil and never enters the interior of the home. Isolating this stream keeps cool air from invading the living space. It also avoids a possible source of humidity from outside and the damp, mold, or mildew that may breed in a crawl space under the house.

In the Habitat for Humanity houses, the heat pump water heater resides in a closet next to the refrigerator, according to Jeff Christian, principal investigator of the Zero Energy House project for Oak Ridge. In the summer, air heated by the refrigerator's compressor passes into the closet, over the water heater's evaporator coil, and comes back into the house as cool, dry air. The system eliminates one source of hot air, at the refrigerator, and gives the water heating system a second role as an air conditioner.

At all other times--during the heating season or at temperate times when neither heating or cooling is called for--a different stream of air passes over the water beater's evaporator coil. In this state, a vent lets out side air into a crawl space below the house. Air from the crawl space enters the closet through a duct, passes the evaporator coil there, and then is vented directly to the outside. Damp air from outside the house or from the crawl space does not enter the living areas of the home.

The streams are controlled by a series of motorized dampers that are directed by the thermostat setting.

When air is pulled through the crawl space, the space is slightly depressurized, a condition that enhances indoor air quality, Christian said. The lowered pressure under the house discourages radon, mold, and mildew from coming up through any cracks in the floor.

"On the first house, we have 16 months of detailed data to review. And we've found that the heat pump water heater costs just $90 [at $0.063/kWhr] to operate for a full year," Christian said. "A conventional electric water heater for a family of four will easily cost $300."

A family of four is currently occupying the first, 1,056-square-foot Zero Energy House.

The houses are very airtight, and must be mechanically ventilated, but a circulating fan on low speed brings in just the right amount of fresh air and has been incredibly efficient and inexpensive to run, Christian said.

He said the house's space heat pump circulates about 700 cfm and introduces 40 cfm of continuous fresh air, which is more than you get most of the year in a standard house. He added that 40 cfm is also much less than you get, and don't want, when it is very cold and windy outside.

The houses have monitoring systems in place to regulate the airflow and make sure it is appropriate to the living conditions. In one of the first houses, the control system contains a C[O.sub.2] monitor. If C[O.sub.2] exceeds 600 ppm, the ventilation system brings in even more fresh air than the standard 40 cfm.

The houses are also equipped with solar collectors on the roof. The photovoltaic system on the first house generated 1,940 kilowatt hours of solar power in its first year of operation. It sold excess capacity to the local power utility, Tennessee Valley Authority, and earned a credit of $291. After receiving this credit, the net cost is already down to 82 cents per day cost for electricity. Not quite zero, but this was only the first test house.

Two more houses have been occupied and have been tracked since December. A fourth house just broke ground in February. After these prototypes, this project might just generate the first house to earn more in energy than it costs to operate.
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Title Annotation:Fluid Handling And Fluid Power
Author:Ehrenman, Gayle
Publication:Mechanical Engineering-CIME
Geographic Code:1USA
Date:Apr 1, 2004
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