The Ecosmart house in Bozeman, Mont., is built to be a near net-zero energy house by combining on-site power generation with efficiency measures to meet most of its own energy requirements. (A net-zero home must produce more energy than it consumes throughout the course of a year.) Testing throughout the next two years using more than 300 sensors positioned throughout the house will tell how close the home comes to near net zero. A team of researchers will collect, interpret and evaluate data generated by systems to determine how they contribute to energy efficiency. Real-time data results will be published at Montanaecosmart.com. The research will determine how the various building systems are best integrated to optimize energy consumption, comfort and life-cycle costs.
Although not certified under any green building programs now, the team will use the research phase of the project to determine the best way to achieve the residential building certification standards of ENERGY STAR, the U.S. Green Building Council’s LEED and the National Green Building Standard.
A radiant heating and cooling system distributes geothermal energy throughout the house. The system circulates a heat transfer fluid through 1/2-inch-thick cross-linked polyethylene pipes embedded in concrete and suspended wood floors. The pipes are connected to the geothermal heat pump system, which provides warm or chilled water depending on the season. The radiant system can reduce fuel consumption by up to 30 percent.
The lower level of the home’s ceiling includes a 12- by 12-foot section of radiant cooling panels installed for monitoring and research for comfort and energy use. Water circulates through the ceiling panels to collect and carry away the heat the panels absorb. The chilled water temperature circulating through the tubing must be 1 to 2 F higher than the dew point of the space; normal design water temperature rises generally are 4 to 5 F. The cooling capacity of the system is dependent on the difference between room air temperature and the mean cooling water temperature. In winter, the radiant ceiling transfers heat to the outer walls, furniture and floors. In summer, a cooled radiant ceiling absorbs heat from these same surfaces and dissipates it to the cooler radiant ceiling.
The lower level also includes the mechanical room, which houses the equipment for the condensing gas boiler, water-to-air forced-air unit, water-to-water geothermal heat pump, heat recovery unit and radiant cooling panels.
Geothermal and the Heat Sink
A ground-air heat exchange system takes advantage of moderate ground temperatures to precondition incoming air. The system can lower air by up to 25 F in the summer and raise it up to 16 F in the winter. The conditioned air will be directed into the heat recovery unit.
For the geothermal ground-loop heat exchange, the house has four 300-foot-deep wells with more than 1,200 feet of 1-inch-diameter PEXa pipe per well with double U-bends in each borehole for optimal energy extraction. In addition, there are three Helix probes, which are placed in the ground at 15 feet deep. Both systems will be connected to the heat pump, which is capable of handling the loads of the radiant system in the floors and latent loads of the forced-air ventilation system.
A solar thermal system provides preheated domestic hot water. Solar collectors on the roof will be coupled to a solar storage tank via a pressurized glycol-filled loop. A heat exchanger will circulate domestic cold water through the solar storage tank prior to supplying the domestic hot water heater. After hot water needs are met, valves will redirect excess solar energy to handle the load of the home’s snow and ice melting system or to be stored in the surrounding geothermal borehole field.
One of the unique concepts of this project is the installation of an experimental heat sink storage. The Ecosmart house will have three thermal solar panels for generation of the domestic hot water needs and supplemental use of the radiant heating demands. Once hot water needs are met, the excess hot water generated from the thermal solar panel in the summer will be directed into a heat sink located on the southwest corner of the house buried 10 feet below the concrete deck of the front patio slab.
The heat sink comprises four zones of 1-inch PEX piping laid in runs of approximately 400 feet each, placed in a loop configuration on top of sheet foil insulation. The loop field was encased in 1 foot of grout. The heat sink then was backfilled with 98 percent compacted road mix base material within the ICF walls, and topped with rigid insulation prior to pouring the exposed concrete patio deck.
The heat sink has sensor monitors located at various heights, as well as temperature and flow sensors to document the benefits of this concept and measure the contribution of the heating requirements for the winter months.