Heating and Cooling Efficiency of Geothermal Heat Pumps
The heating efficiency of ground-source and water-source heat pumps is indicated by their coefficient of performance (COP), which is the ratio of heat provided in Btu per Btu of energy input. Their cooling efficiency is indicated by the Energy Efficiency Ratio (EER), which is the ratio of the heat removed (in Btu per hour) to the electricity required (in watts) to run the unit.
Look for the ENERGY STAR® label, which indicates that the unit meets ENERGY STAR criteria. Manufacturers of high-efficiency geothermal heat pumps (GHPs) voluntarily use the EPA ENERGY STAR label on qualifying equipment and related product literature. Many GHPs carry the U.S. Department of Energy (DOE) and Environmental Protection Agency (EPA) ENERGY STAR label.
Economics of Geothermal Heat Pumps
Although the purchase and installation cost of a residential GHP system is often higher than that of other heating and cooling systems, properly sized and installed GHPs deliver more energy per unit consumed than conventional systems. For further savings, GHPs equipped with a device called a "desuperheater" can heat household water. In the summer cooling period, the heat removed from the house is used to heat the water for free. In the winter, water heating costs are reduced by about 50%.
Depending on factors such as climate, soil conditions, the system features you choose, and available financing and incentives, you may recoup your initial investment in a few years through lower utility bills. And -- when included in a mortgage -- your investment in a GHP may produce a positive cash flow from the beginning. It may also be possible to include the purchase of a GHP system in an "energy-efficient mortgage" that would cover this and other energy-saving improvements to the home. Banks and mortgage companies can provide more information on these loans.
There are also special financing and incentives available to help offset the cost of adding a GHP to your home. These provisions are available from federal, state, and local governments; power providers; and banks or mortgage companies that offer energy-efficient mortgage loans for energy-saving home improvements. Be sure the system you're interested in qualifies for available incentives before you make your final purchase.
Evaluating Your Site for a Geothermal Heat Pump
Shallow ground temperatures are relatively constant throughout the United States, so geothermal heat pumps (GHPs) can be effectively used almost anywhere. However, the specific geological, hydrological, and spatial characteristics of your land will help your local system supplier/installer determine the best type of ground loop for your site.
Factors such as the composition and properties of your soil and rock (which can affect heat transfer rates) require consideration when designing a ground loop. For example, soil with good heat transfer properties requires less piping to gather a certain amount of heat than soil with poor heat transfer properties. The amount of soil available contributes to system design as well -- system suppliers in areas with extensive hard rock or soil too shallow to trench may install vertical ground loops instead of horizontal loops.
Ground or surface water availability also plays a part in deciding what type of ground loop to use. Depending on factors such as depth, volume, and water quality, bodies of surface water can be used as a source of water for an open-loop system, or as a repository for coils of piping in a closed-loop system. Ground water can also be used as a source for open-loop systems, provided the water quality is suitable and all ground water discharge regulations are met.
Before you purchase an open-loop system, be sure your system supplier/installer has fully investigated your site's hydrology, so you can avoid potential problems such as aquifer depletion and groundwater contamination. Antifreeze fluids circulated through closed-loop systems generally pose little to no environmental hazard.
The amount and layout of your land, your landscaping, and the location of underground utilities or sprinkler systems also contribute to your system design. Horizontal ground loops (generally the most economical) are typically used for newly constructed buildings with sufficient land. Vertical installations or more compact horizontal "Slinky™" installations are often used for existing buildings because they minimize the disturbance to the landscape.
Installing Geothermal Heat Pumps
Specialized technical knowledge and equipment is needed to properly install the piping, so a GHP system installation is not a do-it-yourself project. To find a qualified installer, contact your local utility company, the International Ground Source Heat Pump Association, or the Geothermal Exchange Organization for their listings of qualified installers in your area. Installers should be certified and experienced. Ask for references from owners of systems that are several years old, and check them.
The ground heat exchanger in a GHP system is made up of a closed or open loop pipe system. Most common is the closed loop, in which high density polyethylene pipe is buried horizontally at 4 to 6 feet deep or vertically at 100 to 400 feet deep. These pipes are filled with an environmentally friendly antifreeze/water solution that acts as a heat exchanger. In the winter, the fluid in the pipes extracts heat from the earth and carries it into the building. In the summer, the system reverses and takes heat from the building and deposits it to the cooler ground.
Ductwork in the home distributes the heated or cooled air through the house, just like conventional systems. The box that contains the indoor coil and fan is sometimes called the air handler because it moves house air through the heat pump for heating or cooling. The air handler contains a large blower and a filter just like conventional air conditioners.
Contact your insurance provider to ensure that the geothermal heat pump is covered. Even if your provider will cover your system, it is best to inform them in writing that you own a new system.
Benefits of Geothermal Heat Pump Systems
The biggest benefit of GHPs is that they use 25% to 50% less electricity than conventional heating or cooling systems. This translates into a GHP using one unit of electricity to move three units of heat from the earth. According to the EPA, geothermal heat pumps can reduce energy consumption -- and corresponding emissions -- up to 44% compared with air-source heat pumps and up to 72% compared with electric resistance heating with standard air-conditioning equipment. GHPs also improve humidity control by maintaining about 50% relative indoor humidity, making GHPs very effective in humid areas.
Geothermal heat pump systems allow for design flexibility and can be installed in both new and retrofit situations. In addition, the hardware may require less space than a conventional HVAC system, thus possibly freeing up space for other uses. GHP systems also provide excellent "zone" space conditioning, allowing different parts of your home to be heated or cooled to different temperatures.
The underground piping associated with GHP systems often carry warranties of 25 to 50 years, and the heat pumps often last 20 years or more. In addition, the components in the living space are easily accessible, which increases the convenience factor and helps ensure that the upkeep is done on a timely basis.
GHPs have no outside condensing units like air conditioners, so there's no concern about noise outside the home. A two-speed GHP system is so quiet inside a house that users usually do not know it is operating.
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