Geothermal Heat Pumps
Homeowners in virtually every region of the country are enjoying a high level of comfort and significantly reducing their energy use today by heating and cooling their homes with geothermal heat pumps (GHPs).
Geothermal heat pump technology relies primarily on the Earth’s natural thermal energy, a renewable resource, to heat or cool a home or commercial building. The only additional energy GHP systems require is the small amount of electricity they employ to concentrate what Mother Nature provides and then to circulate high-quality heating and cooling throughout the building.
Homeowners who use GHP systems give them superior ratings because of their ability to deliver comfortably warm air, even on the coldest winter days, and because of their extraordinarily low operating costs. As an additional benefit, GHP systems can provide inexpensive hot water, either to supplement or replace entirely the output of a conventional, domestic water heater.
Geothermal heating and cooling is cost effective because it uses energy so efficiently. This makes it very environmentally friendly, too. For these reasons, agencies like the Environmental Protection Agency and the Department of Energy recognize it.
Owners of geothermal heat pump systems can relax and enjoy high-quality heating and cooling year after year. These systems work on a different principle than an ordinary furnace/air conditioning system, and they require little maintenance or attention from homeowners. Furnaces must create heat by burning a fuel--typically natural gas, propane, or fuel oil. With geothermal heating systems, there’s no need to create heat, hence no need for chemical combustion. Instead, the Earth’s natural heat is collected in winter through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond or lake. Fluid circulating in the loop carries this heat to the home. An indoor geothermal heat pump then uses electrically-driven compressors and heat exchangers in a vapor compression cycle, the same principle employed in a refrigerator, to concentrate the Earth’s energy and release it inside the home at a higher temperature. In typical systems, duct fans distribute the heat to various rooms.
In summer, the process is reversed in order to cool the home. Excess heat is drawn from the home, expelled to the loop, and absorbed by the Earth. GHP systems provide cooling in the same way that a refrigerator keeps its contents cool--by drawing heat from the interior and moving it to an outside space.
Geothermal heat pump systems do the work that ordinarily requires two appliances, a furnace and an air conditioner. They can be located indoors because there’s no need to exchange heat with the outdoor air. They’re so quiet homeowners don’t even realize they’re on. They are also compact. Typically, they are installed in a basement or attic, and some are small enough to fit atop a closet shelf. The indoor location also means the equipment is protected from mechanical breakdowns that could result from exposure to harsh weather.
Geothermal heat pumps work differently than conventional heat pumps that use the outdoor air as their heat source or heat sink. GHP systems don’t have to work as hard (which means they use less energy) because they draw heat from a source whose temperature is moderate. The temperature of the ground or groundwater a few feet beneath the Earth’s surface remains relatively constant throughout the year, even though the outdoor air temperature may fluctuate greatly with the change of seasons. At a depth of approximately six feet, for example, the temperature of soil in most of the world’s regions remains stable between 45 F and 70 F. This is why well water drawn from below ground tastes so cool even on the hottest summer days.
In winter, it’s much easier to capture heat from the soil at a moderate 50o F. than from the atmosphere when the air temperature is below zero. This is also why GHP systems encounter no difficulty blowing comfortably warm air through a home’s ventilation system, even when the outdoor air temperature is extremely cold.2 Conversely, in summer, the relatively cool ground absorbs a home’s waste heat more readily than the warm outdoor air.
Studies show that approximately 70 percent of the energy used in a geothermal heating and cooling system is renewable energy from the ground. The remainder is clean, electrical energy which is employed to concentrate heat and transport it from one location to another. In winter, the ground soaks up solar energy and provides a barrier to cold air. In summer, the ground heats up more slowly than the outside air.
Making Hot Water
GHP systems can also provide all or part of a household’s hot water. One economical way to obtain a portion of domestic hot water is through the addition of a desuperheater to the GHP unit. A desuperheater is a small, auxiliary heat exchanger that uses superheated gases from the heat pump’s compressor to heat water. This hot water then circulates through a pipe to the home’s water heater tank. In summer, when the GeoExchange system is in the cooling mode, the desuperheater merely uses excess heat that would otherwise be expelled to the loop. When the GHP unit is running frequently, homeowners can obtain all of their hot water in this manner virtually for free. A conventional water heater meets household hot water needs in winter if the desuperheater isn’t producing enough, and in spring and fall when the system may not be operating at all.
Because GHP systems heat water so efficiently, many manufacturers today are also offering triple function systems. Triple function systems provide heating, cooling and hot water. They use a separate heat exchanger to meet all of a household’s hot water needs.
The Earth Connection
Once installed, the ground loop in a GHP system remains out of sight beneath the Earth’s surface while it works unobtrusively to tap the heating and cooling nature provides. The loop is made of a material that is extraordinarily durable but which allows heat to pass through efficiently. This is important so it doesn’t retard the exchange of heat between the Earth and the fluid in the loop. Loop manufacturers typically use high-density polyethylene, a tough plastic. When installers connect sections of pipe, they heat fuse the joints. This makes the connections stronger than the pipe itself. Some loop manufacturers offer up to 50-year warranties. The fluid in the loop is water or an environmentally safe antifreeze solution that circulates through the pipes in a closed system.
Another type of geothermal heating and cooling is direct exchange (DX), which utilizes copper piping buried underground. As refrigerant is pumped through the loop, heat is transferred directly through the copper to the earth.
To ensure good results, the piping should be installed by professionals who follow procedures established by the International Ground Source Heat Pump Association (IGSHPA). Installers should be certified by IGSHPA or be able to show equivalent training by manufacturers or other recognized authorities at a recognized institution.
The length of the ground loop depends upon a number of factors, including the type of loop configuration used; a home’s heating and air conditioning load; soil conditions; local climate; and landscaping. Larger homes with larger space conditioning requirements generally need larger loops than smaller homes. Homes in climates where temperatures are extreme also generally require larger loops. A heat loss/heat gain analysis should be conducted before the loop is installed.
Types of Loops
Most loops for residential systems are installed either horizontally or vertically in the ground, or submersed in water in a pond or lake. In most cases, the fluid runs through the loop in a closed system, but open-loop systems may be used where local codes permit. Each type of loop configuration has its own, unique advantages and disadvantages, as explained below:
Horizontal Ground Closed Loops. This configuration is usually the most cost effective when adequate yard space is available and trenches are easy to dig. Workers use trenchers or backhoes to dig the trenches three to six feet below the ground, then lay a series of parallel plastic pipes. They backfill the trench, taking care not to allow sharp rocks or debris to damage the pipes. Fluid runs through the pipe in a closed system. A typical horizontal loop will be 400 to 600 feet long per ton of heating and cooling capacity. The pipe may be curled into a slinky shape in order to fit more of it into shorter trenches, but while this reduces the amount of land space needed it may require more pipe. Horizontal ground loops are easiest to install while a home is under construction. However, new types of digging equipment that allow horizontal boring are making it possible to retrofit GeoExchange systems into existing homes with minimal disturbance to lawns. Horizontal boring machines can even allow loops to be installed under existing buildings or driveways.
Vertical Ground Closed Loops. This type of loop configuration is ideal for homes where yard space is insufficient to permit horizontal buildings with large heating and cooling loads, when the Earth is rocky close to the surface, or for retrofit applications where minimum disruption of the landscaping is desired. Contractors bore vertical holes in the ground 150 to 450 feet deep. Each hole contains a single loop of pipe with a U-bend at the bottom. After the pipe is inserted, the hole is backfilled or grouted. Each vertical pipe is then connected to a horizontal pipe, which is also concealed underground. The horizontal pipe then carries fluid in a closed system to and from the geothermal heat pump. Vertical loops are generally more expensive to install, but require less piping than horizontal loops because the Earth deeper down is cooler in summer and warmer in winter.
Pond Closed Loops. If a home is near a body of surface water, such as a pond or lake, this type of loop design may be the most economical. The fluid circulates through polyethylene piping in a closed system, just as it does in the ground loops. Typically, workers run the pipe to the water, then submerge coils of piping under the water. Pond loops are typically used only if the water level never drops below six to eight feet at its lowest level to assure sufficient heat-transfer capability. Properly designed pond loops result in no adverse impacts on the aquatic system.
Open Loop System. This type of loop configuration is used less frequently, but may be employed cost-effectively if ground water is plentiful. Open loop systems, in fact, are the simplest to install and have been used successfully for decades in areas where local codes permit. In this type of system, ground water from an aquifer is piped directly from the well to the building, where it transfers its heat to a heat pump. After it leaves the building, the water is pumped back into the same aquifer via a second well--called a discharge well--located at a suitable distance from the first. Local environmental officials should be consulted whenever an open loop system is being considered.
Standing Column Well System. Standing column wells have become an established technology in some regions, especially the northeastern United States. Standing wells are typically six inches in diameter and may be as deep as 1500 feet. Temperate water from the bottom of the well is withdrawn, circulated through the heat pump’s heat exchanger, and returned to the top of the water column in the same well. Usually, the well also serves to provide potable water. However, ground water must be plentiful for a standing well system to operate effectively. If the standing well is installed where the water table is too deep, pumping would be prohibitively costly. Under normal circumstances, the water diverted for building (potable) use is replaced by constant-temperature ground water, which makes the system act like a true open-loop system. If the well-water temperature climbs too high or drops too low, water can be "bled" from the system to allow ground water to restore the well-water temperature to the normal operating range. Permitting conditions for discharging the bleed water vary from locality to locality, but are eased by the fact that the quantities are small and the water is never treated with chemicals.
Other loop designs are also being used. In a few places, for example, home builders have installed large community loops, which are shared by all of the homes in a housing development.
Purchasing a System
To ensure they receive the highest-quality equipment, system design and installation, consumers should consider the following guidelines when shopping for a system:
Ratings and Certification: Look for equipment that is certified by the Air-Conditioning, Heating and Refrigeration Institute (AHRI), a non-profit organization that rates the performance of heating and cooling equipment. Certified equipment carries the AHRI seal.
Warrantys: Manufacturers’ terms of warranty vary. To assure a high-quality installation, seek a performance guarantee on the installed system, as opposed to coverage limited to the heat pump itself.
Sizing: GHP systems that are too large waste energy and do not provide proper humidity control. Check to see that the contractor carefully determines your home’s heating and cooling requirements using accepted procedures, such as those recommended by the Air Conditioning Contractors Association. The actual size of the system should be within 15 percent of the calculated load.
System Design: While designing a residential heating and cooling system is not particularly complicated, always use experienced contractors. The contractor should carefully select the size of the system, the size and design of the ground loop, and the type of fluid that will circulate through it. The contractor should also examine ways to use the system to provide hot water. Finally, the contractor should examine your home to ensure the ductwork is designed and installed properly to prevent leaks, as well as to ensure it is properly insulated and has window glazings and other energy-efficiency features. Minimizing heating and cooling needs reduces the required size, hence the cost, of the heating and cooling system.
Geothermal heat pump systems are becoming the system of choice in many parts of the United States as consumers learn more about their aesthetic advantages and long-term value, and as they become more widely available.
These systems are no longer just for the affluent, a reputation it once held because typical early buyers were owners of upscale homes. They wanted the quiet comfort GHP systems provide, and they were more than willing--and could afford--to pay the cost premium associated with early systems. This is because the extraordinarily low operating costs of GHP systems more than make up for any higher installation costs within a few years. According to the U.S. Environmental Protection Agency, GHP systems save homeowners 30-70 percent in heating costs, and 20-50 percent in cooling costs, compared to conventional systems, and also save money in other ways. They are highly reliable, require little maintenance, and are built to last for decades. They add considerably to the value of homes.
Today, homeowners in all income brackets can take advantage of the benefits of geothermal heating and cooling. Initial costs have declined substantially as many more builders and heating and cooling contractors nationwide make GHP systems available, and as innovative techniques enable the ground loops to be installed more quickly and for lower cost.
What’s more, some electric utilities around the nation now have incentive programs and low-interest financing programs which can make GHP systems even more affordable. Many financial institutions also now allow home buyers to qualify for larger mortgages if they purchase a house that utilizes a GHP system. The reduction in monthly energy bills more than offsets the slightly higher mortgage payment. With such mortgages, homeowners with GHP systems can begin saving money from day one, then go on saving year after year!
Today, the major barrier to wider use of this marvelous technology is the fact that many consumers simply aren’t aware it’s there.
A Wise Choice
Geothermal heating and cooling is a smart investment for consumers who want a system that provides a high level of comfort and low monthly energy bills for as long as they own their homes.
1 A study by the Environmental Protection Agency, Space Conditioning: The Next Frontier (Office of Air and Radiation, 430-R-93-004), found that geothermal heating and cooling systems are much more efficient than competing fuel technologies when all losses in the fuel cycle, including waste heat at the powerplants during the generation of electricity, are accounted for. High-efficiency GHP systems are on average 48 percent more efficient than the best gas furnaces and more than 75 percent more efficient than oil furnaces. The best GHP systems even outperformed the best gas technology, gas heat pumps, by an average of 36 percent in the heating mode and 43 percent in the cooling mode.
2 Surveys by utility companies indicate a higher level of consumer satisfaction for GHP systems than for conventional systems. Polls consistently show that more than 95 percent of all GHP customers would recommend such systems to a family member or friend.