A geothermal heat pump (GHP) exchanges warmth and cooling with the earth through a device called a ground heat exchanger.
In contrast to the more familiar air-source heat pump, which exchanges warmth and cold with outside air, a GHP takes advantage of the constant underground temperature of relatively shallow earth. The ground at depths below the frost line maintains such a stable temperature that geothermal systems are up to twice as efficient as air-source heat pumps, especially during the coldest weather.
Below the earth's surface, ground temperature remains relatively constant below the freeze line, ranging from 45 degrees Fahrenheit (7 degrees centigrade) to 75 degrees Fahrenheit (21 degrees centigrade), depending on the area. Year-round, this constant ground temperature is either warmer than the surface air during winter or cooler than the air in the summer.
As with any heat pump, GHPs can heat and cool a house, and with proper equipment can also provide hot water. Equipment options on geothermal systems also include two-speed compressors and adjustable fans, which offer more comfort and energy savings.
How geothermal heat pumps work
Every geothermal heat pump (GHP) system includes three major components:
Earth connection subsystem – A network of pipes is buried vertically or horizontally in the ground near the house. The pipe network, or loop, circulates a fluid, which can be water or a mixture of water and antifreeze. Depending on whether the surface air temperature is colder or warmer than the soil, the fluid in the loop absorbs heat from the ground or radiates heat into the surrounding soil.
Pump subsystem – To warm the house, the heat pump removes the heat from the fluid circulating through the loop, concentrates it, and transfers it to the building. To cool the house, the process is reversed.
Distribution subsystem – Duct work is generally used to distribute heated or cooled air from the geothermal heat pump throughout the building.
Consider the entire house
Before installing the equipment required for a geothermal heat pump, the condition and systems of the entire house must be considered. The better the house’s overall energy efficiency condition, the better the GHP will work. Putting an otherwise efficient system in an inefficient house won’t produce the expected results.
A new house under construction would be the ideal situation for installing a GHP, assuming the home is being built with other current energy efficient materials and features such as climate-appropriate insulation, windows and doors.
Many existing houses can be good candidates for a GHP retrofit, especially those that are undergoing major renovations that include upgrades such as appropriate insulation and energy efficient windows and doors.
However, many contractors will recommend upgrading windows, doors and insulation before installing any kind of system. The cost of these upgrades could pay off immediately by allowing installation of a smaller-capacity system which then will pay its own energy-saving dividends.
Other GHP retrofit considerations
Another major consideration in whether an existing house is a good candidate for geothermal is the type of system being replaced. Factors include whether the system is gas-, electric- or oil-powered and related mechanisms, such as whether heating and cooling is delivered with forced air, baseboards, radiators, radiant floors, or other equipment. A contractor may be able to tie a GHP in to some hot-water systems, depending on the type and configuration.
Any type of heat pump requires appropriate duct work. In a retrofit situation, the existing ducts may be acceptable in a house where an air-source heat pump is being replaced. In other homes where a traditional air conditioner (AC) and furnace are being replaced, duct work may need to be upgraded to accommodate a heat pump system. Other homes may need duct work installed from scratch. Such a complete overhaul of ductwork may not be feasible in some older homes.
The feasibility of retrofitting a house with a GHP depends on the mechanical and engineering details, plus a consideration of the expense. A geothermal engineer, building contractor, and local building codes will help you make the right decision.-- http://www.exchangenergy.ca/geothermal-retrofit/
The cost of geothermal for a home
In most cases, initial costs of geothermal heating and cooling are recouped by consumers in energy savings within 5-10 years. System life is estimated at up to 25 years for the inside components and at least 50 years for the underground components.
With all the variables, GHP installers are reluctant to offer ballpark estimates. Regardless, the expense of a GHP is likely to be several multiples of what an air-source heat pump or air conditioner/furnace package of the same capacities might cost. Most of the difference is driven by the extra work to install the earth connection subsystem that other systems don’t require. And as discussed, other costs may include infrastructure and energy efficiency upgrades necessary so the house can take full advantage of the GHP’s potential.
Financial assistance may be available
Consumers may be able to offset part of the initial cost by taking advantage of rebates and incentives from equipment manufacturers and installers and local utilities, as well as tax rebates from local, federal and state governments. Check with your municipal government, local utilities and local installers for incentives. This website lists many incentives for renewable energy by state: http://www.dsireusa.org/.
Several websites offer calculators to help consumers estimate how much installation might cost and what they might save on utility bills by installing a GHP. Here are a few of the websites: