Geothermal Explained

Geo Energy, geo exchange, geothermal heat pump systems; no need to be confused, they all mean the same thing. They all describe a sustainable resource that lies in the ground at your location. Here's how it works.

A typical air conditioning system draws heat from the inside of a residence or commercial building and discharges it to the air outside using a compressor and fan. As you might expect, on hot days when the A/C is used the most, it is difficult to shed heat into the already hot outside air. The system is very inefficient and must run a long time to remove heat.Conversely in the winter, the air source heat pump tries to draw heat out of already cold air. Likewise, it struggles to draw heat from the frigid air, and must also run a long time.

Geothermal is different. At a depth of 4 to 6 feet below the frost line the ground temperature of the earth remains relatively constant throughout the year. Ground source heat pumps (GSHP) are designed to capitalize on this near constant temperature. Using a properly constructed ground heat exchanger (the ground loop or loops), they harness the thermal energy stored in the ground beneath your site.

Geothermal installations are integrated systems consisting of three major sub-systems.

Air distribution within the structure - air ducts and blowers circulate the air throughout to control the climate within the structure.

Ground source heat pump(s) - mechanical device to drive or "pump" the heat between the two other subsystems. GSHP work most efficiently when the entering liquid from the ground exchanger remains within a range of 40 to 80 degrees.

Ground heat exchangers - in-ground piping (ground loop or loops) arranged in a grid that makes contact with the earth to facilitate heat exchange. Typical ground heat exchangers are constructed of plastic tubing encased in grout creating improved thermal conductivity. These are referred to as closes loop systems. An open loop system uses groundwater from an ordinary well as a heat source. The groundwater is pumped into the GSHP unit where heat is extracted and the water is disposed of back into the ground. These open loop systems are commonly called "pump and dump" systems. Poor water quality can cause serious problems in open loop systems. Mineral deposits can build up inside the GSHP.Impurities, particularly iron, can eventually clog a return well. Because of these issues we will concentrate this discussion on the closed loop systems.

Vertical Loops Diagram

Vertical Loops

Vertical loops are used where space is limited or where soil conditions make horizontal loops impractical. Installing vertical loops require the use of a drilling rig. Multiple holes are bored at a minimum 10 feet apart. A double high density polyethylene (HDPE) pipe connected with a U-bend and filled with a mixture of water and food grade anti-freeze is inserted into each hole. The hole is filled with grout to provide good contact around the pipe and to seal the hole. The vertical pipes are then connected to a header system horizontally a few feet below the surface. The depth of the holes is dependent upon soil/rock conditions and size of the system. Although most holes are bored about 100 to 250 feet deep, there's no "magic depth" that needs to be reached. Capacity is not based on depth: rather how much pipe is in the ground and the overall thermal conductivity of the hole.

Horizontal Loops Diagram

Horizontal Loops

If adequate land is available, horizontal loops can be installed. Trenches are dug using a backhoe or trencher. HDPE pipes are inserted and the trenches are back filled. There are various designs (slinky or race track) of horizontal loops using one, two or three circuits per trench. The more pipes in each trench, the shorter the trenches can be. Trenches normally range from 100 to 300 feet depending on the design. A typical home requires 1/4 to 3/4 of an acre for the trenches.

A variation on the horizontal loop is the directional bore loop. This type of loop is most often used in a retrofit situation to minimize disruption to the landscape. It requires special equipment to bore holes under the surface. The operator can "steer" the drill head to go deeper or shallower, or turn right or left. This machine drills at a slight angle down to a typical depth of 15 to 20 feet, then back to the surface, typically 200 ft. away. At that point, two ends of the pipe are attached to the drill bit and pulled back through the hole until the pipe is buried. This technique allows the loop to be placed underneath homes, basements, wooded lots or even swimming pools.

Pond Loops Diagram

Pond Loops

If an adequately sized body of water is close to your home, a pond loop can be installed. A series of sealed HDPE pipes containing a mixture of water and anti-freeze can be coiled and sunk to the bottom. A 1/2 acre, 8-foot-deep pond is usually sufficient for the average home. Ideally, the pond should be close to the home (less than 200 ft.). If the pond is farther from the home, the benefit of using a pond loop is reduced due to added trenching, materials and pumping costs.

Pond loop coils are connected together on dry land, and then floated into location. Once filled with fluid, they will sink to the bottom and remain there. Generally, a 300 ft. coil is used for each ton of capacity. This is less pipe than is used in an earth loop because water is a better conductor of heat energy. Pond loops are a cost effective way to install a geothermal system, because trenching is limited to only the supply and return piping from the pond to the house.

Some frequently asked questions about geothermal.