Direct Exchange (DX) Geothermal Heat Pumps

A direct exchange (DX) geothermal heat pump system, also called "direct expansion" or "DX geo" is a geothermal heat pump system where refrigerant circulates through copper pipes placed in the ground. The refrigerant exchanges heat directly with the soil through the walls of the copper pipes. This direct exchange, or should we say "x-change", eliminates the polyethylene water pipe and water pump to circulate water found in water-source geothermal heat pumps.

Direct Exchange with Earth

Direct-exchange and water-source geothermal heating and air conditioning systems usually have simlar installation costs, though with DX operating efficiencies are considerably better. DX technology circulates refrigerant through highly conductive copper earth loops that are inserted into boreholes of 50 to 100 foot depths, then embedded in a protective thermal grout that enables direct transfer of energy with the earth. The refrigerant moves directly from the loop to the unit's compressor with no stops or intermediate heat exchangers required, enabling efficient transfer of thermal energy.

illustration showing an array or copper pipes going into earth on diagonal angle

Direct Exchange Components

The copper tubing consists of a line set, a pair of manifolds (sometimes called headers), and several earth loops. The line set is the pair of main copper pipes coming from the heat pump compressor unit, usually located indoors. One line is for the liquid refrigerant, the other is for gaseous refrigerant. The line set runs through the building wall and runs underground to the location of the manifolds. Each manifold - one for gas and one for liquid - attaches the main pipe to the earth loops which exchange heat with the ground.

The refrigerant lines are connected to the compressor, which increases the pressure and temperature of the refrigerant. Heat energy released by the pressurized refrigerant may be channeled either to the building if heat is needed or to the earth if cooling is needed. The process is controlled by a thermostat and a reversing-valve.

The refrigerant line may then be run from the compressor to an air handler. The air handler is used to transfer heat from the refrigerant to the air and circulate the heated air through ductwork, or to remove it from the building in summer by exchanging it from the circulating air into the refrigerant line for transport to the earth.

Alternatively the heat can be distributed as radiant in-floor heating by running the refrigerant line from the compressor to a refrigerant-to-water heat exchanger that is used to transfer heat into a hot water circulating system to heat the building.

It can also be used to heat a home's domestic water through integrated full-demand water heating, or through "desuperheating" to capture waste heat from the air conditioning cycle and move it to the water heater.

Diagonal Loops

The loops may be configured as horizontal, vertical or even diagonally. Vertical and diagonal installation is done by drilling several boreholes radiating outward from the manifolds and placing an earth loop into each of the boreholes. Diagonal drilling, only possible with DX systems, requires only a very small area to be affected at the surface while a large volume of soil is used underground as a heat reservoir. For homes where ground space is limited, or especially for existing homes with mature landscaping, this configuration is ideal because all of the small-diameter drilling takes place from a shallow, 6-square-foot pit, with drill holes radiating outward and down at an angle from the base of the pit.

DX manufacturers

The largest manufacturer of DX systems in the United States is EarthLinked Technologies, Inc. (www.earthlinked.com), founded in 1980. Their DX systems are distributed in several countries outside the United States including Australia by EnergyCore, founded in 2007.

EarthSource Energy Solutions (www.advgeo.com) was established in 2006 in Massachusetts, and manufactures the EarthSource Geo Direct® line of Direct Exchange geothermal heat pumps.

American Geothermal (www.AMGEO.com) is another DX company, this one based in Tenessee.

Maritime Geothermal Ltd. began in 1983 in New Brunswick and has made the most of those long Canadian winters developing the well-regarded Nordic line of heat pumps (www.Nordicghp.com).