How is a geothermal heat pump system installed?
Geothermal systems can be installed in an open loop, closed loop, or direct exchange (DX) configuration. Specialized installers decide among the different configurations depending on geological conditions, heating/cooling load, type of space to be heated/cooled, supply temperature from underground, and availability of ground water. Typically, installers will fuse socket fusion fittings such as tees, elbows, and couplings, with piping for residential systems and fuse butt fusion fittings with piping for commercial systems.
In this configuration, a water-antifreeze solution circulates high density polyethylene (HDPE) piping buried underground. HDPE is used because of its durability and thermal conductivity. Multiple pipes are heat fused together at the joints, creating a connection stronger than the pipe itself. Closed loop ground source heat pump systems can be installed in either a horizontal or vertical loop:
- Horizontal Loop: Where adequate land space is available, horizontal loop installations are very common. Installers dig trenches 6-8 feet deep, install geothermal fittings and piping several hundred feet in length, and backfill the trench with a sealing solution. The land area required for horizontal ground loops will range from 1500-3000 square feet per ton of heating/cooling depending on soil properties and earth temperatures.
- Vertical Loop: This configuration is generally used in installations where land area is limited, and almost always in retrofits. Installers will drill boreholes of 150-400 feet in depth, install parallel polyethylene pipes, connected at the terminus of the hole by a U-Bend fitting, and backfill the holes with a sealing solution consisting of grout and sand. Each vertical pipe is then connected to a horizontal underground pipe via a manifold that carries fluid in a closed system to and from the indoor exchange unit. Vertical loops are generally more expensive to install, but require less piping than horizontal loops because the earth’s temperature is more stable far below the surface. Vertical ground loops typically require 150-300 square feet of land area per system ton of heating/cooling capacity.
Open loop systems are used where an abundant supply of water from a well, pond, lake, or river is available. Ground or well water is pumped and circulated directly through the polyethylene piping. Once this water circulates through the heat pump it is discharged either into a pond, river, lake, or stream or re-injected back to the source. Both methods are safe as the difference between the water source and the re-injected water is a minor difference in temperature.