Geothermal heating and cooling systems, also referred to as "ground source", "ground-coupled", or "earth energy" heat pump systems, are "...self-contained, electrically-powered systems that take advantage of the Earth's relatively constant, moderate ground temperature to provide heating, cooling, and domestic hot water more efficiently and less expensively than would be possible through other conventional heating and cooling technologies" according to IGSHPA, the International Ground Source Heat Pump Association.

Introduction
Closed-loop geothermal systems utilize plastic pipes and fittings that are buried in the ground in a variety of configurations, or submerged in water. The network of pipe and fittings, sometimes referred to as the ground-coupled heat exchanger, or simply the ground loop, is usually connected to a mechanical fluid-source heat pump unit. The ground-coupled heat exchanger is the thermal energy source during heating cycles and the thermal sink during cooling cycles. The heat pump transfers thermal energy to or from the ground loop to heat or cool the building as needed.

Materials
The typical plastic piping materials for ground loop piping systems are:

• HDPE: High-density polyethylene
• PEX: Crosslinked polyethylene
• PE-RT: Polyethylene of raised temperature

In addition to these ground loop piping materials, CPVC (chlorinated polyvinyl chloride) and PP (polypropylene pressure pipe) may also be used for interior piping, such as headers and manifolds, when approved. PP pipe can also be used as buried system headers in some instances.

Applications
A closed-loop geothermal system typically includes:

1. Mechanical Components: Packaged mechanical heat pump with integrated electronic controls, circulating pumps and valves, typically installed indoors
2. Ground-Coupled Heat Exchanger Piping: Plastic pipe and fittings which are:
• Buried in a horizontal plane (e.g. trenches),
• Dropped in a vertical configuration (e.g. boreholes),
• Installed in a vertical large diameter hole (e.g. helix),
• Drilled or pushed into an angled configuration (e.g. inclined),
• Submerged in a surface body of water (e.g. pond, lake), or
• Encased in structural building piling systems (e.g. energy piles)
3. Heat Transfer Fluid: Water, water/antifreeze solution (e.g. glycol), or brine solution

Other terms for a ground-coupled heat exchanger are the "ground loop" or the "source heat exchanger."

The design of a system for a specific application, including selection of the proper piping materials and ground loop installation method, is an important part of installing a successful system. Site-specific considerations (soil material, distance to bedrock, water table level, availability of a pond or water retention feature) can be the dominant factor in system design. In any configuration, there is an appropriate plastic piping material that meets the site needs and can be incorporated in a successful system design for the application.

Advantages
The selection of piping material is critical to the overall success of the closed-loop ground-coupled system and must provide corrosion resistance, chemical resistance, flexibility, impact resistance, resistance to slow crack growth, long-term hydrostatic strength (pressure capability), and temperature resistance. In addition, the ground loop heat exchanger materials must provide suitable heat transfer capabilities.

The BCD ground loop piping materials listed above will satisfy these requirements. Although one material may have an advantage in certain applications, each material should be considered as part of the overall design process.

See also
PPI TN-55 Plastic Piping Materials for Ground Source Geothermal Heating and Cooling Applications
PPI Handbook of Polyethylene Pipe (Ch. 13) HVAC Applications for PE Pipe
ANSI/CSA/IGSHPA C448 Design and installation of ground source heat pump systems for commercial and residential buildings
International Ground Source Heat Pump Association IGSHPA
The Geothermal Exchange Organization GEO
NSF 358 Certification Programs for Geothermal Piping Systems
Meline/Kavanaugh Paper: Geothermal Heat Pumps—Simply Efficient