GEOTHERMAL GROUND LOOP PIPING SYSTEMS

Geothermal heating and cooling systems, also referred to as "ground source", "ground-coupled", "geothermal exchange" 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 "geothermal ground loop", "ground heat exchanger", "ground-coupled heat exchanger", or simply the "ground loop", is usually connected to a mechanical fluid-source heat pump unit also known as a ground source heat pump or GSHP. The geothermal ground loop 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 and for the production of domestic hot water and/or other energy needs.

An open-loop geothermal system may utilize ground water or surface water for the purpose of exchanging thermal energy by circulation of the water through a mechanical heat pump.

In certain cases, the temperature of the fluid from the open- or closed-loop ground heat exchanger allows direct heat transfer for heating and cooling a building without the use of a mechanical heat pump. Such circumstances are rare.

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
  • PP: Polypropylene

PP Polypropylene Pipe (types: PP-R and PP-RCT) is typically used as distribution manifolds which may be installed in a vault or indoor mechanical room or as buried system headers.

In addition to these ground loop piping materials, CPVC (chlorinated polyvinyl chloride) may also be used for interior piping, such as headers and manifolds, when approved.

Please see PPI MS-7 Model Specification for Plastic Piping Materials for Ground Source Geothermal Applications for language related to HDPE, PEX, PE-RT, and PP pipe and tubing materials, fittings, and joining procedures. Installation and pressure testing recommendations are also provided.

Note: The term “tubing” typically refers to products where the actual outside diameter (OD) is 1/8 inch (0.125”) larger than the nominal size, the same as copper tube sizes (e.g., ¾ NTS). The term “pipe” typically refers to products where the actual OD matches that of steel pipe of the same nominal size, otherwise known as Iron Pipe Sizing (e.g., 4 IPS), or products in which the actual OD matches the nominal size directly (e.g., 63 mm). In both cases, specified tolerances apply, as per relevant standards.

 

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. Heat Transfer Fluid: Water, water/antifreeze solution (e.g. glycol), or brine solution
  3. Ground-loop Piping: Plastic pipe and fittings which are:
    • Buried in a horizontal plane (e.g. trenches),
    • Dropped in a vertical configuration (e.g. borehole heat exchangers),
    • 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)

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.

Note: In all cases, ground source geothermal systems including piping must be installed, tested, and operated in accordance with local regulations which may include the mechanical code that is enforced at the project location (e.g., ANSI/CSA/IGSHPA C448, IAPMO UMC, IAPMO USHGA, ICC IMC) or other codes or regulations.

 

Advantages of Plastic Piping Materials

The selection of piping material is critical to the overall success of the ground-source heat pump 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 and suitable durability to deliver decades of reliable service.

The 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.

Sustainability Benefits of GSHP Technology

The US Department of Energy (DOE) states that building energy use accounts for 36% of all primary energy used in the US and that 40% of the total energy used in buildings is space conditioning (heating & cooling) and water heating.

The DOE webpage on Geothermal Heat Pumps (GHP) states "The biggest benefit of GHP's is that they use 25% to 50% less electricity than conventional heating and cooling systems. This translates into a GHP using one unit of electricity to move three units of heat from the earth. According to the EPA, geothermal heat pumps can reduce energy consumption and corresponding emissions up to 44% compared with air-source heat pumps and up to 72% compared with electric resistance heating with standard air-conditioning equipment. GHPs also improve humidity control by maintaining about 50% relative indoor humidity, making GHPs very effective in humid areas."

Other sustainability benefits of ground source geothermal systems include:

  • Geothermal is Renewable Energy: The Consolidate Appropriations Act of 2021 modified the definition of Renewable Energy to include Thermal Energy. It also changes the word "generated" to "produced", eliminating the argument that geothermal technology does not generate power and therefore is not renewable energy.
  • Lower Operating Costs: By significantly reducing energy consumption for buildings, operating costs are also significantly reduced, leading to long-term savings for building owners.
  • Reduced Noise: Ground source heat pumps are installed indoors and are quiet.  With no outdoor heat pump or a/c units, the noise of fans and compressors is eliminated.
  • Reduced Air Pollution: By eliminating on-site combustion of fossil fuels, there are no emissions of carbon dioxide or the other byproducts of combustion.
  • Longevity: Buried ground loop pipes are proven to deliver decades of reliable service and ground source heat pumps, which are installed indoors, are also known to last for more than twenty years, reducing the need for frequent replacements.

Related PPI Documents

Case Studies

Industry Associations

Industry Codes & Standards

Research Reports

Geothermal in the Media