It’s important to understand the piping materials used for geothermal jobs.
By Lance MacNevin, P.Eng.
Let’s begin with revisiting the definition of geothermal heating and cooling systems, also referred to as “ground source” or “earth energy” heat pump systems.
Geothermal heating and cooling systems are defined as: “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 the International Ground Source Heat Pump Association (IGSHPA).
The heart of a ground source heat pump system is the electric heat pump or pumps located within a house or building.
The heat pump is connected to a network of piping installed outdoors through which fluid travels and exchanges heat with the earth or a body of water. The fluid is transferred through the pipe by circulator pumps. The majority of systems are closed loop, which means the same fluid stays within the piping network; groundwater is not extracted from the ground.
Ground source heat pump (GSHP) systems do the job of air-to-air heat pumps and fossil-fuel heating appliances with much higher efficiency and lower operating costs.
For example, most GSHP systems deliver a coefficient of performance (COP) of 4-to-1 or better, which means that for every unit of electricity purchased to operate the heat pump, four units of heat energy are delivered to the building. These systems are 400% efficient or better. Also, heat pumps usually contain a heat transfer coil for heating domestic hot water.
As the price of fossil-fuel energy and electricity are increasing unpredictably, GSHP systems can deliver significant cost savings to their owners. Geothermal systems can dramatically reduce carbon emissions and help to flatten the energy demand spike for electricity.
A critical aspect of GSHP systems is the underground network of plastic pipes and fittings that are buried in the ground or submerged in water. The network of pipe and fittings—sometimes referred to as the ground heat exchanger or simply the ground loop—is the thermal energy source during heating cycles and the thermal sink during cooling cycles.
With all these advantages, the popularity of GSHP systems is increasing fast as homeowners, governments, businesses, and universities are turning to geothermal technology. The Plastics Pipe Institute, the Texas-based nonprofit trade association, estimates growth in demand for GSHP systems of 20% per year over the next five years.
Geothermal Ground Loop Piping Materials
The piping material in the outdoor ground loop is critical to the overall success of any geothermal system. Other than direct-exchange systems, where refrigerant is piped through the earth in copper pipes, practically 100% of all GSHP systems have relied upon plastic piping materials for performance and longevity.
With pipes and fittings buried or encased directly in the earth, piping materials must provide corrosion resistance, chemical resistance, temperature resistance, flexibility, impact resistance, resistance to slow crack growth, and long-term hydrostatic strength (pressure capability).
In addition, the ground loop heat exchanger materials must provide suitable heat transfer capabilities, since conducting heat with the earth is the primary function of the ground loop.
Moreover, all ground loop pipes must meet the requirements of standard NSF/ANSI/CAN 61 for drinking water safety to ensure that any aquifer or water reservoir into which the piping system is installed is not contaminated by the pipe itself.
Considering all these challenges, the three types of plastic piping materials which are approved for geothermal ground loops are HDPE, PEX, and PE-RT. Each of these piping materials delivers long-term reliability proven over decades of use around the world.
Geothermal specifiers and installers need to be aware that only certain sub-types of each of these piping materials are recommended and approved for geothermal ground loops.
High-Density Polyethylene (HDPE)
HDPE is the predominant piping material used for ground heat exchangers and is approved in all modern codes for ground loops. It is joined primarily by heat fusion to provide leak-free connections.
HDPE pipes (Figure 1) have improved significantly over the past 20 years, thanks to new bimodal resins that deliver higher strength with greater toughness and resistance to slow crack growth. The highest performing grade, known as PE 4710, is flexible, tough, and strong.
For horizontal piping systems, HDPE is typically supplied in long coils as needed for the specific project. Some coils can be more than 1000 feet long, depending on the pipe diameter. Larger diameter pipes (e.g., 3-inch), which are often used for headers, are typically supplied as 40-foot straight lengths.
For vertical boreholes, pipes can be provided with a molded U-bend fused to two parallel pipes. For example, for a 400-foot-deep borehole, the piping manufacturer can provide two 405-foot pipes which are fused in the factory to a molded U-bend fitting.
This allows the drilling contractor to drop the U-bend assembly down the borehole without having to first fuse pipes to the U-bend fitting. About 5 feet of pipe length from each pipe will remain outside the 400-foot borehole for connection to horizontal headers, in this example.
Ground loops are one of the most demanding applications for pipes, so there are specific recommendations for HDPE piping components. For example, HDPE ground loop piping should have a minimum pipe material designation code of PE 3608 (higher numbers are even better) and be certified to industry standard NSF/ANSI Standard 358-1.
HDPE pipes and fittings are joined using various heat fusion methods: butt fusion, socket fusion, or electrofusion. One bad joint buried underground could create a leak that is expensive and difficult to find, access, and repair. With that, fusion contractors should strictly follow the guidance provided in ASTM F2620 “Standard Practice for Heat Fusion Joining of Polyethylene Pipe and Fittings” that describes proper fusion procedures for butt and socket fusion.
Once installed, ground loops should be tested according to ASTM F2164 “Standard Practice for Field Leak Testing of PE and PEX Pressure Piping Systems Using Hydrostatic Pressure.”
Crosslinked Polyethylene (PEX)
PEX is HDPE that is modified during manufacturing to crosslink the majority of molecular chains. It is a high-temperature, flexible pressure pipe material first developed in the early 1970s. The primary benefit is capability of continuous operation at temperatures of 180°F (82°C) or above.
This higher temperature capability may be necessary in borehole thermal energy storage (BTES) systems where thermal solar energy is pumped into the earth to raise the ground temperature during summer, for example.
PEX tubing (Figure 2) has greater resistance to chemical contact, and it is slightly more flexible than HDPE. It is also more expensive.
PEX tubing cannot be joined using butt fusion or socket fusion. Instead, several types of mechanical compression fittings are approved for underground use with PEX tubing, as are electrofusion fittings.
PEX ground loop piping should have a minimum material designation code of PEX 1206 and be certified to ASTM F876 or CSA B137.5, as well as NSF/ANSI Standard 358-3.
In North America, PEX is available up to 3 inches in diameter and can be supplied as coils, straight lengths, or as prefabricated U-bends for boreholes.
Polyethylene of Raised Temperature (PE-RT)
PE-RT is high-density polyethylene material with enhanced capabilities to withstand higher temperatures up to 180°F. PE-RT may be joined using heat fusion processes, just like HDPE, or using mechanical compression fittings, just like PEX.
PE-RT ground loop piping should have a minimum pipe material designation code of PE 3608 and be certified to ASTM F2623, ASTM F2769, or CSA B137.18, as well as NSF/ANSI Standard 358-4.
Geo piping materials are produced according to rigorous product standards with strict industry certification programs to ensure consistent quality control.
The life expectancy of these plastic piping materials, when specified correctly and installed according to industry and manufacturers’ guidelines, is typically well in excess of 50 years. In fact, many of the earliest geo ground loops installed in the 1970s using earlier generations of plastic piping are still in service today 50 years later.
All ground loop pipes must meet the requirements of standard NSF/ANSI/CAN 61 to ensure that any aquifer into which the piping system is installed is not contaminated by the pipe itself.
Several industry tools are available to assist geothermal designers, specifiers, and installers. For example, PPI document TN-55 “Plastic Piping Materials for Ground Source Geothermal Heating and Cooling Applications” provides even more details about the piping loops.
Designers can also use the free online Plastic Pressure Pipe Design Calculator at www.plasticpipecalculator.com to assist with calculations.
Design and installation training is available through IGSHPA, and CSA/ANSI/IGSHPA C448 is the ultimate code for geothermal design and installation. More information is accessible on Plastics Pipe Institute’s geothermal webpage at www.plasticpipe.org/buildingconstruction.
Lance MacNevin, P.Eng., is the director of engineering for the Building & Construction Division at the Plastics Pipe Institute (PPI). He has been in the plastic pipe industry since 1993, involved with applications such as hydronic heating and cooling, geothermal, plumbing, and fire protection systems. MacNevin earned his IGSHPA installer accreditation in 2008 and has been closely involved with the geothermal industry ever since.