By, Ryan Carda 
With so many available options, why has the geothermal heat pump industry gravitated toward exclusively using polyethylene (PE) - specifically HDPE and PEXa - for ground loop construction, especially considering it is one of the most insulative piping materials available?
Aside from the fact that PE piping accounts for a tiny fraction of the overall thermal resistance in a loopfield1, it offers a lot more benefits than deficits.
Photo courtesy of ISCO Industries.
Industry Standards
HDPE and PEXa are the only materials that IGSHPA formally approves for use in the buried portion of a closed-loop GSHP system. Per Section 1C of IGSHPA's Design and Installation Standards:
The acceptable pipe and fitting materials for the underground portion of the ground heat exchanger are high-density polyethylene (HDPE), as specified in Section 1C.2 and cross-linked polyethylene (PEXa), as specified in Section 1C.32.
These recommendations were born out of a combination of past experience along with the acknowledgement of the many advantages that polyethylene (PE) has to offer. Aside from being the industry standard, here are the top 5 reasons for using PE over the alternatives:
1 Affordable & Available
Polyethylene is used in a wide range of applications such as food packaging, plastic bottles and bags, pool liners, and of course, geothermal piping. It is a commodity plastic and is among the least expensive types to make. Geothermal grade polyethylene pipe is mass produced and readily available in the marketplace at commodity prices.
2 Durability
Geothermal heat pump systems operate under a wide range of temperatures and pressures. It is normal for ground loop temperatures to swing from 25-30F in heating mode to 90-100F in cooling mode. Thermal expansion and contraction of the piping due to temperature swings will cause system pressures to follow suit.
Polyethylene is highly resistant to damage due to fatigue (as well as damage due to abrasion, weathering, corrosion, etc.). It can withstand the abuse of pressure fluctuation due to temperature changes, as well as the abuse of being transported and handled on the jobsite. According to the Plastic Pipe Institute, it can even withstand damage due to an earthquake:
The toughness, ductility and flexibility of PE pipe combined with its other special properties, such as its leak-free fully restrained heat fused joints, make it well suited for installation in dynamic soil environments and in areas prone to earthquakes.
The durability of PE pipe is tough to beat (pun intended).
3 Installation Ease
Mechanical fittings are not necessary when PE pipe is used. Simple heat fusion techniques are used to join pipe and fittings together in a leak-free, virtually fail-proof manner. Even if leaks or other errors occur, they are extremely easy to fix.
PE pipe is also relatively flexible, lightweight and very easy to manage on the jobsite. Pipe coils are generally available for purchase in any 100-ft increment, leaving it to the installer to pick the length that best suits the project without the hassle of a special order.
4 Service Life
The life expectancy of polyethylene is greater than any mechanical component inside of the building, and even the building itself. According to Chapter 7 in the Handbook of PE Pipe:
The service life of HDPE pipe manufactured from today’s materials is expected to exceed 100 years.
In fact, most pipe manufacturers offer a 50-year warranty to guarantee that the pipe will perform according to specifications without failure of the material itself.
5 Maintenance Free
The long service life coupled with the use of heat fusion in lieu of mechanical fittings virtually eliminates the need for maintenance on the pipe itself. Once installed, the buried ground loop will be a permanent fixture on the property for as long as there is a building to heat and cool.
Polyethylene is also corrosion resistant and inert to most chemicals. It does not promote biological growth and helps to minimize the amount of water quality-related issues typically associated with a water-source HVAC system. Alternative piping materials such as steel, copper and galvanized iron are much more demanding from a maintenance point of view.
All things considered, HDPE and PEXa are far and away the most practical choice for geothermal loopfield construction.
Notes:
1GeoPro’s Importance of Grout TC illustrates the fact that pipe is a very small portion of the overall thermal resistance in a loopfield. In fact, LoopLink PRO can be used to show that the thermal resistance of a basic HDPE or PEXa u-bend accounts for only 10%-12% of the overall total.
2Refer to IGSHPA's Design and Installation Standards for further information on pipe manufacturing methods and materials, pressure ratings, dimensions, tolerances, etc.