Polyethylene is the most common piping material used in geothermal heat exchangers. It is flexible and can be heat-fused to form joints stronger than the pipe itself.
Use high quality pipe and insist on a 50-year-life at 100 psi pressure. (These system typically operate at a maximum of 40 psi.) PVC pipe is forbidden for the buried portion of the piping.
Wall thickness and pipe strength is defined in terms of a material designation and a Schedule Rating (SCH) or a Size Dimension Rating (SDR). For the ground heat exchangers, two pipes are recommended:
- Polyethylene: PE3408 SDR 11 / PE3408 SCH 40
- Polybutylene: PB220 SDR 13.5 / PB2110 SDR 17
The designer should select the thinnest wall pipe for loops to promote heat transfer, and thicker wall pipe should be used for headers to provide structural strength.
The selected pipe diameter should be:
- Large enough to keep the pumping power small, and
- Small enough to cause some turbulence inside the pipe.
Keep in mind that large-diameter pipe is more expensive, takes more antifreeze (if required), and more difficult to handle and install.
For example, a typical design for a three-ton system may use 1-1/4 or 1-1/2 inch pipe headers with 400 feet of ¾- or one-inch pipe in each parallel loop.
Pipe depths for horizontal loops are typically three to six feet. A depth of five feet is most common. In the north, excessive depth will reduce the ability of the sun to recharge the heat used in winter. In the south, the depth must be sufficient so the high temperature of the soil in later summer will not seriously affect system performance.
Antifreeze is used in northern climates to prevent winter freezing of the circulating water. A fluid with a freezing temperature below 32 degrees must be used if the evaporator refrigerant temperature approaches 32 degrees in winter.