With the climate crisis becoming ever more urgent, and the price of fossil fuel-based heating systems rising all the time, geothermal heat pumps are becoming a more and more attractive option. But which type of pipe is normally used, and is it the best available?
The type of pipe used for geothermal heating is almost always made of High-Density Polyethylene (HDPE), and for a good reason. HDPE, and other polyethylene forms, are the best materials for heat pumps, especially if the system has a cooling capacity since this means higher temperatures.
There is a pretty simple answer to this question, but there is still a lot more to talk about when it comes to the different materials that have been used and the different configurations of the pipes. Let’s get started!
What Is Geothermal Heating?
Geothermal heating is a form of renewable heating that transfers the latent heat in the ground to homes for space heating and water heating. The name comes from ‘geo,’ which means ‘earth’ and ‘thermal,’ which means ‘heat.’ Geothermal heating is extremely sustainable since it does not require the burning of any fossil fuels or biomass to generate domestic heating. The device most commonly used to provide domestic geothermal heating is called a ‘heat pump.’
In a standard geothermal heat pump, a mixture of water and refrigerant is continually pumped through the pipes, where it heats up. This mixture is then used to boil pure refrigerant, which is boiled then compressed. This is an ‘indirect’ heat exchange since the warmth of the ground does not directly heat the refrigerant. HDPE and PEX are the only materials formally approved for standard geothermal systems by the International Ground Source Heat Pump Association.
Types of Geothermal Heating System
There are a few different setups you can have in a geothermal heating system, and which one you have will, to an extent, determine which type of pipe is best. The main two types are closed-loop and open-loop, but there are further classifications within these categories. Pipes can also be laid vertically or horizontally, with the additional option of using a ‘slinky’ configuration for the pipes.
Closed or Open Loop
In a closed-loop system, the heat transfer fluid (HTF) never leaves the system but is instead reused. This makes it a more environmentally friendly option. An open system, which is usually used in a body of water, pumps the HTF back into the environment. In a closed-loop, the heat transfer fluid in question is a refrigerant that evaporates at relatively low temperatures, whereas in an open-loop system, the fluid is simply water.
Vertical, Horizontal, and Slinky
The piping is laid horizontally 6-10 feet under the ground in a horizontal heat pump system. This takes up more land but does not require expensive drilling equipment. Vertical systems, on the other hand, require deep drilling but take up much less land. One way to reduce the amount of land used by a horizontal system is to employ a ‘slinky’ configuration of piping.
This means that you use pipes curved into a spiral, similar to a slinky toy. This increases the surface area available for heat transfer without the need for taking up huge amounts of land. Slinky configurations have become more and more popular in recent years since they allow you to install horizontal piping in an area as small as ¼ of an acre. Slinky configurations will likely become the norm in the next few years.
Another type of system is a Direct Exchange (DX) geothermal system, in which copper pipes are used. In this system, the refrigerant itself is continually pumped through the pipes. While this can be very efficient due to copper’s high thermal conductivity, the constant supply of pure refrigerant can cause the copper pipe to corrode very quickly, which can lead to a total failure of the system.
Why Use HDPE for Piping?
By far, the best material to use for geothermal piping is HDPE, for several reasons. Here are some of the best reasons to use HDPE rather than other materials:
- High chemical resistance. HDPE is extremely resistant to chemical degradation, which means it will last a very long time in the ground. You will most of the time get a 50-year warranty on HDPE piping, but it should last up to 100 years.
- High-pressure rating. It is recommended that you only use HDPE for vertical wells since the pressures can get very high. HDPE has the best pressure rating of any plastic piping used for heat pumps.
- No mechanical fittings. HDPE can be ‘heat fused,’ which is a simple process for joining sections of pipe without mechanical fittings. Heat fused joints can be even stronger than the rest of the pipe and are 100% leak-resistant.
- Thermal conductivity. HDPE has the best thermal conductivity of any material used for geothermal heat pump piping, meaning that it is the most efficient at collecting heat from the ground.
What Are the Other Options for Piping?
While HDPE is considered the top of the line, it can be quite expensive. As well as there being other types of polyethylene that can be used instead, PVC or CPVC can be used for certain parts of the system. However, it is not recommended to use anything other than polyethylene for the sections of pipe that are underground or exposed to high temperature or pressure. Here are some of the uses for other options for piping in heat pump systems:
Crosslinked Polyethylene (PEX) is the next best thing after HDPE and can operate at temperatures of up to 99℃ (210.2°F). The main problem with PEX is the price. It can cost up to double what you would pay for HDPE, which has better thermal conductivity. Another downside is that PEX is not connected using thermal fusion, which means you will have to use a mechanical or chemical connection, which might not be as strong as a thermal connection.
PVC can be used for certain parts of a heat pump system, for example, transferring the HTF from the condenser to the ground loop and back. However, if used for the ground loop, PVC could easily become degraded by chemicals in the soil and end up leaking refrigerant into the ground, wasting money and harming the environment. PVC is about 40% cheaper than HDPE and has a thermal range of -18 to 60℃ (-4 to 140°F). PVC can be used for interior piping.
As mentioned above, it is also possible to use metal piping if you are using a direct exchange (DX) system, although this can result in high costs and difficult installation. Copper has an extremely high thermal conductivity rating, much higher than any plastic could ever achieve.
Because copper is now less widespread, it is becoming costly to use it in the quantities required for a heat pump system. Copper can also corrode quite quickly when exposed to pure refrigerant for extended periods.
The main take-home should be that HDPE is the ideal material to use for geothermal piping. It is the most commonly used for an excellent reason. You will get the best bang for your buck when it comes to thermal conductivity, and the pipes should last so long that you will not have to replace them over the course of a human lifespan.
Copper is another great option for DX geothermal systems, but the constant supply of refrigerant can cause the pipes to corrode, resulting in a catastrophic failure.
- Copper Development Association Inc.: How Do Copper DX Geothermal Heat Pumps Work?
- Dandelion: Geothermal Ground Loop Frequently Asked Questions
- Dandelion: Direct Exchange Geothermal (DX Geothermal)
- Dig the Heat: Geothermal Heat Pump Loop Configurations
- ResearchGate: Pipe Materials for Borehole Heat Exchangers
- Geoconnected: Geothermal Loops: 5 Reasons for using HDPE & PEXa
- Geothermal Pros and Cons: Geothermal Pipe – What to Use and What to Stay Away From
- Geothermal Heat Pump Systems: Slinky Coils
- Geothermal Heat Pump Systems: Piping Selection
- Houzz: CPVC Pipe for Geothermal
- Md. Hasan Ali, Keishi Kariya, and Akio Miyara: Performance Analysis of Slinky Horizontal Ground Heat Exchangers for a Ground Source Heat Pump System
- OPUS: Material Polyethylene (PE) Pipe
- Oxford International Journal of Low-Carbon Technologies: Comparing the Thermal Performance of Horizontal Slinky-Loop and Vertical Slinky-Loop Heat Exchangers
- Plastics Pipe Institute: Geothermal Ground Loop Piping Systems
- Plastics Pipe Institute: Plastic Piping Materials for Ground Source Geothermal Heating and Cooling Applications
- Plastic Pipe Institute: Plastic Piping Materials for Ground Source Geothermal Systems
- Viessman: How Does a Geothermal Heating System Work?