What is the difference in piping between DX and normal geothermal HVAC systems
What is the difference between the underground piping used for DX geothermal and all other geothermal? DX systems use copper pipes. The pipe used in almost all geothermal ground loops is high density polyethylene (HDPE). The reason for this difference is that DX systems circulate refrigerant in their pipes and conventional geothermal uses water that is commonly mixed with a non-toxic antifreeze.
HDPE is very safe. Plastic milk bottles are made of HDPE because there are no plasticizers in HDPE that could leach out and pollute the milk. The lack of plasticizers assures that this type of plastic does not get brittle over time. HDPE is a long chain molecule that is stable and durable. It is not affected by acidic conditions, and is immune to any type of corrosion. Natural gas and propane underground piping installations are almost all done with HDPE. Advanced scientific testing done on the HDPE pipe used in geothermal systems indicates that the piping will last centuries underground. HDPE does have small insulating qualities and the loop field is designed with this consideration. The primary determination of how large a loop field needs to be is the ability of the soil or rock to conduct heat to and from the pipe, not the pipe itself.
Copper is safe. For years water pipes have been mostly copper, although PEX piping, which is made from HDPE, is now more common than copper for residential water pipes. Copper pipe can have a very limited life. Copper pipe can oxidize, corrode and dissolve under certain adverse conditions. Copper pipe will corrode and oxidize when exposed to acidic conditions, and will dissolve from electrochemical, or electrolytic corrosion. I have replaced copper pipe in houses because they have corroded and dissolved to the point they were weak and developed leaks due to acidic water and/or electrolytic corrosion. In the ground, copper pipes are frequently subject to adverse conditions that will either quickly or slowly weaken them to the point they leak. According to DX manufactures, the copper piping buried in the ground has to be provided cathodic protection, using a sacrificial magnesium anode, to prevent pipe deterioration and failure. This anode must be maintained and replaced on a regular basis because as it is sacrificed, or dissolved, the copper pipe becomes less protected, until the anode can no longer protect it at all.
The water/antifreeze solution circulated in the HDPE closed loops used in normal geothermal systems is non-toxic ethanol or polypropylene. I almost always just use water in the closed loops I install because I design for maximum operational efficiency and the loop water does not freeze, even during the cold winter we just had. If a leak develops, there are no environmental concerns, and the fluid is relatively inexpensive. The refrigerant circulated in a DX system is non-toxic, but in confined spaces, such as basements, it can cause suffocation as it displaces the air since it is heavier than air. If a small amount of the refrigerant leaks, then all of the refrigerant must be removed and replaced since the refrigerant is a blend. This is expensive. There is such a large quantity of refrigerant used in DX systems, that in the case of a leak it could be dangerous to enter the basement. If a DX system is installed, it would be wise to have a refrigerant leak alarm installed in the basement to warn the occupants should a leak occur.
The conclusion I have reached is that DX systems are not well suited for residential applications and the fragility of the copper pipe buried in the ground is one reason for this conclusion. The robustness of the HDPE pipe and its ability to withstand environmental conditions in the ground for centuries is one reason I and most other professionals use this type of pipe for our geothermal HVAC systems. I will be posting more comparisons between DX and conventional geothermal systems in the future and welcome questions and comments concerning this and other posts.