A Warning About Direct Expansion Systems

Direct Expansion (DX) systems (also sometimes given the misnomer "Direct Exhange") work differently than traditional Ground Source Heat Pumps (GSHPs).  In a traditional GSHP, heat is extracted either directly from the ground water (open loop) or from a glycol solution that pumps through closed loops of high density polyethylene pipe (HDPE) that is heated and cooled by the ground.  The extraction process utilizes an intermediate compressor loop to "pump" the heat from either the water or the glycol into the air that will then circulate throughout the home.  There are therefore three separate "loops" involved in the process; the ground loop, the refrigerant / compressor loop, and the loop that delivers the heat to the home, usually via air or hydronic (water) circulation.

A DX system, on the other hand, only has two loops.  This increases its efficiency, but it comes at a cost.  In these systems, the ground loop and the compressor loop are the same.  They work by pumping refrigerant through the ground loop which is directly connected to the compressor.  They use copper pipe for the ground loops instead of polyethylene.  Copper pipe is a better conductor of heat than polyethylene, and it's somewhat cheaper to install because less excavation is required. 

However, the same concepts that give these systems their economic advantages can also make them environmentally hazardous, and potentially much more expensive in the long run.  They're a much newer technology and so they lack the time proven track record of traditional heat pumps.  For these reasons, we at Varna Energy Solutions do not install DX systems.

Critics of this technology point out that copper pipe (or any metal for that matter) will rapidly degrade when in contact with soil whose pH is sufficiently low.  Acid rain, salt from the ocean or street de-icing, and septic systems can all contribute to lower pH values in the soil.  The danger of using a direct exchange system is that the copper pipes in the ground will inevitably break down over time (it's just a matter of when), and when they do there may be a rather serious environmental contamination as the refrigerant that's pumped through the ground loops leaks into the soil.  The amount of refrigerant lost to the environment in these situations can far exceed that lost from a traditional AC leak. 

Even if a (relatively speaking) environmentally friendly refrigerant is used, the pipes still must be dug up and repaired which can be a very expensive process.  And even the most environmentally friendly refrigerants (HFCs) still likely contribute to global warming.  These systems therefore require a far more rigorous site evaluation.  A simple pH test in one location and at one depth level is insufficient, particularly if you have a septic tank.  If you choose to gamble on one of these systems, make sure you take soil samples from multiple site locations and at multiple depths to ensure the longevity of the copper pipes.

Another point worth making is that these systems also require a far more rigorous design process, which if not properly conducted by a highly qualified engineer can lead to dead grass, buckling of the ground, or freezing / rupturing of your water pipes, which would require they be dug up and repaired.  According to the Canadian government (CanREN ):

"the systems require a large amount of refrigerant and, because the ground is subject to larger temperature extremes from the direct-expansion system, there are additional design considerations. In winter heating operation, the lower ground-coil temperature may cause the ground moisture to freeze. Expansion of the ice buildup may cause the ground to buckle. Also, because of the freezing potential, the ground coil should not be located near water lines. In the summer cooling operation, the higher coil temperatures may drive moisture from the soil."

The bottom line: saving a few bucks on the installation is not, in our opinion, worth the added risk.  If, however, you choose to go with a DX system, make sure the installer has not only installed these systems before (he could just be doing them incorrectly over and over again), but that he is a qualified engineer who understands the design considerations.