DX vs. Water-source

I've received a couple questions as to the differences in installation between a DX geothermal setup and a water-source geothermal system. Although this is, by no means, a complete discussion of the installation of either product, this should give you a general idea of what to expect.

Water-source geothermal systems come in two flavors: open loop and closed loop. For our purposes, let's ignore open loop water-source geothermal systems as they are becoming extremely difficult to install thanks to state and federal regulations dictating the collection and return of groundwater. Instead, we'll talk about closed-loop systems, the most common geothermal HVAC solution found in the United States.

Unlike a DX system, heat transfer in a closed loop water-source system occurs between the ground, water in underground loops, and the refrigerant. The heat pump unit itself contains a water to refrigerant heat exchanger which allows the refrigerant in the heat pump to trade heat with the water from the ground loops. On the other side of the heat pump, refrigerant trades heat with air in a refrigerant to air heat exchanger, much like a DX system.

The major difference in installation (and in operation, really) is the ground loops. Closed loop water-source systems use high-density polyethylene piping filled with a water and antifreeze solution instead of copper tubes filled with refrigerant. However, HDPE piping and water are less effective at transferring heat than refrigerant in copper piping, so substantially more piping must be installed to achieve the same tonnage rating for a heat pump. Thus, installation costs tend to be quite a bit higher with water-source systems than DX systems simply because drilling and excavation costs are greater. While there are advantages to having a water/antifreeze solution in the ground over refrigerant (it's generally preferable to leak water and antifreeze than refrigerant), the lower heat transfer coefficient of water combined with HDPE piping reduces system efficiency while requiring a water to refrigerant heat exchanger, further lowering the coefficient of performance.

3-ton closed loop water-source geothermal system HDPE ground loop installation

In short, a DX system requires less drilling and excavation while providing higher efficiencies and lower costs of operation when compared to closed loop water-source systems. This isn't to say water-source systems are a bad option...they, too, provide substantial environmental, financial, and efficiency benefits over traditional heating and cooling solutions.

5-ton DX geothermal system copper ground loop installation (panorama)

Note the differences in pit depth and overall ground loop size, even when comparing a 3-ton closed loop water-source system to a 5-ton DX system.


The Green Home

I completely forgot about one of the other solar-centric items Bill Nye discussed...stranger yet, it's an item I have in my own home, so it's particularly fitting that I should forget to discuss it.

Solar tube lighting. Imagine, instead of using lights during the day in your home, having a bright light provided by the sun, even in rooms without windows or with otherwise poor lighting. The concept is simple, but simplicity is elegance, particularly when it comes to reducing your carbon footprint.

Essentially, a small domed window installed on the roof is connected, via a mirrored tube, to a "light fixture" in the ceiling. Because the tube is mirrored and the dome on the roof is a Fresnel lens, large quantities of sunlight can be funneled into your home, easily lighting most rooms during the day and further reducing home energy needs. Plus, unlike typical skylights, solar tube lighting uses very small openings in your home, reducing energy loss through poorly insulated skylight windows and increasing the efficiency and effectiveness of your home's HVAC system.