A few weeks back we had a very interesting workshop discussion that included the opportunity to hear from two home-owners with ground source heat pumps, also known as a geothermal system.
The basic idea is that the earth’s temperature anywhere below about 6 feet is a fairly constant at around 50F. It doesn’t matter if the outside temperature is 0F or 90F, the ground and the ground water temperature will be about 50F.
With a geothermal system, there are a couple of different mechanisms that can be used to transfer the heat from the ground to your house. One way is to pump water from a deep (and large) aquifer up to the house, extract the heat, then send it back to the same or another near by aquifer. Another method is to use a closed loop of transfer fluid which picks up heat as it is circulated down and back up a very deep well.
Once the 50F source is in the house the heat pump does the rest of the work to get to the desired house temp. Since we extract heat from the source, the return water or transfer fluid is colder than the source by about 10 degrees.
Assuming the ground source of water is very large, or the well is very deep, returning the water a little cooler has no appreciable effect on the temperature of the large body of water underground or the earth itself.
One of the homeowners has been very happy with their geothermal system and has saved quite a bit of money compared to the fossil fuel system they had before.
The other homeowner’s system was installed 15 or 20 years ago, and the body of water that supplies the heat source has cooled down significantly over that time, so it is actually too cold to provide heat to his house. One reason this could happen is if the water source is too small. Another reason might be that the well for the geothermal system is too shallow.
It is very difficult to know when the well is drilled how much water is in the aquifer. The well company can do a water flow and level test which can be a good indicator of the system’s capacity. For a closed loop system the depth of the well determines how much heat you can get.
You can learn more about these systems from many sites online. Here is one from ClimateMaster, a manufacturer of geothermal equipment: