Following are some more detailed explanations of several basic aspects of geothermal heating and cooling.

The Heat Pump
A heat pump is a mechanical device that moves or ’pumps’ heat from one medium to another. In the geothermal case, during the heating season, heat is moved from the fluid that has warmed up in the ground to the air or water that is going to heat the house. This is done using an intermediate fluid called a refrigerant (Note: Refrigerants are now environmentally friendly as opposed to the Freon that used to be used in refrigerators). The refrigerant is compressed to a fluid using a standard compressor whilst it is circulating in a heat exchanger pipe within the air or water that is to be used to heat the house. When it is compressed it gives up a lot of heat and so heats the air or water. It is then circulated through a second heat exchanger where it is allowed to turn into a gas again. In so doing it requires a lot of heat which is taken form the ground loop fluid. It then circulates with this heat back to the compressor where it is liquefied again. The net effect is that heat is taken from the ground loop fluid and delivered to the air or water. The beauty of a ground source heat pump is that it can simply be reversed in the summer and used to ‘pump’ heat from air circulated round the house and inject it into the ground loop fluid which then dissipates it in the ground.

Ground Temperature
A geothermal heating and cooling system relies on the fact that only about 15 feet below the surface of the ground the temperature of the earth remains relatively constant. In the Denver metro area the temperature is about 51°F. In the Grand Lake or Granby area it is about 48°F. The exact temperature depends on the average surface temperature throughout the whole year. This temperature is then maintained down to a considerable depth. This means that the temperature down there is warmer than the air temperature in the winter when we require heating and is cooler than the air temperature in the summer when we require cooling. Above 15 feet the temperature varies between the constant temperature and the temperature at the surface. (Note: If you are considering a horizontal loop you must realize that it is almost impossible to get a surface loop down into the constant temperature zone so a lot more pipe is needed in the ground).

Heat Source and Heat Sink
In the winter we use the earth as a source of heat and in the summer we use it as a depository or sink for heat. A heat pump could put heat into the air and take heat out of the air (in fact air source heat pumps are quite common particularly in moderate climates). But the efficiency of the heat pump cycle is a lot lower if the heat is being taken from cold air. There is a lot less heat in cold air than there is in the warm loop fluid. Conversely heat could be put out into the hot air outside as a conventional air conditioner does but again the efficiency is much lower than if we put the heat into the cool loop fluid. Low efficiency results in high power usage by the compressor and eventually an inability to heat or cool sufficiently.

Of course all parts of the earth are not the same. Geology, weathering and ground water can cause large changes. Of particular interest to us is the thermal conductivity of the earth around the area we are using for the loop field. The thermal conductivity determines at what rate we can get heat to the pipe and hence to the loop fluid in the winter and away from the pipe in the summer. For this reason we have to make some estimate of the soil and rock type that the loop will be in when we design the system. Of even more significance to the conductivity is the presence or absence of water. Completely dry materiel will have a much lower conductivity than wet materiel. In our region the water table is often quite deep. This is a second reason why the use of horizontal loops should be considered very carefully and, if used, plenty of pipe should be put in the ground.

The Loop Field
The loop field which collects the heat from the ground in the winter and puts heat into the ground in the summer is a crucial part of the system. For this reason we have dedicated a page to it. click here.

Heating and Cooling Distribution
The systems we use to distribute the heat and the cooling around the house from a geothermal system are no different to those used by conventional heating and cooling systems. Geothermal units are made with air fans just like gas furnaces and units are made with water heaters just like boilers for hydronic heating systems. There are also units available which will supply hot water and hot and cold air. These are designed for those people who want the luxury of radiant floors but also want air conditioning in the summer. Check out the Waterfurnace range of units here.