Geothermal heat pumps are the stars of the energy efficient heating and cooling world. For years, they have been at the top when it comes to providing their owners with comfort and substantial energy savings. Unfortunately, they are also at the top of the heap when it comes to installation costs…they are substantial.
Geothermal refers to the fact that these heat pumps use the earth as their heat and cooling source. As mentioned in the mini-split article elsewhere in this issue, air source heat pumps use compressors, condensers and refrigerant to move heat from one spot to another. In the summer, they pump the heat inside the home outside. In the winter, the heat in the outside air is captured and pumped inside.
The geothermal system uses the constant temperature of the ground to handle its heat pumping chores. Once you get about 36” below the surface, the temperature remains pretty constant between 50 and 60o, or averaging 55°. The depth and temperature will vary depending upon where you live.
To take advantage of this stable temperature, the geothermal system runs piping filled with a heat exchange medium (usually water) into pipes buried in the ground. These pipes can run vertically or horizontally and form the water loop. The choice is generally dictated by site constraints. For example, a farmhouse with lots of land can use horizontal piping. A big ditch is dug then the piping laid out. This piping is typically in a big coil so it look like a giant Slinky.
In a residential neighborhood with smaller lots, the vertical approach is generally preferred. Holes are drilled and the piping installed as if a water well was being created. The number and length of pipes is dictated by the size of the home being conditioned. The water loop is where the cost of a geothermal heat pump system jumps up. Installing the loops is pricey.
Here’s how it works. The compressor unit sits outside just like a regular heat pump. Inside the home, the A-coil sits in the ductwork, again just like a regular HVAC system. Fans blow air over the A-coil, absorbing either heat or cold from its tubing.
Let’s look at the heat mode first since its winter right now. The pump moves the water in the tubing out into the water loop. There is picks up warmth from the ground around it. The warm water goes back into the system where that warmth is transferred to the refrigerant via a heat exchange mechanism. The heat in the refrigerant is increased by compressor action. This hot refrigerant is pumped into the A-coil where air flows around it, removes the heat and warms your home. The now cool refrigerant goes back to the compressor unit to repeat the process. Because the temperature of the ground is so high relative to the outside temperature, the system can supply heat very efficiently.
When you need to cool your home, the process reverses. The fluid in the piping flows through the earth and is cooled by the 55o temperature around it. This flows into the compressor where it cools the refrigerant. The compressors increase the refrigerant’s coolness and pump it to the A-coil. Warm air flowing over the coil absorbs the cool from the refrigerant and fans send it to your rooms. The now warm refrigerant carries heat captured from your home out to the water loop where it uses the ground temperature to cool it and repeat the process.
This is probably the most expensive residential system you can buy. Still, the investment will be paid back over time and Federal tax credits are available through the end of 2016 to offset some of the cost. If you plan to stay in your home for quite a while or are building a new one, I recommend talking to a qualified geothermal contractor to see how the system would work in your situation. To find the accredited contractors in your area, visit this site, http://www.igshpa.okstate.edu/directory/.