By Rick Clemenzi on 03/06/2012
A geothermal system is one of the most efficient heating and cooling systems available. It harnesses the stable temperature of the earth as an energy source. Figure 1 shows the basic relationship of the temperature of the earth and air in the summer. For example, when it’s 90 degrees in Asheville, the ground temperature is 59 degrees. The opposite is true in the winter: When it’s 20 degrees outside, it’s still 59 degrees in the ground. A geothermal system can also provide much of a home’s hot water for free.
The economic benefit of geothermal systems is significant. In essence, the system “mines” three to four units of heat from the earth for every one unit of electricity used to run the system. That is, you pay $1 and get a total of $4- to $5-worth of heat. This is possible because the Earth is a vast solar collector, and a geothermal system uses your yard as its thermal-energy source.
Geothermal systems for heating and cooling are different from both the “high-temp geothermal” being used in the western United States, and the electricty-generating volcanic-heat collection under way in Iceland. Instead, we are simply mining solar energy stored in the upper layer of the earth. And, while these systems are heat pumps, they are also very different from the air-source heat pumps in current residential use that produce marginally warm air and cannot produce usable heat at all in the dead of winter. A geothermal system will produce toasty warm air all winter because it uses the stable 59 degree-Earth instead of the volatile, cold air.
Figure 2 shows the average economic savings that a geothermal system creates. These charts show the average savings, but it is even more dramatic if you are using propane or oil to heat your house. Both propane and oil are derived from crude oil, and they are subject to the same wild fluctuations as gas prices. A geothermal system can easily reduce your heating cost by 60 to 75 percent if you are now heating your house with propane or oil.
Figure 3 shows a more detailed breakdown of the cost savings available with a geothermal system over the various conventional heating and cooling systems typically in use. As you can see, a geothermal system is an extremely economical heating and cooling method. Along with increased operational savings, geothermal also has a much lower carbon footprint than other heating and cooling options.
Geothermal systems also operate extremely quietly. There is no noisy outdoor equipment, and no equipment exposed to the environment. This also means that the system has a 50 percent longer life expectancy than conventional equipment, and the earth portion will have a life much longer than your home, making it an infrastructure investment in your property.
The challenge with a geothermal system is that they are more complicated to install than conventional heating and cooling systems, involving either digging trenches or drilling wells (called the loopfield). Figure 4 shows a vertical slinky trench in process and a completed loopfield. The trench was dug 7-feet deep and 6-inches wide, then the dirt was fully compacted back into it. After scattering some grass seed, the yard was restored. Other techniques for loopfields include drilling wells, digging large trenches or placing loops in lakes or large ponds.
The complexity of a geothermal system also makes it more expensive to install, but tax credits help make the net cost of a geothermal system about the same as a conventional high-efficiency air source heat pump. There is a 30 percent federal tax credit and a 35 percent N.C. tax credit with some limitations. Most homeowners can have between 55 percent and 65 percent of the cost of the system paid for through these tax credits.
Even with the tax credits, you still have to pay for the upfront cost of the system ranging from $20,000 to $30,000. After tax credits, typically, your net cost is about $7,000 to $12,000. And when you factor in the 50-percent-longer life expectancy of a geothermal system (even longer for the loopfield), the long-term cost relative to other heating and cooling systems is even better.
Geothermal systems are also more challenging to install than standard heating and cooling systems. Some firms ask the homeowner to subcontract a portion of the loopfield installation directly with excavators or well drillers, potentially leaving the homeowner at risk if something goes wrong. We have also seen systems undersized that have left the homeowner without enough heat for the coldest days of winter when it is most needed.
To ensure a successful geothermal system installation, here is a list of key points to consider:
- The geothermal system must be engineered to match your home. The N.C. HVAC Licensing board requires all contractors to perform a formal heat and cooling load analysis on your house before installing any new heating and cooling system, known as a Manual J. Make sure you review this analysis with your contractor so you are confident the system’s capacity properly matches your home’s needs in the both the winter and summer.
Figure 4: Piping laid out like a “vertical slinky” is placed in trenches for this geothermal loopfield
The loopfield must be sized for efficient operation in all seasons. The key here is that the geothermal system will typically “operate” even if the loopfield is undersized. However, the operational cost savings you are expecting will fly out the door if the loopfield is undersized. The contractor should be able to show you design documents predicting a minimal operational efficiency “co-efficient of performance” of 3.8 to 4.0 in the winter. If it is less, the system is not designed correctly. An undersized system might cost less to install, but it will cost you more every season to operate throughout its life.
- All indoor portions of the loopfield piping must be fully insulated. If there is any gap in the insulation on these pipes, they will drip condensation, causing a mess and possibly a mold problem. Proper installation of geothermal and hydronic piping is essential.
- Know all the costs and risks, especially if you are expected to directly pay other subcontractors like excavators or well drillers.
- Expect the system to operate efficiently, quietly, and for many years to come.
Geothermal systems cost less to run than any other heating and cooling system, last 50 percent longer, have the lowest carbon footprint and give you a park-like environment outdoors year round. In all, a geothermal system is an excellent investment.
Rick Clemenzi is a licensed Geothermal HVAC contractor and part owner of Asheville Geothermal Inc. Rick is also involved in geothermal product development, maintaining a legislative and regulatory environment helpful to geothermal system installations, and he teaches CEU classes on geothermal systems for architects and engineers.