Ask the Expert: What to consider before installing geothermal?
Before installing a geothermal heating and cooling system into a new home, there are many factors to consider.
So, ProudGreenHome turned to one of its Approved Contributing Experts to ask for tips for homeowners on this topic.
Ted Clifton, founder of Zero-Energy Plans LLC, and CVH Inc:
There are lots of factors that will enter into the equation of whether or not to consider a ground source heat pump (GSHP) for your heating or cooling needs. I will outline these factors below, in their relative order of importance
The first may surprise you: Does your home have a large enough heating and/or cooling need to justify the extra cost? I would always ask that question before digging into any of the more location-specific questions. The smallest GSHP units are 1.5 tons of capacity (about 18,000 btuh), and some manufacturers do not have anything under 2 tons (24,000 btuh). While the efficiency of these units is at least one and a half to two times higher than the next most efficient systems available, the added cost would not be justified if you either have a super-efficient building envelope, a very small house, or if you are in a very mild climate zone. If your design degree day calls for at least 1.25 tons (15,000 btuh) of heating or cooling, you could consider a GSHP.
Second, you need to decide what you need to do with your system. Are you heating, cooling, or both? Are you also going to heat your domestic hot water with the GSHP system? These decisions will help you select the right type of equipment. If you do not have a cooling load, (or if you are going to deal with cooling in another way) you can consider an in-floor radiant heating system. If you do have significant cooling loads, and want to use your GSHP for cooling and heating, you will probably choose a system with a water-to-air heat exchanger. These units can achieve spectacular COPs in cooling mode. By adding a desuperheater, you can provide domestic hot water at 400 percent efficiency while the unit is in heating mode.
The next question to consider is your soil conditions, and the space available for a shallow system, versus a deep-well system. The cost of a shallow system is usually far less than the cost of a deep-well system, but you will need between 85 and 150 feet of trench per ton of capacity, depending on your climate zone, and the type of system you are using. A coil-loop system needs a wider trench, but it can be shorter; a straight-line trench will be longer, but can be narrower. I have found that a two-ton system will fit on many suburban lots, by following the perimeter of the lot with the trench. Start at the middle of the back, and wrap each way around towards the front, stopping at the driveway entrance. If you can go 150 feet in each direction, you can do a shallow system. Depending on the space between the house and the side lot lines, there may not be enough room to put the backfill until the system is buried, so we sometimes have to install the ground loops before the foundation goes in. A GSHP retro-fit would be very difficult, if not impossible, with narrow side yards.
If you are in a rural location, with an on-site septic system, the shallow system can often be integrated beneath the septic system. Design the two systems together, like opposing forks. The septic system will be above the GSHP system, with its trenches half-way between the GSHP trenches. The septic system will keep the ground moist, assisting in the transfer of energy to the ground loops. Because the two will not be directly over top of one-another, if you ever need to repair one system, you will not have to dig up the other.
Any shallow system requires suitable soils. Gravel is good, clay is better. Sand is somewhere in between. You will need to know how deep to dig. In some areas, the soils stay around 50 degrees yearround at as little as 5 feet of depth. In other places, typically where the winters are colder, the system will need to be twelve or fifteen feet deep to be in soils warm enough to function at higher COPs (coefficient of performance). This can run the cost of the shallow trench system up to where a deep-well system should be considered. Other factors that can move you away from shallow trench systems would be solid bed-rock very near the surface. While you could consider blasting a ditch-line, it will probably be cheaper in this case to use a deep-well system. The temperature of the water and the depth of the aquifer will determine the efficiency of the system. Make sure you know this information before designing the system. Without good information, you can easily make a very expensive mistake.
We have installed a great number of ground source heat pumps, all shallow systems, and all are out-performing our expectations. The units I have heard of that failed to satisfy their owners had one or more of the following problems:
- The ground loops were too short.
- The ground loops were installed too close together, or in too small an area.
- The ground loops were installed too shallow for the soil temperatures.
- The system was not correctly sized for the house.
- An in-floor system was used to cool the house, causing condensation on the floor.
You can avoid all of these problems by hiring an HVAC contractor who has been certified by the International Ground Source Heat Pump Association.
Teena Hammond Teena Hammond has published more than 2,000 articles in People and W magazines, Women's Wear Daily, and in dozens of newspapers and books. She also wrote a home improvement, remodeling and decor column that ran in Gannett newspapers nationwide. She's interested in all things green and would love to hear from you with your story ideas.