Buildings
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Ground Source Heat Pumps by Arnie Katz
Q: My cousin, who drills wells for a living, is all excited about ground-source heat pumps. I'm considering this kind of system for my new house; but the more I read, the more confused I get. Do these systems make sense in North Carolina or only in the frozen north or the deep south? A: Remember playing with a Slinky as a child? Imagine making a Slinky out of flexible pipe, digging a long trench in your back yard, putting the Slinky into the trench, filling it back up with earth, and running water through the Slinky. Hook the Slinky up to a heat pump, and bingo! You have a ground-source heat pump. Most ground-source systems use straight pipes rather than "Slinkies". Sometimes the pipes go deep into the ground and sometimes they even go into a pond or lake, but the basic idea is the same. Ground-source heat pumps, also called water-source heat pumps or geothermal heat pumps, have been around for a number of years. The demonstration solar house over at NC State, for instance, has a ground-source heat pump as the back-up heating system and the air conditioning system. That system was installed in 1980, and the last time I checked, it was still working fine. There are currently several dozen certified contractors in North Carolina. Across the country, there are over 100,000 systems installed and operating in both residential and commercial buildings. Ground-source heat pumps work the same way as air-source heat pumps (the kind we're all used to), but instead of transferring heat from the air outside to the air inside in winter, they transfer heat from the ground to the air inside the house. Since the ground is a lot warmer than the outside air during the winter, a lot more heat can be captured with less energy, making the system more efficient. In North Carolina, once you get a few feet underground, the temperature rarely drops below 50 o F, so a ground-source heat pump can always find heat. Since a heat pump "moves" heat from one place to another, rather than manufacturing it, as a toaster or a furnace does, it's very efficient as long as there is heat for it to move. Similarly, it takes less energy to "pump" heat into the cool ground in the summer than it does to pump it into the hot air. A ground-source heat pump is often 150% to 200% more efficient than an air-source system. Over the long run, savings can be substantial. The U.S. Department of Energy, for instance, estimates that ground-source heat pumps save anywhere from $300 to $1000 per year on typical residential energy bills. The higher first cost — which runs from $2,000 to $4,000 — is paid back in energy savings in three to eight years. Other things people like about them are that they tend to be very quiet (no outdoor unit) and they tend to last longer than conventional equipment, since nothing is exposed directly to the weather. They can also provide most of your hot water at lower costs than other methods (except solar). Since they use less energy than other systems, they also cause less pollution. |
