GEO-Thermal Heat Pumps
How Do They Work?
Remember, a geothermal heat pump doesn’t create heat by burning fuel, like a furnace does. Instead, in winter it collects the Earth’s natural heat through a series of pipes, called a loop, installed below the surface of the ground or submersed in a pond or lake. Fluid circulates through the loop and carries the heat to the house. There, an electrically driven compressor and a heat exchanger concentrate the Earth’s energy and release it inside the home at a higher temperature. Ductwork distributes the heat to different rooms.
In summer, the process is reversed. The underground loop draws excess heat from the house and allows it to be absorbed by the Earth. The system cools your home in the same way that a refrigerator keeps your food cool – by drawing heat from the interior, not by blowing in cold air.
The geothermal loop that is buried underground is typically made of high-density polyethylene, a tough plastic that is extraordinarily durable but which allows heat to pass through efficiently. When installers connect sections of pipe, they heat fuse the joints, making the connections stronger than the pipe itself. The fluid in the loop is water or an environmentally safe antifreeze solution that circulates through the pipes in a closed system.
Another type of geothermal system uses a loop of copper piping placed underground. When refrigerant is pumped through the loop, heat is transferred directly through the copper to the earth.
In nearly all cases the loop piping is made of flexible, high-density polyethylene that is warranted for 50 years and has a life expectancy of 200 years. Its flexibility and lack of “coil memory” also make it easier to install than the polybutylene used just a few years ago. In residential installations, it’s usually 3/4 in. in diameter and is joined with heat-sealed (thermal-fusion) fittings.
When it comes to ground loops, there are two general system types–open loop and closed loop. Closed-loop systems are more common and can be trenched or bored underground horizontally or installed vertically like water wells. If you live next to a private lake, piping can even be laid underwater on the lake bed. You’d need at least 8 ft. of water over the pipe year-round but, if this option is available, it’s far less costly than an underground loop.
The second option, an open-loop installation, is not as popular as it used to be. In this case, a dedicated well with a submersible pump serves as the source of water delivered to the heat pump. Once the water is cycled through the system, it’s returned to the aquifer–typically through a second well drilled specifically for this purpose, or to a nearby stream or lake. While these systems are quite efficient, they tend to be more expensive. Water wells are costly and water quality can be a problem. You’d also have the added cost of running the submersible pump, typically $100 to $160 per year.
The most common installation is a horizontal loop. In this situation, an access pit is dug near the house, so the piping loop can be brought through the foundation wall and connected to the indoor compressor unit. From this pit, several piping loops are bored or trenched at least 5 ft. deep.
On average, a horizontal system requires 220 ft. of piping for every ton of compressor load (12,000 BTUs of heat). A newer 2000- to 2400-sq.-ft. home will require 3 tons of capacity and roughly 660 ft. of piping loop. Two pipes can be installed in each narrow trench or bore–one out and one return–so that’s 330 ft. of trench. If a backhoe is used and a 3-ft.-wide trench is dug, six pipes can be laid in one trench, allowing a shorter trench. Prices vary, but expect to pay around $600 in trenching for every ton of capacity, or approximately $1800 for a 3-ton system.
Horizontal systems have always required lots of unencumbered space, but two recent developments have shrunk the lot-size requirements a little. First, new boring technology allows the operator to accurately steer a 5-in. boring machine under and around common obstructions. Starting from a header pit near the house, the machine can dive under outbuildings, trees and septic systems, and come up 100 ft. away. When finished, two pipes, fused with a “U” fitting on the far end, are pulled through most of the bore. The tail end of the bore hole is then backfilled or packed with a dense grouting material such as bentonite clay.
The other new twist has more to do with ingenuity than equipment. Instead of laying the pipe lengthwise in the bottom of a long trench, it is coiled in 2-ft. – to 3-ft.-dia. loops like a large Slinky toy. The coils are then laid down and covered with soil. This “Slinky” method greatly increases surface exposure and substantially reduces the amount of trenching needed. With these two innovations, a horizontal system can often be installed on a lot as small as 1/4 acre.
When a property won’t accommodate even this much trenching or boring a vertical, closed-loop system is the next best option. In this case, a well driller typically drills several holes without casings 150 ft. to 200 ft. deep. The contractor then drops two pipes joined with a U fitting at the bottom into each hole and joins all pipes from all holes in a common pit 5 ft. to 6 ft. deep. Then the contractor runs a feed line and return line through the foundation wall and connects them to the compressor unit. Before filling the pit, each bore hole is grouted to meet state and local codes.
Vertical, closed-loop systems are actually more efficient, but more piping–typically 300 ft. per ton–is required. The drilling costs are also higher. Expect a vertical, closed loop to run $750 to $950 per ton of compressor capacity, or $2300 to $3000 for a 3-ton system.
A pond or lake can be used as a source for the heat-pump system. Coils of tubing are laid on the lake bed in at least 8 ft. of water.
In an open-loop system, heat source water is not continually recycled but drawn from a well. It’s then immediately pumped back to the aquifer through a second well, lake or stream.
As an alternative to the horizontal-loop system, tubing is coiled in one trench to maximize tube length over a more compact area.
In areas too small for long trenches, tubing is installed in drilled holes and connected near the surface. This system can be more efficient than horizontal installations, but more expensive as well.